ANLYTICAL ATOMIC SPECTROSCOPY IN

FUME AND NON—FLAME CELLS

by

LESLIE COLIN EBDON, B.Sc., A.R.C.S., A.R.I.C.

A Thesis submitted for the Degree of Doctor of Philosophy in the University of London

September 1971

Chemistry Department,

Imperial College of Science and Technology,

London, E.W.7. ABSTRACT

The advantages and uses of non-flame, atom cells in analytical atomic spectroscopy are reviewed. The determination of iron and manganese using a flame cell and a non-flame cell is described. In particular, the determination of sub-microgram amounts of iron and manganese by atomic fluorescence spectroscopy, in the air-acetylene flame using micro-wave excited electrodeless discharge lamps as sources, is reported and compared to atomic absorption and atomic emission methods. The determination of sub-nanogram amounts of iron by'atomic absorption, and manganese by atomic absorption and atomic fluorescence, using a carbon filament atom cell is also described. late effects of concomitant elements on such determinations has also been investigated. An analytical method for the determination of traces of iron in milligram amounts of plastic is proposed, as are a number of suggestions for future work. ACKNOWLEDGEMENTS

The work in this thesis was carried out in the

Chemistry Department of Imperial College of Science and Technology betWeen Ocobter 1968 and July 1971. It is entirely original except where due reference is made and no part has been previoubly submitted for any other work.

I wish to thank my supervisors, Professor T. S. Test and Doctor G. F. Kirkbright for their advice, encouragement and guidance throughout the course of this work. I am also grateful to other members and colleagues of the Analytical Department for many helpful discussions and suggestions.

I would like to thank the Ministry of Defence (Aviation

Supply) for their financial support and the Royal Aircraft

Establishment, Farnborough for the preparation of the samples of carbon—fibre used in this work.

Finally, I would like to thank my wife, Judith, for patient assistance with the preparation and typing of this manuscript.

Z. Cipmzer.... ERRATA

Page (iii), line 3 - for Ocobter read October. Page 1, line 2 - for contunuous read continuous. Page 30, line 10 - for comparitavely read comparatively. Page 30, bottom line - for Massman read Massmann. Page 235, line 18 - for know read known Pate 243, line 11 - for filaemnt read filament CONTENTS

ABSTRACT

ACKNOWLEDGEMENTS

CHAPTER 1 Introduction

CHAPTER 2 Experimental Parameters and Description of Apparatus 67

CHAPTER 3 The Determination of Iron Using a Flame Cell 90 CHAPTER 4 - The Determination of Manganese Using a Flame Cell 115

CHAPTER 5 The Determination of Manganese Using a Non—Flame Cell 144 CHAPTER 6 The Determination of Iron Using a Non— Flame Cell 191

CHAPTER 7 Conclusions and Suggestion for Future Work 223::=

BIBLIOGRAPHY 264 CHAPTTR 1

INTRODUCTION

1.1. History

Sir Isaac Newton (1642-1727) is rightly regarded as having founded the science of spectroscopy when in 1666 he analysed the contunuous solar spectrum. His simple description of his apparatus is famous:

"Having darkened my chamber, and made a small hole in the window shuts to let in a convenient quantity of the sun's light, I placed my prism at his entrance, that it might thereby be refracted to the opposite wall". In 1802 Wollaston repeated Newton's experiment and found that if the sun—light passes not through a circular aperture but through a slit, the solar spectrum is intersected by several dark lines. This discovery was not, however, considered important until fifteen years later Fraunhofer working independently from Wollaston again found these dark lines in the sun's spectrum. These were used as the first precise standards for measuring the dispersion of optical glasses. Later in 1823 Fraunhofer constructed the first transmission gating and he was thus able to measure the exact wavelengths of the lines. In his honour these dark lines are called Fraunhofer lines.

Although early workers had noted the colours imparted to diffusion.flames of alcohol by metallic salts, it was not until the development of the premixed air/coalgas burner of Bunsen that in 1859 2.

Kirchhoff established the origin of the Fraunhofer lines. Kirchhoff showed that the chemical composition of a substance can be determined from its spectrum. Working with Bunsen, Kirchhoff developed a spectroscope of high sensitivity and demonstrated that the visible lines were not due to compounds but to elements. Together they gave numerous examples of 1 2 the use of spectra for determining alkaline metals in a flame ' and thus

Bunsen and Kirchhoff are considered to be the founders of spectrochemical analysis.

Kirchhoff established the presence,of certain elements in the sun's atmosphere from the fact that the emission lines for these elements coincided with the Fraunhofer lines in the solar spectrum. It was in the field of astrophysics and astrochemistry and later in more fundamental spectroscopic and atomic studies, that atomic spectroscopy found its first applications. However, in 1928 Lundegardh with the publication of a series of papers3, in which a pneumatic nebuliser together with an'air—acetylene flame was used for atomic emission spectrometry, revived interest in analytical atomic spectroscopy. Instruments became available in the mid—nineteen forties mainly for the determination of alkali metals with their easily excited resonance lines, and more recently the range of elements determinable by atomic emission spectroscopy has been extended to encompass most of the periodic table.

Although the basic principles of atomic absorption spectro— scopy, the measurement of the absorption of radiation by discrete atoms, 1 were established by Kirchhoff in 1860, and many major contributions to the theory of atomic absorption spectroscopy were made by physicists and 3. astrophysicists, many of these being summarised in an excellent treatise by Mitchell and Zemansky4, it was not until the latter half of this century that a general laboratory technique was devised. In 1955 6 Alkemade and Milatz5 and Walsh independently published papers on the analytical usefulness of atomic absorption spectroscopy, although Walsh had previously in 1953 demonstrated a complete laboratory apparatus in a patent specification7. Later, in 1957, Walsh and his colleagues published the first of many results on the experimental developments of the technique

Atomic fluorescence spectrometry, the measurement of radiation from discrete atoms that are being excited by absorption of radiation from a source which is not seen by the detector, was first reported by Wood9 in 1905 when he succeeded in exciting fluorescence of the D lines of sodium vapour. Again the early fundamental vork of spectro— scopists is summarised by Mitchell and Zemansky4. In 1962 Alkemade used the atomic fluorescence of sodium in flames in a study of the excitation and deactivation of atoms and he was also the first to point out the analytical applications of this technique. Following Alkemade's suggestion, Winefordner and his co—workers outlined the theoretical 11 basis of an analytical method and published in 1964 the first of many 12 papers reporting experimental results .

As the theory and methodology of the three techniques of analytical atomic spectroscopy i.e. atomic emission spectroscopy, atomic absorption spectroscopy and atomic fluorescence spectroscopy has been 13-26 comprehensively described in the literature4'6'11' , in some cases 4.

together with a fuller summary of the development of the techniques, in

the next section an attempt will be made only to ,summarise some of the more

basic points.

1.2. Theory and Methodology

The emission and absorption of light are associated with

and characteristic of the processes of transition of atoms from one steady

state to another. If we consider the case of two steady states i and j,

correspondinF'toenergiesofE.and1 E. where E. j> E.,1 then the transition i j results in the absorption of light and the transition j i in

the emission of light with a characteristic frequency .31v.. where:

vji— = E. — E. 1.1 h

where h is Planck's constant.

Einstein's quantum theory of radiation defines three types

of transition between levels i and j:

1) Emission (j --> i) transitions from the excited state

into a lower energy state, taking place spontaneously;

2) Absorption transitions (i j) taking place in

response to the action of external radiation with a frequency v..;

3) Emission transitions (j i) stimulated by external

radiation With a'frequency v. . J i This third type of transition has not yet been used directly in spectrochemical analysis and the three techniques of analytical atomic 5.

spectroscopy are based on the first two types of transition. Thus

the analytical techniques are closely related. In atomic absorption

and atomic fluorescence spectroscopy the atoms are excited (i j

transitions) by means of an external light soltrce containing radiation

characteristic of the analyte atoms (in this case ). In atomic ji absorption spectroscopy the fraction.of the radiation absorbed by the

analyte atoms as a result of radiational excitation is monitored, whereas

in atomic fluorescence spectroscopy a portion of the radiation resulting

when a fraction of the excited atoms undergo radiational deactivation is monitored. In atomic emission spectroscopy the analyte atoms are excited by means of collisions. with flame gas molecules and a portion of the rad— iation emitted when a fraction of the excited atoms undergo radiational deactivation is measured. The basic instrumental systems used in the

:three techniques is shown in Figure 1.

A detailed study of the rules governing the transitions which give rise to observed atomic spectra would be out of place in this thesis and the reader is referred to a number of texts upon the subject, such as that written by White27 in which the results proved by quantum mechanics are combined with a simple vector model of atomic structure. A most suitable comprehensive discussion of spectroscopic theory and spectro— scopic notation has been given by Mavrodineanu and Boiteux:14 and this will provide the reader with the basic theory necessary for a critical understanding of the experimental work to follow. Thus we will proceed directly to the theory of quantitative atomic spectroscopy.

6.

Figure 1 Basic Instrumental Systems Used in Analytical Atomic

Spectroscopy

A 0

Atom Monochromator Read-out

Cell and Detector System

Atomic Emission

Source Atom Monochromator Read-out Cell and Detector System Atomic Absorption

Atom Cell

MonochromatOr Read-out and Detector System

Source

Atomic Fluorescence 1.2.1. Atomic Emission Spectroscopy

The probability of transitions from given energy levels of a fixed atomic population was expressed by Einstein in the form of three coefficients. These are termed transition p:.obabilities and are usually written A.., B..,ijji B.. which refer to spontaneous emission, absorption and stimulated emission respectively. They-can be considered as representing the ratio of the number of atoms undergoinr a transition to the number in the initial level. The intensity of a spontaneous emission line is related to A.. by the equation: ji

I = A...h P..N. 1.2 em ji ji

When a system is in thermodynamic equilibrium the level populationi.e.thenumberea:,:msN.in the excited state is given by the Boltzmann Distribution Law:

—(Ej/kT) 1.3 N . = N e

o

is the number of atoms in the ground (unexcited state) with an where No energy Eo=0 and gj and go and the statistical weights of the jth and ground states respectively. Thus:

N. = g, exp [—E:i /kT] 1.4 1T'o exp [—E0/kT]

If we e ress N the total number of atoms resent as the sum of the 8. population of all levels i.e. N

N.= g, exp [—E./kT] -2, A 1 N z g, exp [—Ei/k1 J = g exp [ElkT] 1 . 5 F(T)

where F(T) is known as the partition function.

If self—absorption is neglected for a system in thermo— dynamic equilibrium:

I = A..h .vii exp [—E .AT] • em ji 16 F(T)

Thus the intensity of atomic emission is critically dependent on temperature. It also follows that when low concentrations of analvte atoms are used (i.e. when:-self—absorption is negligible) the plot of emission intensity against sample concentration is a straight line.

1.2.2. Atomic Absorption Spectroscopy

Atomic absorption follows an exponential relationship for the intensity I of transmitted light to the absorption path length 1 which is similar to Lambert's law in molecular spectroscopy: 9.

—k v 1 1.7 I = I0 e

where I is the intensity of the incident light beam and k v is the o absorption coefficient at the frequency v . In quantitative spectroscopy a parameter, absorbance A is defined as:

A = loF 1.8

thus from equation 1.7 we obtain the linear relationship:

A = ky I log e = 0.4343 k y I 1.9

Atomic absorption corresponds to transitions from lower to higher energy states and therefore the degree of absorption depends on the population of that lower level. When thermodynamic equilibrium prevails the population of a given level is determined by Boltzmann's law

(equation 1.3 above). As the excited level populations are generally small compared to the ground state (that is the lowest energy state peculiar to the atom) population, absorption is greatest in lines resulting from transitions from the ground state; in atomic absorption analysis these lines are called resonance lines.

Absorption lines, like emission lines, are not infinitely thin lines but have a certain finite width. A common notation is illustrated by Firure 2, where the half—:•ridth Eiv of the absorption line is the width of the profile at which the absorption coefficient k v is halved. The width of a spectral line is governed by a number of

10.

Figure 2 Profile and half-width of an absorption line

Absorption kmax Coefficient (ky )

IL Frequency (v) factors, these will be summarised below; fuller treatments are available

elsewhere 22

Natural Broadeninp- is the result of the finite lifetime (7) of any atom

in an excited state. By Heisenberg's principle this leads to an uncertainty in the energy of the level and hence in the frequency of a

transition to that level. The ground level is stable (T =o0) and thus

the natural width (Al; ) of a resonance line can be defined as: N

H 1 A = 1.10 2 rr

In most cases A>) can be neglected compared to other causes of broad— N ening.

Doppler broadening is associated with the random thermal motion of atoms

relative to the observer. In the case of an atom with a velocity in the

line of sight o? Vx the observed frequency of absorption is displaced by:

= Vx

where c is the spped of light. Assuminr- thermodynamic equilibrium the motion of the atoms

corresponds to a Maxwell distribution and the distribution of the

absorption coefficient Ic.v is found from the equation:

12.

(D) Mc2 ( k = k exp v 1.12 2RT vo

where N is the atomic weight, R the gas constant, T the absolute temperature (D) the absorption coefficient at the centre of the line. If f is and ko the oscillator strength, m the mass of the electron and e the charge of

the electron then:

(D) 2 . Nf ko = 247;75 e 1.13 me AVD

The simplified half—width of the line, A;)D is determined by:

1n2RT) a = 2

-6 = 0.716.10 v 1 .14 0 if

Even,:at low temperatures the Doppler width is much greater than the

natural width. Lorentz broadening is caused by collisions with molecules of foreign gases and increased proportionally to the change in the pressure of the foreign

gas, hence the alternative names of collisional or pressure broadening.

Together with the Doppler effect, Lorentz broadening makes the greatest

contribution to the shape, width and position of the line in the majority

of systems of interest to the analytical spectroscopist. Two theories

are at present advanced to explain the observed phenomena of broadened line

1) •

profiles and shifting of lines as foreign gas pressure is increased.

The profiles of absorption lines for temperatures of

1,000-3,000°K at foreign gas pressures of about 1 atmosphere are determined

by the Doppler and Lorentz effects. The central part of the line

primarily by the Doppler effect and the winr's by the Lorentz effect).

The profile of such lines is expressed by the Voipt equation: o(D) . 2 k a te—Y Y' V = dy 1.15 2 , a r a 4- 04—y)

where = L1 V a L V(In2) 1.16 VD

w 2())— V0) )1(1n2) 1.17 AV D

Y = 6 V(1n2) 1.18 2,%"D

is the half—width due to Lorentzian broadening and (5 the frequency andA))L displacement from1)—Vo.

Other broadening processes which may be encountered are

Holtsmark broadening (also known as resonance broadening as it is caused

by collisions with atoms of the same kind) and Stark and Zeeman broadening

(caused by the splitting of an atomic line by the effect on the energy

levels of electric and mapnetic fields respectively). Neither of these

processes are generally important in atom cells used in analytical 14.

.spectroscopy.

When viewed by an instrument of high resolving power a

spectral line often appears to consist of a number of components, known

as the hyperfine-, structure. These components may be due to the presence

of more than one isotope and, or, the interaction of a nuclear spin with

the spins of the electrons. Each of these components is really a single

spectral line and is broadened by all of the above factors. However, if

the hyperfine splitting (A-A)hfs) is very much less than the width of

the individual components due to the Doppler and Lorentz effects then it

can be ignored, or is very much greater than the other ifhfs broadening effects the hyperfine components can each be treated as simple,

separate lines, but in the complex case where the profiles of the separate

components are superimposed on each other then the overall profile must

be obtained by graphical summation.

Three methods of measuring absorption have been

extensively used in theoretical studies based on the determination of:

1) the integrated absorption coefficient of a resonance line;

2) the total energy absorbed from the continuous spectrum by a resonance

line;

3) the relative absorption of light from a source with a line spectrum. 6 The technique first proposed by Walsh is based on the

third method, the absorption coefficient directly at the centre of a line 15. being: measured. Low pressure hollow cathode lamps with narrow emission line profiles were proposed as sources and flames, where the absorption lines are broadened by Doppler and Lorentz effects, as the atom cell.

The emission line is isolated by means of a monochromator and its intensity with and without the presence of absorbing analyte atoms in the cell measured photoelectrically. The absorbance, A, is related to the • condentration of analyte atoms in the volume of the atom cell examined.

This method holds a considerable advantage over the determination of the integrated absorption coefficient in that only a moderate monochr-omator is required, whereas in the latter case an instrument with a very high resolving power would be needed. In the case where the emission line half—width is negligible compared to the absorption line width, A is linearly related to kol (k0 being the absorption coefficient at the centre of the line) and if the shape of the absorption line is entirely' due to Doppler broadening, from equation 1.13

( 2 k D) = 2 (1n2 re Nf o•-" N) D me

i.e. Ao

It should be noted that the frequency of the absorption line maximum may be slightly displaced from that of the emission line maximum, the half— width of the emission line may not always be negligible, the absorption line profile is not solely determined by the Doppler effect but also by

Lorentz and possibly other types o7 broadening. These factors may 16. cause curvature of the calibration curves (plots of absorbance against concentration of analyte) but overall absorption methods have several advantages over emission methods of spectrochemical analysis; these will be summarised in section 1.2.4.

1.2.3. Atomic Fluorescence Spectroscopy

Although atomic fluorescence spectroscopy has only recently been adapted for analytical use several comprehensive reviews have already 26,28-32 been published23- There are seven types of atomic fluorescence mechanism's which have been observed'10,11, 33, 34 andthese are illustrated in Figure 3. The most intense atomic fluorescence generally occurs from resonance fluorescence and this has.been the mechanism most useful in a majority of analytical studies. Direct—line fluorescence mechanisms have also been of analytical use but step—wise fluorescence has been of little use. Sensitised fluorescence, also known as energy transfer atomic fluorescence, has not yet been of any analytical use.

The intensity, or more strictly the radiance, of atomic 28 35'36'37 fluorescence ' radiation is dependent on the intensity (or radiance) of the exciting radiation (Is), the ratio of the excitation source line half—width (A vs)(or in the case of continuum source the band half—width, in this discussion only line sources will be dealt with as only line sources were used in the following experiments, treatments concerning continuum sources are available elsewhere35136'37),the dimensions of the 17.

Figure 3 Types of Atomic Fluorescence

5?4 A C)

w

Ground (6) State I-1 ( ) (2) (3) (A) (5)

The solid lines represent radiational processes, the dashed lines

non—radiational processes, in the latter case a single—headed arrow

represents non—radiational deactivation and the double headed

arrow a thermal activation process.

Types of Fluorescence

1) resonance; 2) normal direct—line; 3) normal stepwise—line;

4) thermally assisted resonance; 5) thermally assisted anti—Stokes '

6Y -thermally assisted direct—line; 7) sensitised fluorescence is not ill— * ustrated here, the mechanism can be summarised as: i) A + by —4 A ; + * ii) A* +M-4- A+ M* — energy; iii) M M+ hv whereAis an atom

in high concentration (the donor) and M is an atom in low concentration

(the acceptor).

(The term anti—Stokes is used when the radiation emitted is of

shorter—wavelength, i.e. greater energy than that absorbed). 18. absorption cell (1 by 1 by 13), the solid angle over which 1 2 excitation occurs (Q), the atomic concentration of absorbing ground state atoms (n ), the efficiency of the absorption and o fluorescence processes, the transition probability and the extent 28 of line broadening. Winefordner has derived the following expression connecting at low concentrations these parameters with the,fluorescence intensity (IF) emitted at right angles to the source:

(D) 1 1 1 ) I = k X,fS. Y' (S2 )( 1 2 3 F 0 n0 ljs 1 19 4r AR where the symbols have the significances described above and Xi

= fraction of n involved in the absorption transition, f is the o absorption oscillator strength, is a factor to account for the 8i. finite half—width of the source compared to the absorption line,

Yt is the fluorescence power yield and AR is the total surface area of the irradiated atom cell. At high concentrations this expression must be modified to allow for reabsorption of fluorescent radiation in the atom cell.

Thus at low concentrations I is proportional to F n (i.e. for practical purposes the concentration of the analyte), o 1 at high concentrations I is dependent on n This behaviour F o 37 is reflected in the growth curves of log I vs log N . It should F be noted too that I is directly related to I and work on the F s development of intense, stable sharp—line sources is an essential 19. feature of attempts to improve sensitivity using atomic fluorescence spectroscopy.

1.2.4. Comparison of the Analytical Usefulness of Atomic Emission,

Atomic Absorption and Atomic Fluorescence.

Increasingly attention is being drawn to the need to regard all three techniques as complementary. While each technique has at present a group of elements for which it provides the most sensitive method of analysis there are also many elements for which two or even all three methods are equally sensitive. In different situations the instrumental advantages or disadvantages of one technique may favour its use as opposed to another. Thus it is useful to make a summary of some points:

1) It has frequently been shown that the probability of the super— imposition of resonance lines of different elements is extremely small, even compared to the probability that in emission methods an emission line will be superimposed on the• resonance line. Even when workers have drawn attention to the occurrence of spectral interferences38 it has been pointed out that such interferences are easily overcome by use of another resonance line.

2) In atomic emission the intensity of radiation measured depends on the population of the excited level, small variations in temperature (e.g. in a flame atom cell fluctuations in the fuel flow rate) have a great effect on the population of excited atom; as determined by 20.

the Boltzmann distribution (equation 1.3), and hence on the

analytical signal. Indeed the analytical signal is critically

affected in emission by any inter—element effect which reduces

the excited population. In atomic absorption it is the number

of atoms in the unexcited state that is important, and this ground

state population is not greatly altered by temperature variations

and it is this which gives atomic absorption its main analytical ,

advantage. As atomic fluorescence depends on the re—emission of

absorbed radiation, it too is related to the ground state population

and to a great extent shares this advantage of atomic absorption, although

certain flame species which quench fluorescence may contribute to

a loss in sensitivity.

3) Several. workers have published studies comparing the signal

strengths and detection limits theoretically. obtainable from atomic

emission, atomic absorption and atomic fluorescence spectrometry;

particular attention is drawn to the work of Alkemade36'39 in which

equivalent noise levels are assumed in each of the techniques and 28 40 41 Winefordner (and co—workers) ' ' who give a complex treatment,

in which the theoretical noise levels are first evaluated. Only.

a few points from the extensive and sometimes controversial

literature will be noted here:

a) atomic emission is most sensitive at higher wavelengths (i.e.>

350 nm), this is because shorter wavelengths correspond to higher energy transitions and even high temperature flames, such 21. as the nitrous oxide—acetylene flame, do not possess enough thermal energy to produce sufficient atomic populations of higher excited states. b) Alkernade39 has derived a ratio for the relative intensities of atomic ahsorption (AI) and atomic emission (Iem) neglecting any differences in noise levels in the two methods:

. Q As 1.20 A I = Js em A X F (T) Jpl

is the spectral radiance of the source (atomic absorption) where Js (T ) the spectral radiance of a blackbody at the atomic and Jpl F cell (in this case a flame) temperature, at the wavelength X 0

of the line centre (this is given by the Planck radiation law),

and AXs and AX F are the spectral width of the source and

emission/absorption line respectively. Thus if 40's A F

atomic absorption is only more sensitive if the spectral radiance

of the source exceeds that of a blackbody at the temperature of

the flame and at the wavelength of the analysis line. This

condition is met in a number of situations e.g. for the ultra—

violet region of the spectrum in a hollow—cathode lamp and in gas

discharge tubes.

c)It has been shown36 that, particularly for relatively high 22. energy states, i.e. from which transitions Five rise to resonance lines in the spectral region below 300 nm, the excited state population in atomic fluorescence can under suitable conditions

(a source with high integrated radiance at the wavelength of interest) considerably exceed the thermal value, i.e. the effective population for atomic emission.

Thus if the same atomic resonance line is measured using the same instrumental system by all three methods it can be 28 40 41 shown r ' that atomic emission should result in lower limits of detection than atomic fluorescence or atomic absorption for resonance lines above ca. 400 nm, between 300 and 400 nm all three methods should give similar limits, but below 300 nm atomic absorption and atomic fluorescence should provide lower limits of detection. Published experimental results42 tend to confirm this generalisation.

4) In absorption methods the ratio of the unabsorbed signal to the absorbed signal is measured. In that this removes the poss— ibility of systematic errors arising from small shifts in the monochromator setting, or small variations in the sensitivity of the detection system between optimisation and the completion of the analysis, this is an advantage of the atomic absorption technique.

However, it is experimentally difficult to moasure a small difference in two large quantities and this places a practical limit on the sensitivity of absorption techniques, whereas the 23. sensitivity of atomic emission and atomic fluorescence are not limited in this way, and their sensitivity can be increased by the use of atom cells with higher temperatures (e.g. radio—frequency plasmas), in the case of atomic emission, and the use of brighter sources in atomic fluorescence.

Finally we can add that atomic emission, because it requires no spectral source, is more easily adapted to non—standard and qualitative determinations, whereas atomic absorption requires less operator skill and less expensive instrumentation to give results of high reproducibility and good sensitivity. Atomic fluorescence spectroscopy remains an under—exploited technique but given the development of suitable high—intensity sharp—line sources and a non—flame cell of low background with the minimum of quenching of fluorescence radiation, it cuuld rival either of the more established techniques. In the next section we shall review the development of non—flame atom cells.

1.3. Non—Flame Atom Cells

Historically the atom cell, or atom reservoir, used in analytical atomic spectroscopy has been a flame cell. Probably because of its long use in atomic emission studies early workers in both atomic absorption spectroscopy and atomic fluorescence spectroscopy used flame atom cells. Flames are still the most 24. widely used method of atomization and all commercial instruments are_:,equipped with flame atomiser facilities. Flames have a number of advantages as atom cells but also a number of dis— advantages, particularly in atomic absorption spectroscopy and atomic fluorescence spectroscopy; these will be discussed with especial reference to these techniques.

1.3.1. Advantares of Flame Cells

The main advantages of flame atom cells briefly summarised are:

1) The large amount of data about flame cells which has been built up because of the long history of their use already referred to above.

2) Most elements and salts can be easily atomised by the appropriate flame. A solution containing the analyte is nebulised into the flame as a fine aerosol,i;the droplets of solution evaporate to form solid clotlets, which by fusion and evaporation or sublimation are converted to the gaseous state, these gaseous molecules then dissociate to give a gaseous dis— persion of the rround state atoms. The flame temperature is high compared to the boiling point of the solvent and in most cases sufficiently high to rapidly evaporate the 'clotlets'.

The enthalpy of the flame is sufficient to provide the molecular 25.

dissociation energy for most species and the temperature high enough to produce a large partial pressure of analyte atoms.

The time of residence of the atoms in the light beam will depend upon the burning velocity of the flame. The atomic population may be reduced by ionisation, Amos and Willis44 have stated that calcium and strontium are approximately 43% and 84% ionised respectively in the nitrous oxide—acetylene flame. However this ionisation can be suppressed by the addition of excess of a more easily ionised element, such as caesium or potassium, to the flame. The atomic population may also be reduced by compound formation especially for those elements which form thermally stable oxides. The nitrous oxide—acetylene flame introduced by WilliF45 has been shown to greatly decrease inter— ference from compound formation, and this has been attributed to its high temperature (ca. 3,000°K) and the presence of large concentrations of the highly reducing CN radical.

3) As has been suggested above, a wide variety of flames is available to allow the selection of optimum conditions for many different analytical purposes.

4) Flames can be adapted to provide long light path lengths, especially for use in atomic absorption spectroscopy. Clinton° first described the now popular long—path slot burner. Fuwa and Vallee47 described a lonp—tube method in which a flame from a total consumption nebuliser—burner was passed down a metre—long 26.

tube, Rubeska and Moldan48 amongst others have reported the use

of heated 'Puwa—tubesl. Robinson49 used a shorter 'IL-piece

adaptor above' a similar burner system. Although these devices

permit increased sensitivity it should be noted that such absorption—tube methods have a number of disadvantages also, such

as memory effects and interference from molecular absorption.

5) Flames are convenient to use, reliable and relatively free from memory effects. Most burner systems are small durable and inexpensive and with relatively simple nebuliser assemblies most

samples are conveniently and rapidly handled. Most flames in common use may be made auiet and safe to operate.

6) The signal to background and signal to noise ratios obtainable

are sufficiently high to allow adequate sensitivity and precision

to be obtained in a wide range of practical analyses at different wavelengths between 200 and 800 nm.

1.3.2. Disadvantages of Flame Cells

Some fundamental and possible practical disadvantages of flames for analytical atomic spectroscopy are:

1) The conventional indirect nebuliser and flame system requires relatively large volumes of solution to operate. The nebuliser is designed to supply a fine aerosol of solution to the burner at a uniform rate and in these systems larger sample droplets condense 50 on the walls of the spray chamber, hence only 3-12% of solution 27. uptake is delivered to the flame. Attempts to improve the efficieny of premix nebulisers by using ultrasonic nebulisation 51-54 heated spray dftambers55'56 or hot gases57 have been reported but generally the apparatus used tended to lack simplicity.

2) Atom concentrations in flames are limited by the dilution effects of the relatively high flow-zrate of unburnt gas used to support'the flame and the flame gas expansion which occurs on combustion. It is estimated that the atoms spend 10 4 seconds in the analysis volume58 while it is practically difficult to record the equilibrium signal from a sample solution in less than 10 seconds. Even the low—burning velocity of the fuel— rich nitrous—oxide acetylene flame45, although often cited as an advantage of this flame, does not challenge this fundamental objection to flame cells.

3) Solutions with high analyte or sample matrix concentrations may undergo incomplete solute vaporisation and gaseous dissociation because of the short transit time in the flame ce1159. Some elements are prone to compound fomation, and although this can be overcome by use of the hot, reducing fuel—rich nitrous oxide— acetylene flame45, even then sufficient partial pressures of oxygen containing species may be available to reduce the concentration of some elements which form stable monoxides, e.g. Zr, Hf, Si, B.

Some elements are strongly ionised in flames, e.g. Na, K, Rb, Cs and a more readily ionised element must be added to suppress this 28. ionisation if the atomic population is not to be seriously reduced.

4) Althour'h solutions are in many ways ideal analytical matrices in certain situations the analysis of solid samples may be preferred.

It is difficult to nebulise viscous oils, and certain other common organic solvents may extinguish the flame when sprayed.

To overcome some of the above disnAvanta4-es a number of modifications to•=the flame cell have been proposed. 60 Kahn and co-workers developed a system whereby nebuliser inefficiency was avoided, by using a tantalum sampling boat from which the sample 61 was evaporated by placing it in the flame. Delves has described a technique for the determination of lead in micro-samples of whole blood, in which the sample is vaporised from a nickel crucible into an absorption tube placed in the flame. Both these technique can be used for solid samples, as can a method proposed by

Venghiattis62 in which the solid sample is mixed with solid propellant powder, the mixture is then compressed and ignited and the flame passed into the absorption light path of a con- ventional atomic absorption spectrophotometer. There are, however, further disadvantages of flame cells that these methods, which are mainly applicable to volatile elements, do not overcome. 29.

5) Flames exhibit a background radiation and absorption which consists of banded and continuous spectra. The banded spectra arise from the excited molecules and radicals in the flame gases and the continuous spectra from the dissociation, ionisation and recombination of these species. Care must be taken that the line of analytical interest is not in a reFion of high flame background. Problems associated with flame background can be reduced by separating the secondary diffusion flame from the primary reaction zone and viewing the low—background interconal zone. The use of separated flames in analytical flame spectro— 63 scopy has recently been reviewed . Infflames considerable reduction in radiation intensity at wavelengths below 200 nm is caused by absorption of radiation by flame gas products, whereas flames also produce appreciable thermal excitation of elements having resonance lines greater than 300 nm (the effects of such emission signals can be minimised by: synchronous modulation and amplification13). Thus flames possess considerable disadvantages for the atomic absorption or fluorescence of elements whose strongest resonance line are below 200 nm or above 300 nm.

6) The flame which produces the greatest freedom from inter— element effects may also in atomic fluorescence produce lower fluorescence ef'iciency than cooler hydrogen based flames, because of quenching of radiationally excited analyte atoms by flame gas molecules.

7) Particulate matter in the li7ht path may scatter radiation,' 30. total consumption burners with hi{-h--burning velocity flames, such as oxy—hydroren, are peculiarly prone to scatter effects and pre— mixed indirect—nebuliser nitrous oxide—acetylene flames least prone to scatter effects.

8) In certain situations flames are inconvenient. Either the flame or the associated high—pressure cylinders may present a hazard. It is not advisable to spray radioactive solutions into a flame, nor to leave flames unattended, i.e. they are not readily automated.

9) Flame gases can be comparitavely expensive.

10)Explosion hazards are always present with flames of high burning velocity and flame products may be toxic. Hot flames besides often being unpleasant to use, can effect the auxiliary equipment, e.g. cause monochromator drift.

11)Precise control over species in the flame is limited. The chemical composition of the flame or its temperature can be only controlled over a very small range and generally only by altering other parameters. It is difficult to envisage the use of flame cells in absolute analytical work.

1.3.3. Advantages of Non—Flame Cells 64 Winefordner has demonstrated that in a graphite cell, 65 similar to that of Massman which will be described later, a

31.

substantially higher peak concentration of atoms may be expected

than in a flame.

The concentration, n (atoms cm-3)% of analyte

atoms in a flame, is related to C, the concentration of analyte -1 66 (moles 1 ) in the solution aspirated into the flame by the

equation: 19 n = 1 x 10 F 6 C 1.21 Q?:f

where F is the solution transport (flow) rate (em3min 1), E and 0

are the aspiration and atomisation efficiencies respectively,

Q is the flow rate of unburnt gases into the flame (cm3sec 1) and

o is the flame gas expansion factor, due to the rise in gas f temperature and the increase in number of moles of flame gas

products. The corresponding peak atomic concentration in the

graphite cell, where V cm3 of solution are introduced using- a

pipette is given by: 20 n = 6 x 10 VCe0 1.22 Vc

where V is the inner volume of the graphite cell. c A similar relationship for a non—flame cell

first reported by West and Williams67 which consists of an

electrically heated carbon filament shielded by an inert gas

flow is given by:

n = 3.6 x 1019 VC E0 1.23 V gr 32. where V is the volume of sample plaCed on the filament (cm3),

V -1 s the shielding gas flow—rate (1 min ), g is a geometry factor to account for the small area on which the sample is deposited compared to the area over which the shielding gas flows, and r the time taken for the sample to vaporise (sec).

We can compare these peak atomic concentrations by substituting typical values for the experimental systems:

F = 2 cm3min-1, Q = 100 cm3sec-1 , of 10, V = 0.005 cm3, -1 3, V = 3 , g = 0.02, r = 0.1 sec and taking Vc = 0.5 cm s 1 min ( E 13 )f = (E )g (co )01, = 1, where the subscripts refer to the flame (f), the graphite cell (g) and the carbon filament (cf).

In this case the ratio of atomic concentrations Rn is (Rn)f:(Rn)g:(Rn)cf

= 1:300:1500.

Thus the carbon filament'is many times more efficient at atom production than the flame because the atomic vapour occupies a far smaller volume and the flame dilutes the atomic concentration by expansion. This advantage is decreased by the use of long—path lengths, but one fundamental advantage remains dominant for the carbon filament and graphite cell, i.e. only

5 µl of sample is required complared to at least 100 or

1,000121 for the flame. It has been assumed that the efficiency of aspiration—atomisation of these systems is equal but for many 6869 is considerably less than unity and we have analytes (E fl )f discussed the inefficiency of many flamef,systems above (section 1.3.2.) 33.

Considerable quenching of atomic fluorescence

occurs in most flames used in analytical spectroscopy (mainly

N2, CO, CO2 etc.), but in graphite cells, or around the shielded. carbon filament, the argon atmosphere present contains few active

quenchers and a signal increase ratio R.Y, corresponding to decreased quenching, can be determined• 64 . (Ry)f:(Ry)g:(Ry)cf is of the order of 1:10:10 for atomid fluorescence, this signal increase does not of course occur in atomic absorption or emission. 64 For the graphite cell Winefordner has derived a geometry factor, RG, for atomic fluorescence, which allows for the reduced signal obtained for his confined system (graphite cell) to the conventional open system (flame). For the carbon filament, where the atoms are viewed above the filament, this geometry factor is less important. (RG)f:(RG)g =

In practical situations noise from flame flicker is often greater than shot noise in atomic fluorescence and source flicker noise in atomic absorption. When using properly shielded graphite cells and carbon filaments the latter should be the major source of noise.

Thus we can see that two typical non—flame cells appear to have several basic advantanes compared to flame cells, principally a gain in analytical signal (Rn for atomic absorption and RII.Ry.RG for atomic fluorescence)i the use of much smaller 34. samples (5 Al for the carbon filament compared to 500 /Al for the flame) and reduced background noise levels. Over recent years an increasing number of non-flame cells have been reported and in the next section several of these cells will be reviewed.

1.3.4. Non-Flame Cells in Analytical Atomic Spectroscopy

The development of non-flame cells for analytical atomic spectroscopy has, with two possible exceptions, been a comparatively recent one but already a large number of techniques 70 71 has been proposed, some of which have been recently reviewed ' .

The two exceptions are for the determination of mercury by atomic absorption, which presents something of a unique situation because of the relatively high vapour pressure of mercury at room temperature, and the ease of reduction of mercury compounds to give elemental mercury, and atom cells for atomic emission studies. A large number of papers proposing schemes for the flameless atomic absorption determination of mercury have been published. Manning72 has reviewed many of these and commercial equipment is now available for such determinations73. It is not proposed here to review thoroughly non-flame cells in atomic emission studies but some mention will be made of some practical systems using the plasma-jet and high-frequency plasmas. 35.

Plasmas

Two kinds of plasma flames for atomic-emission have recently been studied; they are the plasma-jet flame (or d.c. plasma- jet) and the high-frequency plasma flame. The plasma-jet flame has been used by a number of workers74-78, but the high-frequency plasma torch has been more extensively used in both emission and absorption studies, 'because it is free from the high background associated with the d.c. arc in the plasma-jet flame. Greenfield et a178'79 have shown that the induction-coupled, or high frequency, plasma can provide an atomic vapour of even the most refractory compounds and overcome inter-element interferences, since the electronic temperature can approach 16,000°K. Other workers have found such inert or diatomic high-frequency gas plasmas, usually argon-nitrogen mixtures, efficient 51 8o-86 sources for the spectral excitation of metal atoms . Methods have been published both for the analysis of solutions51,78-86 and 78'808487. more recently the analysis of powdered samples Low wattage microwave induced argon plasmas have also been proposed for use in emission spectroscopy and two groups of workers have 8889 reported very sensitive determinations of several metals 90 Wendt and Fassel have described the use of induction coupled plasmas for atomic absorption spectroscopy.

An ultrasonic nebuliser was used together with a triple-pass optical system. It was shown that the sensitivity obtained for aluminium, niobium, rhenium, titanium, tungsten, yttrium and 36.

vanadium was similar to that obtained using the nitrous oxide—

acetylene flame, despite a sample consumption rate for the,

plasma (0.12 ml min-1) an order of magnitude lower than that for the flame. It was concluded that the degree of free—atom formation was increased or that the formation of monoxides was decreased. They also reported a marked reduction in chemical interferences. It should be noted also that whereas an atom spends about 10-4 sec58 in the analysis volume of a flame, the

optimal flow rate of the argon bearing the aerosol in the plasma -2 86 gives a residence time of about 10 sec .

Wendt and Fassel's multipass system was neces— sitated by the small path—length available in the plasma; Greenfield et al91 increased the path—length of a high—frequency plasma by banding the plume through a right—angle. With this system copper was determined by atomic absorption spectroscopy. 92 Veillon and Margoshes have evaluated the

induction—coupled radio—frequency plasma torch for both atomic

emission spectroscopy and atomic absorption spectroscopy. They

obtained relatively poor detection limits by absorption and note

the following disadvantages of the plasma torch: the need to use

bright line sources, an expensive power supply and an elaborate

sample introduction system (owing to the sensitivity of the

discharpe to molecular gases, especially water vapour), a flame

was regarded as more convenient to use. 37.

O3 Friend and Diefenderfer' determined elements which form refractory oxides (vanadium and aluminium) by atomic absorption spectroscopy using a plasma jet. Brandenburger94 also reports the use of a radio—frequency plasma in atomic absorption and Biancifiori and Bordonali95 have used an argon induction plasma in atomic absorption trace analysis.

The advantages of plasmas for atomic absorption studies lie in the considerable freedom from matrix interference, the greater formation of free atom populations for certain elements and the slow sample take—up rate.

Furnaces

The first graphite furnace used in atomic spectroscopy was designed by King96, it consists of a praphi-,:e tube, in a vacuum chamber heated by a.c. to 3,000°C. The element under investigation was placed in a special boat in the furnace. King's furnace was used for theoretical invest— igations and is not applicable to analytical use because the number of atoms which enter the analysis volume depends only on the saturated vapour pressure of the element at the furnace temperature. A large number of modified King furnaces have been described, especially for atomic absorption spectrometry.

Vidale's modification97198 enabled the determination of sub—nanogram amounts of sodium by atomic 38.

absorption. The sample is placed in a sealed quartz cell

within a quartz tube which is itself purged with argon, evacuated

and placed in an electrically heated furnace at ca. 1100°C.

However, the precision was only ca. 20% and the method is

cumbersome and of limited application.

Tomkins and Ercoli's modification99 was a

miniaturisation consisting of an inductively heated, small

diameter tantalum tube supported in a radiation shield ( a

double walled fused silica tube filled with finely divided,

degassed, carbon under vacuum). Useful metal vapour absorption spectra have been obtained (using samples weighing only a few milligrams) for barium, calcium, thallium and radium but a serious limitation is the maximum temperature of ca. 1,400°C. 100 Mislan detervined cadmium, using a heated fused silica tube, down to levels of less than 10 ppb using the 228.8 nm absorption line. The sample is introduced, via a sidearm at 1,000°C, as a fine mist into the absorption tube which was heated up to ca. 1250°C by a wire wound resistance furnace. Serious interference problems were encountered and the method is limited to elements whose salts have relatively low dissociation energies. The analysis of cadmium in heavy water was, however, reported and a scheme 101 for the analysis of cadmium in uranium proposed. Hudson 39.

has used a stainless steel absorption cell heated by a resistance

wire for the atomic absorption measurement of sodium vapour. 102 Burgess and Donega have reported on the determination of sodium,

and other contaminant vapours, in furnace atmospheres by atomic 103 absorption spectroscopy. Choong and Loong—Seng have also

reported the use of a furnace in atomic absorption studies as 104 105 have Laurent and Winiger and Edelhof et al althowth in

these latter cases the studies were of a theoretical nature.

The earliest and best known modifications of the

Kin furnace for analytical use is that first described by 107,108. L'vov106' A pulsed method of atomising samples in a

Fraphite furnace is used. The sample applied to the tip of

a carbon electrode is introduced into the heated fitrnace throu-h the transverse aperture at the centre of the tube. To accelerate sample atomisation the sample is pre—heated, in the early experiments this was achieved by using a powerful d.c. arc but this has been replaced by the more effective, 22. and technically simpler, resistance method of preheating109'

The furnace in L'vov's apparatus is not the means of atomisation but merely acts as the atom cell hindering the loss of atoms, hence the term 'graphite cuvette' by which the apparatus is called.

The shape of the cuvette and electrode may vary 40. according- to the purpose of the measurements, but the cuvette is a graphite cylinder usually 30-50 mm long and of internal diameter 2.5-3 mm. The cuvette is heated by a.c. from a 4 KW transformer at 10 volts and the electrode, which is introduced into a transverse aperture in the wall of the cell, (the electrode head being tapered at 30° or 60°), is heated from a 1 KW trans— former at 15 volts. . The temperatures of the furnace and electrode are regulated by altering the voltage in the primary circuits of the transformers. Air is removed from the chamber in which the cell is placed and the fill—gas is either argon or nitrogen, pressures up to 10 atmospheres can be used. In L'vov's early work the cuvette was lined with tantalum foil to eliminate vapour diffusion throu-h the porous walls, but pyrographite, which has low gas permeability, high heat conductivity a high sublimation point (3,70000 and resistance to oxidation, is now used.

The experimental procedure is to place the sample onto the electrode either in solution or powder form. The electrode is positioned below the cuvette, the chamber closed and purged. The cuvette is heated to the desired temperature

(10-20 seconds). The electrode is introduced into the aperture and immediately heated, the sample is vapourised within ca. 2 seconds and the absorption signal is either recorded by the peak or 110 integrated absorption method . The atoms remain in the

Guyette for relatively long times, 4.3 and 23 seconds for cadmium 41.

111 at respectively 1 and 9 atmospheres L'vov has also employed 112 a two-channel optical system , a single beam system with

simultaneous recording, thus a continuum source may be used to

correct for scattering and molecular absorption, or a second

atomic spectral source used to enable one element to be used

as an internal standard. L'vov reports22 that it is possible

to make up to 30 analyses an hour with his apparatus.

L'vov and other workers have reported detection limits for 46 different elements using the cuvette with absolute -13 g sensitivities largely in the range 10-9 to 10 rams. This includes the determination of traces of mercury, iodine, phosphorus and 113 sulphur.using resonance lines in the vacuum ultra-violet . The technique has been used for a number of practical analyses, for 114-116 the analysis of zinc and aluminium in metallurgical samples radioactive samples containing zinc and cadmium117, for the deter- 118 119 mination of traces of cadmium and molybdenum and in the measurement of a number of important theoretical parameters, the 120'121, absolute oscillator strengths for resonance lines the 122'123 Lorentz width of resonance lines and the coefficients of 106 124 diffusion of atoms '

The reader is referred to LIvov's own reviews of 22'125 the work carried out with the graphite cuvette and various 126-128 other more detailed descriptions of the experimental apparatus

The graphite cuvette is a versatile method of analysis, which 42. enables a wide range of elements to be studied reportedly without 116 serious interference effects115' with increased sensitivity 129 and precision equivalent to that obtained with a flame . The technique is not, however, suitable for atomic fluorescence measurements and is considered by sow: to be cumbersome. 65'130 Massmann has described the-use of a modified graphite cuvette in both atomic absorption and atomic fluorescence spectroscopy, there is no auxiliary electrode and the sample is micropipetted directly into the graphite tube via a small orifice.

The absorption tube is 55 mm long, 6.5 mm internal diameter and

1.5 mm wall thickness, the orifice is of 2 mm diameter. The fluorescence cuvette is cup—shaped 40 mm lone, 6.5 mm internal diameter and 1.5 mm wall thickness, the sample, and source radiation enter via the open top and the fluorescence radiation is viel,ed via a slit cut into the wall. Using a power supply with a maximum of 400 amps at 10 volts, the cuvette can be heated up to 2,600°C in a few seconds. The absorption tube is purged with argon but the optical path remains open to the atmosphere, whereas the fluorescence radiation is viewed via a silica window in the side of the enclosing chamber, again argon is used to purge the cuvette. Solution sample volumes of 5 to 200 /41 were used in absorption studies and 5 to 50 p1 for atomic fluorescence, solid samples of up to 1 mg weight were also acceptable. Hollow cathode lamp sources were used for both techniques, but in the 43. fluorescence mode the lamp was operated at high current for short 131 periods of time. Massmann also employed a two—channel spectrometer and corrected for background absorption by monitoring a nearby non—resonance line.

Massmann obtained detection limits of between 10 9 and 10 14 g for 16 elements by atomic absorption and for 9 elements by atomic fluorescence. Only for two elements, zinc and cadmium were the detection limits lower with atomic fluorescence, but this was attributed to the lack of suitable high intensity primary light sources. Standard deviations between 4 and 12%, depending. on the matrix element and its concentration, were obtained, although higher precision was possible by use of an internal standard. Lvov has 22 noted that despite the considerable uses of the Massmann simplification for volatile elements, absolute sensitivity should be lower and interference effects higher, also problems of engineering and continuous background from the cuvette walls limit the temperature available in the Massmann cuvette.

A graphite tube furnace of similar design to that of Massmann has recently been introduced as an accessory to a commercially available atomic absorption spectrometer. Using this furnace Manning and Fernandez have determined copper and strontium 132 in milk , the milk (25 Al) is applied directly to the furnace and first charred to remove volatiles before measuring the analytical signal. These workers have also noted some unexpected results from 44. the use of different purge—gases, report a coefficient of variation of 5, absolute sensitivities for 26 elements and matrix interference results which are not present in flame atom 64 cells. Winefordner has also reported the construction of a graphite cell similar to that of Massmann but designed for atomic fluorescence studies using a 150—W Xenon arc lamp as continuum source, as yet no analytical results have been published. 133-136 Woodriff and co—workers have developed a well—insulated, constant temperature furnace as an absorption tube. The tube is machined from graphite with a wall thickness of about 1 mm, the length is 150 mm and the internal diameter

7 mm. The tube is heated by a current of 100-150A from an a.c. arc welder, temperatures of up to 3,000°C can be reached but most elements give maximum 5ensitivity below 2,500°C.

Samples can be introduced either by nebulising solutions with argon as carrier—gas using both pneumatic and ultrasonic nebulisers, or as solids using a carbon cup. The cups are twisted onto a graphite rod and inserted into the furnace through a sample port in the side of the furnace, the cup making a tight contact with a constriction in the sample port just below the optical path of the absorption tube.

Using these simple cups volatilisation is rapid and enables samples to be introduced almost as fast as solutions can be 45.

changed using a flame cell. Other advantages claimed for the

technique include simplicity, easy replacement of graphite tubes,

a high enough heat capacity to volatilise most samples rapidly

and linear calitration curves. No significant matrix effects were

observed from the presence of relatively large weight excesses of

Al, Cr, Cu, Fe, Ni, Mn, Zn and Mg on the atomic absorption signal 9 135 observed for 10 g of silver . Fifteen elements have been to determined with detection lithits in regions of 10-10 10 11g 135.

A serious disadvantage of the system does appear to be the memory

effects reported and non—specific absorption, although this

latter effect has been minimised by use of a dual—channel optical system.

An induction furnace for the determination of

cadmium in solutions and zinc—base metals. by atomic absorption

spectroscopy has been described by Headridge and Smith137. A 71 furnace temperature of 1350°C was used and 5-400 Ag.g of

cadmium in zinc was determined with a precision of 6.9%. The use of this furnace to determine trace elements in volatile materials 7-1 in the 10-1,000 ng.g range appears promising but calibration

curves so far reported exhibit marked curvature.

Morrison and Talmi138 have reported the micro— analysis of solids by atomic absorption and emission spectroscopy.

An induction heated graphite crucible (22 mm outer diameter, 16 mm 46. internal diameter and 110 mm length) serves as a thermal means of atomisation, while the excitation is achieved using a helium plasma formed by the R. F. field The furnace consumes 4.5 KW reaching• temperLtures up to 2,5000.. The system is protected by a helium atmosphere. For absorption measurements the light beam from a hollow—cathode lamp passes through the plasma plume located over the mouth of the graphite crucible. The samples are introduced into the crucible by deposition on graphite cups or tantalum foil discs which are then dropped from a

Teflon sample introduction chamber into the crucible. Nineteen -11 elements were determined in the range 10 to 10-8 g by emission and Ag, Bi, Cd, Hg, Mg and Zn in the same range by absorption. The method was applied successfully to the emission aralysis of freeze—dried liver samples and the preliminary results reported indicate the possibility of rapid solid sample analysis, particularly by emission.

Winefordner has briefly reported43 a metal tube furnace that .Shull and Winefordner are developing. The heating element is made of platinum and the sample is introduced as an aerosol, there is an alumina tube heat—shield. Although less porous than graphite platinum has an upper temperature limit of

1500°C, compared to about 2700°C for graphite, tantalum tubes enable this latter temperature to be reached but even in the presence of high flow—rates of argon oxidation problems with 47. tantalum are severe. The ".Shull metal tube furnace is particularly designed for use in atomic fluorescence spectrometry.

Filaments

- Atomisation techniques in which a wire loop or a sample boat carrying the sample, usually in the form of a solution residue, is introduced into the hot flames gases above the primary reaction zone have been in common use since the 139 original work of Bunsen . Several non—flame techniques have recently been developed in which the filament is electrically heated. Such devices are open and transient analytical signals are obtained as the atomic vapour passes through the absorption or fluorescence light path. A large temperature gradient exists between the hot filament and the cooler region above it and this may lead to increased interference from matrix elements. The unconfined nature and the high filament temperature minimises memory effects in such cells. The combination of low background and an open cell is also particularly useful in atomic fluorescence spectroscopy.

Several workers14°-145 have published methods for the determination of nanogram amounts of mercury by collecting it as an amalgam on a wire, and then heating the wire to vaporise the mercury into the optical path of an atomic 48. absorption spectrometer. Brandenberger and Bader have used both a dynamic method141'142 and a static method143 in which the vapour was released from the copper wire into an absorption tube. 144 Wheat also used a copper coil, the mercury being isolated by spontaneous displacement from acid solution, to determine mercury in radioactive samples.

Brandenberger145 as well as depositing mercury on a copper wire by spontaneous amalgamation or electrolysis has isolated Cd, Zn, Pb, Ti, Cu, Ag, Au and Pt on platinum or tungsten samplinp- wires. Again static and dynamic systems have been used, in the latter 'free—sample wire' technique a coated platinum spiral is placed in the light beam of an atomic absorption spectrometer without using a cell. The absorption signal is produced mainly by the vapour within the heated spiral. In the 'enclosed sample wire' technique the coated sample wire is placed in a silica cell before atomisation, thus tungsten wire can also be used. An argon atmosphere is recommended. For all the elements studied the method was claimed to be from 100 to 10,000 times more sensitive than atomic absorption flame spectroscopy. A fast response recording system was necessary. 146 Bratzel, Dagnall and Winefordner have described the use of a platinum wire (30 guage, 1/32" diameter) atomiser for determining cadmium, mercury and gallium by atomic fluorescence spectroscopy. For these three elements detection limits of 10—1 4g, 49.

8 -7 10 g and 10 g respectively have been reported, with standard deviations of about 5% when 1f.4.1 samples were applied by syringe, and 8%, when the sample was simply applied by dipping the loop into the solution. Argon was preferred as shielding gas, which was introduced from a 10 mm internal diameter tube, 25 mm below the loop, at a flow of about 3 1 min-1. Using an electrodeless discharge lamp as excitation source, and measuring the analytical signal with a d.c. electrometer, linear calibration curves over

3 to 4 orders of macnitude were obtained. The interference of

1,000 fold excess of several anions on the signal obtained from -1 0.1 /4gm1 of cadmium was studied, carbonate and silicate gave no interference but large enhancements were obtained from sulphate and phosphate.

This loop device has recently been adapted for emission studies147. The atomic vapour produced from a platinum or tungsten loop is excited at the mouth of a- quartz tube by a micro— wave induced argon plasma of power ca. 40W (2450 MHz). The determination of twelve elements is reported using a simple rapid response amplifier—integrator detection system. Interferences on the detection of cadmium were investigated, no spectral interference, was observed at 228.8 nm from cobalt or antimony and a method was proposed for eliminating that from arsenic, no chemical inter— ference was observed from 1,000 fold molar amounts of phosphate, silicate or aluminium, but ammonium metavanadate caused a signal 50.

intensity reduction of 40%. Detection limits using samples of -13 0.121 were typically in the range 109 to 10 g. Woodriff and Siemer148 have excited silver by a

R.F. helium discharge after evaporation from a filament. A -11 sensitivity of approximately 2 x 10 g and a. coefficient of

variation of about 7% was achieved.

The construction and operation of a carbon filament

atom reservoir for both atomic absorption and atomic fluorescence

spectroscopy was first reported by West and Williams°. A graphite

filament (2 mm diameter and 40 mm long), supported by water cooled

stainless steel electrodes, may be heated to 2,000 to 3,000°C

within 5 seconds by passage of a current of ca. 70 amps at up to

12 V_149. Small liquid samples (1to 5/4,1) are pipetted onto a depression on the filament. The original filament assembly was enclosed in an inert gas—purged pyrex chamber with quartz windows, but more recent designs, in which the shielding gas flows around the filament from beneath, have dispensed with this chamber

Because of the rapid cooling of the atomic vapour noted above, it has been found that limited field viewing of the atomic cloud immediately above the filament gives greatest sensitivity and reduces, 151'152,154. interferences from matrix elements Fourteen elements have now been determined by atomic fluorescence spectrometry with detection limits in the range 10-9g to 10-14g and 7 elements by 51.

atomic absorption with detection limits in the range 10 9g to 10 11g 67,149-155 The work with non—flame cells presented

in this thesis was carried out using this device and therefore

a full account of the construction and operation of the apparatus,

-and a critical discussion of its analytical potentialities will

be given later. 156 Amos has described two modifications of the

carbon filament atom reservoir. In the first a hole (1.5 mm diameter)

is drilled into the rod to form a sample cavity into which a

smaller liquid sample (0.5 to 1 /4.1) may be introduced for

absorption measurements. In the second the argon or nitrogen

shielding gas is replaced, or augmented, by hydrogen which

ignites to form a diffusion flame when the rod glows. These

modifications are claimed to improve atomisation and to minimise

inter—element effects, although obviously several of the

advantages of the original device are sacrificed. Amos and 157 his fellow workers have compared the carbon rod atomiser, as

they term the device, with inert gas and hydrogen shielding,

with a carbon tube device. They obtained greater sensitivity in

determinations of elements using atomic fluorescence and the

carbon rod, as opposed to using the tube and atomic absorption,

however interferences were greater using the rod than the tube

but when the rod was shielded by the flame interferences were

greatly reduced and absorption sensitivities enhanced. The 52. application of the technique for the direct analysis of lead in blood and urine by atomic fluorescence, with a detection limit of 0.005 vebm1-1 of orifrinal sample, was also described. A 158 commercial instrument company has published a list of detection limits for 34 elements in the range 1010 to 10 14g for atomic absorption determinations using a carbon rod with a radial hole as described above. 159 Recently Dipierro and Tessari have reported the determination of nanoram amounts of nickel using a mod— ification of the early type of West—Williams filament. _ Graphite was preferred to platinum, molybdenum or tungsten for the filament, although at wide slit—widths some interference from C2 absorption, or scatter from soot. was observed. The effect of different nickel salts and their decomposition in argon and argon—hydrogen was studied and a detection limit of 0.1 ng reported. 160 Donega and Burgess have reported a filament device in which five parameters may be varied, i.e. temperature, the material of the filament (in this case a boat shape), the chemical composition of the sample, the pressure of the filler—pas in the surrounding sealed tube and the gas composition. The sample boats are cut from graphite sheet (5 mil) or high purity tantanlum or tungsten foil (1mil) so as to be 50 mm long and 6 mm in width. The boats which take 100 or 50/41 of solution, are heated with a current of 30-50 amp at 12V, thus reaching a 53.

temperature of ca. 22000C in less than 0.1 second. The two

copper rods which support the sample boat are insulated and pass through a brass baseplate which contains an 0-ring seal,

and to which a quartz window is also attached. The filament

assembly is enclosed in a quartz tube 50 mm in diameter which has an optical quartz window sealed.on one end and an 0-ring flange on the other. The chamber is purged with inert gas

and used at any pressure, usually between 1 and 300 Torr. It is estimated that the time for one analysis is 2 minutes, but the

measured atomic absorption signal is of the order of tenths of

a second as the atomic vapour enters a hollow-cathode light

beam, modulated at 1,000 Hz directly above the boat. A fast response recording system is needed to measure peak-heights, because of the short residence time of atoms in the light-beam using such a reduced pressure system. Detection limits and

conditions are reported for ten elements,'including such involatile elements as V, Si and Mo which are of the order -12 of 10-9g to 10 g for a few microlitres of solution.

Hollow-Cathode Sputtering Cells

Hollow cathode discharges have been widely

accepted as a source of excitation in emission spectrographic analysis, particularly for the determination of sulphur and 54.

the halogens, refractory oxides, metallurgidal'samples and vases in

metals, and it is not proposed to review their use here. The

interested reader will find several useful references in two 161'162. recent papers Hollow—cathode lamps are also of course

widely used as spectral sources in atomic absorption spectroscopy.

.The sputtering action which produces the atomic vapour within such

lamps can also be used as a method of atomisation in the same 163 164 technique, as was first demonstrated by Walsh and co—workers '

Metal samples were analysed after machining into the shape of an

open—ended, cylindrical, cathode which was clamped in a sputtering

chamber. The chamber was then evacuated, purged and filled with

argon at the required pressure and a conventional hollow—cathode

discharge initiated. The sputtering chamber fitted with silica

vindows, was placed in the light path of an atomic absorption spec— 164 165 trometer. The analysis of Ag in copper was reported . Walsh

subsequently described an improved flow—through cathode sputtering

chamber and the analysis of Ag and P (using the 177.5 urn vacuum

ultra—violet resonance line) in copper and Si in aluminium and

steel. The great advantage of the method lies in the absence

of air in the atom cell enabling the use of the vacuum—ultra,-

violet region of the spectrum, e.g. the atomic absorption of 165 P at 177.5 nm and atomic emission determination of S, Cl, 166. F in solid samples at 90.68, 134.7 and 95.48 nm respectively 55.

However, a number of problems such as lack of reproducibility and preferential sputtering in certain alloys have been identified.

Goleb and Brady164 overcame the need for solid metallic samples by evaporating sample solutions onto the inner wall of an • a,luminium

Schiller—Gollnow hollow—cathode. This was then used as part of a water—cooled demountable hollow cathode lamp with a continuous flow low—pressure system. Only small sample volumes were required and 1pLg quantities of Na, Ca, Mg, Si and Be were detected, however lithium and magnesium were found to suppress the sodium absorption values. The coefficient of variation for sodium was found to be 8%. Goleb has also appliei this device in the isotope 168 169 determination by atomic absorption of uranium and lithium , in both cases lineLbroadening was minimised by water—cooling the sputtering chamber. 170 Ivanov et al combined some of the advantages of the graphite cuvette technique with those of a sputtering cell by using a graphite hollow cathode in a sputtering cell. Sample solutions were evaporated directly onto a fine molybdenum wire which was then placed_ along the central axis of the cathode.

Yokoyama and Ikeda171 have produced atomic vapours' of copper and mapnesium in a neon—filled (1.4 Torr) hollow cathode by a pulsed discharge, which enabled measurements to be made without interference from the emission spectra. Atomic absorption 56.

spectroscopy was used to measure the half—lives of the atomic

vapours, to prove ground state atoms survive for a considerably

long duration after the disappearance of emission spectra of

the same element.

Massmann172 has described the atomic absorption

-analysis of solid samples using a heated graphite hollow cathode.

A graphite tube (30 mm length, 7 mm internal diameter, 9.3 mm

external diameter) is supported on a small cylindrical graphite

electrode passing through the wall of the cathode tube. The

solid sample is,held by this carrier electrode in a suitable

depression (usually 2 mm diameter and 3 mm deep). The cathode

assembly is mounted on a molybdenum rod (2 mm diameter) whose insulated-

base passes through the water—cooled, earthed, brass baseplate of

the assembly housing. This ezrthed housing acts as anode. The

cathode assembly is covered by a cell with quartz windows which

is purged with argon. The discharge is then operated at an argon

pressure of 1-10 Torr, at a power of up to 1 KW. When used

in atomic absorption the cathode continuum radiation and emission

from the sample, in such a hollow—cathode atom cell, might be

expected to seriously interfere with the measurement of the

attenuation of the primary light source radiation. Massmann

pointed out that while with cool cathodes this interference is

difficult to avoid, the atomic vapour persisting only as long as 57. the discharge current is applied, except for volatile elements, if a hot cathode is employed, with suitable conditions, the atomic vapour may persist after the discharge current has fallen to zero.

In the apparatus described by Massmann the hollow—cathode discharge is operated with a 50 Hz half—wave current and this discharge, which is thus present for only half the operating periodl is shielded from the detector system by use of a rotating sector, driven in phase with the discharge current cycle. The attenuation of radiation from a primary hollow—cathode lamp, modulated at 450 Hz, by a second rotating sector between the source and atom cell, is measured while-the discharge current is at zero. When the peak absorption signal is measured linear calibration is possible only over a narrow working range (e.g. 1 to 10 p.p.m. silver in a 30 mg sample of lead). This linear working range was greatly improved

(e.g. 1 to 300 p.p.m. silver in lead) by using signal integration.

In the analysis of lead, zinc and aluminium samples (30 mg) coefficients of variation of 8 to 15% were obtained, and for the determination of silver in lead, using the integration method, a standard deviation of 4%.

The graphite cathode heated by an electrical discharge at reduced pressure can not reach temperatures higher than 2,000°C and therefore only relatively volatile samples such as lead, zinc and aluminium can be analysed. Because of the low pressure: in the hollow—cathode the residence—time of the 58.

atomic vapour in the absorption volume is short and therefore the

method is not suitable to detect small amounts. If however low

concentrations are to be determined, samples of large size can be

evaporated without serious interferences. Unddr such conditions

the power of detection is good and in most cases better than that

.commonly observed using flame absorption cells. The detection

limits for the determination of Ag, Sb, Zn, Cu, Cd, Mg, Mn and

Cr in 30 mg samples of relatively volatile matrices by atomic

absorption have been reported172 andl with the exception of

Sb and Cu, compare favourably with detection limits obtained for

the same samples by emission spectrography with a hot hollow— 173 cathode and a medium quartz spectrograph . Because of the

small background absorption interference (less than 1% at

wavelengths greater than 220 nm) in the analysis of large samples

(100 mg) hot graphite hollow—cathodes may be superior to graphite

furnaces.

Arc and Spark

The use of arcs and sparks for atomisation and

excitation in emission spectrochemical analysis has a long

history174. Robinson49'175 first used the d.c. arc for

atomisation of solutions for atomic absorption. The sample

was nebulised by a conventional total consumption burner which

was insulated, the aerosol (no visible discharge occurred when 59.

the electrodes were put across a flame, the flame being a good

conductor)' passed between two electrodes between which the d.c.

arc was struck. Atomisation took place in the spark. For -1 aluminium 20% absorption was obtained from 10fGgml at the

394.4 nm resonance line. The method was reported to be efficient .at producing atoms of those metals which form refractory oxides

in flames, e.g. Al, Ti, W and Mo, but the spray became highly

charged and strong electrical and magnetic fields affected the

detection system of a normal spectrophotometer and therefore a

quantometer was used. Conventional hollow—cathodes were used and the

high emission signal from the discharge was troublesome. 176 Belyaev and his colleagues have used the d.c.

arc from a carbon electrode, the samples being placed in a channel

in the electrode, for atomisation. Using a current of 5-1C amps

measurements were made over the time taken to completely consume

the sample, for volatile elements., e.g. Zn, Cd, Pb and Bi, about

20 seconds and for other elements, e.g. Mo, Cr and Sn, about 60

seconds. Measurements were made by integrating the signal using

charge storage by capacitor, the integrated signal being expressed

in intensity difference units (I° — I). For the elements listed

above relatively high sensitivities were obtained for determination

of traces in carbon powder (104 to 10 60)with a variance of 15%.

Unfortunately no matrix effects were investigated. 177 Kantor and Erdey have used a time resolved 60. a.c. arc spectra for atomic absorption spectroscopy. An a.c.,arc and a rotating sector to shield the detection system from emission being used. High intensity Osram lamps or tungsten filament lamps were used as primary sources the light, being focussed onto the arc gap. Absorption spectra from Cd, Na, Al and Pb were obtained using a graphite electrode and an argon atmosphere, rapid oxidation in air appearing to seriously deplete the atomic populations especially of Al and Pb. 178 Other workers have also reported the use of a d.c. arc in atomic absorption studies.

Lasers

The use of laser energy to atomise solid samples offers the possibility of the examination of surfaces by atomic 179'180. 181 182 spectroscopy Karyakin et al ' have described in detail the atomisation of samples by pulsed laser beams. A parallel beam of light generated by a laser is focussed, by means of a lens, on the surface of the sample. Normally the pulse energy is between a few joules and as few tens of joules. Emission or absorption is observed in the atomic vapour leaving the surface of the sample as a result of the pulsed heating of a small area of the surface, diameter ca. 0.1 mm, to 5,000 to 10,000°C. This technique has been used in spark emission spectroscopy, a spark 61. is passed across electrodes placed just above the surface so that it passes through the plume of atomic vapour produced by 183-5, the laser the method is particularly useful in the detection of micro-concentration gradients on the surface of 186 solid samples . Most quantitative applications of laser-spark atomisation are restricted to one type of material where it can be assumed that a reproducible amount of sample is atomised, although techniques have been proposed187'188 to over-come this, but as they involve appreciable sample preparation this prevents their use in situ for analyses of microsamples such as inclusions and 189 surface deposits. Recently Webb and Webb have proposed a semi-quantitative laser-spark microprobe using recorded excitation spectra.

The same problems were found in the laser micro- probe atomic absorption apparatus devised by Mossotti, Laqua and 190 Hagenah . A ruby laser was used which consumed between 0.1 and

10 /g of sample per pulse. The absorption .•ras followed with a fast response recording system, i.e. oscillographically, the absorption pulses were found to represent symmetrical peaks with a total duration of 100 /.sec. The peak photo-electric method of recording absorption was used and the total amount of sample atomised determined from the emission from the principal component of the sample. The primary radiaticin source was a.xenon arc- 62. lamp and a multi—pass optical system was used. Reproducibility was very poor, scatter was 30-60% and the absorption was markedly dependent on sample matrix, e.g. the absolute sensitivity for Mn, Cu, Ag and Cr varied by three orders of magnitude, between -6 10 and 10-9g, depending on the base substance, iron, zinc or lead. Because of the fast response recording system, circuit time constant 15p. sec, the shot noises were 4% and absorptions less than 10% could not be measured. 191 Kaxyakin and Kaigorodov used an impulse laser in the atomic absorption analysis of copper in potassium bromide, alumina and. silica. Between 0.3 and 10% copper was determined with a precision of 7% and a reported sensitivity of 100 ppm.

Pulsed Lamps 192-195 Nelson and Kuebler used pulsed lamps to analyse elements with average or low volatility. Solid samples such as wire, ribbon or powders were placed on a tungsten grid, or quartz or graphite strips, supported in an enclosed, inert gas purgsd, quartz tube. This tube is housed in a spiral capacitative discharge tube. A powerful source of light with 2 a flash energy of 30 5/cm , the flashes lasting about 3 m sec, enables the grid or strips to be heated to several thousand degrees centigrade, and the applied sample is atomised. A light 63.

pulse from another Lyman—type lamp emitting a continuous

spectrum is passed through the tube with a time—lag of 1 m sec

relative to the start of the main pulse. The spectra of these

illuminating pulses, lastin,- 20 pLsec are photographed with a

grating spectrograph. The advantages of the technique appear

to be: isolation of the sample from the heating device, easy

sample interchange and an inert atmosphere stabilises the neutral

atomic vapour produced. Although the method has not been used

quantitatively Nelson and Kuebler195 have reported absorption from: Au, B, Cu, Dy, Fe, Ph and W produced from the solid element, Al and Eu from their chlorides deposited on'graphite strips and Ag, Al, Ca, Cu, Fe, Mg and Zn from impurities in tungsten. Strong absorption was reported from only p.p.m. amounts of some tungsten impurities, i.e. Al (10 p.p.m.), Cu (50 p.p.m.),

Mg (50 p.p.m.) and Zn (300 p.p.m.).

The major requirements of a non—flame atom cell for atomic absorption and fluorescence spectroscopy can be summarised as:

1) efficient atomisation, for this high temperature and a reducing atmosphere are to be preferred;

2) freedom from background emission or absorption over the 64.

spectral region which may be used for analytical measurements,

i.e. ca. 90 to 900 nm;

3) relative freedom in atomic fluorescence from radiation quenching species;

4) freedom from matrix interferences;

5) a relatively long residence time for the atomic population

in the optical path, unless fast response recording systems are available;

6) the geometry should be such that for atomic fluorescence

the cell should enable irradiation and collection of fluorescent radiation over a large solid angle and in atomic absorption the cell should provide a long absorption path—length;

7) the cell must atomisesamples reproducibly, be relatively inexpensive, convenient and simple to use.

No non—flame atom cell yet reported meets all these requirements, but perhaps three based on heated graphite (which can be heated up to ca. 3700°0 at atmospheric pressure, in an inert atmosphere, and provides a strong reducing environment) 06 65 appear to offer most promise: the furnaces of Livovl and Massmann 67 and the filament of West . The major disadvantage of the first two of these techniques lies in applying their essentially closed systms to fluorescence, whereas the third might be expected to suffer more from matrix effects as the atomised vapour cools above 65.

the rod. A modification of West and William's carbon filament

atom reservoir was chosen as the non-flame cell for work

presented in this thesis.

1.4. Scope of 'Tork presented in this thesis

Recent advances in analytical chemistry have often

occurred in two apparently divergent fields. Trace analysis has

been concerned with a very minor constituent in a large sample,

whereas micro-analysis has been concerned with a major constituent

in a very small sample. Because of the large samples required for use in flame cells analytical atomic spectroscopy has been identified

with trace analysis, however the development of non-flame cells such

as the carbon filament atom reservoir which require only 0.5 to

5 /41 of sample should now enable trace analysis of micro-samples to be accurately performed.

The analysis of trace metals in plastics provides such a problem. Techniques are now available to ash ca. 1 mg samples of plastics using low temperature RF ashing without loss 196 of even volatile elements such as sodium . If such plastics, however, contain less that 10 3% of trace elements even a few microlitres of solution from its ash will contain considerably -1 less than 1 pt-gml of such trace elementS. The work in this thesis was directed towards the analysis of traces of iron and 66.

manpanese in carbon—fibre composites. An improvement in the

sensitivity of a flame—cell by using atomic fluorescence

spectroscopy with microwave excited electrodeless discharge

lamps as spectral sources will be described, as will the

development of the carbon filament atom reservoir as a practical

analytical tool and its application 'to the determination of traces of iron in carbon—fibre composites. 67.

CHAP ER

Experimental Parameters and Description of Apparatus

2.1. Experimental Parameters

The basic instrumental systems used in analytical atomic spectroscopy have been shown in Figure 1. Care must be giVen to the design of the basic units: the source, the atom cell (or atom reservoir), the monochromator, the detector and the read—out system. A number of theoretical and practical factors affecting their design will be discussed belmi.

2.1.1. The source

No spectral source is required in atomic emission spectrometry but the design of the spectral source in atomic absorption and fluorescence spectrometry is a critical parameter.

Atomic line sources are generally used in atomic absorption measurements as they proVide radiation characteristic of the element to be determined. The monochromator only serves then to isolate this line from other source lines and unwanted backgroul,d, therefore a monochromator of poorer resolution (see section 2.1.3.) can be used, compared to the high resolution monochromator required (a spectral band pass 68. of < 0.05 nm) for a continuum source. To obtain linear working— curves it is essential that the emission line—profile from the source is appreciably narrower than the absorption line—profile in the atom cell197. It has been noted previously (section

1.2 2.) that for temperatures of 1,000 to 3,000°K and foreign gas pressures of about 1 atmosphere the spectral line profiles are determined by the Doppler and Lorentz effects, and in part— icular the central line—profile is determined primarily by the

Doppler effect Thus the width of both the absorption and emission lines are proportional to the square root of the absolute trAlperature. The temperaturesof most analytically

,useful flame cells are typically greater than. 2,300°K but the temperature of +he atomic vapour in the carbon filament atom reservoir may be significantly less than 2,000°K. It follows that the line profiles in the latter case will be considerably narrower and in consequence the requirement for a narrow line source will be stricter. The other major requirement for the source is that it should be stable both with respect to long and short term fluctuations. The sensitivity in atomic absorption is independent of the intensity of the source and therefore the source need only provide an adequate current at the photodetector with the optics employed, and permit a narrow slit—width to be' used in order to resolve the line of 69.

analytical interest.

A hollow—cathode lamp most closely meets these requirements for a spectral source in atomic absorption. The

luminescence in a hollow—cathode during a d.c. glow discharge 198 was first discovered in 1916 by Paschen and a low—pressure

hollow—cathode lamp was used by Walsh6'7 in the earliest

analytical atomic absorption measurements. Since that time

considerable advances have been made in the manufacture of 199 hollow—cathode lamps8 ' particularly with regard to

increasing the stability, working—life and intensity of the

lamp without increasing the self—absorption of resonance lines,

e.g. the high intensity lamps of Sullivan and Walsh200 where secondary electrodes are used to boost the spectral output,

and the 'Intensitron' ceramic shielded hollow cathodes recently designed by an instrument company, in which attention

is also given to optimising the cathode size, shape and material, the fill gas and its::pressure, and some elements are run in

a 'molten' state

Sources in atomic fluorescence spectrometry should

be:

1) intense at the principal resonance lines, the intensity of

atomic fluorescence being directly proportional to the source

intensity (see section 1.2.3.) and be essentially free from 70.

self—reversal at these lines;

2) stable both over the long term (i.e. no drift) and over

the short term (i.e. no flicker);

3) available for a large number of elements;

4) of low cost, safe to operate and with both a long working

and shelf—life.

Both continuum and line sources have been widely used, but

as the intensity of line sources over the absorption line

is generally higher than that of a continuum source, and line

sources are more convenient to use and generally result in fewer

spectral interferences, in the work described here the use of

line sources was preferred.

Electrodeless discharge lamps can be prepared

to emit sharp and intense spectral lines and good stability can

be obtained. Electrodeless discharge lamps are operated in a

microwave field which causes excitation of the atomic vapour

contained in the discharge lamp. The lamp normally consists

of a sealed quartz tube ca. 3-4 cm long containing a fewpg

or mg of a metal, or a metal salt, under an inert gas pressure of

a few Torr. Jackson203 in 1928 prepared a:,caesium electrodeless 204 discharge lamp for spectral studies. Ninefordner and Staab first used commercially available electrodeless discharge lamps for analytical atomic fluorescence studies and Dagnall, Thompson 71.

205 and West were among the first to realise the potential of

suitably prepared electrodeless discharge lamps for atomic

fluorescence. The preparation of electrodeless discharge

tubes and their use in atomic fluorescence spectrometry :rill

be described later in this thesis. The spectral output and

stability of such lamps compared to other available primary sources

strongly favour their use in the atomic fluorescence spectrometry

of iron and manganese.

2.1.2. The Atom Cell

A discussion of atom cells has been given in

Chapter I. In this thesis both a flame cell, the air—acetylene

flame, and a non—flame cell, the carbon filament atom reservoir

were used.

2.1.3. The Monochromator

Although the isolation of the spectral line of

analytical interest may be made by use of an absorption or

interference filter, indeed many inexpensive flame photometers use absorption filters for atomic emission spectrometry and

the use of interference filters in absorption spectrophotometers 207 and more particularly atomic fluorescence spectrophotometers

has also been reported, the use:of quartz prism or grating 72. monochromators is generally preferred. This is because the much greater resolution of even a poor monochromator greatly reduces spectral interferences. In atomic emission studies the monochromator should be of sufficient resolution to exclude emission from interfering species in the flame, and exclude as-::much of the flame background as possible (the radiation reaching the detector from a spectral line is directly proportional to the spectral band—pass of the monochromator, whereas background radiation from the flame continuum is proportional to the square of the spectral band—pass). Similar considerations apply in atomic fluorescence except that in cells of high background synchronous modulation of the excitation source should remove most background inter— ference. In atomic absorption the requirement is that the monochromator should isolate the spectral line of analytical interest from other lines in the source. There are several examples of analytically useful resonance lines near to which lie non—resonance lines (or resonance lines with a different transition probability) of the analyte element, ionic lines or fill—gas lines, e.g. near to the highly sensitive iron 248.33 nm resonance line lies the 248.42 nm iron line, and failure to resolve such lines results in curved calibration curves and reduced sensitivity. In a few—rare instances poor resolution in atomic absorption and 73.

38 atomic fluorescence may result in spectral interferences .

In atomic emission measurements the optical systems employed with the monochromator are simple, greatest sensitivity is obtained by collecting the maximum possible of the emitted radiation, that emission being, of course, directions. This is usually achieved by focussing an image of the flame on the monochromator entrance slit or by placing the flame as close to the entrance slit as possible. The same considerations apply in atomic fluorescence, with the additional need to illuminate as brightly as possible the atom cell with the source. This can be achieved by focussing the source on the cell or placing the source as close to the cell as possible.

In both these techniques a monochromator optical system with a large aperture should be employed. Three optical systems are commonly employed in atomic absorption: 1) the image of the source is focussed at the centre of the atom reservoir and this may then be focussed on the monochromator slit; 2) the source is focussed on the monochromator slit the atom reservoir being irradiated by a converging beam; 3) the atom reservoir is irradiated by a parallel beam which is then focussed on the monochromator slit. Quartz is used throughout the optical system as the majority of useful resonance lines lie in the ultra—violet region. 74.

In this study a modified Unlearn SP 900A

(Unlearn Instruments Cambridge Ltd.,) spectrophotometer with

a quartz prism monochromator was employed.

2.1.4. The Detection and Read—Out Systems

A photomultiplier is almost- rithout exception used as the detector but the methods of amplifying and displaying

the photomultiplier signal have varied greatly. A particular

concern in the work described in this thesis was to optimise the recording circuit to give the highest possible signal—to— noise ratio, particularly for highly transient signals.

Three methods of measuring the signal in analytical atomic spectroscopy have been proposed: the equilibrium method, the peak method and the integration method22. The equilibrium method

of measurement is commonly used with flame cells where sample is

1: 2/f 4:1, where introduced to the flame at a constant rate and • 'Ic1 -c 1 is the overall .length of time during which atomization occurs is the mean length of time spent by an atom in the and T.2 4 58 analysis volume (for flames t2 = ca. 10 sec ) In non—flame cells where the sample may not be atomized at a uniform rate, e.g. thermal atomisation where evaporation occurs at a constantly increasing evaporator temperature, and t1 may be less than

-C the peak method of measurement is preferred. The 2 75. integration method of measurement is applicable to both types of cell and produces greater sensitivity, especially when 1;2 is large, however more complex instrumentation is required to use 22'129 this method. The reader is referred to L'vov's work for a more comprehensive treatment of the theory of the measurement of analytical signals.

The recording of peak signals requires that the _ response time of the recording circuit be sufficiently small to enable the peak concentration to be recorded without any great distortion. For this to be so the response time should be 208. less than the life—time of the signal129;

When used with a flame and the equilibrium method of measurement the a.c. amplifier of the SP 900A spectrophotometer with a chart—recorder met the above requirements. The largest time constant in the system came from the time needed to re— establish equilibrium conditions, this was a function of the aerosol spray chamber, and this ,was measured as ca. 16.5 seconds full scale deflection (half—full scale deflection was ca. 3 seconds).

When using the carbon filament atom reservoir because of the non— uniform rate of atomisation the peak method of measurement was used. In this case the response time of the system of the

SP 900A amplifier and a chart recorder was too large to be employed, except for the slow atomisation at low temperatures of metals of high volatility. For elements of average volatility, 76.

such as manganese, it was still possible to use the SP 900A

amplifier but the signal was read—out on a storage oscilloscope,

this involved a time constant of ca. 0.6 sec for full—scale

deflection (for half full—scale deflection ca. 0.2 sec). For

elements of high volatility, such as iron, a faster response system was required and this was obtained by modifying the

photomultiplier detector system of the Unicam and feeding the

photomultiplier output direct to the amplifier of the oscil—

loscope, thus by—passing the SP 900A amplifier. The response

time of this system was found to be ca. 700 Itsec for full—scale

deflection (ca. 150 r sec for half full—scale deflection). This

latter system was not only necessary to separate the analytical

signal from the thermal emission continuum of the glowing filament

at high filament temperatures, it was also found to give highest

sensitivity, presumably by allowing higher temperatures to be used and Z1 to be reduced, which increased the instantaneous atomic

concentration, and by reducing signal distortion.

2.2. Description of Apparatus

Commercially available hollow—cathode lamps and

electrodeless discharge lamps were used as spectral sources.

The preparative scheme for the electrodeless discharge lamps 77. used is described below.

2.2.1. Preparation and Operation of Electrodeless Discharge Lamps

The preparative scheme outlined here follows closely that reported by Dagnall and West- . The discharge lamps were made from transparent, vitreous, qtiartz—tubing supplied by

Messrs. Jencons Ltd., (liemel Hempstead). The internal diameter was 8 mm with ca. 1.1 mm wall thickness. A length of tubing of about 12 cm, sealed at one end and with a constriction at

2-4 cm from this seal was prepared (4 cm bulbs being preferred for volatile elements and 2 cm bulbs for less volatile elements).

The quartz was thoroughly washed in successively distilled water, dilute Lissapol detergent solution, distilled water, acetone and distilled water. The tube was then attached to a vacuum line via a short piece of medium pressure tubing and a B-10 socket. The vacuum—line used is of simple construction consisting merely of a two—stage air—ballasted rotary pump

(Edwards High Vacuum Ltd.), two silica gel moisture traps, two two—way taps as a means of introducing argon from an argon cylinder, a three—way tap for evacuation, maintenance of vacuum and introduction of air to the pump or the line, three

B-10 outlets and a vacuostat guage (Edwards High Vacuum Ltd. -2 Crawley, 10 — 10 Torr). Argon was introduced into the line • -1 and then the line evacuated to a pressure of below 10 Torr, 78. argon was re—introduced and re—evacuated. The lower half of the quartz tube was then heated to just below its melting—point for about three minutes, the pressure being maintained below 1 10 Torr. This degassing process was repeated after flushing with argon and re—evacuating.

After cooling the tube was disconnected and the required amount of metal or metal salt added. This varies from a few pbg for volatile salts, such as zinc chloride, to a few mg for involatile salts such as transition metal chlorides (especially chlorides which irreversibly decompose in the tube). The tube was evacuated and flushed twice with argon (if a hydrated salt is used it should by now be dehydrated in situ), and the metal or salt sublimed onto the walls of the tube, the constriction being kept hot to prevent any condensation. there which would prevent efficient sealing. The tube was disconnected from the line and any unsublimed compound knocked out. This simple procedure removes many impurities and leads to extended lamp — life.

The tube was reconn-'cted to the line, evacuated ( and flushed with argon twice. When an iodide was to be prepared in situ a few rg of: iodine were now added and sublimed and the iodide formed, otherwise the pressure was set to 3 Torr argon and a lamp bulb formed by sealing the tube at the constriction. 79.

A 'Tesla' vacuum leak—tester being used to confirm the quality

of the seal.

The pressure of argon fill—gas is critical, tubes

with argon pressures below 2 Torr are difficult to light (an

electrodeless discharge lamp is lit by placing it in a microwave

field and initiating a discharge with a spark from a 'Tesla Coil')

and when lit are unstable. Intensity decreases with increased

argon fill—pressure however, and an argon fill—pressure of 3 Torr proved

•most satisfactory.

The electrodeless discharge lamps used in this

study were operated in a g wave Broida—type resonant cavity

(No. 210L, Electro—Medical Supplies, London). The power being

supplied by a 'Microtron 200 Mark II' microwave power generator

(Electro—Medical Supplies) capable of supplying 20-200 watts

at am- operating frequency of 2450 ±25 Hz (12.5 cm). The operation

of electrodeless discharge lamps and the 210L cavity have been 209 adequately described elsewhere .

When necessary the lamp output was modulated

at 50 Hz using a 'Microtron modulator Unit Mark II' (Electro—

Medical Supplies Ltd.). The modulation is effected by introducing

a modulation signal to the anode voltage of the magnetron in the

microwave generator. The SP 900A amplifier is then triggered

by the synchronisation output from the modulator unit and not 80.

by the phasing coils of the standard absorption rotating

sector. The use of such electronically modulated microwave-

excited electrodeless discharge lamps has been reported by

Browner, Dagnall and West

2.2.2. The Nebuliser and Air-Acetylene Flame

The standard nebuliser and air-acetylene burner

of.the Unicam SP 900A was used during the work with flame cells

described in this thesis. The concentric jet nebuliser consists

.of a metal capillary tube (80% Pt, 20% Ir) which receives sample

through a polythene capillary and the resultant spray is spun

in a 1 litre glass expansion chamber. This cyclone nebuliser

acts in such a manner that larger droplets are removed by centri-

fugal force and only a fine homogenous mist reaches the burner. 1 The maximum take-up rate of sample is about 3 ml min . 211 Gaydon and Wolfhard have calculated the maximum

temperature of the air-acetylene flame to be 2250°C and it is the

generally accepted flame cell for the atomic absorption analysis

of iron and manganese. The low flame background in the ultra-

violet region is an additional advantage and even this low

background can be reduced by separating the flame. Separation

proved advantageous in the determination of manganese where the

analytically useful line at 279.5 nm lies close to the 281.1 nm

OH bands. 81.

For atomic emission and atomic fluorescence the circular stainless steel Meker—type Unicam burner was used.

The acetylene and air/sample mixture is ignited through a series of concentric holes in the burner head. The flame was separated when desired by sheathing it with a laminar flow of inert gas which caused the secondary diffusion zone to 'lift off' the 63 primary cones . The design of the separator was the same as that described by Hobbs, Kirkbright and West212 using a coil of alternate layers of corrugated and flat cupro—nickel strip.

For atomic absorption measurements. a long path burner is provided.

This is fitted merely by unscrewing the round burner—head and screwing onto the burner—stem the long-path Meker type burner.

2.2.3. The Carbon Filament Atom Reservoir

A diagram of the modified carbon filament atom reservoir used in the work described here is sho:•m in Figure 4. The reservoir is fixed rigidly to the SP 900A via the stem, a collar, and an optical base held by the former burner support screws.

One of the electrodes is isolated from the base by means of a mica collar. Screws in the collar permit ready adjustment of the height and orientation of the filament with respect to the primary source and the monochromator entrance slit. The electrodes are water—cooled to provide reproducible electrical contact and to facilitate speed of analysis. The inert gas flow 82.

Figure 4 Diagram of Carbon Filament Atom Reservoir

carbon filament

water-cooled Titanium electrodes laminar-flow box for .shielding gas water-cooling link

Brass base-plate

I .11 inlet for shielding gas

transformer terminals

water inlet and outlet

-'\support stem for reservoir 83.

provides a sheath around the filament to prevent oxidation,

-the filament being heated without any other protection.

Laminarity of the gas flow is provided by packing the copper—

box through which the gas enters with alternate strips of

corrugated and plain cupro—nickel foil. When the gas—flow

was monitored visually by adding ammonium chloride (by

bubbling the gas successively through concentrated hydrochloric

acid and concentrated ammonia solution) increased laminarity

appeared to be provided by means of a packing of small

capillary tubes (ca. 1 mm internal diameter held together

by tAralditet), but such a packing failed to shield the ends

of the filament close to the electrodes which consequently burnt

away and reduced the useful life of the filament. Reproducible

electrical contact is maintained by the two tightly fitting

electrode caps each of which are held in place by two screws.

Titanium-was used for the electrodes and their caps to prevent

contamination from iron which might have arisen from the use

of stainless steel electrodes. The filament is cut from 2 mm

diameter carbon (graphite) rod (,Morganitet electrodes, type

WW5 from Morgan Crucible Group). At the centre of the filament

a small notch is filed, 4 mm long, 2 mm wide and 1 mm deep, onto

which a 5 rl sample (a smaller notch being needed for 1 .„..1

samples) will fit reproducibly. The drop being held in place 84. by surface tension in a notch so dimensioned. This notch halves the thickness of the rod, therefore at this point resistance is greatly increased and when the filament is heated the filament temperature is highest at the notch (tungsten powder placed on the filament at this point was observed to melt, m.p. W=3370°C, at a filament voltage of 12V).

Current was supplied to the filament from the a.c. mains via a 1.2 stepdo—n transformer and Variac control which permits the voltage across the filament to be continuously varied from 0 to 12 V; the maximum load current being estimated to be ca. 70A. Calibrated capillary glass pipettes of 1 or 5 ti,1 volume (Drummond Scientific Co., U.S.A.) treated with

Silicone 'Repelcote' (Hopkin and Williams) water repellant, to enable reproducible delivery, were used for sample delivery.

Using the operating conditions which will be described in later chapters one rod could be used for 200-300 determinations.

No memory effects were observed except in the instance of

2 mg Ca which required two pulses of the filament for complete removal. Scatter problems did not become serious even using disadvantageous matrices until the 1 mg level using the absorption technique and the ca 0.25 mg level using the fluorescence technique. 85.

2.2.4. The Monochromator

The Unicam SP 900A monochromator was used which is fitted with a 45° suprasil prism, which gives good transmission over the range 210 to 850 nm. The monochromator has a high light— gathering power with an aperture of f 4.5 based on the Czerny compensated spherical mirror system. The entrance and exit slits of the monochromator are ganged and may be simultaneously and continuously adjusted between 0 and 2 mm.

2.2.5. Unicam SP 900A Detection and. Amplification System

The normal EMI 9529E photomultiplier supplied with the instrument was replaced by an EMI 9601B photomultiplier to obtain higher sensitivity in the•wultraviolet region. The photo— multiplier tube, which acts as detector, recieves radiation at

100 c.p.s. In the 'emission' mode, used for atomic emission studies, the radiation leaving the exit slit of the monochromator is modulated by a rotating sector (100 c.p.s.) whose phasing coils trigger synchronously the amplifier. Hence both the analytical signal and flame background are amplified by the a.c. system. With the 'absorption mode' of operation this sector is held in the open position and the continuous background radiation is not amplified. A second rotating sector (100 c.p.s.) is pos— itioned between the spectral source and the atom cell and thus 86. an a.c. absorption or fluorescence signal reaches the detector and is amplified. When a modulated electrodeless discharge lamp is used the phasing coils of the absorption sector were removed and the SP 900A amplifier triggered from the synchronisation output from the modulator unit. Using the absorption mode the atomic emission signals at the analytical line, which using flames are often significant, are not amplified. The absorption mode when using the carbon filament atom reservoir was not used because this atom reservoir has been found to possess, insufficient energy to excite atomic emission, and modulation at 100 c.p.s. does not efficiently reduce the continuous thermal emission background of the glowing filament, because this glow has a mains

50 c.p.s. ripple superimposed on it. Therefore in instances where the 'Unlearn amplifier was used with the carbon filament the emission mode was used.

The Unlearn SP 900A photomultiplier and detection unit are shown in Figure 5. The a.c. signal from this unit is converted to a d.c. voltage by a homodyne rectifier system.

This voltage is then applied to a conventional mirror—type galvonometer with a translucent scale provided with the SP 900A, or to an external pen recorder, or to the Y—axis amplifier of a ,-_, storage oscilloscope. A servoscribe RE 511 chart recorder

(Smith's Industries Limited) and Telequipment Storage Oscilloscope 87.

(Type DM53A, Telequipment, London) with K-type (20V/cm to 0.1 mV/cm) and JD-type (50 V/cm to 0.1 V/cm) plug in amplifiers were used.

2.2.6. Modified Detection System and Amplifier

Because of the relatively long response time of the

Unicam amplifier as described above (section 2.1.4.) and the possibility of distortion of transient signals (<0.1 sec) by

100 c.p.s. modulation, the Unicam amplifier was replaced. The detection unit was modified to receive a d.c. signal as shown in Figure 6. The negative B.H.T. supply was a Brandenburg

Model 470 Regulated Power Supply (2.5 Kv and 5 mA) and the signal was led straight from the photomultiplier output to

-;:he K—type amplifier of the storage oscilloscope. 88.

Figure 5 Unicam SP 900A Photomultiplier and Detection Unit

Signal (a.c.) . 0.1 FF .H.T.(4-ve) OX 33K 33K 33K 33K C 33K 33K EMI 9601B 33K Photomultiplier 33K 33K -C 33K 1Mx = 0.25FF vws. 33K 1Mx 71:ala ( 33K .0.25FF 89.

Figure 6 Modified Photomultiulier and Detection Unit

©Signal (d.c.)

on.Nv 1Mx 33K C 33K C 33K C 33K 33K -c 33K C 33K EMI 9601E C Photomultiplier 33K

33K 11ix _L 0 . 2 5p.F 33K 1Mx 33K7,4 1Mx --0.2•pF 40.24F C T.W-ve) 90.

CHAPTER 3

The Determination of Iron Using a Flame—Cell

3.1. Introduction

The analytical atomic spectroscopy of iron has 213 been extensively studied in flame cells. Allan first determined iron by atomic absorption, using a hollow cathode lamp as the source. Allan obtained absorption at 11 wavelengths,

248.3 nm being the most sensitive wave—length. Since this early work considerable improvements have been made in the sensitivity of iron determinations. Slavin214 reported a -1 sensitivity for 1% absorption of 0.15rgm1 using an air— acetylene flame and the 248.3 nm iron resonance line, with a detection limit (signal to noise 2:1) of 0.01 p.gml-1. Slavin also reports sensitivities of below 1 itgm1-1 for 8 other iron lines. The air—acetylene flame is reported to offer four times the sensitivity of the nitrous oxide—acetylene flame. 215 Robinson and co—workers confirm that of the following flame cells — oxy—hydrogen and oxy—acetylene total consumption burner flames and premixed air—acetylene and nitrous oxide— acetylene flames — air—acetylene offers the greatest sensitivity. 216 De Galan reports that iron is almost completely atomised 91. in the air-acetylene flame and that this flame provides optimum sensitivity, because in comparison it has the smallest volume. 217 Smith and Frank have reported 31 usable resonance lines of iron for atomic absorption spectroscopy but in common with other workers prefer the 248.3 nm line. Two major manufacturers of atomic absorption spectrophotometers both claim a detection -1 218 219 liMit of 0.005 fkgmf at 248.3 nm using aqueous solutions ' 218 in one case this corresponds to a sensitivity for 1% absorption of 0.062rgm1-1.

Iron has been determined in a large number of matrices using atomic absorption spectroscopy. In particular, 220 iron has been determined in plastics. Druckman determined iron in polypropylene by ashing the plastic in a muffle furnace and dissolution in molten sodium carbonate followed by 221 dissolution into sulphuric acid. Farmer analysed for iron in Nylon 66 after ashing, dissolution into hydrochloric acid and an ammonium pyrroldine dithiocarbamate - methyl isobutyl ketone extraction system to remove titanium dioxide. 222 Olivier determined iron and other trace metals in polymers by ashing with sulphuric acid or hydrogen peroxide and'by analysis of a 2% polymer solution.

A number of reports of chemical interferences on the determination of iron in air-acetylene flames by atomic absorption spectroscopy have been reported. Platte and Marcy • °2/ •

-1 . reported that the interference of up to 200 ag ml sllica on -1 1 jig ml iron (a negative interference of over 20% at some —1 levels) could be removed by the addition of 50 rg ml calcium chloride. Ourtisfound224 interferences could be reduced to a negligible level if the flame was fuel lean and the light beam passed through the hottest portion of the flame only. 225 Ottaway et al found interferences similarly dependent on flame parameters and on the anion; they used 8—hydroNyquinoline to remove interferences. Roos and Price have reported the inter— 226 227 ference of silicate and citrate . The silicate interference was removed by the addition of lanthanum as releasing agent and that of citrate by the addition of. phosphoric acid or sodium chloride. 228 In a recent paper Roos has, on the basis of a flame atomisation model, suggested that the effect of chemical interferences directly on elemental iron, rather than oxide species, is important. Sachdev et a1215 found no interference from fifty fold excess of 54 ionic -1 species on the atomic absorption of 5 fig ml iron, however, a 5% suppression from 5% sulphuric acid (attributed to the increased viscosity of the sample solution) and 50% and 20% suppressions form silicon and titanium respectively were found. The silicon interference was removed by the addition of EDTA. The depression of signal 229 by aluminium and silicon and by acids230 (with the exception 93. of hydrochloric acid, where it was postulated that the signal enhancement observed was due to the chlorinating effect producing volatile iron chlorides) has also been reported.

Passel et a138 have reported a spectral interference from the strong overlap of the Pt 271.9038 nm line on the iron

271.9025 nm line. The 271.9 nm iron line is rarely, however, used for atomic absorption measurements.

In most studies of the atomic absorption spectroscopy of iron, a hollow—cathode lamp has been used 231 as the primary. light source. Human et al have reported the use of a gas stabilised arc as primary light source and reported their technique as two thirds as sensitive as the conventional technique using_a hollow—cathode lamp. Marshall 232 and West report the determination of iron by atomic absorption spectrophotometry using both a hollow—cathode lamp and a microwave—excited discharge tube, they reported improved sensitivity and range of linearity and this is discussed further below (section 3.5). A report of an atomic absorption analysis with a high—frequency excited iron iodide lamp has also been published

An increasing number of investigations of the

atomic fluorescence spectroscopy of iron at the 248.3 nm resonance line have been published. In the first reports continuum light 94.

sources were used. Dagnall, Thompson and West234- used a

150W xenon arc source and obtained a detection limit of 1 51...I.gml in both an air—propane flame and an air—hydrogen flame. With an air—hydrogen flame and a total consumption

burner, Bratzel et al235 obtained the same limit using a

150W Eimac xenon arc source with a colour temperature of

6,000°K. Using a 450W xenon arc source and hydrogen— 236 entrained air flame Demers reported a detection limit -1 237 of 1.8/4gml Omenetto and Rossi used a 90W mercury

discharge lamp as the source and obtained a detection limit — 1 of 114gml for iron. Manning and Heneage238 have compared

the use of a 150W xenon arc and a high brightness hollow

cathode lamp and reported detection limits of 1 and 0.4rgml

or iron respectively. More recently several workers have reported the use of a hollow—cathode lamp as the excitation

source. Using a hot hollo-T—cathode lamp Dinnin239 reported —1 a detection limit for iron of 5Iugml whilst Rossi and

Omenetto240 used a demountable water—cooled lamp run at

500 mA to obtain a detection limit of 2r-gml-1. Using a

high—intensity hollow cathode lamp of the Sullivan and Walsh 200 241 type Matousek and Sychra made a detailed study of the

atomic fluorescence of iron reporting detection limits at

five wavelengths, of which 248.3 nm was the most sensitive. 95.

At this wavelength five different flame cells were compared

and air—hydrogen was preferred as this gave a detection limit -1 of 0.02 fcgml . A solvent extraction system was also proposed.

Mitchell and Johansson207 have reported the use of pulsed iron

hollow—cathode lamp and a rotating filter wheel in simultaneous

multi—element atomic fluorescence spectrophotometry, and the

use of a solar—blind photomultiplier with a high—intensity

hollow cathode242 and a modified high—intensity lamp243 have

also been reported.

However, in the most sensitive determinations micro—

wave excited electrodeless discharge lamps have been used.

Winefordner and co—workers244, using such a lamp, containing

iron and ferrous iodide prepared in situ, obtained a detection — 1 limit of 0.25mgml iron using an air—hydrogen flame. Dagnall, 245 Taylor and West used a lamp prepared from ferrous chloride —1 and obtained_detection limits of 0.008 and 0.50 r.gml iron at

248.3 and 327.0 nm respectively using an air—propane flame.

Bratzell, Dagnall and Winefordner246,247 have

attempted to evaluate the flame cells used in iron atomic fluorescence spectroscopy. Of air—hydrogen flames they

preferred turbulent flames of hydrogen based flames they recommended +he use of nitrous oxide—hydrogen.

Atomic fluorescence has been used in few practical 96. analysis, but Smith et al248 have determined 0.1 ppm iron in lube oils using a 900W Xe arc and electrodeless discharge lamps.

Cotton and Jenkins249 have determined 0.04 ppm iron in kerosine using a 450 -W Xe arc and a novel burner, however the limit of detection obtained by them using aqueous solutions was only .

0.16 itgml .

The determination of iron by flame. emission spectroscopy has been an established analytical technique for several years. Conventionally, an air-acetylene flame has been used as the atom cell although recently the emission of iron from the inter-conal zone of an oxy-acetylene flame has -1 been used250 to obtain an improved detection limit of 0.7p.gml

of iron at 372 nm. The use of an oxygen enriched air-acetylene flame has also been reported251 with a detection limit of 1 252 0.2jLgml iron at 372-nm. Pickett and Koirtyohann have used a nitrous oxide-acetylene flame on a slot burner to obtain -1 a detection limit of 0.05 rgml iron at 372 nm and a reduction

of interferences. Hobbs, Kirkbright and West212 investigated

a nitrogen-separated air-acetylene flame and, although the emission intensity of iron at 372 nm was reduced, on separation the reduction in flame noise observed enabled a detection limit -1 -1 of 0.0314gm1 to be obtained, compared with 0.5/4gml in the

conventional flame. 97.

In this chapter a study of the analytical atomic

spectroscopy of iron using an air—acetylene flame is presented.

In particular the optimum conditions for the sensitive and

selective determination of microgram amounts of iron by atomic

fluorescence spectroscopy and the preparation and properties

of iron microwave—excited electrodeless discharge lamps for use

as excitation sources are described.

3.2. Reagents -1 Iron stock solution — A 1,000/igml stock solution of iron was

prepared Tiy dissolving 7.136 g of iron (II) ammonium sulphate

(A. R. grade) in 1 1 of 0.5 M sulphuric acid. This solution was

diluted as required before use.

Diverse ions — Solutions of diverse ions were prepared from

analytical reagent grade salts.

3.3. Preparation and Operation of Electrodeless Discharge Lamp

The preparation of iron electrodeless discharge

lamps from the element and iodine232, the element and chloririe2091

iron (II) chloride245 and iron (II) iodide with iron244 has

previously been reported. These and other methods of preparation were examined in an attempt to produce the most stable and intense lamps which had extended operating life—times. o8 •

Using the preparative scheme outlined earlier

(section 2.2.1.) the first lamps in our study were made from iron filings and iodine. However, the life—time of these lamps was less than 30 minutes. The appearance of the 'dead' lamps indicated the presence of oxygen and it was thought that this might arise from an oxide layer on the large surface area of the filings used. Therefore the filings were replaced by small pieces of iron—wire which had been 'pickled' by successively dipping the wire into acetone, water, concentrated hydrochloric acid, 1:1 phosphoric acid—water, water and acetone etc. Lamps made using wire had life—times of about two hours.

Anhydrous ferric chloride was also used but this was found to be difficult to'Awork with because of rapid hydrolysis. Attempts to make lamps from ferrous chloride bound by oxygen free organic ligands were unsuccessful. The most successful lamps were obtained using ferrous chloride.

Anhydrous ferrous chloride could either be placed directly into the silica blank or hydrated iron (II) chloride was dehydrated in situ in the blank, by repetitive gentle heating at reduced pressure. The most satisfactory results were obtained for tubes containing ca. 2 mg of chloride. If more than 2 mg was used there occurred some sublimation on:the walls of the lamp during running and this caused a drop of intensity. 99.

Measurements also indicated that lamps containing more iron

exhibited self-absorption at the resonance lines.

The lamps were operated in the -Z wave resonant

cavity, described above (section 2.2.1.), at 60 watts with

modulation at 50Hz and moderate air cooling to the length of

the tube. .It was found the moderate cooling increased stability

at high intensity. After an initial 'running-in' for between

one and two hours, only 5 minutes 'warm-up' period was required

after-. initiation of the discharge. With these operating con- ditions, the line-to-background ratio at 248.3 nm was typically greater than 100:11 while the short term output stability of the radiation at this wavelength was ± 3% over a period of several hours.

The discharge obtained exhibits the resonance lines of the iron atom spectrUm. The relative intensities of the most useful lines obtained with the above operating conditions are shown in Table I.

3.4. Atomic Pluorescence Spectroscopy of Iron

The apparatus used in this study consisted of the

Unicam SP 900A flame spectrophotometer and circular stainless steel Meker-type burner. The source used was an electrodeless discharge lamp prepared as described above. The lamp was 100

TABLE I. Iron Atomic Fluorescence: Relative Source and Fluorescence Intensities and Detection Limits.

Wavelength Transg 4 Relative Relative Limit of nm. (Ref. ' ' ) Intensities Fluorescence Detection fir (a) from source Intensities Iron (pgml ) (b) (d) (b) (d)

386.0a5 D --z5 Do 50 , 23 15 4 4 5 5 382.0 c a F5--y D4o 127 193 1.2 5 5 o 382.4 c a D --z D 4 3 5 o 382.6 c a5F --y D 4 3 372.0a5 D --z5 F o 135 328 0.84 4 5 356.5 c OF --z304 8 54 4.2 3 357.0 c a5F --y5 D0 4 3 5 o 302.1 c a5D3--y D3 24 85 6.0 302.06 c a5D4--y5D°4 302.05 c a5D2--y5 D2o aD5 5D o 23 70 3.6 298.4 4-7Y 3 272.1 c a5D3--y5P 2 31 50 3.0 5 o 271.9 c a D —:--y-5P 4 3 252.7 c a5 D 3--x5 D3o 35 518 0.015 5 5 o 252.3 c a D --x D 4 4 5 5 249.1 c a D 2 --x -W3 5 5 o 249.0 c a Do--x F1 248.8 c a5D --a5F° 3 4 248.3 c a5D --x5F5 100 1000 0.009 4 See overleaf for footnotes ... 101.

Footnotes for Table I:

a. lowest state is a5D4

b. relative to 248.3 nm.

c. unresolved lines.

d. uncorrected for response characteristics of photomultiplier. 102.

modulated at 50Hz and the SP 900A operated in its 'absorption'

mode. Details of the apparatus and its operation have been

described ih Chapter 2.

The burner unit was placed as close as possible

to the monochromator entrance slit; i.e. 50 mm in front of the

slit, and so that the slit viewed the flame in the first instance

just above the primary reaction zone. The.isouXce was positioned o at 90 to the burner—monochromator axis and in the same horizontal

'plane. Again the source was positioned as close to the flame

as was possible without excessive heat transfer from the flame

to the lamp, which was found to affect the stability of the

source.

Using the above arrangement six wavelength scans

were made in the region 400 nm to 230 nm. The emitted radiation

from:the flame was recorded over these wavelengths with the source on

and with the source off and while spraying into the flame successively

solutions containing 100 and 10 ppm of iron and distilled water.

By comparing the emission at various wavelengths under each of

these conditions it was possible to attribute the emitted radiation

to iron atomic fluorescence, iron flame emission or emission

from other species in the flame. The wavelengths at which atomic

fluorescence was observed are listed in Table I.

The strongest fluorescence was observed at the 248.3 nm

iron resonance line and because of the low flame background and 103. very small iron thormal emission at this wavelength, 248.3 nm was selected as the line at which the analytical parameters were optimised.

3.4.1. Optimisation of operating conditions

It was found that the strongest fluorescence signal was observed when the lamp was run at 60 watts with air—cooling, as described above.

The effect of variation of the acetylene flow—rate between 1.0 and 1.5 1/min on the fluorescence intensity is shown in Figure 7. The burner height was adjusted so that the . monochromator entrance slit viewed the interconal zone of the flame between 4 and 24 nm above the primary zone. The most intense fluorescence was obtained with an-acetylene flow—rate of 1.1 1/min, while the air flow—rate was maintaind constant at 7.0 1/min.

With the optimum acetylene flow—rate, the effect of variation of the height of observation in the flame on the atomic fluorescence intensity at 248.3 nm was investigated. The result of this study, shown graphically in Figure 8, indicated a steady decrease in analytical signal as the burner was lowered in relation to the monochromator entrance slit. The burner was therefore positioned so that the slit viewad the emitted radiation immediately above the primary reaction zone without 10. Figure 7 Variation of Iron Atomic

Fluorescence Intensity at 248.3 nm

with Acetylene Flow—Rate

9.0

Flu Drescence Intensity (ar itary 7.6unts)

Acetylene flow rate (1 min 6.0 1) 1.0 1.10 1.20 1.30 1.40 1.50 105.

Figure 8 Variation of Iron Atomic Fluorescence Intensity at 248.3 nmlwith Burner Height

350 1

300

Fluo scence Inter ity (arb tart' uni

200

100 t< >; portion of flame viewOd ,when 1y cones just ;below base of entrancd ;slit

5 10 15 20 25 30

Burner height (mm below centre of entrance slit) 106. receiving radiation from this zone itself. The monochromator slit—width was varied under otherwise optimum operating con— ditions. The slit—width which gave the most favourable signal— to—backgraundand noise ratio at 248.3 nm was found to be 0.2 mm.

By scanning the iron lines in this wavelength region it was found that 0.2 mm corresponds to a spectral band—width of ca. 1.0 nm.

3.4.2. Calibration Data and Detection Limits

With the optimum operating conditions the calibration curve shown in Figure 9 was obtained for the atomic fluorescence of iron at 248.3 nm. Linearity was obtained over the range 0.1-40/kg ml-1t however as the 248.3 nm line is a resonance line arising from a ground—state transition it is a strong absorption line. At concentrations greater than 1 40 itg m1 the calibration curve at 248.3 nm exhibits curvature towards the concentration axis because of self—absorption. The

248.3 nm line is the strongest iron absorption line and linear atomic fluorescence calibration graphs may be obtained for higher concentrations of iron when the less strongly absorbing

372.0 nm line is employed.

The detection limits at each of the nine lines for which atomic fluorescence was observed are shown in Table I.

The detection limit was defined as that concentration of iron in aqueous solution which produced a signal equivalent to twice

107.

Figure 9 Calibration Curve for Iron

1,000_ Atomic Fluorescence at 2/+8.3 nm

750 -

Fluorescence Intensity (arbitary units)

500 _

0

20 • 40 60 -1 Concentration of Iron (fig ml ) 108. the standard deviation of the background noise measured near the limit of detection.

3.4.3. Interference Studies

The interference effects on the atomic fluoreScence 1 intensity from a 10[tg ml iron solution at 248.3 nm produced by various cations and anions were examined. -The results of this study are shown in Table II. No interference was caused by 1000—fold amounts of Co, Hg, Ta or by 100—fold amounts of

Al, Cd, Co, Cr(VI), Hg, Mo, Pb, Ta, Ti or Zn. Negative errors of 5% or less were caused by 1000—fold amounts of Al,

Cu, Mo, Ni or Pb. Positive errors of less than 5% were caused by 100—fold amounts of Th or 200—fold amounts of W. Negative errors of 10% or less were caused by 1000—fold amounts of Th,

Cd or PO 3- or by 100—fold amounts of Cu, V or Mn. Zirconium 4 in 100—fold excess caused a positive error of ca. 9%. Large negative errors were obtained in the presence of 1000—fold amounts of V, Mn and Zn.

It is probable that the slight reductions in signal for iron obtained in the most concentrated solutions -1 (10 mg ml ) may be caused by their reduction of the nebuliser efficiency. Very little positive interference was caused by scat— tering of the radiation from the source by particulate matter in the flame. Only in the case of the most refractory elements 109.

TABLE II

Effects of Foreign Ions on Iron Atomic Fluorescence at 248.3 nm

. % change in .Fluorescence Signal for 10/4g m1-1 iron Foreign Ion 1000—fold weight excess 100—fold weight excess

Al.3+ -5 0 . Cd 2+ -8 0 Co 2+ 0 o cr 6+ - 0 G\I 2+ -4 -9 Hg 22+ 0 0 Mn 2+ —28 —10 Ivlo 6+ -3 0 2+ Ni —2 -4 Pb 2+ -4 0 Ta 4+ 0 0 Th 4+ -6% +4 Ti 4+ — 0 V 5+ —26 -9 W6+6+ +4 a)

Zn 2+ —13 0 Zr 6+ +9 a)

P043- —10 —10

a) 200—fold weight excess 110.

(Th, W and Zn) was appreciable scatter_ detected at 248.3 nm.

3.5. Atomic Absorption Measurements

A brief study of the atomic absorption spectro—

photometry of iron was made with the same apparatus as described

for fluores.cenee measurements For atomic absorption measurements,

however, the modulated electrodeless discharge lamp source was

employed with the SP 900A flame spectrophotometer in the

'absorptions mode with the source radiation focussed onto

the entrance slit of the monochromator. In this way a

convergent beam of radiation was made to pass through an air—

acetylene flame using the standard 10—cm path absorption burner

fitted to the commercial instrument. The calibration curve for

the determination of iron by AAS is shown in Figure 10. A slit

width of 0.02 mm was used, which corresponds to a half—intensity

spectral bandwidth, obtained by scanning the iron lines in this

spectral region, of ca. 0.2 nm. The curve was linear between -1 1 and 40 111g ml and the extrapolated concentration required

to produce 1% absorption, i.e. the sensitivity for 1% absorption

was 0.9/xg ml-1. The limit of detection, that concentration

of iron in aqueous solution which produced a signal equivalent

to twice the standard deviation of the background noise measured -1 near the limit of detection, was found to be 0.1p-g ml .

It was noted that the calibration curve is Figure 10 Calibration Curve for Iron

Atomic Absorption at 243.3 nm

0 . 3 0

0.20-

Absorbance

0.10

-1 Concentration of Iron (µp ml ) 7- 0 20 40 60 80 100 112.

232 similar to that obtained by Marshall and West . They noted that the range of linearity was extended by using a microwave excited electrodeless discharge lamp as the source as compared to a hollow cathode lamp. This.was attributed to the higher intensity of the discharge lamp employed which enabled the monochromator entrance slit to be closed down as compared to when a hollow cathode lamp was employed, thus improving the resolution such that the 248.3 nm iron line could be separated from neighbouring lines. In this present study it was also possible to isolate the 248.3 nm line because the intensity of the source permitted the use of a narrow slit width. Recent studies2551 however, have suggested that electrodeless discharge lamps may exhibit broad resonance line profiles at other than extremely precisely controlled operating conditions and this has been shown to be a cause of curvature in atomic absorption 197 calibration graphs .

3 6. Flame Emission Measurements

Flame atomic emission measurements were made for iron with the burner assembly employed for fluorescence measurements but with no source. The SP 900A was used in the 'emission' mode, i.e. with a rotating sector interposed between the flame and the monochromator, to modulate the iron atomic emission and produce an a.c. current from the photomultiplier suitable for 113. amplification (see Chapter 2). This signal was not amplified when fluorescence measurements are made with the source and detector operating at 50Hz. The obtainable detection limits -1 for iron, uder optimum conditions, were 314g ml and 0.4tLg ml-1 at 248.3 and 372.0 nm respectively.

'Me detection limit data for the three techniqus are summarised in Table III. 114.

TABLE III

Detection Limits for Iron Using a Flame Atom Reservoir

Detection Limits (pg m1 1) Wavelength Atomic Atomic Flame (nm) Fluorescence absorption emission.

248.3 0.009 0.1 3

372.0 0.84 1.5 0.4 115.

CHAPTER 4

The Determination of Manganese Using a Flame—Cell

4.1. Introduction

Many analytical studies of the atomic spectroscopy of manganese have been reported. Allan213 first determined manganese by atomic absorption using a hollow cathode lamp.

Absorption at 4 wavelengths was reported, 279.5 nm being the most sensitive wavelength. The useful manganese resonance lines are grouped in two triplets at 280 nm and 403 nm, i.e the atomic spectrum is simpler than that of iron. Since

Allan's early work considerable improvements in the sensitivity of manganese determinations by atomic absorption 214 have been reported. Slavin reported a detection limit -1 of 0.00514g ml manganese using the 279.5 nm resonance line, this corresponds to a sensitivity for 1% absorption of —1 0.1 /.4 g ml using a spectral band—pass of 2.0 nm. When the spectral band—pass was reduced to exclude the 280.1 nm line this sensitivity was doubled, but the subsequent loss of light flux increased the noise on the signal. Similar care must be taken at 403 nm with regard to the spectral band—pass; 116.

-1 Slavin2'4 reported a sensitivity for 1% absorption of 0.8 tig ml

at the 403 nm line if the 403.31 nm manganese line was excluded

Using the nitrous oxide-acetylene flame and the 279.5 nm line -1 Slavin reported a sensitivity for 1% absorption of 0.31.1g ml 215 manganese. Other workers examined the oxy-hydrogen and

oxy -acetylene total consumption bUrner flames, the air-acetylene

and nitrous oxide-acetylene premixed flames and confirm that

air-acetylene offers the greatest sensitivity. De Galan and 216 Samaey report that manganese is almost completely atomised

in the air-acetylene flame and that this flame provides optimum

sensitivity, because in comparison it has the smallest volume.

These same authors have reported the bending of manganese atomic

absorption working-curves when more than one absorption line 256 falls within the spectral band-pass of the monochromator .

At a spectral band-pass of 1.6 nm the manganese 280 nm multiplet is unresolved and these workers report that in this case working- curves will be significantly non-linear above 0.5 absorbance units. A spectral band-pass of 0.04 nm is recommended if linear working-curves are to be obtained.

A number of workers have reported chemical inter- ferences in the atomic absorption spectroscopy of manganese.

Interference from silicon has been reported several times. Platte 223 1 and 1Iarcy found that the addition of 501ag ml of calcium

117.

as the chloride removes depression of absorbance caused by -1 226 200/4g ml of silicon. Roos and Price suppressed silicon

interference by the addition of lanthanum as releasing agent.

Again for manganese, as for iron, Roos228 has postulated the formation of atoms in the flame without an intermediate oxide 215 stage. Sachdev et a1 examined the effect of fifty fold weight

excesses of 57 ionic species on the atomic absorption signal -1 for 51ag ml of manganese and found only four slight depressions of signal. Sulphuric'acid (5%) increased the sample solution viscosity which led to a signal suppression of

5% and the slight signal suppressions eaused by Ti, V and Zr were removed by the addition of fluoride ions.

Da Waele and Harjadi257 have found that the concentration of manganese affects the height of the zone of maximum atomic population and that this influenced the shapes of their calibration curves.

A spectral interference in manganese atomic absorption has been reported by Allan258. He reports mutual interference from the overlap of the manganese 403.3073 nm line and the gallium 403.2982 nm line in a number of flame cells.

It is pointed out that these lines are considerably broadened by hyper—fine structure and collisional broadening.

Several reports have been made in recent years of the atomic fluorescence spectroscopy of manganese at the 279.5 nm 118. line. In the first reports continuum light sources were used. 234 With a 150—W Xe arc source Darnall, Thompson and West obtained -1 a detection limit of 0.3pg ml manganese in an air—propane flame 235 and 0.15 pg m1-1 in an air—hydrogen flame. Bratzel et al used a 150—W Eimac Xe arc, with a colour temperature of 6,000°K, and -1 obtained a detection limit of 0.2,4g ml manganese in a hydrogen—argon—entrained air, unpremixed flame 'on a total 236 consumption burner. Demers has reported a detection limit -1 of 0.004/4g ml manganese in a hydrogen—entrained air flame, using a 450—W Osram high—pressure Xe arc as light source. 259 Dresser and West studied interferences in atomic, fluorescence using a d.c. high pressure 500W Xenon arc. Detection limits -1 -1 of 2.5)Kg ml manganese at 279.5 nm and 414g ml at 403.1 nm were reported using an air—acetylene flame. The fluorescence at

279.5 nm was found to be virtually free from radiation interferences, 1 but at 403.1 nm 100Org ml potassium gave a 12% enhancement -1 in the determination of 20jug ml manganese (there is a potassium resonance line at 404.41 nm) and the same concentration excess of gallium gave less than a 5% enhancement (there is a gallium resonance line at 403.30 nm). A 90—W high pressure mercury discharge lamp was employed with a total consumption burner by

Omenetto and Rossi237 The continuum from this source was used to excite manganese fluorescnece at 279.5 nm, a limit of —1 238 detection of 0.5flg ml being obtained. Manning and Heneage 119. have compared the sensitivity obtained using a 150—W Xe arc

and high brightness hollow cathode lamps; at 279.5 nm -1 -1 detection limits of 0.5pg ml and 0.05pg ml for these

sources respectively were obtained with an air—hydrogen flame. 239 Other sourc?s have also been used. Dinnin reported a -1 detection limit of 5.0rg ml at 279.5 nm using a hot 240 hollow—cathode lamp. Rossi and. Omenetto used a demountable

water—cooled hollow—cathode lamp, run at 500 mA, and obtained -1 a detection limit of 0.05rg ml for manganese. The use of an RF

plasma as the primary light source for the determination of 260 manganese by atomic fluorescence has also been reported .

The most sensitive reported determinations of manganese

by -atomic fluorescence have used electrodeless discharge lamps 261 as sources Bratzel and Winefordner employing such a lamp

in a tapered rectangular micro—wave cavity obtained a detection -1 limit of 0.006/1g ml manganese, using an argon—hydrogen

diffusion flame and a Zeiss total consumption nebuliser—burner.

In a further report244 this same detection limit was reported,

in this case the lamp was made using sublimed manganese (II)

iodide and the metal in a lamp 3.7 cm long under 1 Torr Argon fill—gas pressure. The lamp was operated at 100 watts.

Dagnall, Taylor and West245 using a lamp containing manganese

(II) chloride and an air—hydrogen flame obtained a detection 120.

-1 limit at 279.5 nm of 0.0141 g ml . Two reports have been made of the optimal parameters for the preparation of manganese electrodeless discharge lamps for atomic fluorescence spectroscopy. 01262 Mansfield et preferred tubes containing the iodide, in lamps of 2.0-2.5 cm3 volume, with an inside diameter of 9 mm, under 2.0 T6rr argon. They report the absolute values for the intensity and noise obtained from such lamp's and that there was no measurable self—reversal, although lamps containing a small quantity of manganese mercury amalgam showed measurable self— reversal. However, because of the noise of the source a -1 detection limit of 1.01Ag ml was obtained. Silvester and

McCarthy263 investigated the effects of the weight and nature of the material in the lamp, the nature and pressure of the fill—gas and the applied micro—wave power on the emission intensity of a demountable manganese electrodeless discharge lamp at 403 nm.

The use of 5 mg of manganese (II) chloride under 10 Torr of helium using 240—W applied microwave power was preferred. It should be noted that both these studies refer to lamps powered by antennae.

Crosser and West264 developed a novel dual element manganese— chromium lamp for use in multi—element atomic fluorescence studies -1 and obtained a detection limit of 0.1pg ml manganese. This value, obtained using an oxy—hydrogen flame, is slightly higher than expected and led the authors to conclude that there may be 121.

self—reversal taking place.

The recent advances in flame cells for the deter—

mination of manganese by atomic emission spectroscopy follow

closely those for iron described previously (section 3.1.). The

detection limits obtained, by the workers mentioned in that section, -1 in various flames using the 403.1 nm line are: 0.1 ml from the 250 -1 inter—tonal zone of a premixed oxy—acetylene flame , 0.025 lag ml 251 -1 using a premixed oxygen enriched air—acetylene flame 1 0.005 pgml 252 -1 using a premixed nitrous oxide—acetylene flame and 0.01 jig ml 212 using a nitrogen separated air—acetylene flame .

In this chapter a study of the analytical atomic

spectroscopy of manganese using an air—acetylene flame is

presented. The preparation and properties of manganese micro—

wave excited electrodeless discharge lamps, as sources for the sensitive and selective determination of manganese by atomic fluorescence spectroscopy in a nitrogen shielded air—acetylene flame,

will be described. In particular, a comparison using the same

detector system with atomic emission spectroscopy will also be

made.

4.2. Experimental

4.2.1. Apparatus

The Unicam SP 900A flame spectrophotometer described 122.

in Chapter 2 was employed. The indirect cyclone nebuliser

unit of this instrument being employed without modification.

The manganese electrodeless discharge tube was

operated at 2450 MHz with a 200 watt Microtron 200 Mark II

power. generator and three—quarter wave Broida—type resonant

cavity (Electromedical Supplies, Ltd., Wantage, Type 210L),

see also Chapter 2. The cavity was modified by the addition

of a side—wall tuning stub. The tube output was modulated at

50Hz as previously described.

A premixed air—acetylene flame was used throughout

this investigation. Nitrogen shielding was used to separate the

secondary diffusion zone from the primary reaction zone, using 212 the type of Meker burner described by Hobbs, Kirkbright and West . The burner head was placed 50 mm in front of the monochromator

entrance slit. The rim of the flame shield device was raised

so that the monochromator did not view the primary zone. In fluorescence measurements the source was positioned at 900 to

the burner—monochromator axis and in the same horizontal plane, so that the distance'between the electrodeless discharge tube

and the centre of the flame was 50 mm.

4.2.2. Reagents

Manganese Stock Solution — A 1,000 ppm stock solution was prepared 123.

by dissolving 3.60 g of crystalline manganese (II) chloride

(AR grade) in one litre of 0.511 HCl. This solution was diluted as required immediately prior to use.

Diverse Ions — Solutions of diverse ions were prepared from

analytical reagent grade salts.

4.3. Atomic Fluorescence Spectroscopy

4.3.1. Preparation of Electrodeless Discharge alhe

The preparation of manganese electrodeless discharge tubes from: manganese (II) iodide and manganese244 manganese (II) 262 iodide 1 manganese (II) chloride245 and the element/ and chlorine has previously been reported. In this study these methods of preparation were examined in an attempt to produce the most stable and intense tubas which had good operating life times.

The tubes were prepared by the general procedure described previously (section 2.2.1.). Investigation was also made of the optimum filler gas pressure and the suitability of both argon and helium as filler gas. Argon filled tubes were found to be more stable and to have a longer operating life time than those containing helium. The most satisfactory results were obtained for tubes which contained ca. 1 mg of manganese (II) chloride (AR grade MnC12.41120 dehydrated under vacuum) and an argon filler—gas pressure of 3 Torr. 124.

4.3.2. Operation and Spectral Characteristics of Manganese

Discharge Tube

The electrodeless discharge tubes were operated in the three—quarter wave resonant cavity with a power input of 50 watts. After amiinitial 'running-in' period of between one and two hours, only 5 minutes ,warm—up, period was required after initiation of the discharge. After this 'warm—up' and under these operating conditions, the line—to—background ratio at 279.5 nm and 403.1 nm was typically greater than 100:1, while the short term output stability of the radiation at these wavelengths was ± 3%. The discharge obtained exhibits the lines of the resonance spectrum of manganese. The tubes also emit the most intense lines of the MnII spectrum, i.e. from 5 1 7 the 3d 4s a S ion. The presence of these ionic lines does 3 not appear to impair the analytical usefulness of the tubes for atomic fluorescence work. The relative intensities of the most useful manganese lines obtained with the above operating conditions are shown in Table IV.

4.3.3. Atomic Fluorescence Measurements -1 -1 When 10 fug ml and 100 Ng ml manganese solutions were nebulised into the air—acetylene flame with the electrodeless discharge tube in operation, atomic fluorescence signals were

125.

TABLE IV

Manganese Atomic Fluorescence: Relative Source and Fluorescence

Intensities and Flame Detection Limits

Wavelength Spectrum Tran.p, Relative Relative Limit of nm (d) (refs-J/ -i ) Intensities Fluorescence Detection for rranesr. (a) from Source Intensities Man- -1 (b) (c) (b) (c) pg ml

257.6 II -z7p° 34 12 1.5 a7S34 , II a7S -z7P0 33 8 12 259.4 3 3 7 7 0 32 8 12 260.6 II a s3-z P2 6 6 279.5 I a S2ry 13.3..?2) ) 6 6 0 ) e e 279.8 I a s,.1.-y psl. 100 100 0.001 e ..2- c-2, 6 6 0 280.1 I, a S2i7y pa ) 5 5 90 e 293.3 II a S2-z P1 2 4 e 5 5 0 293.9 II a S22-z P 5 2 5 5 o go 94.9 II a s23-z P 6 "3 6 6 0 304.5 1 a 1-v P 1 4 11 3o D2,z 3-if 6 6 0 357.8 I a 1-x P 1 8 7 50 D2i,1 31t- I a6Dn1-z6F, 01 6 8 67 383.4 .7;-7,

403.1 I -z6P°1 ) a6S2ii32 ) 6 6 0 ) 312 e 49 e 0.02 e 403.3 I as2rz p21 ) z - ) i a6S -1,:-z°P?1 30 f 403.4 cf_T 1-,71 contd/ 126.

TABLE IV (contd)

6 a lomst state of MnI is a S 1 and of MnII it is a7S3 2y b relative to 280 nm triplet

0 'uncorrected for response characteristics of photomultiplier 5 26 d normal state of valence electrons MnI 3d 4s S2i=0 5 17 MnII 3d 4s s =0 3 e unresolved lines f relative intensity at 403 nm with 280 nm source rad. filtered

out. 127. observed at each of the wavelengths;.shown in Table IV. The most intense fluorescence was obtained from the 280 nm and 403 nm triplet lines which arise from transitions to the ground state.

The individual lines of these triplets were unresolved with the spectral band-pass used. The use of an optical wide-band filter to prevent irradiation of the flame by the 280 nm radiation from the source revealed that the fluorescence emission signal at 403 nm originated from both resonance fluorescence and some step-wise 6 6 o fluorescence from de-activation of the excited y P3/20, y P5/2 6 o and y P7/2 state atoms by radiationless transitions to the 6 6 o 6 o z P3/2 z P5/2 and z P7/2 excited states. This is most clearly shown on the simplified Grotrion diagram, Figure 11. The use of an optical filter to prevent irradiation of the flame by the

403 nm radiation from the source revealed, as expected, that excitation at 403 nm did not contribute to the fluorescence radiation observed at 280 nm.

The suitability of both fluorescence signals

(280 and 403 nm) for the analytical determination of manganese was investigated. Lower flame background intensity, lower thermal emission intensity for manganese and freedom from spectral inter- ference258 is observed at 280 nm, however, and all analytical atomic fluorescence measurements were made at this wavelength.

The emission at 260 nm from the most intense

128.

Figure 11 Simplified Grotrian DiaKram for Manganese

4„,--Ionisation -a— Potential Siy 1—Piod& RSV.. 8 P14._414. 1D0f,.St 7 - 4-e' / -T- 7.434 V

6 TT\ 304.46 5 3906 353.1(20-.21) 332.35 354. (60-.92) 39 35695 (6.99-7.00) volts 383.44 4 475.40 4781.34 .492.35 3 279.48 27 9.83 220.11 2 40308 403.31 03.45

IMO 539.47 543.25

0

14 (Adapted from Mavrodineanu and Boiteux , wavelengths at which fluorescence observed shown in bold) 1 29. lines of the MnII spectrum was observed to stimulate ionic 5 7 resonance fluorescence from manganese in the 3d 4s1 a S ionic 3 state in the air—acetylene flame, the calculated degree of ionisation produced when a 10iug ml 1 manganese solution is nebulised into a nitrogen shielded air-acetylene flame at

2450°K is 0.6 x 10-3%. Although the population of ions is low, the gf values (see section 1.2.) and source intensities of the

257.6, 259.4 and 260.6 MnII ground state lines are high. The net result is a resonance ionic fluorescence signal of appreciable intensity. Although the gf values of the MnII lines at 293.3,

293.9 and 294.9 nm are high, the source intensities at these lines is low and they are not ground state Mull lines. For this reason, and the relatively high background at these wavelengths, only very weak ionic fluorescence was observed at these wavelengths.

The fluorescence from manganese ions was suppressed at all of the observed lines when 1000 ppm of potassium as potassium chloride was added to the solutions nebulised.

4.3.4. Optimum Operating Conditions The use of both conventional unshielded and nitrogen— shielded air—acetylene flames was investigated. At 280 nm the atomic fluorescence signal intensity was reproducibly 30% greater in the shielded flame and the signal noise levels obtained were decreased by a factor of 2 on shielding. The nitrogen shielded air— 130. acetylene flame was chosen for all further measurements and a nitrogen flow—rate of 14 litres/min. was employed

The effect on the fluorescence signal at 280 nm of variation of the air and acetylene flow—rates between 5 and

7.5 litres/min. and 0.9 and 1.4 litres/min.respectively was investigated. The variation of analytical signal with acetylene flow—rate is shown in Figure 12. The most intense fluorescence was obtained with an air flow—rate of 6.5 litres/min. and an acetylene flow—rate of 1.1 litres/min. 257 Me Waele and Harjadi have reported the variation of the optimum burner height with concentration of manganese in its determination by atomic absorption spectroscopy. Careful study was therefore made to determine the optimum height of observation of the atomic fluorescence in the flame with several different concentrations of manganese introduced into the flame. Although a clear optimum height of observation was found, its concentration dependence did not appear marked. The variation of the atomic fluorescence signal at 279.5 nm with burner height is shown in

Figure 13. With the optimum burner height the monochromator views the interconal region of the flame between ca. 6 and 26 mm above the burner head.

The effect oftvariation of the monochromator slit— width on the atomic fl.)orescence signal to background noise ratio 131.

Figure 12 Variation of Manganese Atomic Fluorescence Intensity at 279.5nm with Acetylene Flow-Rate

75-

Flucrescence Intensity (Artitary units)

70_

65 -

Acetylene Flow-Rate (1 min..1) 6o I 1 I 1 I 0.9 1.0 1.2 1.4 132.

Ficure 13 Variation of Manganese Atomic Fluorescence Signal at -1 279.5 nm with Burner Height (for 1 pg 1 Mn)

Fluor escence Inte sity (arb tary uni s) 150

10 portion of flame viewed when ly cones just below base of entrance slit

0 5 10 15 20 25 30 Burner height (mm below base of entrance slit) 133.

was investigated under otherwise optimum conditions. The optimum

slit—width at 280 nm was 0.4 mm; this corresponds to a spectral

band—width of approximately 2.6 nm, as measured from the resolution

of lines at this wavelength. The use of a lens to focus radiation from the source into the flame did not improve the attainable detection limit for manganese.

4.3.5. Calibration Data and Detection Limits

With the optimum operating conditions established, linear atomic fluorescence calibration graphs were obtained at 280 nm over -1 the range 0.0025 to 10)ug ml of manganese in aqueous solution.

The fluorescence at 280 nm suffers self absorption above 10/Ag ml-1 andabovetlhis;,concentration the calibration graphs exhibit curvature towards the concentration axis. The working curve at 280 nm over /4 --1 the range 0.0025 to 1.0 g m1 isshown in Figure 14 and the fluorescence growth curve at this wavelength in Figure 15. Linear atomic fluorescence calibration graphs were obtained over the -1 range 0.1 to 15/Ag ml manganese in aqueous solution when fluorescence measurements were made at 403 nm.

The limits obtained at each of the ten wavelengths for which atomic fluorescence was observed are shown in Table IV.

The detection limit was defined as that concentration of manganese in aqueous solution which produced a signal equivalent to twice 134.

Figure 14 Manganese Atomic Fluorescence Calibration Curve (0.0025 -1 to 1.0pg ml ) at 279.5 nm

100

75

Fluo escence Inte sity (arb tart' uni s) 50

25

0 0.2 0.4 0.6 0.8 1.0

2.1 Manganese Concentration (fag ml ) 4.5- 135-

Figure 15 Growth Curve for Manganese Atomic Fluorescence at 4.0 280 nm..

3.5 -

3.0_ log10 signal (arbi ary units) 2.5_

2.0_

1.5-

1.0

0.5 7

-1 log10 manganese ,concentration (in lag ml ) 1 -3 -2 -1 0 1 2 3 136.

the standard deviation of the background noise measured near

the limi-Lof detection.

4.3.6. Interference Studies

Theeeffect on the atomic fluorescence intensity -1 at 280 nm produced by a 0.5pg ml manganese solution of the

presence of a 1000-fold and 100-fold excess by weight of a range

of cations and anions has been examined. The results of this investigations are summarised in Table V. Of the 26 ions

studied at this concentration only 4 produced serious interferences2

three of the most refractory elements (Thl Si and V) gave positive

interference by particulate scattering of incident radiation from the source. Two of these three elements (Th and Si) gave

negligible interferences when a 100-fold weight excess was

employed. Magnesium seriously reduced the fluorescence signal

even when only a 100-fold weight excess was present. A similar suppression interference from magnesium-on the atomic absorption signal obtained for manganese was also found, using a 5/Ag ml 1

manganeseesolution2 and an electrodeless discharge lamp as the' -1 source, 500 pg ml gave a depression of -40% on the manganese absorption signal in the separated flame. Thus it would appear that the magnesium interferes by reducing the population of manganese atoms in the flame. In the unseparated air-acetylene flame this -40% interference was reduced to -25%2 indicating that 137.

TABU: 1r

Effect of Foreign Ions on Manganese Atomic Fluorescence at 280 nm

% change in Fluoresce ice signal from 0.5 gig ml Mn Ion Salt 1000—fold 100—fold weight excess weight excess

Al3+ AlC1 0 0 3

Bat* BaC12 N.I. 0 2+ Ca . CaCO#HNO3 N.I. 0

Cd2+ CdC12 0 0 2+ Co CoC12 0 0

Or3+ CrC1 0 0 3 Cu2+ CuSO 0 0 4 Fe3+ Fe/HC1 0 0

H HgNO N.I. 0 gt 3 . + K KNO N.I. 0 3 2+ Mg MgSO -39 -11 4 6+ M0 0 0 (NH4) 6111°7°24

Na NaCl 0 0 2+ Ni Ni01 2 0 0 2+ Pb PbNO 0 0 3 t 138.

TABLE V contd.

% change in Fluorescelre signal from 0.5 jig ml Mn

Ion Salt 1000—fold 100—fold weight excess weight excess Si4 + . Si/HF +5 . N.I. 2+ Sn SnC12 N.I. 0

Ta5+ Ta/HF N.I. 0 4+ Th m(No ) 0 3 4 +4 v5+ V/HC1 +16 +8

W6+ Na2 W0 N.I. 0 4 2+ Zn ZnSC4 0 0

NH 4. NH ci 0 0 4 4 Cl— HC1 0 0

NO HNO 0 0 3 3 PO 3— (NH ) HPO • 0 0 4 4 2 4 5042— H2SO4 0 0

Ga Ga/HC1 N.I. 0

N.I. = negligible interference, i.e. <3% 139. the hotter unseparated flame was reducing the interference.

Chemical interferences in flames and in particular the formation of stable monoxides are often less in fuel-rich flames, and in this case further evidence of the chemical nature of the inter- ference was obtained from the reduced interference noted (-18n in the fuel-rich air-acetylene flame. The interference in the atomic absorption measurements was found to be independent of whether an electrodeless discharge tube or a hollow-cathode lamp was used as the spectral source. Demers and Ellis265 have reported serious chemical interference (-27n by manganese on the atomic fluorescence of magnesium using a hydrogen- entrained air diffusion flame. The interference of silicon in the atomic absorption spectrophotometry of manganese has previously been discussed (section 4.1.).

4.4. Atomic Emission Spectroscopy When spraying 10 and 100 vg m1-1 manganese solutions into the air-acetylene flame atomic emission signals were observed at each of the wavelengths shown in Table VI.

The optimum operating conditions for the detection of manganese in both the conventional and nitrogen separated flames were investigated. The greatest emission intensity was observed

t 403 nm and investigations were made, in the first instance,

at this wavelength. The effect of variation of acetylene flow- 140

TABLE VI

Manganese Atomic Emission: Summary of Detection Limits

and Optimum Slit—Width

Wavelength Undeparated Flame Separated Flame nm Slit width Limit of Slit width Limit of (mm) Detecti (mm) Detect4nn (jig ml's) (lag ml )

279.5 a 279.8 a 0.175 0.08 0.19 0.05 280.1 u"

354.78 a 354.80 a 0.08 1.5 0.13 1.2

356.95 a 356.98 a 0.08 1.0 0.12 1.0

'403.1 a 403.3 a 0:04 0.02 0.09 0.01 403.4 a

a unresolved lines 141.

-1 rate between 0.9 and 1.2 1 min was investigated. The 1 optimum acetylene flow-rate was found to be 1.07 1 min for

both the separated and unseparated flames. The air-flow rate -1 was varied between 6.5 1 min and 7.5 1 min-1. For the 1 nitrogen shielded flame an optimum flow-rate of 7.5 1 min air

was observed while for the unshielded flame an optimum flow- -1 rate of 7.0 1 min was observed. The optimum flow-rate for 1 the nitrogen shielding gas was found to be 14 1 min . With

these optimum conditions the effect of variation of the height

of observation the flame was investigated. In the conventional

unshielded flame the optimum height of observation was found

to be when the monochromator viewed the flame in the region

between 13 and 33 mm above the burner head. The same optimum

::eight of observation was established for the nitrogen shielded

flame.

At each emission wavelength the optimum slit-

width which produced the most favourable signal: background noise

ratio was established for both the unshielded and shielded flames.

Separation of the flame allowed in every case larger slit--,idths

to be used before equivalent noise levels were obtained, the

greatest advantage being derived at the longer wavelengths where the flame

background is markedly reduced on separation. Table VI shows the

detection limits for manganese obtained in this study, together 142. with the optimum slit—width used. The use of the most intense line emission at 403 nm and a separated flame give rise to the lowest -1 detection limit (0.01 jig ml manganese).

4.5. Atomic Absorption

A brief study was made of the atomic abscirption spectrophotometry of manganese, using the same apparatus as described here for fluorescence measurements, adapted as in the case of iron (section 3.5.). The manganese electrodeless discharge lamp was used as the source. The limits of detection -1 for an aqueous manganese solution were found to be 0.1 pig ml -1 and 0.4 dig ml at 279.5 nm and 403.1 nm respectively, —1 corresponding to sensitivities for 1% absorption of 0.2 fig ml at 279.5 nm and 1 tag m1-1 at 403.1 nm.

The detection limit data obtained by the three techniques are summarised in Table VII. 143.

TABLE VII

Detection Limits obtained for Manganese

Wavelength Detection Limits jig m1-1 (nm)

Atomic Atomic ' Flame Fluorescence Absorption Emission

280 0.001 0.1 0.05

403 0.02 0.4 0.01 144.

CHAPTTR 5

The Determination of Manganese Using a Non—Flame Cell

5.1. Introduction

The aomisation of manganese for atomic absorption and atomic fluorescence analytical measurements has, with few exceptions, been achieved using various flame cells. A few of the non—flame atom cells described previously (section 1.3.4.) have been used to atomise manganese for atomic absorption analysis at the 279.5 nm line. Lvov using a graphite cuvette (2.5 mm internal diameter) has reported the determination of manganese with high absolute sensitivity. The practice is now established of quoting sensitivities and detection limits using non—flame cells which require only micro—litre amounts of solution in terms of the appropriate amount of sample in grams i.e. the concentration of the solution multiplied by its volume; these are then often referred to as 'absolute detection limits( and 125 absolute sensitivities'. L'vov reports an absolute sensitivity 13 12 for 1% absorption of 2 x 10 g manganese, 2.5 x 10 g being observed to give an absorbance of 0.048 at 279.5 nm. The optimum parameters are 2 atmospheres argon pressure in the 133 cuvette and a temperature of 2,000°C. Woodriff et a1 report 145.

that using their furnace at an optimum temperature of 1250°C, and nebulising manganese in methanol, a sensitivity for 1% — 1 absorption of 0.008 Fg ml 1 corresponding to an estimated detection limit of 0.002 Fe ml 1 manganese was obtained.

When using a carbon cup and a furnace temperature of 1200°C Woodriff et a1135 report a sensitivity for 1% absorpticin of 11 8 x 10 g manganese, with an estimated detection limit of 2.7 x 10— 10 g, and linear calibration curves from 1 x 10-9 g to 20 x 10-9 g (ca. 0.9 absorbance). Massmann65 obtained a 12 detection limit of 8 x 10 g manganese using his modified graphite cuvette. Using a heated grephite atomiser, based on 132 Massmannis cuvette, Manning and Fernandez I found that the sensitivity obtained in manganese determinations depended on the

gas flow rate (e.g. 0.004 rg manganese gave an absorbance of ca. 0.3 with 1 1 min 1argon flow—rate but this dropped to

ca. 0.2 at 2.5 1 min-1 flow—rate) but the choice of argon or nitrogen was not critical. An absolute sensitivity of 7 x 10-12 g was reported. These same workers have recently 266 reported the determination of manganese in test solutions and river water samples. An absolute sensitivity for 1% 12 absorption of 11 x 10 g was reported and a limit of -11 — 1 detection of 1 x 10 g, i.e. 100 Fl of 0.1 Fg1 manganese solution. A coefficient of variation of 3.5%; using 50jul of 146.

-1 sample containing 2 pg 1 manganese was claimed. The absence

of matrix interferences for manganese in the river water samples 1 1 was reported and in two samples 2.7 pg 1 and 1.3 pg 1 manganese ,1 -1 were found, corresponding to values of 3 ig 1 and 1 1ig 1 respectively obtained using a flame cell following solvent 152 extraction. West and co-workers report a detection limit

of 5 x 10-12 g using their carbon filament device and atomic fluorescence spectroscopy at the 279.5 nm wavelength, in this report interferences and atomic absorption spectroscopy were 156 not studied. Amos reports an absolute detection limit of -13 5 x 10 g, using 1 Fl of sample, with a commercially available hydrogen shielded modified carbon filament. Donega and Burgess 12 obtained an absolute sensitivity of 3 x 10 g manganese by atomic absorption using their filament device with a tantalum boat 172 filament and 300 Torr argon fill-gas pressure. Massmann reported a detection limit of 2.2 ppm manganese in a 30 mg metal sample using a heated hollow-cathode.

Manganese is typical of a number of analytically important elements of only moderate volatility, having melting

and boiling points at 1247 and 2030°C respectively. While a number of reports of the determination of manganese using non-flame cells have been made, as yet no systematic examination has been published. In this chapter the determination of manganese 147. by atomic absorption and atomic fluorescence spectroscopy will be described, with particular reference to interferences in atomic absorption determinations.

. Atomic Absorption Spectrometry

5.2.1. Apparatus The monocbromator, detection and amplification system of the Unicam SP 900A spectrophotometer were employed.

It was found to be possible to observe signals from manganese when the recorder output was led to a Servoscribe RE 511 chart recorder, but because of signal distortion and insufficient separation of the signal and the filament continuum (see Chapter 2) this system was unsatisfactory. Therefore the recorder output from the spectrophotometer was led directly to the Y—axis of a storage oscilloscope, as described previously (section 2.2.5.).

A manganese hollow—cathode lamp (Atomic Spectral Lamps Pty. Ltd.) was used as the spectral source, because of_the need in absorption methods of a narrow line source. The construction of the modified carbon filament atom reservoir has been described in section 2.2.3. A number of optical systems were investigated in an attempt to find the optimum arrangement which would also enable observations to be made 0 to 1 mm above the filament/ 148.

to obtain maximum sensitivity and to minimise matrix interferences.

Atomic absorption measurements with the carbon filament under conditions such that an area of 0.5 mm high above the filament 154 was irradiated have been described . This system will here be termed 'limited field irradiation', to distinguish it from

'limited field viewing' where only a small area above the filament is viewed, a horizontal slit preventing radiation which has passed through cooler areas from reaching the monochromator.

Limited field irradiation was investigated. In this case a small horizontal slit was placed near to the lamp, the radiation from the lamp was focussed just above the rod and then re—focussed by a second lens on to the monochromator slit. It was difficult to obtain sufficient light intensity to avoid using high amplifier gains by using this system, and consequently only signals with high noise levels were obtained. When a narrow horizontal slit was placed as closely behind the filament as possible and limited field viewing employed more of the source radiation reached the monochromator. Therefore an optical system whereby light from the source was focussed so that a circular image (diameter ca. 2.5 mm) was formed at the filament, with the sample platform just below the horizontal diameter of the circle, was used, and immediately after passing above the filament the light beam traversed a horizontal slit with a fixed 149.

width of 0.7 mm. Optimum results were obtained with the

carbon filament backed up as close as possible to the spectro—

photometer, (i.e. ca. 45 mm from the entrance slit) rather than with a second lens to refocus the light beam onto the monochromator

slit. No advantage was derived from placing a second horizontal

slit in front of the filament, in an attempt to ensure that no

light entering at a wide angle could reach the monochromator

without passing close to the filament, and this second slit being un—

necessary was dispensed with. A nickel foil screen was placed

between the filament and the slit to prevent as much as possible

of the radiation from the glowing filament from directly entering

the slit. A small copper tube (4 mm internal diameter, 40 mm in

length) was positioned between the horizontal slit and the mono—

chromator slit to act as collimator. This greatly decreased

noise from stray light. The relative positions of the apparatus

are shown in Figure 16. The calibrated capillary glass pipettes

described in section 2.2.3 were used for sample introduction.

New carbon filaments were found to contain little

manganese, and needed only to be heated at the operating voltage

for 3 seconds to remove all the manganese so that there was no - 'blank' signal.

5.2.2. Reagents

All reagents were of analytical reagent, grade 150.

Figure 16 Diagram of Apparatus for Atomic Absorption Measurements

Using the Carbon Filament

Monochromator entrance slit

collimator

horizontal slit nickel foil screen carbon filament

shielding-gas box focussing lens combination (f=35 mm) Hollow cathode lamp

optical axis 151.

and the water used was glass—distilled and then deionised. The

water used to prepare the solutions, and the reagents used in the

interference studies, were periodically checked for contamination

by manganese.

•. The sample pipette and all glassware were treated with 'Repelcote'.

'.2.3. Procedure .

The following procedure was used in routine

determinations: The wavelength, electronic gain, slit—width,

shielding gas flow—rate, water—cooling and hollow—cathode lamp

current were adjusted to previously determined optimal values.

The filament was then heated to the working temperature. After

30 seconds a 5 p.l sample was placed on the prepared surface,

and the water was then driven off slowly by heating the rod to

ca. 100°C. The filament was then switched off and the Variac

set to the value previously determined for atomisation. Nhile

waiting 30 seconds the 0 and 100% transmission signals were

stored on the screen of the oscilloscope. The filament was

switched on for two seconds; the signal from the SP 900A being

recorded on the oscilloscope and stored using the 'single—shot'

facility. The percentage absorption was then calculated from

the traces stored by the oscilloscope. The use of this regular 152.

timing sequence improved reproducibility. In particular, if

sample is placed on-the filament before it has cooled to below

100°C some solution is lost as the sample boils, this is also

so if the water is driven off too rapidly in the drying process.

Using this timing sequence ca. 30 analyses an hour can be

accomplished.

5.2.4. Effect of Filament Voltage on Absorbance due to Manganese

The manganese analytical signal varies with the

applied filament voltage. This relationship is illustrated in

Figure 17, from which it is evident that the optimum applied

voltage is ca. 6.4 V. Manganese is completely removed from the filament at the vonage and heating time employed. This is

demonstrated by the absence of a second manganese absorption signal (memory effect) when the filament voltage was applied for

a second time.

5.2.5. Effect of Shielding Gas, its Flow—rate and Height of

Observation on Absorbance

Figure 18 shows the variation of manganese

absorbance with the nitrogen shielding gas flow—rate, using a filament voltage of 6.4 V and a height of observation 0-1 mm

above the filament. The optimum flow—rate was observed to be

153•

Figure 17 Manganese Atomic Absorption: Variation of Absorbance at 279.5 nm with Filament Voltage

o.16o _ 4

0.140 _

Absorba ce

0.120 _

Ow

0.100 -- 0.095 1 1 1 5.0 6.0 7.0 Filament voltage (volts) 154.

Figure 18 Mananese Atomic Absorption: Variation of Absorbance

at 2791.2 nm with Nitrogen Shielding-Gas Flow-Rate

0.30

Abso bance

0.2

0.1

1.0 2.0 3.0 4.o 5.0 6.o Flow-Rate (1 min-1) 155.

4.1 1 min-1 of nitrogen. This is high compared to previously 150-152,154. reported flow—rates using this cell However, it would appear that less volatile elements require higher shielding gas—flow rates for optimum results. The main effect of increasing the shielding gas flow—rate was to produce sharper absorption peaks, thus improving sensitivity. Rod life—times were also enhanced. For some elements some workers154'132 using heated graphite non—flame cells have reported that absorbance is markedly dependent on the nature of the shielding gas, either nitrogen or argon A careful study of the effect of replacing nitrogen by argon was therefore made, a graph essentially the same as Figure 18 was obtained. The optimum argon flow was also — 1 4.1 1 min and no significant enhancement of absorbance was observed. This is in agreement with results for manganese 132 obtained using a heated carbon tube . Therefore nitrogen was used in this study.

The effect of height of observation above the filament on the manganese absorbance can be followed by raising or lowering the filament with respect to the light beam, and altering the slit height so that it remains in the centre of the light beam. With the optimum filament voltage and shielding gas flow—rate, the sensitivity obtained for manganese was observed to be relatively independent of the height of observation above-the 156. filament between 0 and 2 mm, although above 2 mm sensitivity began to be severely reduced. However, it will be noted later that the effect of foreign ions on the absorbance observed for manganese was critically related to height of obServation and that least interference was obtained in the region 0 to 0.7 mm above the filament. Therefore measurements were made 0 to 0.7 mm above the filament.

5.2.6. Effect of Monochromator Slit—Nidth and Hollow Cathode Lamp

Current on Absorbance.

The need to resolve the principal manganese resonance lines from their neighbours in order to obtain linear calibration curves and good sensitivity has already been referred to (section

4.1.). To obtain good resolution using the SP 900A prism mono— chromator it is necessary to use very narrow monochromator slit— widths. An approximate value for the spectral band—width to which different slit—widths correspond was obtained by scanning the spectrum emitted by the lamp in the 280 nm region. The scan was made by driving the wavelength control with a synchronous motor (Unicam Instruments Cambridge Ltd.) and presenting the signal, obtained from the monitoring of the hollow—cathode radiation, on the oscilloscope screen. The peak to peak distance of the

280.11 nm and 279.48 nm lines was measured, and if this separation 157.

is assumed to be 0.63 nm, the width of the peaks of the three

lines at half intensity can be given in nm. These peak half—

widths were taken as an approximation to the spectral band—pass

at the given slit—width. Greatest sensitivity in manganese

atomic absorption determinatinns was obtained with a slit—width

of 0.01 mm, but at this slit—width very little light reached

the detector and the resultant signal after amplification was

too noisy to be analytically useful. As the sensitivity only '

dropped slightly with increasing slit width (ca. 15% between

0.01 mm and 0.03 mm) the optimum monochromator slit—width for

.the measurement of manganese absorption at 279.5 nm was 0.02 mm

(which corresponds to a spectral bandwidth of ca. 0.2 nm at this

wavelength). When. strictly limited field viewing was employed

monochromator slit—widths of 0.025 and 0.03 mm (spectral band—

width ca. 0.25 nm) were often employed to improve the signal to

noise ratio.

Similarly, lower hollow—cathode lamp currents gave

greater sensitivity but the signal to noise ratio increased4.as

the lamp current increased. Greatest sensitivity was obtained

with a lamp current of 5 mAl but the amplified signal was then

too noisy and the optimum current was 10 mA. When only that

fraction of light which passed extremely close to the rod was

viewed, a current of 30 mA (the manufacturer's r'3commended 158. maximum) was preferred. Again the fall in sensitivity involved was only very slight (ca. 12% between 5 and 30 mA). While noting that these parameters did not critically affect the sensitivity obtained it should be pointed out that they will be discussed later as possible causes of non—linearity in manganese calibration graphs.

5.2.7. Detection Limits and Nature of the Signals

With the established optimum conditions the detection limits (that concentration of manganese which gave a signal to noise ratio of 2:1) and sensitivities (for 1% absorption) were determined At 279.5 nm and 403.1 nm. Using a 5 111 sample -1 an 0.01 ig m1 manganese solutinn could be detected at 279.5 nm, 12 this corresponds to an absolute limit of detection of 50 x 10 g. — 1 The sensitivity for 1% absorption was also 0.01 Fg ml using a

5 fal sample, indicating the relatively high noise levels using limited field viewing and a fast response system. At the 403.1 -1 resonance line using az5 Fl sample 0.05 iu.g ml could be detected, -11 an absolute limit of detection of 25 x 10 g. This corresponded -1 to a sensitivity of 0.10 jig ml . The intensity of the lamp at

403.1 nm is stronger than that at 279.5 nm and therefore smaller amplifier gains were needed, and the resultant reduction in noise enabled amount smaller than 1% absorption to be detected by using scale explansion. The concentration at the limit of detection of —1 0.01 jig ml at 279.5 nm compares favourably to typical values 159. obtained using flame cells. The carbon filament, however, is suitable for the detection of much lower absolute amounts of manganese. The limit of detection with this system for 1 1 12 samples was found to be 0.03 pg ml i.e. 30 x 10 g, but when the same monochromator was used for the atomic absorption of manganese in an air—acetylene flame (see section 4.5.) it was necessary to use a sample volume of 2 ml to obtain a useful signal at the detection limit of 0.1 pg ml-11 i.e. 2 x 107g.

Thus the advantage in terms of absolute amounts of sample is several orders of magnitude. This advantage was exploited by placing 20 5 pl samples of a very dilute solution on the filament and carefully driving the water off between applications. -1 In -this way 0.0005 pg ml manganese were detected. Twenty applications of 5 pl samples of blank solutions gave no signal.

The detection limits obtained are summarised in Table VIII.

The reproducibility of thl signals was investigated and checked regularly. At the one nanogram level using twelve

5 p.l samples a coefficient of variation of 6% was obtained. All signals quoted here represent the average of at least six values in good agreement.

The possibility that the absorption signals might arise from scatter of the incident source radiation was excluded by using the lead line at 283 nm. No signal was obtained from —1 5 p1 of a 10 lig ml manganese solution using a lead hollow cathode

TABLE VIII: Detection Limitsa for the Determination of Manganese

. Atomic Absorption Spectroscopy Atomic Fluorescence Spectroscopy 5 Atom Cell (Hollow Cathode Lamp Source) (279 nm) 279.5 nm 403.1 nm Hollow Cathode Electrodeless Lamp Source Discharge Lamp Source b c Carbon Filament: 5 p1 sample 0.01 11 0.05 • (5 x 10- g) (2.5 x 10-10g) 1 Fl sample 0.03 -11 (3 x 10 g) 20x5 p1 sample 0.005 11 (5 x 10 g)

Carbon Filament:5 Fl sample 0.001 d in 0.001 ' 40 ' 0.0002 in (modified d.c. (5 x 10 1`g) (5 x 10 1'g) (1 x 10-'-g) detection system) am 1 ill s ple 0.003 in 0.003 10 0.0006 -13 ' (3 x 10-'-g) (3 x 10-.g)' (6 x 10 - g) Air-Acetylene flame in this 0.1 e h 0.4 f h 0.001 a study (Chapter 4) (2 x 10-7g) (8 x 107g) (2 x 10 'g) • 214 Air-Acetylene flame 0.005 g

1 a Detection limits quoted in F g ml (with absolute limit of detection in parenthesis) 1 -1 Sensitivities (for 1% absorption):. b 0.01 ig ml i manganese; c 0.10 pg mli manganese; d 0.001 pg ml__i manganese; e 0.2 Fg ml manganese; -1 f 1.0 Fg ml manganese; g 0.05 pg ml manganese; h using electrodeless discharge lamp source. 161. lamp.

The calibration curve at 280 nm is shown in Figure

19. A number of factors may account for the non—linear nature of the working curve; an investigations of these factors is presented in section 5.2.10. The analytically useful working range is from 0.05 to 10 nanograms.

5 2.8. Interference Studies

An.extensive examination of the effect of foreign ions on the abSorbance recorded for manganese was made. Matrix interferences more severe than those reported as typical for flame cells were observed. Sodium was a typical case and was studied in some detail. First the nature and flow—rate of the shielding gas was found not to affect -Vie degree of interference. Other workers have previously concluded that interferences in the carbon filament technique largely arise from vapour phase inter— 151,. 152 151 actions Aggett and West produced strong evidence for this theory by using two filaments mounted in parallel, the sample was placed on one filament and the interference on the other. The observed interference was as great as when a mixture of sample and interference were placed on the same filament. Accordingly it seemed likely that interferences would be least close to the rod where the atomic vapour still 1.0 Fijure 19 Manganese Atomic Absorption at

279.5 nm (Using a.c. Detector): Calibration

Curves

0.8

Abso bance

0.6

0.4 _

a

Manganese Sample (a-10-10g) (b-10-901 5 10 15 20 25 163.

remains hot. A small radial hole drilled in the centre of a 156 filament (1 mm diameter), similar to that described by Amos was studied as a possible new sample site, the sample being inserted from a further hole above. The results from this device were not encouraging and strictly limited field viewing seemed to offer the best results. The vapour in the cooler regions, i.e. above the rod and at the edges of the sample platform, was progressively, and as far as possible, excluded from the field of view and the interference from sodium was found to decrease. The results are summarised in Table IX.

TABLE IX

Effect of limited field viewing on % interference on manganese

(2 ng) from 2,000 ng sodium

Height above filament viewed (mm) 0-2.0 0-1.5 0-1.0 0-0.7

% Interference (% decrease in Mn signal) 66 53 47 40

The interference from sodium and other matrix elements appears to

be dependent on the absolute amount of concomitant element present rather than its weight excess ratio, e.g. less severe interference 164. was observed from a 1,000 fold excess of sodium ions when the manganese concentration employed was lower.

Table X lists the effects of 1,000, 100 and 10- fold weight excesses of 24 different foreign ions ot 5 jZl -1 aliquots of 0.4 lig ml manganese (2 ng). The effects of the interference's are listed as percentage suppressions or enhancements of the percentage absorption signal. In a 150 previous report concerning interferences using the carbon filament cell considerable irreproducibility of interference effects was claimed and attributed to the slow response recording system used. In this study it was found that, provided the height abo-e the filament viewed remained constant, interferences came within the limits of reproducibility given earlier. No comments on the height of Observation were made in the earlier 150 report and this may also be a factor in this reported irreproducibility. In this study only interferences which result in a change of=signal of 5% or more were taken to be significant.

At the 10-fold weight excess level (20 ng) no interference was obtained from any of the ions studied. However, at the 100-fold excess level (200 ng) interference was observed from the relatively volatile elements. At a 1,000-fold excess

(2 jig) most of the cations investigated produced serious 165.

TABLE X

Interferences in the Determination of Manganese Using the Carbon Filament

% change of Signal from 1,000, 100 and 10—fold wt. excess in Atomic Absorpt;On in Atomic Fluoresnce Signal from 2x10 9g Mn Signal from 2x10 g Mn Ion Salt 1,000 100 10 1,000 100 10

3+ AVs0 ) —8 N.S. M.S. Al 4 3 N.S. M.S. N.S. 2+ Ca Ca(NO3)2 +30 N.S. N.S. N.S. N.S. N.S. Cr3+CrC1 —70 —67 . M.S. N.S. N.S. 3 —13 Fe3+ Fe/HC1 —45 —15 N.S. N.S. N.S. N.S.

K+ KNO3 —60 —40 N.S. —10 N.S. N.S. Mg2+ MgC12 —100 —80 N.S. —88 N.S. N.S.

Nai. NaC1 —40 —20 N.S. —10 N.S. N.S. 2+ Ni —60 N.S. N.S. N.S. Ni(NO3)2 • N.S. N.S. Si4+ Si/HF +16 N.S.--- N.S. N.S. N.S. N.S.

V4+ VIM —20 417 N.S. —7 N.S. N.S.- - 3 Cl— HC1 N.S.. N.S. N.S.' N.S. N.S. N.S.

HNO NO3 3 N.S. M.S. N.S. N.S. N.S. N.S. PO 3— (NH ) 4 4 2HP04 N.'--s. N.S. N.S. N.S. N.S. N.S. 5042 H2SO4 N.S. N.S. N.S. N.S. N.S. N.S.

Contd 166.

TABLE X (contd.)

Interferences in the Determination of Man,,anese Using the Carbon Filament

% change of Signal from 1,000, 100 and 10—fold wt. exce:

in Atomic Abeorpt4on in Atomic Flnore.wce Signal from 2x10 'Er: Mn Signal' from 2x10 g Mn Ion Salt 1,000 100 10 1,000 100 10

3+ Ga Ga/HC1 —50 N.S. N.S.

H HgNO gt 3 N.S. N.S. N.S. 6+ Mo (NH4)21400 4 N.S. N.S. N.S. 2+ Pb Pb(NO ) 2 N.S. N.S. N.S. . 3 TO; Ta/HF —20 N.S. N.S. m4+ Th(NO ) 420 N.S. N.S. 3 4 + 11 T1NO N.S. N.S. 3 —7 W 6+ WHN0 /HP N.S. N.S. N.S. 3 2 Zn+ N.S. N.S. N.S. ZnS(34, • + N.S. N.S. M.S. NH4 lalOH

N.S. = not significant change in signal < 5% 167. depressions; no interfer'ence was noted from the anions studied.

It can be seen that volatile elements tend to interfere more than involatile elements. This is in contrast to our study of interferences in the atomic fluorescence of manganese in the air—acetylene flame (see section 4.3.6.), but tended to:confirm the theories of vapour—phase interactions. All the interferences observed were depressions of signal, except in the case of calcium -1 and silicon, where 400 lig ml solutions of these cations produced an absorption signal which appears to be caused by scatter of the source radiation by particulate material produced above the filament. Figure 20 shows some reproductions of oscilloscope traces illustrating interference.

It may be observed from Table X that where serious interference was observed in many cases the cations were aided as their chlorides. It was therefore thought that the more volatile chlorides might lead to more serious interference than if a less volatile compound was present. In a separate experiment it was -1 found that the interference on a 0.4 jig ml manganese solution by 1,000—fold weight excess of iron present as its sulphate was only —12%, whereas the interference from the same weight of iron present as its chloride was —45% (see Table X). This observation might be construed to provide support for this suggestion. It has, however, been found that the atomic absorption signals obtained 168.

Figure 20 Traces Showing Interferences in Manganese Determinations

100% transmission

- 0.4 pg ml_1i Mn +1_ 0.4 pg m1-1 Mn 400 ptg ml_l Na 0.4 pg ml Mn -1 .4 pg ml_i Mn +t 400 pg ml Ca f

100% abs rn+inn

Sodium Interference (absorption) Calcium Interference (absorption)

100% transmission -1 0.4 pg ml_i Mn 400 pg ml Fe 0.4 pg m1-1 Mn

-1 0.04 pg Mn + 40 pg ml Cr

100% absorption o A

Iron Interference (absorption) Chromium Interference (fluorescence) 169. in the determination of iron using the carbon filament technique are identical for given concentrations of iron whether it is applied to the filament as the,chloride or sulphate (see Chapter 6).

This indicates that the interferences observed may be more complex than can be explained by Simple consideration of the volatility of the matrix compound. In particular iron and nickel seemed to displace the manganese absorption peak as well as to reduce it, whereas other interferences tended to reduce it. It is possible that these elements represent an 'atomisation' interference.

In the peak method of measurement (see section 2.1.4.) it is' necessary to assume that TIPC2 is constant, where'Ci is the overall atomisation time andT2 the residence time, as in this case 2.22 Thus if matrix elements slow the rate of atomisation C1 the peak absorption will be reduced, the integration method of absorption measurement may be used to overcome such interferences. This will be further discussed in Chapter 7. In the• study of interferences on the determination of manganese in the air—acetylene flame it was noted (section 4.3.6.) that magnesium depressed both the atomic fluorescence and absorption signals of manganese, and this was ascribed to a chemical effect.

In this study magnesium was again the most serious interference, depressing the manganese absorption when present at both 1,000 and 100—fold weight excess. 170.

5.2.9. Sensitivity of Absorption Measurements Using the Modified

Detector System

The modification of the Unicam SP 900A detection

system and the direct input of the d.c. signal to the amplifier

of the oscilloscope has already been described (section 2.2.6.).

Some preliminary studies of the atomic absorption of manganese

using this system were made. This rapid response system enabled

higher filament voltages to be used (up to 12 volts) which

produced sharper absorption peaks. There was also less dis—

tortion of the signals and it was no longer necessary to convert

the signal to an a.c. level. Thus the sensitivity was improved

by an order of magnitude. At an optimum voltage of 12 volts and -1 a -nitrogen flow rate of ca. 4.1 1 min an absolute detection •••••••• limit of 5 x 1012g manganese (5 Fl,of 0.001 jig m1-1 manganose)

at 279.5 nm.was obtained. This corresponded to a sensitivity -12 — 1 for 1% absorption of 5 x 10 g (5,.x1 of 0.001 tig ml manganese).

.5:.2.10. Calibration Curves

The calibration curve for manganese atomic absorption

at 279.5 nmi using- the unmodified a.c.) detection system,

is shown in Figure 19. It was thought that a number of factors

might account for the non—linear nature of the working curve and

these were investigated: To remove any possibility that signal 171. distortion might account for the non—linearity the modified d.c. detection system was used in the investigations. Three possible causes of the non—linearity were given particular attention.

These can .be summarised as: a) Spectral — Non—linear calibration curves have been shown197 to arise in atomic absorption spectroscopy if the emission source width is not narrower than the width of the absorption line, if the, source light beam traverses the portions of the atom cell of different atomic concentrations and if non—resonant light is received by the detector. Using flames with temperatures of about 2,250 C and low—pressure gas discharge sources with temperatures of only ca. 300°C, the width of the absorption line profile tends to be two or three times the width of the Doppler broadened emission line profile of the gas discharge. However, the temperature of the atomic population above the;:carbon filament may be as low as 400°C (as will be- Shown in Chapter 7) and, in addition, the source emission line profile may, by self— absorption (e.g. if a hollow—cathode lamp is lover—runt), be considerably broadened. Thus the source emission line profile may be7of the same order as the width of the absorption line profile. Secondly, the nature of carbon filament atomisation causes the atomic concentration to fall as the atomic vapour cools and expands as it leaves the filament. Hopefully, the 172.

problem of falling atomic_concentration can be minimised by

observation of only atomic vapour close to the rod. Mention

has already been made (section 4.1.) of De Galan and Samaey's 256 study of the bending of manganese atomic absorption analytical

curves unless the manganese multiplet at 280 nm is resolved, the

third possible spectral cause. Non-linearity above 0.5 absorbance

is predicted if a spectral band-pass of 1.6 nm is used, and

it is shown to be necessary to use a spectral band-pass of 0.04 nm

to obtain linear analysis curves.

Non-linearity would also be produced if the absorption

line profile were to change with increasing atomic concentration.

This would be so if resonance broadening (see section 1.2.2.)

was comparable to Lorentz broadening. It has been sho;•m39 that

in flames, because of the great dilution of analyte atoms by the

flame gases, resonance broadening is insignificant compared to

Lorentz broadening, at least up to molar solution concentrations.

It has already been shown in this thesis (section 1.3.3.) that

the atomic vapour above the carbon filament is not as dilute as

the atomic vapour in a flame. Consequently it might be thought

that collisions with atoms of the same kind (resonance broadening)

might betas important as collisions with foreign gas molecules

(Lorentz broadening). However, calculations made to determine

the ratio of the Lorentz to resonance broadening by substitution 173.

into the appropriate formulae4 showed conclusively for manganese, under these analytical conditions and over normal working ranges,

that resonance broadening compared to Lorentz broadening was

insignificant.

b) Atomisation - Reference has alreafiy been made to the need

to maintain the ratio 17(T 2 constant (section 5.2.8.); The residence time for atoms in the light path (T2) should be

independent of concentration, but it might be expected that

greater concentrations would require longer times for complete, atomisation (t1). Thus it might be expected that larger manganese concentrations would cause peak broadening as well as increasing

the peak height. Integration of the signal would overcome this difficulty, more simply the problem would be minimised by shortening

1C and heating the filament more rapidly, i.e using a higher 1 filament voltage.

c) Vapour-Phase Interaction - It has already been suggested

(section 5.2.8.) that interferences in the carbon filament technique are caused by vapour-phase interactions. Several mechanisms for this process are possible, the formation of molecular species as the atomic vapour cools and the inclusion of manganese in these species either by direct chemical bonding or by physical occlusion being the most probable. Both these mechanisms could also reduce the population of free manganese atoms as the manganese population increases when calibration curves are plotted. Limited field 174. viewing might again be expected to minimise these effects.

The linearity of calibration curves was studied while a number of parameters were varied. The effects of varying the filament voltage, height of observation above the filament and monochromator slit—width, while holding other parameters constant, are presented graphically in Figures 21,

22 and 23. It was found that greatly increased linearity, to above 0.5 absorbance, was obtained using a high filament voltage

(10.7 V), a narrow monochromator slit—width (0.02 mm corresponding to a spectral band—pass of ca. 0.2 nm) and viewing only the region immediately above the filament (0-0.7 mm above). It was not possible using these conditions to use a hollow—cathode lamp current below 25 mA, because of the noise at high gains produced using this rapid response detection system. However, should improved optics make this possible further improvement in linearity might be expected. The results obtained in this study indicate that non—linearity is probably caused by a combination of the factors listed earlier, but that if careful attention is given to experimental parameters a range of linearity may be obtained similar to that for calibration curves using flame cells and the same source and monochromator.

0.7 Fimire 21 'Manganese Atomic Absorption Calibration Curves —

Effect of Varying Filament Voltage 0.6

0.5

0.4..

Abs rbance 0.3

0.2

0.1

0 ng Manganese 0 0:5 1. 0 1:5 21.0 2:5 3.0 0.8- Figure 22 Manganese Atomic Absorption Calibration Curves - Effect of Varying Height of Observation

0.7_

cr"\, 0-1.0 •rnm above filament o.6- c.5_

Abs rbance -1 .5 mm 0.4

1.0-2.0 mm above filament 0.3

0.2

0 ng Manganese C 0.5 11.0 1.5 2.0 2.5 1.0 0.8 Figure 23 Manganese Atomic Absorption Calibration Curves -

Effects of Varying Monochromator Slit Width 0.7

0.6

0.02 mm

0.5 - 0.03 mm Absor ance

0.4

0.3

0.2

0.1 -

np Manganese 0 0.5 1.0 1.5 2.0 2.5 178.

5.3. Atomic Fluorescence Spectroscopy of Manganese

- 5.3.1. Apparatus

The carbon filament atom reservoir and. Unicam

SP 900A monochromator described previously were used with the

modified fast response d.c. detection system (see Chapter 2).

When the signal was fed to the oscilloscope via the SP 900A

amplifier fluorescence signals app,aared only as shoulders on

the peak caused by the glowing filamentl even when the monochromator

was shielded as much as possible from this glow. Therefore, use

of the faster response system to separate these two signals was

essential. Both the manganese hollow—cathode lamp, used in

the- work described previously inthis chapter, and an electrodeless

discharge lamp similar to that used in the studies in Chapter 4

were employed as spectral sources. The optical system used for

the atomic absorption studies Was used with the modification

that the source and lens. were moved to be at right—angles to the

optical axis. This was achieved by placing the source and lens

above the filament and focussing the light from the source onto the

filament. This arrangement, although not ideal as the lens has to

be periodically cleaned to remove deposited carbon, was the

most suitable to enable limited field viewing. 179.

5.3.2. Use of a Hollow—Cathode Lamp as Excitation Source

Usually hollow—cathode lamps run at normal

operating powers would not be regarded as sufficiently intense

for atomic fluorescence studies. However, because of the

• low background of the carbon filament atom reservoir at the

time of measurement of the manganese fluorescence signal," appreciable

manganese atomic fluorescence was observed, using a manganese

hollow—cathode lamp as source.

The optimum parameters at 279.5 nm were established in the usual manner. The variation of fluorescence signal, for

2 ng manganese, with the filament voltage is shown in Figure 24.

The optimum filament voltage is ca. 11 volts. The variation

of signal, for 2 ng manganese, with argon shielding gas flow—rate — 1 is shown in Figure 25. The optimum argon flow rate is 3.8 1 min .

Nitrogen being more efficient at quenching the fluorescence was

not used as shielding gas. As was expected the fluorescence

signal showed a marked increase as the hollow—cathode lamp

current was increased, and the maximum recommended current (30 mA)

was used. The fluorescence signal also showed a marked increase as

the monochromator slit was opened. The continuum increased as

the slit was opened and therefore the optimum, 0.35 mm, was chosen

to give the most favourable signal to background relationship.

This slit—width was found to correspond to a spectral band—pass

of ca. 2.4 nm. The optimum value of the E.H.T. was found to be 180.

Figure 24 Effect of Filament Voltage on Manganese Atomic Fluorescence Signal at 279.5 nm Using a Hollow Cathode Lamp Source 200

150

Fluo scence Sign (mV)

100 _

50 _

4 5 6 7 8 9 10 11 12 Filament Voltage (volts) 181.

Figure 25 Effect of Argon Flow-Rate on Manganese Fluorescence Signal at 279.5 nm Using a Hollow-Cathode Lamp Source

525

500

475

Fluo escence Signal (mV) 450

425

400

375

360 0 2 3 4 Argon Flow-Rate (1 min °) 182.

—900V, and the time base used on the oscilloscope was usually -1 100 m sec cm .

With these established optimum conditions the

detection limits (signal to noise ratio 2:1) at 279.5 nm were

.0.001 .ig m1-1 manganese using 5 pl of samples and 0.003 p.g m1-1

.for 1 pl samples, i.e. an absolute limit of detection of -12 3 x 10 g. This absolute limit is of the same order as -12g)152 that previously reported (5 x 10 for manganese atomic

fluorescence using a hollow—cathode lamp and the carbon

filament atom reservoir, but using different instrumentation.

Linear calibration curves were obtained in the 12 -12 region 5 x 10 to 300 x 10 g manganese. The growth curve

for manganese atomic fluorescence under these conditions is

shown in Figure 26. Above about 250 ng the growth curve shows

that scatter of the source radiation occurs. By using a lead

lamp it was shown that scatter was not the cause of the signals

noted above and no signal was observed from even 50 ng Mn at

the 283 nm lead line.

p.3.3. Use of an Electrodeless Discharge Lamp as Source

The preparation, operation and characteristics

of manganese electrodeless discharge lamps have already been

described (Chapter 4). The optimum argon shielding—gas flow—

183.

Figure 26 Growth Curve for Manganese Atomic Fluorescence at 279.5 nm Using a Hollow-Cathode Lamp_E2urce

3.0-

2.0_

Log1 Fluorescence Sign 1 (mV)

1.0 -

0 1 2 3 4 5 6 7 -12 log10 Manganese Sample (10 g) 184. rate, filament voltage and monochromator slit-width were found to be the same as those reported above with the hollow-cathode lamp source. The optimum E.H.T., with regard to signal to noise ratio, was found to be -950V. The optimum lamp power was found to be 50W, as previously. The limit of detection

(signal to noise ratio 2:1) using a 5 fa sample was found to be -1 0.0002 jig ml manganese, i.e. an absolute limit of detection of 12 1 x 10 g. When a 1 Fl sample was used an 0.0006 Fg m1-1 manganese solution corresponded to the limit of detection, -13 i.e. an absolute limit of detection of 6 x 10 g. The reproducibility of the signals obtained was poorer than those obtained for manganese atomic absorption using • the filament. It would appear the short-term noise on the source contributes an additional factor. The reproducibility of the signal obtained from any single solution was within ± 5% of the mean at a 95% confidence level, and usually somewhat more reproducible, i.e. a coefficient of variation never larger than 10%. However, the analytical range was ca. one order of magnitude lower than that obtained using the absorption technique.

This is illustrated by Figure 27, a calibration curve for manganese -11 - 1 atomic fluorescence in the range 1 x 10 g to 5 x 10 0g sample.

This calibration curve is linear. The growth curve for manganese atomic fluorescence using an electi-odeless discharge lamp is 1,500 Figure 27 Manganese Atomic Fluorescence Calibration Curve (0.i-9x10-412s) at 279.5 nm Usinp an Electrodeless Discharge Lamp Source

1,250

1,000

Fluor scence Siena (mV) 750

500

250

Manganese Sample(10-10p) 1 2 3 186. shown in Figure 28. Again scatter was observed above ca. 250 ng manganese but the possibility of the analytical signals being caused by scatter was eliminated.

5.3.4. Fluorescence Signals at Other Wavelengths

Attempts to observe manganese atomic fluorescence at wavelengths other than 279.5 nm using both sources were unsuccessful. Particular attention was paid to examinations at all lines at which atomic fluorescence was observed using a flame cell (see section 4.3.3.). It was not unexpected that fluorescence from the two Mn(II) triplets was not observed . As the ion line— emission from the electrodeless discharge lamp source was present it must be assumed that the carbon filament provides insufficient energy to ionise manganese (the first ionisation potential for manganese is 7.43 eV). It was not possible to observe atomic fluorescence at 403 nm because the continuum from the glowing filament increases with increasing wavelength and any fluorescence signal could not, with the present apparatus, be distinguished from the continuum. This problem will be discussed at greater length later in this thesis. Study of the simplified Grotrian Diagram for

Manganese (Figure 11) shows that the fluorescence observed in the flame cell at 304.5, 357.8 and 383.4 nm probably arises from thermally assisted resonance fluorescence (see Figure 3). In the 187.

Figure 28 Growth Curve for Manganese Atomic Fluorescence at 279.5 nm Usinkan Electrodeless Discharge Lamp as source

3.5 -

3.0 _

2.5 • log10 fluorescence signa L (mV)

2.0 _

1.5 .

1.0

1 6 1 2 3 4 56•12 7 log10 Manganese Sample (10 g) 188.

air—acetylene flame sufficient energy exists to populate the

lower energy state in question. It would appear, however, that

the carbon filament is unable to produce sufficient energy (over

2 eV is required) to significantly populate this level. This

conclusion is in agreement with the estimated temperature above

the filament (see Chapter 7) and results obtained for iron atomic

absorption (see Chapter 6).

5.3.5. Use of Nitrogen as Shielding gas

The use of nitrogen as shielding gas was investigated

and the signals obtained compared to those obtained using argon.

The intensity of atomic fluorescence using argon was found to be

ca. 16 times greater than that obtained using nitrogen. This

is to be expected from the much smaller quenching cross—section 157 of argon. It has been suggested in the determination of lead

in matrices prone to scatter and using a carbon rod, where the fluorescence intensity ratio Ar:N2 was 13:1, that to a good approximation the scatter signal could be nullified by subtracting the signal obtained in nitrogen from that obtained in argon. The results obtained with such matrices in this study would strongly support use of such an approach for manganese determinations.

5.3.6. Interference Studies Using Atomic Fluorescence

The effect of 14 foreign ions on the atomic fluorescence of 5 pi aliquots of 0.04 'g m1-1 manganese (2 x 1010g) 189.

at 1,000,100 and 10—fold excess were studied. An electrodeless

discharge lamp was used as spectral source, because it offered

highest sensitivity. Only the fluorescence signal occurring 0 to

0.7 mm above the rod was viewed by using a slit in the manner

previously described. The effect of these ions are listed as

percentage suppressions or enhancements of the fluorescence

signal in Table X. Only interferences which result in a change of

signal of 5% or more were taken to be significant.

It should be noted at the 10 and 100—fold weight

excess level no interference was obtained from any of the ions

studied. This would agree with the conclUsion reported earlier

(section 5.2.9.) that the absolute amount of sample is important, rather than the weight excess ratio. The 100—fold weight excess

level in this study corresponds to 10—fold weight excess at the

2 ng level used for absorption studies, at which level no inter— ference on the manganese absorption was reported. At the 1000— fold excess level interference was only reported for five cations,

and only that of one, magnesium (the most serious interference in

flame atomic fluorescence and non—flame atomic absorption), was

serious. Why the interference from the other cations studied

was lower in the atomic fluorescence determinations than in

atomic absorption was interesting. It is possible that the higher filament voltage used in these fluorescence measurements and the 190.

faster response recording system may be a contributory factor.

In particular, it was noted that certain oscilloscope traces

indicated interferences from elements less volatile than manganese

were more serious after the signal peak had been passed, e.g. the

interference of chromium illustrated in Figure 20. The reduction

in signal by chromium can be clearly seen but it occurs largely after

the peak fluorescence has been recorded. Therefore with the peak method

of'measurement chromium is a less serious interference than if the

integration method of recording is used. Thus with the fast

response system such interferences can be minimised. Advantage

might also therefore be expected to be derived from this fast

response system with regard to interferences in the absorption

technique.

From this brief interference study it would appear

feasible to improve upon the determination of manganese in the

presence of concomitant elements which interfere in atomic absorption,

by diluting the solution ten fold and determining the manganese using atomic fluorescence spectrometry, provided that a slightly

higher coefficient of variation can be tolerated. 1 9 1 .

CHAP'T'ER 6

The Determination of Iron Using a Non-Flame Cell

6.1. Introduction

A few of the non-flame atom cells described previously (section 1.3.4.) have been used to atomimiron for atomic absorption spectroscopy and in one case atomic fluorescence spectroscopy, although flame cells have been by far the most 22 popular atomisers in such analyses. L'vov has determined iron by atomic absorption at the 248.3 nm resonance line using s graphite cuvette (2.5 mm internal diameter). At the optimum pressure, 2 atmospheres of argon, and with a temperature of 2,100°C, -11 2.5 x 10 g of iron gave an absorbance of 0.11. This corresponds -11 to an absolute sensitivity for 1% absorption of 1 x 10 g.

Woodriff et a1135 when using a carbori cup and their furnace at

2,200°C report a sensitivity at 248.3 nm of 1 x 10-10g iron and estimate the detection limit using their device to be 2.8 x 1010g iron.:-+A linear calibration curve from 1 x 10-9g to 20 x 10 9g 65 (ca. 0.65 absorbance) was also reported. Massmann studied both the atomic absorption and atomic fluorescence of iron at 248.3 nm using his graphite cuvette technique. Detection limits of •192.

-1 2 x 10 1g iron using absorption and 3 x 10 9g using fluorescence were reported. In Massmann's study iron atomic fluorescence determinations appeared to be the least sensitive of the nine elements 266 studied in fluorescence. Fernandez and Manning have determined iron in water using a commercially available,heated graphite atomiser, based on that of Massmann. An absolute sensitivity for x 10 1% absorption of 2.5 11g iron using the 248.3 nm line was - 11 reported and a limit of detection of 4 x 10 g, i.e. 100 Fl of 1 0.4 Fg 1 iron solution, a coefficient of variation of 4.7% was 1 claimed from 15 determinations of 100 Fl samples containing 10 jig 2 156 - 12 iron. Amos reports a detection limit of 3 x 10 g iron

(i.e. 1 Fl of 0.003 jig m1-1 solution) using a hydrogen-shielded modified carbon filament. Using a commercially available carbon filament 157 11 device Amos and co-workers report detection limits of 1 1. 10 g and 2 x 10 12g iron in the atomic absorption and atomic fluorescence modes respectively, using argon as shielding gas.

Iron is an important trace element in a large number of matrices and therefore of particular interest. It is

somewhat less volatile than manganese, m.p. 1535°C, b.p. 3,000°C,

and additionally the formation of iron carbide (Fe3C m.p. 1837°C)

is well known. These facts may explain why few reports of the

determination of iron using graphite non-flame cells have been

made. In this chapter a study of the determination of iron by

Atomic absorption spectroscopy will be described, with particular 193. reference to interferences from diverse ions. An analytical

procedure for the determination of trace amounts of iron in small

samples (ca. 1 mg) of carbon—fibre composite will also be outlined.

6.2. Atomic Absorption Spectroscopy

6.2.1. Apparatus

The optical system, monochromator, d.c. rapid response detection system and atom cell described prw.riously (section 5.2.1.

and 5.2.9.) were used. An iron hollow—cathode lamp, (Pye Cathodeon

Limited, Cambridge) was used as the speltral line source. It was found to be:possible to observe iron atomic absorption signals using the a.c. amplification system of the SP 90CA spectrophotometer

and feeding the recorder output directly to the Y—axis of the

storage oscilloscope, but as will be shown later, the modified d.c. detection system (see section 2.2.6.) was preferred for

these studies. No analytical signals were obtained for iron using the SP 900A amplifier and a chart recorder, it being impossible

with such a system to separate the absorption peak from the glowing filament continuum. Limited field viewing was found to

be essential if analytically useful signals were to be obtained.

The arrangement of viewing 0 to 0.7 mm above the rod, described

previously (section 5.2.1.), was again adopted in order to maximise

sensitivity and minimise interferences. 194.

6.2.2. Reagents and Procedure

All-reagents were of analytical reagent grade and

the water used was glass—distilled and.then deionised. The water used to prepare the solutions, and the reagents used in the

interference studies, were periodically checked for contamination

by iron.

The sample pipette and all glassware were treated

with 'Repelcotel.

The procedure followed in routine determinations

was the same as that described for manganese (section 5.2.3.).

Additionally the EHT was set to a previously determined optimum

voltage.

6.2.3. Preparation of filaments for analytical studies and nature

of the signals

The carbon used to prepare the filaments was found

to contain a large amount of iron, it was necessary to remove this

iron from new filaments before they could be used in analytical

studies. This was .achieved by repeated heating of the filament

for 3 seconds at 12 volts. A glass dome which fits tightly onto

the base—plate of the filament apparatus, and with a tap at the

top for the shielding gas exit, was used. This completely

isolated the filament from the atmosphere, as otherwise slight 195.

oxidation at the ends of the filament occurred, which reduced the useful life of the filament. Despite this precaution the filaments which were freed from iron in this way showed an increased porosity, presumably as a result of the fierce heating necessary to remove all the iron. The use of a solution of polystyrene in 22 benzene which Lvov has reported prevents the seeping of solution into the electrode heads used with his cuvette, was investigated.

When filaments so treated were heated strong absorption was recorded, measurements at other nearby lines which were not iron resonance lines, confirmed that this was band and not line absorption.

It seemed likely that these bands arose from benzene being liberated from the degradation of the polystyrene. The use of a double=-beam background correction system might enable the use of polystyrene, but with the apparatus used in this study polystyrene could not be used. In a further attempt to prevent the solution seeping into the rod, tungsten metal powder was spread over the filament notch and then melted by heating the rod to above 3,3700C (3 seconds at 12 volts). After this process had been repeated several times a layer of tungsten was formed at the centre of the sample notch, it did not however spread onto the walls and edges of the notch and thus the solution still came into contact with porous portions of the rod. For this reason this approach was also unsuccessful.

Best results were obtained when the filament was 196. freed from iron by heating in the enclosed dome, which was fitted

with silica windows so that the attenuation of the light beam by

iron atomic absorption could be monitored, and using a-nitrogen -1 flow—rate of ca. 2 1 min . Filaments treated in this way gave

very high sensitivity for the first fifteen determinations after

which the sensitivity decreased, possibly because solution was

seeping into the rod; the reproducibility of the signals, however,

increased. A coefficient of variation of 6% for 5 F' samples was

obtained after the first twenty—five determinations and this was

maintained during the useful filament—life. Using these conditions

and switching the filament on for 1.5 seconds at 12 V for each

determination, useful filament—life times were about 200 determinations.

The possibility that the absorption signals obtained

might arise from scatter of the incident source radiation was

excluded by using the cobalt lines at 242.5 and 252.1 nm. No —1 signal was obtained from 5 pl of a 10 p g mliron solution using

a cobalt hollow—cathode lamp. The possibility that the signals

might arise from residual iron being eluted from the filament was

eliminated when no signal was obtained from blank solutions of

deionised water or acid.

It was thought that the differing volatility of

various iron salts (e.g. iron (III) chloride b.p. 315°C and iron

(III) sulphate which decomposes on heating to iron (III) oxide

m.p. 1565°C) might mean that equivalent amounts of iron in 197.

different salts might not give similar analytical signals.

However, no difference was observed in the atomic absorption signal

when equal concentrations of iron were applied to the filament

whether it was applied to the filament as the sulphate or the

chloride. This finding was confirmed by interference studies

(see Table XIII) and presumably arises from the strongly reducing

nature of heated carbon.

6.2.4. Optimisation of Experimental Conditions

The relationship of filament voltage to the

analytical signal from 5 ng iron at 248.3 nm is shown in Figure 29.

It can be seen that higher filament voltages can be used and

increased sensitivity obtained with the d.c. detection system. The

optimum filament voltage was 12 V. The graph of absorbance for 0.5 ng

iron at 248.3 nm against height of observation, shown in Figure 30,

indicates that at 12 V the iron atomic vapour may exist for a few

mm above the rod but that greatest sensitivity is obtained

immediately above the rod (0 — 0.7 mm).

Figure 31 shows the variation of the absorbance

due to 0.5 ng iron at 248.3 nm with the nitrogen shielding gas

flow—rate, using a filament voltage of 12 V and a height of observation

0 — 0.7 mm above the filament. The optimum flow—rate was found

to be 3.9 1 min-1. As for manganese no enhancement of absorption 178.

Figure 29 Iron Atomic Absorption: Variation of Absorbance at

248.3 nm with Filament Voltage

7 8. 9 10 11 12 Filament Voltage (Volts) 199.

Figure 30 Iron Atomic Absorption: Variation of Absorbance at 248.3 nm with Height of Observation

0.4_,

0.3

0.2 -

Absor ante

• • 0.1 - •

• • •

• • • • . i 0 I 1 1 1 I 0 1 2 3 4 5 6 Height of Observation (mm above the filament) 200.

Figure 31 Iron Atomic Absorption: Variation of Absorbance at 248.3 nm with Nitrogen Shielding Gas Flow-Rate

Nitrogen Flow-Rate was obtained when argon was substituted as shielding gas, in this 1 case the optimum argon flow—rate was 4.0 1 min . Nitrogen was used as shielding gas in these studies.

Greatest sensitivity and linearity of calibration curves was obtained with a very narrow monochromator slit—width (0.01 mm), ,which gave the highest resolution of the 248.3 nm line from neighbouring lines. Similarly, greatest sensitivity was obtained at low hollow—cathode lamp currents: However, in order that sufficient light intensity reached the detector to prevent the need to use high amplifier gains, which with this fast response system result in high noise levels, a wider slit width and higher lamp current were preferred. The optimum settings were a slit width of 0.02 mm (corresponding to a spectral...band—pass of 0.15 nm at this wavelength, as determined by scanning the iron lines in this region) and a hollow—cathode lamp current of 15 mA (the manufacturers recommended maximum). The optimum EHT was found to be —950 volts.

6.2.5. Detection Limits and Calibration Data

With the established optimum conditions the detection limit (that concentration of iron which gave a signal to noise ratio of 2:1) and sensitivi y (for 1% absorption) were determined at 248.3 nm. Using a 5 F1 sample an 0.002 lig ml-1 202.

iron solution corresponded to the limit of detection, i.e. an -12 absolute limit of detection of 10 x 10 g The sensitivity

for 1% absorption was 5 pl of 0.001 p.g ml—1 solution, i.e. 5 x 1012g iron, the sensitivity was lower than the limit of detection because of the high noise levels using limited field

viewing and this fast response detection system. The limit of

detection could be lowered considerably if a more intense narrow—line .

source was available, or the optical'system could be improved. The

limit of detection with this system for 1 p.l samples was found to be -1 -12 0.006 pig ml i.e. 6 x 10 g, but when the same monochromator

was used for the atomic absorption of iron using an air—acetylene

flame (see section 3.5.) it was necessary to use a sample volume

of 2 ml to obtaima useful signal at the detection limit of -1 0.1 pg ml i.e. 2 x 10 7g. Thus, as for manganese determinations

using the carbon filament, the advantage in terms of absolute

amounts of sample is several orders of magnitude.

The absolute limit of detection using the unmodified

SP 900A amplifier (a.c. detector) and the oscilloscope was found 11 to be 1 x 10— g10 iron corresponding to a sensitivity of 5 x 10 g.

The modified fast—response system (d.c. systeM) thus enables an

improvement of an order of magnitude in sensitivity'to be made.

This is because the fast—response system enables a higher filament

voltage to be used, 12 volts is the optimum for the d.c. system and

.7 volts the optimum for the a.c. system, resulting in sharper peaks 203. and also because the d.c. system causes less distortion of signals.

The detection limits obtained for iron atomic absorption at 248.3 nm are summarised in Table XI.

Linear calibration curves were obtained for iron atomic absorption up to high absorbances when narrow monochromator slit— widths and low hollow—cathode lamp currents were employed with limited—field viewing. It has already been noted that with limited— field viewing the optimum slit—widths and currents for the optimum signal to noise ratios were somewhat larger, i.e. 0.02 mm and

15 mA, these optimum settings increased non—linearity in the calibration curve and slightly reduced sensitivity. For practical analyses the effects were quite tolerable and a calibration curve for'iron atomic absorption, over the analytical working range 0.05 to 5 nanograms, using the previously established optimum conditions is shown in Figure 32.

6.2.6. Determination of Iron at Other wavelengths

Iron exhibits a relatively complex atomic spectrum with a large number of resonance lines and lines which arise from transitions with lower levels lying very close to the ground state.

Thus there are many lines at which iron atomic absorption may be observed, although the sensitivities of these lines vary greatly, largely because of their differing transition probabilities. 217 Smith and Frank have reported 31 usable lines for iron atomic 204.

ABLE XI

Iron Atomic Absorption: Detection Limits and Sensitivitiesa at 248.3 nm

Atom Cell Detection Sensitivity Limit (for 110 absorption

Carbon filament with d.c. detection system: 5 p1 sample 0.002_1/ 0.001_1 2 (1x10 )g (5x10 )g

1 pl sample 0.006_,, 0.00 3-12, (6x10 i )g (3x10 )g

Carbon filament with 11 a.c. detection system: 5 111 sample 0.02 la 0.01 (1x10 )g (5x10 )g

Air acetylene flame in this study 0.1, 0.9 (see section 3.5.) (2x10 7)g

218 Air—acetylene flame 0.005 0.062

-1 a) Detection limits and sensitivity quoted in lig ml (with absolute

limits and sensitivities in parenthesis) 205.

Figure 32 Iron Atomic Absorption at 248.3 nm:: Calibration Curve (0.05-5 ng)

Iron•Sample (ng) 206.

absorption spectroscopy using a flame cell. In the study

presented in this thesis iron atomic absorption was observed at

27 different lines, these are listed, together with the energy

level transitions to which they correspond, in Table XII: Also

listed are the limits of detection obtained in this study using

the carbon filament and the d.c. detection system and the sensitivities

for 1% absorption. The sensitivity is. largely dependent on the

transition probability and the population of the level from which

the transition arises, but the detection limits are also dependent

on a number of other factors including the intensity of the hollow—

cathode lamp at that line (which determines to some extent the

amplifier gain needed) and therefore the relative source intensities

are 'also listed. For comparison purposes the sensitivities obtained 17?19'215,217 by,four other workers using air—acetylene and oxy— hydrogen flame atom cells are also shown. In a few instances

at lines reported by these workers the particular line could not

be isolated from the hollow—cathode source, and in two instances where a reported line was isolated no absorption was observed.

These two lines are included in Table XII for comparison purposes.

It can be seen from Table XII that no absorption was observed from lines resulting.from transitions from the 0.11, and 0.858 eV (888 and 6928 cm-1) energy levels. Further, the sensitivities obtained from lines resulting from transitions from the 0.052 and 0.082 eV (416 and 704 cm-1) energy levels using the 207.

TABLE XII

Iron Atomic Absorption: Sensitivities and Limits of Detections)

at various wavelengths

Reported Sensitivity Carbon filament in Flame Cells

Wave Transition Relative Sensit- Detection Airre2-H2 02-H2 Length (eV) Source ivity Limit (nm) Ref:253'254 Intensity (217) (17) (19) (215)

248.33 0 -4.99 100 0.001 0.002 0.01 0.1 0.10 0.8 248.82 0.052-5.04 6o 0.0014 0.004 0.02 0.2 1.2 252.29 0 -4.92 70 0.0012 0.0026 0.2 0.21 1.25 252.74 0.052-4.95 44 0.01 0.02 0.6 271.90 0 -4.55 124 0.002 0.0026 0.04 0.4 0.34 3.5 275.01 0.052-4.55 50 0.12 0.26 275.63 0.052-4.548 5o 0.15 0.32 279.50 0 -4.43 7o 0.2 0.8 293.69 0 -4.22 44 0.03 0.08 0.47 2.35 294.79 0.052-4.25 46 0.03 0.08 0.64 296.69 0 -4.17 140 0.02 0.016 0.07 1.2 0.82 9.0 297.32 0.052-4.22 90 0.2 0.26 0.28 298.36 0 -4.15 72 0.13 0.2 0.23 299.44 0.052-4.195 72 0.03 0.04 0.24 300.10 0.082-4.21 76 0.24 0.26 0.48 302.06 0 -4.10 392 0.003 0.003 0.03 0.5 0.37 2.8 Contd 208.

TABLE XII (cont.)

Carbon Filament Reported Sensitivity -An Flame Cells

Wave Transition Relative Sensit- Detection Airte2-H2 02-H2 Length (eV) Source ivity Limit (nm) Ref:'253,254 Intensity (217) (17) (19) (215) 304.76 0.082-4.15 96 0.02 0.008 0.29 344.06 0 -3.60 940 0.03 0.02- 2.8 1.65 8.0 349.06 0.052-3.60 256 0..23 0.3 367.99 0 -3.36 188 0.2 0.8 8.9 370.56 0.052-3.39 304 0.33 0.3 371.99 0 -3.34 26,400 0.006 0.0016 0.05 1.0 0.67 3.3 373.71 0.052-3.36 22,000 0.033 0.015 6.1 374.56 0.082-3.36 1,480 0.066 0.02 382.44 0 -3.24 860 0.66 0.2 _8.7 385.99 0 3.21 1,680 0.009 0.004 0.13 2.0 1.12 6.0 388.63 0.052-3.24 76o 0.25 0.08

273.73 0.11 -4.64 42 - - 1.61 358.12 0.858-4.32 52o - - 0.55

a) Sensitivities (for 1% absorption) and limits of detection (signal: -1 noise 2:1) quoted in rg ml . The values quoted for the carbon filament are those obtained in this study using 5 i1 samples. 209.

carbon filament atom cell are generally relatively lower than those

reported using flame cells. This is consistent with the lower

temperature of the atoms in the carbon filament atom cell. If the

temperature of the air—acetylene and oxy—hydrogen flame cells are o o 211 assumed to be 2250 C and 2810 C respectively and the temperature

of the atoms in the carbon filament atom cell is assumed to be 900°C,

and if thermodynamic equilibrium prevails, then, from the Boltzmann

Distribution Law (equation 1.3.), there will be a marked difference in the populations of various energy states. At these temperatures the iron atomic populations in the air—acetylene flame at 0.052,

0.082, 0.11 and 0.858 eV (4162 704,888, and 6928 cm-1) will be ca. 1.3, 1.6, 1.75 and 82 respectively times greater than the populations of these levels in the carbon filament atom cell. The respective ratios for the oxy—hydrogen flame and carbon filament are ca. 1.5, 1.7, 1.9 and 165. A comparison of the sensitivities obtained for iron atomic absorption at different lines using the same instrumentation but with different flame cells and the carbon filament atom reservoir could, with careful control of experimental parameters, yield useful information as to the energy level distribution of the iron atoms above the filament, and hence their environment. This possibility will be discussed further in Chapter 7.

From this study it was confirmed that 248.3 nm was the most sensitive iron atomic absorption line and was preferred 210. for most analytical measurements. However, the emission from the hollow—cathode source at 372.0 nm was ca. 264 times more intense, which enabled higher—scale expansion factors to be used before equivalent noise levels were obtained. This resulted in a lower detection limit, 0.0016 pg m1-1 of iron (i.e. 8 x 10-12g using a 5 p sample), being obtained at this wavelength. Different wave—lengths having differing sensitivities for iron atomic absorption offer varied analytical working ranges and in certain situations measurements may be made at a line less sensitive than 248.3 nm to avoid the necessity for sample dilution.

6.2.7. Interference Studies

An extensive examination of the effect of foreign ions on the absorbance recorded for iron was made. Five pl aliquots -1 of 0.4 fig ml solution of iron containing. 1,000, 100 or 10—fold weight excess of 22 different foreign ions were employed. The previously determined optimum conditions and the d.c. detection system were employed with limited field viewing 0 to 0.7 mm above the rod. The effects of these foreign ions are listed as percentage suppressions or enhancements of the percentage absorption signal in

Table XIII. Only interferences which resulted in a change of signal of more than 5% were taken to be significant.

Two interesting interference phenomena were noted. 211. TABLE XIII

Interferences in the Determination of Iron by Atomic Absorption Using the Carbon Filament

% change in Signal from 2x10 9gFe by 1,000, 100 and 10—fold weight excess.

Ion Salt 1,000 100 10 Al AI2(SO4)3 —24 —14 N.S. Ca CaCO3/HNO3 N.S. N.S. N.S. a)Co CoC12 —85 —45 —20 Cr Ore13 —80 —45 —20 Cu Cu(NO3)2 —50 —40 N.S. K KC1 —20 N.S. N.S. Mg Mg012 —70 N.S. N.S. Mn MnC12 —70 N.S. N.S. Na NaC1 —50 —15 N.S. a)Ni NiC12 —50 —45 N.S. Pb Ph(NO3)2 N.S. N.S. N.S. Si Si/HFLHNO3 N.S. N.S. N.S. Th Th(NO3)4 N.S. N.S. N.S. Ti Ti/HF N.S. N.S. N.S. V V/HNO3 N.S. N.S. N.S. W W/HF/HNO3 N.S. N.S. N.S. Zn Zn SO N.S. N.S. N.S. b)NN4 + 2 4 NH4OH/xsHcl N.S. N.S. N.S. Cl HC1 N.S. N.S. N.S. No3 HNO3 N.S. N.S. N.S. PO —• +10 4 H3PO4 N.S. N.S. SO 2— H SO N.S. N.S. N.S. 4 2 4 a) see Table XV; b) see Table XIV; N.S.=not significant, change in signal < 5%. 212.

Aqueous ammonia solutions were found to enhance iron absorption.

This was shown not to be the result of molecular absorption or -1 iron impurity in the solutions employed, as 400 j'g ml of the ammonia solutions produced no signal when added to the filament alone. It is also difficult to explain the interference by the vapour—phase model (see Chapter 5) as heating of the filament to rid heat (ca. 1,400°C as measured by an optical pyrometer) did not remove the effects of the ammonia despite certain degradation or evaporation at this temperature. This interference was, however, removed by the addition of excess acid to the test solutions, suggesting that there was an ,atomisation interference, possibly involving a hydroxide intermediate. The effects of various solutions containing ammonia on iron absorption signals are summarised in

Table XIV.

TABLE XIV

Effects of Various Ammonia Solutions on Iron Atomic Absorption

% change of signal from 1,000, 100+ and 10—fold weight excesses of NHA

Solute 1,000 100 10

NH 0H +50 +50 +40 4 (NH )HPO +70 +18 +13 4 2 4 (NH )SO \ 0 0 0 4 2 4 NH OH + excess HC1 0 ' 0 0 4 (NH )PO + excess HC1 0 0 0 4 2 4 213.

Nickel ions added as the nitrate produced at 100—fold excess a suppression of signal of —50% but at 1,000 fold excess an enhancement of +70% was observed. - 'It was shown that the pure nickel nitrate 1 solution gave no signal when 400 'g ml solutions were applied to the filament. Further nickel chloride at 1,000—fold excess gave a suppression of signal of —50%. This anomalous pattern was followed to some extent by cobalt where the suppression of signal at

1,000—fold excess was less than at 100—fold when the nitrate was added, but greater when the chloride was used. The interference effects of cobalt and nickel solutions are shown in Table XV. These obser-

-vations indicate once again the difficulty of applying simple models to explain interferences in the carbon filament technique.

TABLE XV

tffect of Nickel and Cobalt Solutions on Iron Atomic Absorption

% change of signal from 1,000, 100 and 10—fold weight excesses of Ni2+ and Co2+

Solute 1,000 100 10

Ni(NO3)2 +70 —50 N.S. NiC12 -50 -45 N.S.

Co(NO3)2 —40 —50 —20 CoC12 —85 -45 —20

214.

, 2+ From Table XIII it should be noted that only two ions k. Co and

Cr3+)seriously interfere at the 10—fold excess level (20 nanograms), the interference from phosphate and ammonia at this level being removed by the addition of excess acid. At the 100—fold excess level (200 ng) several cations interfered, largely those of equivalent volatility to that of iron. At a 1,000—fold excess

(2 micrograms) most of the cations studied, with the exception of the most involatile elements, produced serious depressions; no interference was observed at this level from any of the anions studied. A few elements, notably magnesium, silicon, titanium and tungsten produced peaks which appeared to be caused by particulate matter at the 2 lig level. These scatter peaks, however, did not interfere with the iron determination as they did not occur at the same time as the iron absorption peak; the fast response d.c. detection system being able to separate the absorption and apparent scatter peaks. As usual all solutions of diverse ions were examined for iron contamination and tendency to produce apparent scatter peaks at the concentration levels employed.

As was observed for manganese less severe interference was observed for 1,000—fold excesses of foreign ions when the iron concentration employed was lower.

6.2.8. Determination of Trace Iron Levels in Carbon—Fibre Composites

The method outlined above for the determination of iron 215.

by atomic absorption using the carbon filament was applied to the

determination of trace iron levels in four different carbon fibre — Friedel Grafts resin composites. The preparation and low— temperature ashing of the samples was performed independently elsewhere (at the Royal Aircraft Establishment, Farnborough). The samples (ca..1 mg were ashed by low—temperature RF ashing196 for

ca. 16 hours at 100 W and 20 hours at 200 W, with 60 ml min-1 oxygen and at 0.8 Torr pressure. The residues were then taken up -in 2 drops

of hydrochloric acid (Aristar grade) and evaporated to dryness before being delivered. Nominal percentages for the iron (III) chloride content of the samples were known.

Procedure

A.series of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8 p.g mliron—1 solutions were prepared from stock iron (III) chloride solution and from these a calibration graph for iron atomic absorption

at 248.3 nm was constructed. The iron absorbance was measured using 51u1 aliquots of solution and viewing 0 to 0.7 mm above the rod with the optimum conditions previously described. The sample boats and blanks were placed in small beakers and deionised water added to take up the sample. To the blanks and the samples with nominal percentage of 0.007 iron (III) chloride (i.e. ca. 2.5x10-8g)

were added 200111 water:a-M7to the other samples (nominal percentages

0.3, 0.7 and 1.8) 1 ml of water was added. The iron content of 216. the solutions thus formed was found by measuring the iron absorbance at 248.3 nm for a 5 ial aliquot of each solution and interpolation onto the calibration graph. These solutions were then diluted by the appropriate amounts of deionised water such that the concentration of all the sample solutions fell in the central portion of the calibration range. The concentration of the sample solutions was now determined accurately from at least six determinations in good agreement of the iron absorbance made simultaneously with the replotting of the calibration graph. The simplest procedure was to determine sequentially the absorbances from the solutions first in order of increasing concentration, then in order of decreasing concentration etc. The iron (III) chloride content of each sample was determined by interpolation and calculation from the calibration graph. It was assumed all the iron found came from the iron (III) chloride.

Finally, the possibility of matrix interference on the iron determinations was investigated by the addition of an equal volume of iron (III) chloride solution of a known concentration td each sample solution. The iron absorbance of these solutions was measured and the iron content calculated from interpolation as before. The percentage recovery of the added iron (III) chloride was calculated.

Results

Table XVI lists the percentages of iron (III) chloride TABLE XVI

Percentages of FeC13 found in Carbon—Fibre Composites

Description of Carbon—Fibre Composite Percentages Found in samples Average % FeC13

ca. 0.007 g Pea 3Nog (Diphenyl Friedel Crafts Resin) 0.032(10.001) <0.0006 0.034(±0x01) 0.033(to.001)

Ca. 0.3 g FeC13/100g .(Terphenyl Friedel Crafts Polymer) <0.003 <0.003 <0.003 not known

ca. 0.7 g FeC1 /100g . 3 (Siloxane—Phosphate Friedel Crafts) 2.20(10.06) 1.57(10.05) 1.72(10.05) 1.9(10.3)

ca. 1.8 g FeC1 /100g 3 (Xylene Freidel Crafts Resin) <0.6 1.50(t0.05) a 1.30(±0.05)a 1.4(±0.1)

Blanks (boats with no sample treated b • as above)

a=from recovery of added FeC13; b;:per supposed 1 mg of sample. 218. found in the samples of carbon—fibre composite, of which three samples of each were supplied Certain samples gave anomalously low results, the reason for this is not clear, possibly sample was lost in transit or preparation, in the case of the Xylene Friedel

Crafts resin the low iron content required that the iron be determined by the recovery of added iron (III) chloride solution to compensate for over—dilution. The recovery of added iron (III) chloride for the three other sample'solutions was 1000, within the limits of experimental error, indicating that there were no serious matrix interferences. Although particular results appear to be somewhat anomalous this preliminary study would appear to indicate that with simple sample preparation the carbon filament technique offers the . possibility of determining traces of iron, and presumably other elements, in milligram samples of plastics.

6.3. Atomic Fluorescence Spectroscopy

The apparatus and optical system described for the atomic fluorescence spectroscopy of manganese (section 5.3.1.) was used. Both the iron hollow—cathode lamp, used in the work described previously in this chapter, and an iron electrodeless discharge lamp, similar to that used in the studies described in Chapter 3, were employed as spectral sources. Reference has been made previously to the continuum radiation from the glowing carbon filament and 219. this background radiation proved to be a serious problem in iron atomic fluorescence studies using this apparatus.

The glowing filament can be to an approximation regarded as a black body. The energy distribution for black-body radiation is given by Planck's Radiation Law:

E (y) dy = 871- hy3 . dy .... 6.1. c3 le)Fp (hy /kT) - where E (y) dy is the energy density betweenVandV+ dV, h is

Planck's constant, c the speed of light, k is Botzmann's constant and T is the absolute temperature. The maximum intensity at temperatures such as those associated with the carbon filament occurs someway' from the ultra-violet region of the spectrum, e.g. the maximum intensity at 2,000°K falls at ca. 1,500 nm and at

11 650°K at ca. 1,700 nm. There is a rapid fall-off of intensity in the ultra-violet region, such that below 300 nm the intensity of radiation from a black-body at 2,000°K is extremely small. In absorption measurements a small spectral band width is used (d1)) ancV;the attenuation of a large source signal is monitored, therefore the small signal from the continuum does not distort absorption peaks in the ultra-violet region for either iron or manganese atomic absorption. The effect:of the continuum radiation can be seen as an increase in the 100% transmission signal but this occurs sometime after the analytical signal has been recorded (when the 220. filament has reached a higher temperature and before the -current is switched off) and does not reduce the peak absorbance. In fluorescence measurements a small signal is being monitored and this is amplified both electronically and by increasing the mono— chromator slit—width, thus dv becomes larger and the continuum poses a greater problem. The fraction of the continuum detected can be reduced by shielding the monochromator as much as possible from the glow and using a small monochromator entrance slit (the radiation reaching the detector from a spectral line, e.g. atomic fluorescence, is directly proportional to the spectral band—pass of the monochromator, whereas background radiation from the MIR continuum is proportional to the square of the spectral band—pass).

The intensity of the continuum is also less at a given wavelength the lower the filament temperature, particularly in the ultra— violet region, this parameter cannot, however, be varied at will because the temperature at which a given element atomises from the filament is determined by its volatility. The fast response detection system does, however, enable this volatilisation event to be distinguished from the continuum at higher filament temperatures which with slower response systems was not possible.

In practice it proved impossible to separate any iron atomic fluorescence signals from the filament continuum. Measure— ments of the time taken to atomise the iron for atomic absorption measurements at different filament voltages and measurement of the 221. continuum signal at these voltages and at 248.3 nm, using mono- chromator slit-widths and electronic gains favourable to atomic fluorescence measurements, showed that the continuum would effectively mask any atomic fluorescence. Attempts to efficiently shield the monochromator from the filament glow were unsuccessful.

In a further experiment a clear glass filter which transmits a negligible intensity of radiation below 350 nm was placed immediately in front of the monochromator entrance slit; with the wavelength control set at 248.3 nm a signal, somewhat reduced, from the filament glow was still observed. This would indicate a light leak within the monochromator and reception by the photo- multiplier of light from the visible region of the spectrum, even when the wavelength is set in the ultra-violet. A similar light leak has been found independently in another SP;900A prism mono- 267 chromator . If a monochromator without such a leak were to be used, perhaps in conjunction with a solar blind-photomultiplier, such photomultipliers do not respond to radiation of wavelength longer than ca. 310 nm, advantage could be taken of the extremely low filament glow at 248.3 nm and possibly iron atomic fluorescence observed. Synchronous modulation is often employed in flame cells to reduce the effect of background in atomic fluorescence measurements

(see Chapters 3 and 4). Attempts to observe iron atomic fluorescence in this case using 100 c.p.s. modulation were unsuccessful, because of the 50 c.p.s. (mains frequency) ripple on the filament glow. 222.

Therefore, modulation should be at a frequency of which 50 c.p.s.

is not a harmonic, or the filament heated by d.c. current. The

modulation frequency would also need to be extremely high to prevent

distortion of the transient signals as at 12 V the iron atomic absorption signal is of ca. 0.03 sec duration. 223.

CHAPTER 7

Conclusions and Su,Tgestions for Future Work

7.1. Conclusions

This study indicates that analytical atomic spectro— scopy can be used to determine iron and manganese with high sensitivity and good selectivity. Usingthe air—acetylene flame cell, and an intense and stable microwave discharge lamp as source, the sensitivity for manganese and iron is improved by ca. one order of magnitude by atomic fluorescence spectroscopy compared to either atomic absorption or atomic emission spectroscopy. In both oases the atomic fluorescence technique offered good selectivity, similar to that obtained for the atomic absorption method. Of the few observed interferences in iron atomic fluorescence most could be attributed to reduction in the nebuliser efficiency by highly concentrated solutions. In only the case of the most refractory elements was appreciable interference by scatter observed. In the determination of manganese by atomic fluorescence spectroscopy only one serious chemical interference, that of magnesium, was observed, and physical scattering was again only observed in the presence of high concentrations of elements which 224. form refractory oxides in the air—acetylene flame. It would also appear that the atomic fluorescence technique offers extended linear calibration ranges compared to those obtained by atomic absorption.

It has also been shown that using the modified carbon filament atom reservoir it is possible to determine solutions of comparable concentrations to those used with the most sensitive flame cells. The carbon filament is, however, suitable for the detection of much lower absolute amounts of iron and manganese and the advantage of the technique in terms of absolute amounts of sample is several orders of magnitude. Thus the technique is particularly suited to the analysis of trace metal levels in very small samples, and a method for the analysis of trace iron levels in 1 mg samples of carbon—fibre composite has been outlined. There would appear to be no reason why this method could not be applied to the deter— mination of other trace metals, particularly manganese, in other plastics. At this stage of development of the carbon filament technique the two major( limitations of the technique appear to be interference from concomitant ions, and the high noise level of signals using limited—field viewing and fast response detection

Systems. In this study interferences have been minimised in a number of ways, particularly byLlimited field—viewing and for many practical analyses the interference levels observed should be tolerable. In analyses of more complex matrices it may be 225. necessary to employ methods such as that of standard additions to obtain the necessary selectivity. Interference phenomena will be further discussed later in this chapter. Workers using flame cells generally obtain limits of detection for atomic absorption determinations signficantly lower than the sensitivity for 1% absorption, because of high noise levels it was not possible to do this ..using the carbon filament. Improvements in source intensity and stability, optical arrangements and detection systems should, however, make this possible. These would enable the improved sensitivity of the carbon filament cell over flame cells for both atomic absorption and atomic fluorescence measurements, which is ca. 50 compared to the most sensitive reported flame cells and ca. 1,000 compared to flame cells using this apparatus

(see Tables VIII and XI), to be utilised in terms of sample con— centration. Improved sensitivities of the same order as those predicted in section 1.3.3. could then be exploited.

7.1.1. Atomic Fluorescence Spectroscopy

As indicated above, when using a flame cell and the apparatus described, atomic fluorescence spectroscopy was found to be the most sensitive of the three techniques of analytical atomic spectroscopy for the determination of iron and manganese. For both these elements the development of an 226.

intense line source was described and for both elements the

most favourable line for analytical measurements lies below 300 nm.

Therefore, the advantage of atomic fluorescence is in agreement with

•that predicted in such circumstances in section 1.2.4. The selec—

tivity of atomic fluorescence spectrometry also appears comparable

to that of atomic absorption when a line source is used and

superior to that usually obtained in atomic emission spectrometry.

The SP 9n0A spectrophotometer was originally designed for flame

emission spectroscopy and consequently the attainable sensitivities

for absorption measurements are poorer than those reported by other

workers using instrumentation specifically designed for atomic

absorption work, but advantage is gained in emission measurements

(both atomic fluorescence and atomic emission) over instrumentation

designed for absorption measurements. The growth curves for

atomic fluorescence, plots of log. atomic fluorescence signal

intensity Vs. log. concentration, have been prddicted theoretically 35 36 27268 by several workers ' ' Those obtained in this study

(Figures 15, 26 and 28) are similar to the theoretically predicted

curves when allowance is made for incomplete collection of

fluorescence radiation and illumination of the cell, self—absorption,

and the possibility of the scatter of source radiation by very

high concentrations of analyte. The data produce• linear

analytical working ranges over several orders of magnitude. These 227. wide linear working ranges are considerably greater than those obtained with the same apparatus for atomic absorptioni and give a practical advantage to the fluorescence technique.

7.2. Suggestions for Future Work

The suggestions for future work fall into two categories, those which seek to investigate further phenomena or problems which have arisen in this study and possible future projects which are suggested by this work. The majority of suggestions will naturally - fall into the former category but two suggestiond for more fundamental studies will also be made. From this study it would also appear feasible to.ii.extend further the range of elements which may be studied by flame atomic fluorescence and, by use of the carbon filament atom reservoir, and mentinn will not be made of this most obvious future line of enquiry.

7.2.1. Flame Atomic Fluorescence

Considerable attention has been paid to the development of suitable stable and intense line sources for atomic fluorescence both in flame and non—flame cells. Microwave—excited electrodeless discharge lamps would appear to be the most promising type of source for atomic fluorescence, the preparation of two such lamps is described in this thesis, but it has been reported 228. that it is extremely difficult to prepare suitable electrodeless 269 discharge lamps for certain elements . The use of an electrode— less discharge lamp containing one element to excite atomic fluorescence

of another element in the atom cell has been reported in cases where there is a strong overlap of suitable lines. The iodine non— resonance line at 206.163 nm has been used33 to excite the 206.170 nm

bismuth line, and using an iodine source bismuth has been determined

at 206.170 nm by atomic absorption and atomic fluorescence, and more

sensitively by direct—line fluorescence at 269.7 nm and 302.5 nm, -1 (in this latter case a limit of detection of 0.05,1g ml was obtained). 209 The arsenic non—resonance line at 228.812 nm has been used to

excite cadmium atomic fluorescence at the cadmium 228.802 nm line,

and using a carbon filament cell luminescence at the cadmium 326.1 nm 150 line was also observed as a result of atomic phosphorescence

In the case of iron and manganese it would seem possible to use the

strong spectral overlaps reported in atomic absorption studies38'258

of platinum and gallium respectively, i.e. excitation of iron atomic fluorescence at 271.9025 nm by the platinum line at 271.9038 nm and

of manganese atomic flUorescence at 403.3073 nm by the gallium line

at 403.2982 nm. The manganese—gallium overlap would appear to be

particularly promising but at 403 nm no enhancement of manganese

Stokesf direct—line atomic fluorescence can be expected. In

resonance fluorescence the possibility of errors arising from 229.

scatter of incident radiation is present, using direct—line

fluorescence stimulated by.a source which does not contain the

line of analytical interest, i.e. because the source contains a

different element, such errors can be eliminated. The use of

such spectral overlap also enables elements for which suitable

sources do not exist to be determined by atomic fluorescence.

The small number of such overlaps has prevented extensive use

of such techniques. However, the excitation of manganese ionic

fluorescence by intense ion lines in manganese electrodeless

discharge lamps, reported in Chapter 4, indicates that electrodeless

discharge lamps may contain-more lines capable of exciting

fluorescence than previously thought. A careful study may reveal

more suitable overlaps, particularly at shorter wavelengths, from

ion lines. Such overlaps will be more useful in flame cells where

atomic absorption profiles are more broadened by Doppler and

collisional broadening than in cooler non—flame cells.

It would seem that manganese ionic fluorescence in

itself is analytically interesting rather than useful. It has

been pointed out earlier (section 4.3.3.) that the gf values (i.e. products of the statistical weights and oscillator strengths)

and the source intensities of the 257.6, 259.4, and 260.6 manganese

ion lines are high, and it is this, rather than any large degree of ion—

isation which enables resonance ionic fluorescence of appreciable

intensity to be observed. Assuming that thermal equilibrium 230. prevails in the flame cell, the calculated degree of ionisation 1 produced when a 10 pg ml manganese solution is nebulised into a nitrogen-shielded air-acetylene flame at 2450°K is 0.6 x 103%.

In the hotter nitrous oxide-acetylene flame it has been estimated that manganese may be ionised to the extent of 5% or more42, in which case the limit of detection for manganese by ionic fluorescence at 257.6 mm (1.5 pg ml-1 in air-acetylene) might be expected to approach the limit of detection for manganese by the.most sensitive -1 atomic fluorescence line at 279.5 nm (0.001 rag ml in air acetylene). However, this assumes equal fluorescence yield and flame concentration of manganese ions and atoms, these are Dnt usually obtained in the nitrous oxide-acetylene flame compared to the air-acetylene flame, further the free flame electron concentration is much higher in the hotter flame and this might suppress the manganese ionisation.

The determination of manganese by ionic fluorescence would also be very susceptible to matrix interferences, any matrix element with a lower or similar ionisation potential to that of manganese (7.434 eV) might be expected to suppress the ionic fluorescence signal by reducing the degree of ionisation. Such depressions would render the 257.6 nm line useless for most practical analyses and rather it would seem best in hot flames, to add an excess of potassium or caesium to suppress the ionisation such that the manganese atomic population is not depleted. 231.

7.2.2. Thermal Environment of Atoms in Carbon Filament Cell

Some mention has already been made as to the environ— ment of the atoms in the carbon filament cell, it has been pointed out that the atoms leave the hot, reducing carbon filament and enter immediately into a cooler inert gas stream. It would be useful, particularly in the study of interferences, to obtain more information about the environment and temperature of the atoms, particularl at the time of analytical measurement.

It is' comparatively simple to measure the equilibrium temperature of the filament at given applied voltages using an optical pyrometer. Figure 33 shows such measurements using an optical pyrometer (Leeds and Northrup Limited, Birmingham). The determination of filament temperature by observing the melting of different metals placed on the filament also yields useful information. It was found in this study that tungsten (m.p. 3370°C) would melt at the centre of the filament when 12 volts was applied. Such methods, however, only determine the equilibrium, or maximum temperature of the filament, which may not be reached until 5 seconds or more after the voltage is first applied In most cases the element of interest will bey atomised some time before this temperature is reached. 270 Ellingham has represented diagramatically the standard free energies of formation of the oxides and sulphides of metals and of carbon. From these diagrams it is possible to read 232. Figure 33 Variation of Filament Equilibrium Temperature with Applied Voltage

3,500

3,000

2,500

Tempera Lure (oc)

2,000

1,500

1,000

500

Applied Voltage (volts)

6 8 10 12 233.

'off the tendency of oxides to be reduced in the presence of o carbon. At temperatures above ca. 1350 manganese oxides

would tend to be reduced to the element and above ca. 75000

iron oxides would tend similarly to be reduced. Thus we might

expect both iron and manganese to be reduced to a large extent

on the carbon filament. If the behaviour of the analyte atomic

vapour above the filament follows approximately the behaviour of

an ideal gas, a value can be obtained for the partial pressure

of iron or manganese above the filament under typical conditions

by substitution into the ideal gas equation:

PV = nRT 7.1.

where P is the pressure, V the volume and T the temperature of the

gas, R the universal gas constant and n the number of moles of

gas. The following are typical values: a temperature of 1,700°C -1 with a flow—rate of 3 1 min of shielding gas modified by a geometry

factor of 0.02 (section 1.3.3.) into:mhich the atoms escape, with

atomisation over 0.1 second and a maximum of 10-7g sample.

Evaluation of this expression gives a partial pressure of ca.

0.003 atmospheres (2.2 Torr) iron or manganese, a value which might

be expected to be reached above the melting—point of these elements.

Thus the following atmisation model would appear feasible: the 234.

-reduction of oxides or salts to the element and vaporisation - of

the element, at temperatures above the melting-point of the

element when an appreciable vapour pressure of the element is

reached. It would appear most elements would be vapourised

before their respective boiling points are reached and that

given the rapid cooling of the atoms as they leave the filament

considerable condensation or super-saturation might be expected.

This model further questions the usefulness of equilibrium

filament temperature measurements. It would be preferable to

measure the temperature of the filament at the time the sample'is

vapourised from the filament. One method by which this could be

done would be to monitor, with the fast response detection system

described in this thesis, the intensity of the filament radiation,

at.the time which the sample is atomised, at two separate wavelengths.

The energy distribution for black-body radiation, given by Planck's

Radiation Law (equation 6.1.) is given, to a close approximation,

by Wien's Law:

I?, = Aci —5 exp. (-02/ 7.2.

where EA is the emissivity at wavelength ?1 and c1 and c2 are the

first and second radiation constants. IA is the intensity in 1 ergs sec between A and A +01 per unit solid angle normal to the

surface of area A. This expression is =suitable as an approximation 235. for emission from the carbon filament, except at long wavelengths or very high temperatures. A derivation of this expression is given by Gaydom.and Wolfhard271. For a black body the emissivity,

is equal to 1 for all wavelengths, if E A is less than 1 but constant for all wavelengths we have a grey body. Assuming the carbon filament to be a grey body the ratio of measured intensities of emitted radiation I and 1 at two wavelengths )i and 1 2 1 2' using the same spectral band—widths at each wavelength can be expressed as:

21 = )%2 5 exp c2( )1— .)2_.) .. 7.3. 12 >,1 ( >1 '2r2

Thus taking c2 to be 1.438 x 107 nm deg, T, the absolute temperature of thefilament, can be calculated If the emission signal is displayed on a storage oscilloscope using a fast response detection system the emission at any given timeafter first applying the filament voltage can be measured. Thus the filament temperature at the time of peak analyte atomic concentration in the atom cell can be determined. Simple apparatus could be used provided the response characteristics of the detector at different wavelengths is know, and that it is possible to exclude the intense radiation of the filament at all other wavelengths other than that being measured. It was not possible to use this method for the measurement of filament temperature with the apparatus used in 236.

-this study because of the light leaks in the monochromator

referred to previously (section 6.3.).

A number of techniques have been developed for the 271 measurement of temperatures in flames, GaTdon and Wolfhard have

presented a useful concise review of several methods. Such methods

fall into four types of measurement by: solid thermometers, gas

properties, spectroscopy and chemical or ionisation equilibria.

The application of all these methods to the measurement of the

temperature of the atoms and their environment above the carbon

filament present considerable problems. The most popular solid ther—

mometers are thermocouples, these however have a definite response

time because of thermal inertia and the gloWing filament micht be

expected to introduce an error arising from radiation heating.

The accurate measurement of gas density, refractive index, the

speed of sound in the gas or other gas properties can be used to

determine gas temperatures, but problems arise from boundary conditions

and sophisticated equipment is necessary. The use of the two most •

popular spectroscopic means of measurement, the spectrum—line

reversal method and the comparison of the emission of two lines

of the same element cannot be used in the case of the carbon

filament, because no atomic emission can be observed from this

atom cell. Attempts in this study to observe sodium emission

using the carbon filament atom reservoir were unsuccessful, no 237.

-6 signal was observed at 589 nm when up to 3 x 10 g sodium, as

sodium chloride, was placed on the filament. Similarly.it has

been demonstrated in this study that the carbon filament atom reservoir possesses insufficient energy to appreciably ionise elements and enable the Saha equation to be used for temperature

measurements?

The use of a thermocouple would seem to be the

simplest of the above methods and in this study an estimation of the temperature of the environment of manganese atoms in the atom $ cell was made using a thermocouple A platinum—platinum 13% rhodium thermocouple (Johnson, Matthey and Co., London) was used,

(diameter 0.25 mm, length 457 mm), the diameter was as-small as practicable to minimise errors. It was still necessary to correct. for errors caused by radiation heating (solids are heated by radiation heating to a much greater extent than gases) and to correct for the thermal inertia of the thermocouple. These corrections were made empirically in the following. way. At applied voltages of

12 volts, 10.7 volts and 6.22 volts it was observed that manganese absorption peaks occurred 0.5, 0.6 and 2.0 seconds respectively

after the voltage was first applied. The e.m.f. generated in the thermocouple above the filament was also read directly on the oscilloscope and if the filament was switched off immediately the manganese absorption peak was reached, within 3 seconds the e.m.f. 238. had ceased to rise, and began to fall. This peak e.m.f. was measured and taken as corresponding to the temperature of the atoms at peak absorbance with a cooling correction applied.

For the cooling correction it was assumed that the rate of cooling of the thermocouple after the peak e.m.f. was recorded was equivalent to that before the peak, the rate of fall of the e.m.f. was measured and extrapolated back to the instant of switching off the filament. Temperature measurements were made at three heights above the filament, and a radiation correction applied by measuring the apparent temperature recorded by the thermocouple at the same distances away from the filament but with the filament turned through 90°, i.e. the e.m.f. generated when measurements were made with the filament and thermocouple in the same horizontal plane (radiation heating and negligible heating of shielding gas) was subtracted from that generated when both were in the same vertical plane (radiation heating plus heating of shielding gas). Practical results seemed to justify both these empirical corrections. The temperature profiles of the manganese environment above the carbon filament are shown in Figure 34. This study provides some justification for assumptions as to the relatively cool environment of atoms in this atom—celll .and indicates that more accurate thermocouple measurements might yield further useful information. 239.

Figure 34' Variation of Temperature of Manganese Environment with

Height Above Filament

1,250

1,00

12V (Applied Filament Voltage)

750 10.7 V

6.22 V

500

Tempe ature (oc) 250

1 2 3 Height above Filament (mm). 240.

Two spectral phenomena which might, in conjunction with a good monochromator, yield a useful comparison of the temp— erature of the atoms in the carbon filament atom reservoir to the temperature of atoms in various flame cells have already been briefly mentioned. In the study of iron atomic absorption using the filament absorption was noted (section 6.2.6.) from a number of lines corresponding to energy transitions from lines lying close to the ground state. The population of non—groUnd state lines is determined by the Boltzmann Distribution Law (equation 1:3.) and thus related to the temperature. Appreciable absorption has been reported in flame cells for certain elements arising from transitions corresponding to lower energy levels up to ca. 0.5 eV above the ground state level, absorption at such lines 'in the carbon filament cell miptit be expected to be much less because of the much cooler nature of this cell. A comparison of the sensitivities of various non—ground state absorption lines in various flame cells and the carbon filament atom reservoir might, with careful control of experimental parameters, yield useful. temperature information. Suitable elements for such a study would appear to be Co, Fe, In, Pd, Se, Sm, Sn, Tb, Ti, V and Zr.

The broadening of atomic absorption profiles is a function of temperature and sophisticated apparatus has been developed to measure such broadening. A simpler, less quantitative, 241. measurement of broadening might be made by an examination of the extent of spectral overlaps, the absorption by one atomic species of radiation characteristic of another, in the carbon filament atom reservoir compared to those in various flame cells. The extent of such overlaps depends on the broadening of the absorption profile

(see section 1.2.2.) which is temperature dependent. A study of, spectral overlaps caused particularly by collisional broadening might be expected to show decreased interference from spectral overlap in the cooler carbon filament atom reservoir. The mutual interferences of the iron 271.9025 nm line and the platinum 271.9038 nm line38 and also the manganese 403.3073 nm line and the gallium 258 403.2982 nm line have been previously mentioned, of these the first pair would appear to give the stronger overlap. Other spectral line interferences which might be studied to advantage 38 have been reported elsewhere, particularly by Passel et a1 .

7.2.3. Interferences in the Carbon Filament Cell

There are four major types of interference in atomic absorption and atomic fluorescence spectrometry by which concomitant elements, or ions, may alter the analytical signal

Observed as compared to that obtained from standard solutions of the pure element. These are usually referred to as: i) spectral, measurement of radiation or absorption characteristic of the interferent as well as that of the analyte; ii) physical or 242.

chemical, reduction or enhancement of the concentration of analyte

atoms in the atom cell; iii) atomisation, a change in the degree

or rate of atomisation; iv) scattering of radiation or band

absorption. The extent of such interferences will be different

in the carbon filament atom cell compared to typical flame cells.

Spectral interferences should, as explained above, because of the

cooler nature of the carbon filament cell, be less in the carbon

filament atom cell than in most flame cells. Physical and chemical

interferences are largely a functinn of the temperature and reducing

environment of the atom cell, in the hot reducing nitrous oxide—

acetylene flame such interferences are minimised. It has been

shown that, because of the cool, inert atmosphere which surrounds

the atoms above the carbon filament, physical and chemical inter—

.eerences in the carbon filament cell are more severe than generally

encountered in flame cells. This typo of interference in the

carbon filament atom reservoir is usually referred to as 'vapour—

phase interaction'. Atomisation interferences in flame cells

can arise because of nebulisation effects or changes in the thermal

or chemical environment in the flame, in the carbon filament cell

such interferences are of a different nature and concern the

vaporisation of the analyte from the filament. In situations

where -scattering of radiation or band absorption are troublesome,

and-again it might be expected that the cooler carbon filament

cell might be more prone to such interferences, suitable optical 243. systems can be used to minimise. such interferences. A single—beam system, in which beams of light from a line and a continuum (a deuterium lamp) source are recorded simultaneously, to allow automatically for interference from molecular absorption and 112'125 scattering, has been described by L'vov for use with his cuvette. Such a system could be adapted for use with the carbon filament atom reservoir. Thus it would appear that vapour—phase and atomisation interferences in the carbon filament cell most merit further investigation':

The realisation that many interferences in determinations using the carbon filaemnt arise from vapour—phase interactions, as shown by Aggett and West151 using two filaments (described in section 5.2.8.), has led to the modifications of limited field- - 151 156 viewing radial—hole sample cavities and hydrogen—argon shieldingl56 discussed previously in this thesis. Mention has been made (section 5.2.8.) of investigation using the first two modifications, the practicability of augmenting the argon and nitrogen shielding gas flow—rate with hydrogen was also investigated. It was found that as the filament heated the hydrogen ignited and shielded the filament in a diffusion flame the flame was extinguished by switching off the hydrogen supply. Thus in operation it is simple, and preliminary investigations7lindicated that interferences could be reduced and some elements, particularly those which readily form oxides, such as sodium could be more easily atomised. However, 244. the technique is of limited potential as the reintroduction of a flame reintroduces many of the disadvantages of a flame cell dis— cussed earlier (section 1.3.2 ) and was not further investigated.

Limited field viewing does not eliminate all vapour phase inter— ferences and further methods of reducing such interferences would be a considerable improvement to the carbon filament atom reservoir.

Some evidence has been obtained (see especially section 5.3.6.) that vapour phase interferences can be reduced by use of a fast response detection system. Most severe interference would appear to come from elements of equivalent volatility, and in some cases it may be possible to record a signal peak from the analyte before appreciable release of interferant from the filament reduces it.

Such observations strongly commend the use of a fast—response detection system and signal peak measurement, rather than signal integration. Interferences might also be reduced by raising the temperature of the atoms, either by keeping them in contact with the heated filament for a longer period, perhaps by reversing the shielding gas flow, or heating the shielding gas. It is difficult to appreciably raise the temperature of large volumes of gas by radiation heating, e.g. heated grids in the gas stream, or multiple filaments, a more successful method might be to use the exhaust—gas of an argon or argon—nitrogen plasma. However, the difficulties of using simple models to explain interferences has been noted in 245. this study and it would seem profitable to base further work on a more fundamental approach. A double—filament approach could be used to distinguish vapour—phase and atomisation type interferences, if the interference was repeated when the analyte is placed on one filament and the interferant on the other a vapour—phase interference is indicated. If molecular species are being formed and reducing the concentration of analyte atoms it may be possible to identify these species. The formation of molecular species when thermodynamic equilibrium prevails is governed by the Law of Mass Action. In this case the reaction of analyte A and interferent X can be represented by the equation:

nA-4. mX 47-± A X n m 7.4. then, if K is the equilibrium constant, which is temperature dependant:

K = LTA] n.P LI.1 ) .1 7.5. P L Anxin

Where P represents the partial pressure (in atmospheres) of the components. The ratio of the partial pressure of free analyte to combined analyte is: 246.

;) 1/n P [A] = Kp 7.6. P [AnXd P [411.P [A X n-1 n m

The partial pressure of A and X will be extremely small and compounds

where m and n = 1 will be favoured in which case equation 7.6.

reduceS to: .

P[A] = Kp 7.7. P pug P [x]

Mass spectrometric methods have been developed for the identification 271 of transient species in flames and it might be possible to adapt

these to identify transient molecular species in the carbon filament

cell. Tables have also been published272 which enable the ident—

ification of molecular species from their spectra and it would

certainly seem possible to attempt to identify molecular species

in the carbon filament atom reservoir by their absorption spectra.

The identification of such species together with eauilibrium

constants for their dissociation, where these have been reported,

should enable a more informed adjustment of experimental parameters

' to reduce chemical..interferences.

A brief treatment of the atomisation process in

flaMes and the carbon filament cell was given in Chapter 2. The

247. peak method of recording analytical signals has been preferre0.- in this study when using the carbon filament, but in Chapters 5 and 6 mention was made of possible disadvantages.of this method.

It is profitable to consider the atomisation process more deeply to understand these- The balance of the number of atoms entering the analysis, volume n1(t) and escaping from it n2(t) at any instant t is given by:

dN = n1(t) — n2(t) 7.8. dt

Where the previous notation is used, i.e. N is the total number of atoms in the analysis volume, No the number of atoms of analyte in the sample, T 1 the overall time of atomisation and 1:2 the residence time of the atoms in the irradiated analysis volume. For the carbon filament with a constantly increasing evaporation temperature:

1 t = NR n1(t) = and n2 2 7-9-

Therefore: dN = N — N 7.10. dt -2 "C2 r1

Integrating 7.10 and substituting for the moment when all the sample

and N reaches its peak value); has'been atomised (t = -C1

248.

= N 1:2 11 exp (— Ti/r2)1 .. 7.11. peals o [ r 1

« as Ltvov22 shows holds for his cuvette, then: If, T1 Pr 2 1

1 — exp 2 ^-=•• T1/12 7.12.

• • • • • • • • • and peak = .17o 7.13.

However, in carbon filament atomisation 't 1 may be as large as 0.1 sec but 2 very much smaller. If the analyte atoms only leave the analysis voluem with the speed of the shielding gas, ca. 25 cm sec—1 (it would be expected that the thermal energy acquired from the filament would give the atoms a velocity above this) thent1/1:2 ca. 25. Thus a simple relationship is possibly only if "t 4/1C2 is constant then:

N ak = kN o 7.14. where k is a constant less than unity. Thus an atomisation inter— ference could be of two types, either a reduction in the degree of atomisation from the filament or an alteration in the ratio'Cl/1:2.

It would seem likely that large amounts of involatile species might increase 1: 1 without a corresponding alteration in 1:2. In 249. which case a broader signal peak with a smaller peak height would be obtained in presence of the interferent. If the signal was inte- grated, rather than peak height measured, such an interference would be eliminated, as the integrated signal would be independent of 1:1 and dependant only on No and 1722, provided the integration period 22 is greater than 1:1+4T2 . Signals in the carbon filament technique could be integrated in a number of ways but attention would need to be given to possible increased interference from vapour-phase effects and distortion of the signal by the radiation from the glowing filament.

In summary, it should be noted that a great deal of work can be proposed for the further investigation of inter- ferences in the carbon filament technique, and that attempts to identify the nature of possible chemical interferences and signal integration appear to be two of the most promising suggestions for study.

7_.2.4. Solid samples

Solutions because of their homogeneity and ease of sampling equal aliquots are in many ways ideal analytical samples.

In some circumstances, however•, the analysis of solid samples may be preferred, e.g. when the need for extensive sample preparation is Obviated, provided errors associated with sample heterogeneity can be avoided. The method described previously for the analysis 250.

of iron in carbon—fibre composites (section 6.2.8.) involves an initial time consuming ashing stage and it was thought that the

carbon filament mifrht be adapted to analyse solid samples. This

would establish a considerable advantage of the filament over flame

cells, where it is difficult to deal with solid samples. Solid

composite samples were placed on a filament adapted for the purpose,

by forming a central hollow to hold the sample rather than a central

platform. The filament was heated and the signals measured in the

normal way. With samples both as flakes of composite and as a fine

powder, strong absorption (ca. 95) was observed at the iron resonance

line 248.3 nm. This absorption, however, appeared to be largely due

to band absorption, presumably from products of the decomposition of the organic matrix, as similarly strong absorption was observed at

non—resonance and filler—gas lines, and also 5 mm above the rod where

the iron atomic population has been shown to be vanishingly small

(see Figure 30). If this absorption does arise from decomposition

products from the composite then it should prove possible to remove

these before atomisation of the iron, either by a preliminary ashing

step or by pretreatment of the composite on the filament with

oiidants. Alternatively, a background correction system such as

described in the previous section could be used. It should also

be noted that the iron content of the samples is relatively high for determinations at the 248.3 nm line, for even the sample with 251.

lowest iron content (0.007%) an 0.03 mg sample would be needed

to give an absorption of ca. 50%, and less than 0.01 mg of the highest iron content sample (1.8%) to give an absorption'af. ca. 50% at the least sensitive iron atomic absorption line

(see Table XII). Therefore the use of solid samples would enable extremely low trace concentrations in milligram amounts of sample to be determined and would appear worthy of investigation although the samples and apparatus used in this study proved unsuitable.

Useful results were, however, obtained in a separate series of experiments concerned to determine the relative zinc contents of lake—water sediments collected on.:2,-glass—fibre filters.

The atomic absorption apparatus previously described and a copper— zinc (brass) hollow—cathode lamp source were used. When small pieces of the filter paper (ca. 1 mm x 2.5 mm) were placed on the rod and heated using a low filament voltage (4.2 volts) strong zinc absorbance signals at 213.8 nm and 307.6 nm were observed, and using the fast rDsponse detection system these were well s-)parated from a later signal apparently caused by band absorption or scattering. That the measured peaks were due entirely to zinc atomic absorption was confirmed when no signal was observed at filler—gas lines near the 213.8 nm and 307.6 nm zinc resonance lines, or the copper resonance lines at 324.7 nm and 327.4 nm 252. although the second band absorption or scatter peak was still observed at these lines. The samples analysed contained ca. 108g zinc and the 213.8 nm line was found to be too sensitive and:the markedly less sensitive 307.6 nm zinc resonance line was preferred.

The particular advantage of the carbon filament atom reservoir for the analysis of such samples was illustrated by the fact that useful information concerning the ratio of zinc—contents of twelve different filters was obtained using the filament, but when an air—acetylene flame cell was used, both in conjunction with a 61 sampling cup for solid samples (as described by Delves and in section 1.3.2.) and after wet ashing and nebulising the resultant solution, it was not possible to obtain useful information. A further advantage of the filament technique was in overcoming the silicon interference observed in the flame cell measurements.

7.2.5. Determination of Atomic Fluorescence Power Efficiencies

and Quenching Cross Sections of Atoms

The intensity of atomic fluorescence is directly proportional to the atomic fluorescence power efficiency (or yield

11 in equation 1.19) and hence is important both in predicting absolute signal levels and in the selection of optimum experimental conditions for the atomic fluorescence method of analysis. A,,7. simple kinetic approach allows expressions to be derived for Ye 253. in terms of radiational and radiationless rate constants, which account for the various means of activating and deactivating the energy level resulting in the atomic fluorescence process. Lack of accurate information as to gas temperatures, collision species and their cross—section, and the activation energies and quenching efficiencies for the collisions of concern in typical anaytical atom cells requires the measurement of power efficiencies experimentally.

However, such experimentally measured values can be used to obtain values of the cross—section for the quenching of excited metal atoms by flame, or shielding, gas molecules. At present, the only reported studies of quantum efficiencies have been made using flame 101273,274 cells. Alkemade and fellow workers measured the resonance yield factor (quantum efficiency) for the sodium D doublet and the 275-277 infrared and blue potassium doublets. Jenkins has reported the measurement of fluorescence yields for sodium, potassium, rubidium, caesium, thallium and lithium. These workers calculated quenching cross—sections from the experimental values obtained. Pearce,

De Galan and Winefordner278 experimentally measured power efficiencies for eleven elements in fuel—rich oxyhydrogen, fuel—rich oxyacetylene, and fuel—rich hydrogen, argon, entrained air turbulent flames in an evaluation of the analytical usefulness of these flames. The complex nature of flame gas mixtures causes considerable difficulty in the measurement of auenching cross—section for different gases. Boers, 273 Alkemade and Smit suppose that air flames burning in air consist 254.

275-277 of nitrogen in their calculations. Jenkins was able to calculate cross—sections for collisions involving eight different gases and each metal, however, this required a large number of experiments with flames of different stoichiometry and an inter— 278 pretation from a complex graphical treatment. Pearce et al found that the values for fluorescence power efficiencies in flames were approximately equal for different flames,because of the entrainment of ambient air into the turbulent flames used. Thus the use of flames as the atom cell in such studies would appear to introduce a number of problems and the carbon filament atom cell might, because of the ease with which the shielding gas environment of the atoms can be controlled, be more suited to such investigations. An additional advantage of the carbon filament technique might also be that less intense spectral sources can be used to excite atomic fluorescence in this low background cell than is practicable for flame cells (see section 5.3.2.). A particular requirement of these studies is a source which produces an intense unreversed resonance radiation, with a line width not very different from the absorption line width of the element in the atom cell.

The theoretical basis for measurements of power efficiencies and quenching cross—section has been treated fully by the previous workers referred to above 273—X78. The atomic 255. fluorescence power efficiency, Y', is defined as the ratio of the. number of watts of radiant power emitted per unit time as atomic fluorescence, to the number of watts of radiant power absorbed per unit time by the sample atoms (the quantum efficiency is the ratio of the number of fluorescence quanta emitted per unit time to the number of absorbed quanta per unit time). The experimental arrangement used in all the reported investigations is essentially similar. The same detector is used for both absorption and emission measurements, the optical bench is pivoted at a point beneath the centre of the atom cell, the source radiation is focussed at the centre of the atom cell and an image of the centre of the atom cell is focussed on the monochromator entrance slit. The absorbed intensity, AP, is measured by the atomic absorption method and then the fluorescence power emerging from the atom cell, Pfl.is measured with the same detection system after the sensitivity has been increased by a known factor. Using flame cells, because of strong thermally excited atomic emission, it is necessary to modulate the beam of exciting radiation and to use synchronous a.c. detection and amplification. This modulation has been performed by all the above workers by means of a rotating sector. It would not be necessary to modulate the source radiation using a carbon filament cell because of the absence of such thermal 273'274 emission. Alkemade and fellow workers and Jenkins used narrow—line sources which met the stringent requirements given 75 276 above, Pirani—type lamps were found to be most suitable. Jenkins2'-' 256. has described the modification of suitable commercially available lamps for Na, K, Rb, Cs and Ti and the preparation of a suitable 277 lithium lamp . These workers used flames which consisted of an inner flame shielded from the atmosphere by a colourless burning 275-277 mantle of the same composition, temperature and velocity. Jenkins used a series of suitably placed apertures to control the solid angle of the radiation viewed, and the detection system viewed the flame through a telecentric optical system and an interference filter.

The photomultiplier was preceded by a further series of apertures, light stops am:Ca ground quartz 'diffuser' plate. For this system, the fluorescence yield, Y', is given:by:

Yi = 47 • Sf 7.15 AS where w is the solid angle of acceptance of the detector (and is introduced here to allow for the radiation of fluorescence in all directions) and S and AS are the signals obtained from the . fluorescence and absorption measurements respectively. The fluorescence power, Pi., emerging from the flame is reduced by reabsorption in the flame and it is necessary to introduce a self— o o = a P where P fis the power absorption factor a defined by Pf f which would emerge in the absence of self—absorption. Thus if

4P is the absorbed power then:

257.

o yt = a P 7.16. A P

Jenkins 275derives a from a theoretical approach because of the complexity of his flame systems, however, with the carbon filament cell it should be possible to measure the fluorescence yield at various low concentrations and extrapolate to zero. From kinetic theory275:

A.. Y1 a. • :1 1 7.17. A.. + Ek ix] 31 where A..ji is the spontaneous emission coefficient (see section 1.2.1.) and Ek Ed is the'rate of the quenching reactions summed for all species of X in the atom cell (a complex summation in a flame cell but essentially the shielding gas in the carbon filament cell), i.e. the excited atoms which do not make spontaneous transitions are assumed to undergo radiationless transitions, being 'quenched' by collisions of the second kind. Thus from measurements of Y1, known, the effective cross—section andprovidedavalue-forAA.ji 275 278 for quenching collisions can be calculated273' Pearce et al preferred to use a continuum source, a 900 W Xenon arc lamp, to avoid problems concerning emission and absorption line profiles.

The difficulties previously mentioned (section 5.2.10.) that arise from the narrow absorption line profiles in the carbon filament atom 258. cell might suggest that a continuum source would be preferable in investigations of fluorescence power efficiencies using the carbon filament. Therefore it is useful to note the fii'al expression 278 derived by Pearce et al :

V = Sf . 3H3 ( 4% Ar 7.18. 4'. S W303/., st A2 v

where the additional parameters are instrumental,H3/ 31 3FW and H refer to the width and height of the monochromator exit (and 3F entrance) slit for the absorption and fluorescence measurements respectively, AF, is the entire fluorescence area and A2' is the area of the xenon arc on the entrance slit of the monochromator.

Some preliminary investigations of the measurement of fluorescence power efficiencies using the apparatus described in this study were made. The atomic fluorescence power efficiencies in nitrogen and argon were compared for mercury and manganese. The first measurements for mercury were made using a simple atom cell, an upright silica tube, flushed with argon or nitrogen and then sealed by taps, the mercury atomic vapour was provided by a small pool of mercury at the base of the tube. Using this cell measure— ments were made of the mercury atomic absorption and fluorescence at 253.7 nm by focussing an image of the source onto the atom cell and an image of the atom cell onto the monochromator entrance slit. 259,

In absorption measurements the source was positioned along the

optical axis of the monochromator and in fluorescence measurements at

right angles to the optical axis but in the same horizontal plane;

Suitable apertare stops were used. As the source a mercury electrode—

less discharge tube, prepared to give the minimum of self—absorption

by sealing a small amount of mercury in a quartz tube 20 cm long

under 3 Torr argon was operated at very low power, 2 watts, using

a 'Microtron 200 Mark I' microwave power generator (Electro—Medical

Supplies Ltd., London). Using this source the ratio of atomic

fluorescence power efficiencies for mercury at 253.7 nm in Ar:N2 was found to be ca. 50.3:1. This is close to the value to be expected 273-277. from the studies of other workers Using a mercury hollow—

cathode lamp (Perkin Elmer Ltd., 'Intensitron' Lamp), operated at,

10 mA, the same ratio Ar:N2 was found to be ca. 11.7:1, suggesting

that the electrodeless discharge lamp was a more suitable source

for these studies. This may be because the lamp current required

to obtain appreciable mercury atomic fluorescence may cause some

broadening of the line profile. Thus the electrodeless discharge

lamp was used in experiments designed to compare mercury atomic

fluorescence power efficiencies using the carbon filament atom reservoir. The same optical system was used and the previous atom

cell was replaced by the carbon filament atom cell. The filament was positioned at 45° to the optical axis. The ratio of atomic fluorescence power efficiencies for mercury at 253.7 nm and in 260.

Ar and N2 was found to be ca. 5.5:1. This value is somewhat lower than expected and may be due to poor optics or the entrainment of air, as measurements were made a few mm above the rod to prevent interference from the electrode heads and with a shielding gas—flow rate of only 2 1 min-1. In a similar experiment using manganese and the 279.5 nm line (see section 5.3.4.) it was found that the ratios of the atomic fluorescence power yields in Ar and N2 at this line were ca. 16:1. In this latter case measurements were made 0 to 0.7 mm above the rod by moving the hollow—cathode lamp source in the vertical, rather than horizontal plane, of the optical axis, and using flow—rates of ca. 3.8 1 min-1.

It would appear that further studies of fluorescence power efficiencies using the carbon filament could, with more sophisticated optics and probably more highly purified gases, yield useful information as to the quenching of atomic fluorescence by different gases and gas mixtures, and prove a useful tool for the determination of cross—sections for the quenching of resonance radiation of metal atoms.

7.2.6. Sensitised fluorescence The mechanism of sensitised fluorescence, also known as energy transfer atomic fluorescence, impact fluorescence, activated fluorescence, and indirectly excited fluorescence, has been given earlier

(section 1.2.3.). Such fluorescence was first predicted by Franck279, 261.

from an extension of the 'Principle of Microscopic Reversibility'

that an excited atom might collide with a normal atom or molecule

to give up a quantum of emrgy by a radiationless transfer. The

prediction was later verified by experiment by Cario and Franck280'281

on mercury and thallium vapours. In their experiments, a heated

quartz tube containing mercury and thallium vapours was irradiated

by a well—coold quartz mercury arc lamp, not only was intense

mercury fluorescence at 253.7 nm observed but also the fluorescence

of a number of thallium lines. Both lines with 'energies which lie above and below 4.9 eV, the excitation energy of mercury, were observed, the kinetic energy obtained through high thermal energies co—operating

to excite higher energy states. Mitchell and Zemansky4 have given

a good summary of the early work in enclosed systems where mercury was

reported to excite fluorescence fronveight other elements. It was found that most foreign gases quenched the sensitised fluorescence.

However, Donat282 and Loria283 found that argon and nitrogen enhanced

the sensitised fluorescence, apparentlylby raising the population of 282 the long-lived metastable 63 P mercury level. Donat found that o even at an atmospheric pressure of argon the intensity of sensitised fluorescence in argon was greater than that in vacuum. Loria283 found that merely a trace of oxygen neutralises this enhancing effect

and quenches the fluorescence.

The collisional deactivation of the donor atoms (mercury 262.

in the reports above) by high concentrations of quenching species

in flame cells, and also because of the relatively low concentrations

of donor atoms that can be achieved in flame cells, prevent the

observation of sensitised fluorescence in flames. In the carbon

filament atom reservoir calculation shows that it is possible to

achieve the concentration levels of donor and acceptor atoms used

in the successful closed—cell experiments of the early Workers4.

Attempts in this study to observe sensitised fluorescence for

thallium and cadmiuni using mercury atoms as donors were ei,ther unsuc—

cessful or inconclusive. The analyte, thallium and cadmium salts

and metal, was placed on the filament in the usual way and the mercury

atoms provided either by placing a concentrated aqueous solution on

the filament or by using an enclosed system. In this latter case

the -dome with silica windows, tap and tight fitting base described

earlier (section 6.2.3.) was used, with a pool of mercury to provide

the necessary donor atoms. Argon shielding was used to prevent

quenching species diffusing into the atom cell and a narrow—line

mercury electrodeless discharge lamp (described in section 7.2.5.) was used to irradiate the cell.

Although the attempts described above were unsuccessful, and this may be because it is impossible in practical analytical systems to exclude all possible quenching species, the practical advantages of sensitised fluorescence could be very great. The 263.

determination of a number of elements for which it is difficult to prepare suitable primary fluorescence sources might be enabled, and also the possibility might arise of using a single mercury source for the determination of several elements.

Suggestions for future work must by their nature be speculaAivel but it is to be hoped that future work will exploit the increased versatility that atomic fluorescence spectroscopy and the carbon filament atom reservoir bring to analytical atomic spectroscopy. 264.

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