ANALYTICAL ASPECTS OF ATOMIC SPECTROSCOPY

AT WAVELENGTHS IN THE FAR

by

ARTHUR FREDERICK WARD B.Sc.,M.Sc.,D.I.C.

•

A Thesis submitted for the Degree of

DOCTOR OF PHILOSOPHY

of the. University of London

Department of Chemistry Imperial College of Science and Technology London S.W.7 July 1973' ABSTRACT

The development of the radiofrequency induction coupled as a spectroscopic source for the excitation of elements whose resonance lines lie in the far ultraviolet is described. The use of sample introduction systems for aqueous solutions based upon indirect nebulization is investigated. The determination of sulphur,phosphorus, iodine, carbon, , arsenic and selenium is described at atomic lines of these elements, in the far ultraviolet using the plasma as an emission source. The technique is applied to the determination of phosphorus in soils using a simple extraction and ion exchange procedure. Some observations about plasmas and flames and a comparison of the two sources as excitation cells for atomic spectrscopy are made. The plasma temperature is determined and the effect of the operating parameters upon the temperature considered. ii ACKNOWLEDGEMENTS

The work described in this thesis is entirely original except where due reference is made. No part of it has been previously submitted for any other degree. The work was carried out in the Chemistry Department of the Imperial College of Science and Technology, between October 1971 and July 1973. I would like to thank my supervisor, Dr.G.F.Kirkbright, and Professor T.S.West for their advice and encouragement. I should like to thank Mr. B. Bach of British.Steel Corporation for the loan of the Radyne . plasma unit used in this work and the Macaulay Institute for Soil Research [Aberdeen] for the provision of soil samples for analysis. I am also indebted to the Science Research Council for the provision of a studentship during this period. lit CONTENTS

Abstract Acknowledgements ii Chapter 1- Introduction 1 Chapter 2 The Induction Coupled Radiofrequency plasma 44 Chapter 3 An investigation of some of the operating parameters of the induction coupled radiofrequency plasma 91 Chapter 4 An investigation of the emission spectroscopic properties of sulphur in an induction coupled radiofrequency plasma 136 Chapter 5 An investigation of the emission spectroscopic properties of phosphorus . in an induction coupled radiofrequency plasma 155 Chapter 6 An investigation of the emission spectroscopic properties of the halogens in an induction coupled radiofrequency plasma 177 Chapter 7 An investigation of the emission spectroscopic properties of carbon in an induction coupled radiofrequency plasma 192 Chapter 8 An investigation of the emission spectroscopic properties of mercury in an induction coupled radiofrequency plasma 204 Chapter 9 An investigation of the emission spectroscopic properties of arsenic and selenium in an induction coupled radiofrequency plasma 214 iv

Chapter 10 Line profiles in the radiofrequency plasma 230 Chapter 11 A comparison of the induction coupled radiofrequency plasma and the nitrogen shielded nitrous oxide-acetylene flame as spectroscopic emission sources 255 Chapter 12 Conclusions and suggestions for further work 299 Appendix Principal symbols and abbreviations used 307 References 311 CHAPTER 1

Introduction. • 1 Although the far ultraviolet region of the electromagnetic spectrum was first explored before the turn of the century, its application has been developed far less rapidly than the other regions. Boyce [1] has suggested the sub-division of the far ultraviolet into smaller regions, fig.l.1. As oxygen is the principal absorbing gas in this region and vacuum techniques are most commonly used to remove it, Boyce suggested renaming the region 0.1-200 nm the Vacuum Ultraviolet. The region 100-200 nm is the range over which radiation can pass through windows and prisms and is often called the Schumann Ultraviolet in honour of the pioneer of far ultraviolet spectroscopy. The wavelength range 0.1-100 nm Boyce suggested calling the Extreme Ultraviolet. It can be seen that this region overlaps with the soft x-ray region of the electromagnetic spectrum and soft x-rays have, in fact, been detected with a vacuum spectrograph at wavelengths above 0.47 nm, while extreme ultraviolet radiation of wavelength 0.11 nm has been detected using x-ray optics. Although these regions overlap, the transitions which produce the radiation are quite different. Ultraviolet radiation is produced by electronic transitions in the outer orbital of atoms or molecular bonds, while x-rays are produced by • inner electron transitions.

Fig.1.1 The Electromagnetic Spectrum

1021 Freq. kHz 103 tcP 109 1012 1015 ids I I II i 1 1 NP.- -v.-- Radio,TM ,Radar i UV. Ylrays. AC. § Infr.SLIstqjLi ,LC:Fal..1_-, i I 1 r 1 I , i i 1 1 i Wavelength m.105 102 10 10-1 10-2 10-4 107 109 10-11 101210-14

Far Ultraviolet Spectrum

200, 100 30 01 nm

SdIumann U.V. Soft --4 X-rays Extreme U.V.

Vacuum UV. Schumann [2] built the first vacuum spectrograph in 1893 using a fluorite prism as the dispersing medium. The linear dispersion of a prism, D, depends,on the prism angle, 2a, the focal length of the spectrograph, f, the wavelength of incident radiation and constants of the prism material.

[X-K1 ]2[1-sin2a] D ' [1.1] 2K2f sins Thus the linear dispersion of a prism depends upon the square of the wavelength of the incident radiation which makes precise wavelength measurement difficult. When he built the first spectrograph, Schumann was unable to make wavelength measurements as the constants for fluorite were unknown. He was, however, able to show that oxygen was responsible for the atmospheric absorption below 200 nm. Rowland [3] described the concave diffraction grating in 1882 and showed theoretically that the linear dispersion depended only on the radius of curvature of the grating, Re, the. groove separation on the grating surface, d, and the angle of emergent radiation, 3. nR c 9 [1.2] D = acospxl° The great advantage of this grating is that the linear dispersion is independent of the incident radiation 4 and is uniform over the whole wavelength range. The resolution of the grating can be doubled by use of the second order spectrum and, as the radiation is diffracted by the front surface of the grating, there is no limit to the transmission of the grating as there is for a prism. The prism, however, has its greatest linear dispersion in the Schumann ultraviolet. Lyman [4] constructed a vacuum spectrograph in 1906 using a concave diffraction grating as the dispersing medium. He was able to accurately measure the absorption spectrum of molecular oxygen with this instrument and also showed that fluorite would not transmit below 125 nm. Lyman also investigated the far ultraviolet spectra of, several sources and developed a far ultraviolet source, the flash tube. Since these early developments, vacuum spectrometers and Tuantometers have replaced spectrographs for most applications of far ultraviolet spectroscopy. Diffraction gratings are almost exclusively used as the dispersing medium for the reasons outlined above. Various methods for mounting diffraction gratings have been employed and Samson [5] has given a detailed account of the various types of monochromator and diffraction grating mounting used in far ultraviolet spectroscopy. Although vacuum techniques are common, they 5 are not essential for the detection of far ultraviolet radiation, as many gases are transparent in the Schumann ultraviolet. In 1961 Kaye [6] described the modification of a Beckmann DK solution spectrophotometer to extend the working range down to 170 nm. He investigated helium, argon, nitrogen and hydrogen as possible purging gases and found that commercially available argon and helium were completely transparent over the range 170-200 nm and were the best gases with which to purge, although he recommendednitrogen containing less than 10-3: of oxygen on economic grounds. Hydrogen was not recommended because of its explosive nature although it possesses high transparency in this region. The detection of far ultraviolet radiation The earliest method of detecting far ultraviolet radiation was the photographic process. In 1892 Schumann [7] demonstrated that the gelatin used in the emulsion to support the photosensitive grains on the plate was responsible for the absorption of radiation below 220 nm. To improve the sensitivity of the photographic plates and to extend their working range down into the far ultraviolet, he removed most of the gelatin from the plates. While these Schumann plates will detect radiation in the far ultraviolet, the plates are very sensitive to 6 abrasion and are-slow to develop; this may result in a poor quality of the photographic reproduction. In an attempt to improve the sensitivity of photographic plates to far ultraviolet radiation without imparing the quality of the photographic reproduction, it has been suggested that thin films of fluorescent laquers or oils be applied to the surface of the emulsion to act as sensitizing agents. The disadvantage of such a treatment is that the photographic plates have to be washed with an organic solvent to remove the sensitizer before the plate can be developed. salicylate is the most commonly used sensitizing agent [5] because it has an almost uniform quantum efficiency over the range 85-200 nm. This is usually applied by brushing a concentrated alcoholic solution of sodium salicylate onto the surface of the emulsion and allowing the solvent to evaporate. The great advantage of this sensitizing agent is that it is water soluble and is thus removed when the plate is immersed in the developer. The main disadvantage of all these sensitizers is that their application is a delicate and skilled operation which must be performed on each photographic' plate prior to its being loaded into the camera. The main difficulty in applying these sensitizers is obtaining a film that is both thin and uniform. Thin films are required to minimize scatter and uniform films are required to retain the uniform response of the photographic plate without which quantitative analysis is impossible. Photography is now not so common in* practical spectroscopy and the most common detectors used now are based upon utilization of the photoelectric effect. A photon incident upon the photosensitive material causes an electron to be emitted from the surface provided that the wavelength of the photon is less than that given by equation 1.3 where W is the work function of the photosensitive material in . hc 9- 3] X max = x10 [1. The energy of the emitted electrons, E, is not uniform over the whole wavelength range,

E = hc[X-1— Xma1x— ]x109 [1.4] but the number of electrons emitted from the surface and hence the current flowing through the photoelectric device depends on the number of photons striking the photosensitive surface, which is the intensity of the radiation. In the simple photocell shown on the left of fig.1.2, the electron ejected from the photocathode is collected by the anode which is maintained at a slight positive potential with respect to the cathode Fig.1.2 Photo electric Detectors.

—ye RI

41••=11■11 il•Me The Photocell \

•• R5

ANODE CATHODE DYNODE GALVANOMETER R9 RESISTOR AMPLIFIER The Photomultiplier at about 100 V. The current flowing through the galvanometer is usually less than 1 11A so that a sensitive, low noise and distortion free amplifier is required to obtain any useful analytical signals. In the photomultiplier tube shown on the right of fig.1.2, the electron emitted from the cathode surface is accelerated by a slight positive potential, about 100.V, onto -the first dynode where it releases several secondary electrons. These secondary electrons are then further accelerated onto the second dynode and so on up the dynode chain, usually nine or thirteen stages. Thus amplification factors of up to a million are obtained inside the tube so that the external amplification need not be say sensitive. The main problem in using photomultipliers in the far ultraviolet is obtaining a window which is transparent in this region. Normal ultraviolet photomyltipliers have windows made of Corning glass which will only transmit down to about 185 nm. In 1949, Parkinson and Williams [8]'used a scintillator to extend the sensitivity of a photomultiplier down to 145 nm. They used a manganese activated willenite but the main sensitizer used now is sodium salicylate. Plastic scintillators such as cellulose acetate and vinyl acetate provide a more convenient method of obtaining a flat, thin film but the 10 quantum efficiency of plastic materials is generally low. Photomultipliers whose windows are made from materials transparent in the far ultraviolet give higher sensitivity than these sensitized photomultipliers. Melvin [9] investigated various materials for their fax ultraviolet transmission and found that lithium fluoride would transmit radiation down to 104 nm and this was the shortest wavelength that could be transmitted through any of the materials he investigated. Melvin also measured the limit of transmission through sodium fluoride to be 108 nm, magnesium fluoride 112 nmo .calcium fluoride 122 nm, strontiuM- fluoride 128 nm, barium fluoride 134 nm, sapphire 141 nm and synthetic fused quartz 160 nm. The application of these materials to the manufacture of photomultiplier windows is limited by the cost, availabilty and working properties of the material as well 'as its transmission. The most common windows used in photomultipliers for far ultraviolet spectroscopy are synthetic fused quartz and lithium fluoride. Sapphire is sometimes used as a window and calcium fluoride is frequently used as a lens material, but the other fluorides are rarely used. Photomultipliers of the window type are 11 only useful in. the Schumann ultraviolet although the .photocathode is often capable of detecting radiation well down into the extreme ultraviolet. The use of windowless• photomultipliers'has been suggested to allow detection of far ultraviolet radiation [10], but photomultipliers will only function if there is a vacuum between the electrodes as otherwise the gas will discharge. This type- of photomultiplier can therefore only be used in conjunction with a vacuum system. The techniques commonly used for the detection of soft x-rays can also be used in the extreme ultraviolet. These detectors incTude ionization chambers, proportional counters and scintillation counters. The latter type of detector is a modification of the phosphor-activated photomultiplier. A screen of sodium, potassium or caesium iodide crystals activated with a trace of thallium is fitted onto the window of a conventional photomultiplier. This screen absorbs the short wavelength radiation and re-radiates this absorbed energy as visible or near ultraviolet radiation which is detected by the phOtomultiplier. The ionization chamber, fig.1.3, consists of a central cylindrical electrode which has a thin central wire cathode and so a very high electric-

Fig.1.3 The Ionization Detector.

Ionizable Gas

Insulator

: \ , Lithium

----T.7 Window .1 .

Electrode Guard Ring

Filling Tube •1

jet-al Body • H fN) 13 field is obtained. The chamber is filled with a gas such as nitric oxide or argon at a pressure of about 20 torr and a potential of about 700 V maintained between the electrodes. The incident radiation causes ionization of the gas and, because of the high , the ions are accelerated and upon collision with neutral•atoms or molecules induce further ionization of.the gas. In this way a cascade process. is established and so amplification factors of up to 100 are attainable with this type of detector. The Geiger or Proportional counter is essentially the same type of detector but operated in the a.c. mode. • Far ultraviolet continuum sources One of the oldest continuum sources is the hydrogen glow discharge which produces a continuum over the wavelength range 160-500 nm. Hopfield [11] modified this type of discharge to generate a continuum over the range 60-100.nm from helium by usivz a repetitive spark. The spark was produced by charging a 2000 pF capacitor to a potential of about . 10 kV and discharging this through helium at a reduced pressure in a discharge tube. Neon and argon when excited under similar conditions produce continua which are about double the intensity of those produced by microwave excitation of these gases. Wilkinson [12] 14 has described the produc-qion of continua from xenon, argon and kyrpton by microwave excitation of the gases which were sealed into a pyrex tube fitted with's. calcium fluoride window. The tube was placed in a microwave cavity and power supplied at 2450 MHz. Lyman [13] produced a hydrogen continuum from a flash tube. The flash was produced by . discharging a capacitor through a narrow capillary • tube filled with hydrogen and maintained at a low pressure. The continuum produced extended down to about 20 nm when high powers were used. The disadvantages of this type of discharge are that the capillary rapidly erodes and may damage the system'and, also, that the'discharges are not very reproducible. Garton [14] modified the flash tube by increasing the bore of the capillary and then improving the electronic and instrumental design so that high current densities of about 0.3A m-2 couldbe attained. The Garton tube,* fig.1.4, normally operates by charging a 10 12P 'capacitor to a potential of about 7 kV and • rapidly discharging this across the electrodes sealed into the tube which is evacuated to a pressure less* than the breakdown pressure of the gas'in the tube. The continuum produced extends down to about 100 nm and the line spectra of ,the elements in the ceramic: and gas extend down to about 10 nm. The advantages Fig.1.4 The Garton Flash Tube.

TEFLON INSULATOR • PLEXIGL ASS WATER JACKET NYLON SCREW, CERAMIC CAPILLARY IrOw•••■•• /RIGGER PIN

PYREX "0". RING JOINT

tr.'..•••■•—• n••••■••••••■••••••••••)t. *GAS IN

...,...... PYREX r,,,, . TUBE .:-,% 0- RING ?I; )'' HOUSING TUBE - RING PLATE :....• ij V) .4 WATER IN 2 •1 . 13 Scala = 1" '2) , 9k • .

PARALLEL PLATE TRANSMISSION LINE' 16 of the Garton tube are that the continuum produced is of a high and reproducible intensity and that the capillary erodes less quickly. Other continuum sources have been suggested [5] such as radiofrequency excitation of the noble gases and the production of synchrotron radiation from the motion of_free electrons in electromagnetic fields. Par ultraviolet line sources A traditional source of line spectra is the discharge. Two electrodes are placed in a system which is evacuated to a pressure of less than 2 torr and the discharge is struck by applying a potential of 2 kV. Operating the discharge at ea 600 V with currents of 100-500 mA, a characteristic line spectrum of the cathode material is obtained. The hollow cathode lamp is commonly used as a line source in atomic absorption spectroscopy. The element under investigation is either used as the cathode or plated onto a nickel br molybdenum sleeve used as the cathode. The lamp is filled with argon or neon at a pressure of a few torr and the discharge initiated by a potential of about 600 V. In operation, the line spectrum of the cathode material is obtained with a lamp current of only about 5-25 mA. 17 Lamps which give high intensity at the arsenic atomic resonance lines at 189.0 nm, 193.7 nm and 197.2 nm and the selenium resonance line at 196.0 nm are available commercially.. The far ultraviolet emission spectrum of a phosphorus hollow cathode lamp has been investigated over the range 175-180 nm [15] and found to give g-very poor intensity compared to the non-resonance lines between 200 nm and 260 nm: The negative glow of hollow cathodes has been used by Paschen [16] and Schuler [17] to produce atomic line spectra of the filler gas. The Penning discharge tube, however, produces a more intense ion spectrum of the filler gas than the hollow cathode [18]. In the Penning tube, a 1 kW discharge occurs across electrodes in a tube filled with hydrogen or at a pressure of about 10, 6torr while a magnetic flux density of 0.1 T is maintained around the discharge. Hartman [19] used a hot filament arc discharge,shown in fig.1.5. The filament, a coiled nickel ribbon dipped in barium carbonate, is maintained in a gas atmosphere at a pressure of about 1 torr. The filament is heated by a current of about 12 A while the discharge potential is about 90 V and the arc current 3 A. Thus the operating power of the discharge is only 270 W and with this,

FLANGE TO MONOCHROMATER • rig.1.5 'The Hot Filament Arc Discharge.

TEFLON GASKETS NICKEL INSULATOR: HEAT SHIELD

WATER COOLED [FILAMENT ANODE Tx.„:37,...... ,------v‘s.4.... --=,==cm. -.0..,===•:•_zr.-_,_-----. • COOLING WATER ,:i.I.S.1..1.',,..!1—.) FINS :.--&:.,:\;,;;;4. • I- .0.::t y,. 2277 1:27:11:trla.7-7.1.2\:. ..,,, s:,...;..\: CCX7.. ..r.Zr=2.1:73S2:17 • QUARTZ • , • .' ' • '.', ,A:i. *1 , ..i. • 6 ; 4' //. • 4? rt.c../.z...: . , //,.',. ,...—..._...: • .., : .LL_L.. . ..1 r, \

• 7777,;;77.-77-- ;••.; .g L- IU1IL

VA

!;,/ GLASS TO =•Z 1:1.1■ 7.; ,ss ======• • METAL SEAL ;;;;;‘,4: • WATER

r-7-1.77...a.:z.z.z.1..1 ....,e• I.i 22=2:2==-...r.: --- z,...Nss •,..y .-zr,-.7 ..nr.-:-22= ■, '..!;-";;',/.77..,//,7-1 '. 0 .';,:.k.S.:>,,, 'i._;..■ ..,-t v WATER k; ...t.,Zi..1;2•"ts .1->2.:7..Z .Zti ' • /77,, GLASS to ./. / I/ A...' ' ' ''''''.-".".-..."7.....%-'"A".rrg7.7,..., '',. ,.•..., ,-.--4 ,..., . METAL SEAL e''''ty,v.,..../...• i'' ., ' • ',Al ■•;.\\:\,„,,,....) . •.N. ••••,,,. , ...s. 74.."•;/,;,•:::/./ 4 . .;,". •f/1 •-e-- WATER • • WiA TRIGGER -./,,.,'• PIN y.7.;„.30. . . ,..,;,,....,..„h..- .„A Scale • t hr. • 19 intense line spectra of the filler gas are readily obtained. The Duoplasmatron shown in fig.l.6 has been used as a source of line spectra in the far ultraviolet [20] although it was initially designed as a proton source. Essentially it is a low pressure discharge similar to the hot filament arc except that there is a -physical constriction between the anode and cathode caused by the baffle plate. An axial magnetic field of about 25 A m—lfurther constricts the gas plasma so that the energy is condensed in its distribution and more intense line spectra of the filler gas are obtained. A plasma is generally defined.as a mass of luminescent gas in which a significant proportion of the gas is ionized. This definition does not necessarily preclude the products of very high temperature flames, but.plasmas are most commonly gases which are excited by electrical means. The most common type of plasma used in spectrochemical analysis is the microwave plasma. Perhaps the first use of microwaves to generate line spectra dates back to 1948, when Meggers [21] used a 110 MHz generator to produce line spectra from Hg198 Since then, many between 10 MHz and 3000 MHz have been employed but the 2450 MHz •

. . Fi g .1. 6 The Duo plasmatron, AIR OUT

CERAMIC PERMANENT SOURCE FLANGE MAGNETIC RINGS (MILD STEEL) ;- --1-. COVER FLANGE (MILD STEEL)

BAFFLE ELECTRODE (MILD STEEL)

. AIR COOLED COPPER AIR COOLING ANODE " ' • FILAMENT FEED THROUGH .2.5 V AT tOA MONOCHROmETER ENTRANCE SLIT I/0"x.050" GAS FEED - 7

LA R COOLING CERAMIC RING

STAINLESS STEEL RETAINER RING •

.o o p.f i I 0 --• • 500.8 • 4 KrL •

- 42V • -BOV 2i generator is now'the most common. A typical flow— through microwave light source is shown in fig.1.7. The gas, usually nitrogen or a rare gas, is fed continuously through the. quartz tube so that there is no danger of contamination of the electrodes. The gas discharge is initiated by a tesla discharge and the line spectra of the gases are obtained. Other gases or vapours can be introduced into the carrier gas flow so that line spectra of many elements can be obtained with this discharge. An electrodeless discharge lamp is essentially the same as a microwave plasma except that the quartz is sealed at both ends and the plasma composition is fixed. These lamps are prepared by charging a quartz tube with a few nanograms of the element or compound of the element whose line spectrum is required and then filled with argon or neon to a pressure of • about 3 torr. As microwave plasmas are usually contained within a quartz tube, the main application of these sources is in the Schumann ultraviolet. The other common line source is the vacuum or hot spark which was first devised by Nillikan and Sawger [22] in 1918. In their system, two electrodes were placed less than 1 mm apart and the system evacuated to a pressure of less than 10-5 torr. The discharge was initiated J:7 an electric field of about '"

....

Fig.l.7 The Microwave Plasma Discharger COOLED

DISCHARGE

.; --- GAS !N

~u s S \ S S S s S s s S S \ S S 5 S \ S:S::::S::S::S::XX::S·Y~~~ . . \ ti , . .,._1\/- 4 Z!JAflTZ DISCHAf1GE TUSE

B ROI DA T,YPE CAVITY r .

POWER IN " I --I Scale: tln.

. . 23 1501rm-1 and the characteristic line spectra of the electrode material was obtained. Molecular spectroscopy in the far ultraviolet In molecular spectroscopy, electronic transitions occur in the u-orbitals of molecular bonds or orbitals in the individual atoms in a molecule and are characterized by a quantum of radiation. Equation 1.5 shows the relatiohship between the energy of the electronic transition in joules and the wavelength of the characteristic radiation in nm. he 9 AE Ek - E. = --x10 [1.5] Band spectra are obtained from molecules because there are several vibrational and rotational states associated • with each electronic state and so each electronic transition is accompanied with one of an almost infinite number of vibrational and rotational transitions. These other transitions produce small changes in the value of LE which broadens the line unless high resolution is employed when the fine structure is visible. Only very strong molecular bonds can exhibit molecular spectra in the far ultraviolet as a wavelength of 185 nm corresponds to an energy of 6.8 eV. This wavelength is thus more energetic than the bond energies of many molecular bonds. 24 A Molecular emission spectroscopy Molecular emission has little application in the far ultraviolet as whole molecules are often fragmented before a sufficient population of excited molecules is obtained. If the Boltzmann distribution, equation 1.6, is applied to a molecule which has a bandhead at 185 nm, then only about 10-32: of the molecules will be in the excited state at 1000 °K compared to about 10-5X for caesium at the same temperature and this temperature will often be sufficient to decompose many organic compounds.

Et = Et exp — Ek/kT [1.6]

The emission spectra of stable molecules and radicals such as carbon monoxide, nitrogen, nitric oxide and oxygen have, however, been recorded [23] in the far ultraviolet using discharge sources to excite the gases. B Molecular absorption spectroscopy The absorption of light by a species is governed by the Beer—Lambert law which is shown in equation 1.7. The absorbance or optical density* A, depends on the intensity of incident light [Is]and the intensity of the transmitted light. A = log[Io/I] = KvIA:= [1.7] • Thus molecular absorption spectroscopy can 25 be used analytically in two modes. Firstly, if the concentration is kept constant and the wavelength varied, the absorption spectrum of the species is obtained, i.e. the variation of Kv with wavelength. Secondly if the wavelength is fixed and the concentration varied, a quantitative analysis is possible. Most of the applications of far ultraviolet molecular absorption spectroscopy have been qualitative so far. The sources used in molecular spectroscopy are continuum sources. Various types of source described previously have been used. The absolute intensity of the source is immaterial provided that it is intense enough to be detected because only a ratio measurement is required. Kaye [6,24] modified the conventional ultraviolet source, the hydrogen lamp, on a Beckmann spectrophotometer by fitting windows: of a synthetic fused quartz, Suprasil, to extend the continuum down to 160 nm. Most molecular absorption spectroscopy is performed at ambient temperatures, so that only transitions from the electronic ground state are observed. When vapour phase and high resolution spectroscopy are used, the vibrational fine structure can be detected. Methyl iodide has a strong absorption bandhead at 201.2 nm which corresponds to the electronic transition from the ground to 26 first excited state. Vibrational fine structure associated with this transition extends down to 185 nm in the vapour phase, but disappears in heptane solution where the vibrations are quenched [25]. This loss of fine structure is common to all solution spectrophotometry so that functional group analysis is possible but no fingerprint region exists as in infra-red spectrocopy. Several functional groups, however, have their characteristic absorption bands in the far ultraviolet s all alkenes absorb strongly around 180 nm, alkynes 173 nm, aldehydes and ketones 195 nm, carboxylic acids 185 nm, amines and amides 185 nm, organic iodides 200 nm, organic bromides 185 nm and aqueous inorganic iodides 195 nm and 225 nm. Several reference spectra are available for different compound types [24,26]. The first absorption spectrum recorded was that of molecular oxygen [4], and since then the absorption spectra of all the other atmospheric gases have been recorded. The far ultraviolet absorption spectra of several other molecular gases such as chlorine, sulphur dioxide, nitric oxide, hydrogen, etc.; organic solvents; liquids such as bromine and vapours such as iodine, phosphorus and sulphur have also been recorded..A comprehensive set of 27 inorganic absorption spectra in the far ultraviolet have been tabulated by Kaye [24] and Sponer [25]. While qualitative analysis by molecular absorption, in the far ultraviolet is now a well established technique, the quantitative applications of molecular spectroscopy in this region are much more limited. The determination of ammonia in air has been reported using the bandhead at 204.3nm [26]. The ammonia bandheads at 197.2 nm and 193.6 nm afford a greater sensitivity, but these bands overlap with the oxygen absorption spectrum, so they can only be used to their full advantage in oxygen free atmospheres; . The determination of water vapour at the.121.6 nm bandhead.has been suggested. The molar absorptivity of water at this wavelength is 38.7 m2 mol-1 while that of oxygen is 0.03 m2 mol 1 but as oxygen has a non-discrete absorption spectrum around this region it must be excluded when this determination is performed [24]. C Molecular spectroscopy Molecular fluorescence has not been widely studied in the far ultraviolet although the fluorescent spectrum of iodine has been determined [23]. For most molecules the excited 'electronic state has a very short lifetime and energy tends to be lost by non-radiative means. 28 Atomic spectroscopy in the far ultraviolet Atomic spectroscopy is concerned with the generation of line spectra. Any of the line sources described previously are- suitable for atomic emission spectroscopy, although some are of a very limited application. The elements which exhibit strong line spectra in the fr ultraviolet are mainly the non—metals, noble gases, arsenic, selenium and mercury. These elements possess atomic spectra in this region and the main resonance lines of these elements are to be found in the far ultraviolet. Many of the common metals such as iron, copper, , aluminium, nickel, silicon and manganese also have ionized species whose electronic transitions produce far ultraviolet radiation. Thus the far ultraviolet can be used for the detection and determination of all the elements that can be determined by atomic spectroscopy. A Atomic emission spectroscopy The number of atoms in any excited state can be calculated from the Boltzmann equation for any temperature. From table 1.1, it can be seen that a temperature of about 5000 °K is required to obtain a high population of excited atomsif the resonance line is in the far ultraviolet. This precludes the use of a flame as the excitation source in this region Table 1.1 Values of. loeNk(Noi for different resonance lines at various temperatures.

T, °K Cs 852 Na 589 Ca 423 Zn 214 Hg 185 C 156 Cl 135 A 88 500 -14.40 -20.66 -28.67 -57.58 -66.75 -79.37 -91.38 -140.78 1000 - 7.20. -10.33 -14.33 -28.79 -33.38 -39.67.: -45.69 -70.39 2000 - 3.60 - 5.17 - 7.16 -14.39 -16.69 -19.83 -22.85 -35.19 3000 - 2.40 - 3.44 - - 9.60 -11.13 -13.22 -15.23 -23.46 4000 - 1.80 - 2.58 -.3.58 - 7.20 - 8.34 - 9.92 -11.42 -17.60 5000 - 1.44 - 2.07 - 2.87 - 5.76 6.68 - 7.93 - 9.14 -14.08

600t - 1.20 - 1.72' - 2.39 4..80 - 5.56 . - 6.61 - 7.62 .-11.73 7000 - 1.03 - 1.48 - 2.05 = 4.11 . 7;4.77 - 5.67 - 6.52' -10.06 8000 - 0.90 - 1.29. - 1.79 - 3.60 - 4.17 - 4.96 - 5.71 - 8.80 9000 - 0.80 - 1.15 - 1.59 - 3.20 - 3.71 - 4.41 - 5.08 - 7.82 10000 - 0.72 - 1.03 - 1.43 - 2.89 - 3.34 - 3.97. - 4.57 - 7.04 11000 - 0.65 - 0.94 - 1.30 . - 2.62" - 3.03 - 3.61 - 4.15 - 6.40 12000 - 0.60 - 0.86 - 1.19 - 2.40 - 2.78 - 3.31. - 3.81 r. 5.87 13000 • . 0.55 - 0.79 - 1.10 - 2.21 .- 2.57 - 3.05 - 3.51 5.41 14000 . 0.51 - 0.74 - 1.02 - 2.07 - 2.38 - 2.83 -3.26 - 5.03 . 15000 - 0.48 - 0.69 - 0.96 - 1.92 - 2.23 2.64 - 3.05 - 4.69 30 as the temperatures of conventional flames are usually less than 3500 °K. The intensity of a spectral line can be calculated from the Einstein-Boltzmann equation.

T . = . i u e_ gk Aki109exp-Ek/kT [1.8] Thus the line intensity depends upon the particle density of atoms, N, the temperature, transition probability and the spectral region. The determination of non-metals in iron, steels and associated steelworks materials by emission spectroscopy is often carried out using the resonance lines of these elements in the Schumann ultraviolet. The most common source used is the vacuum spark and the problems involved with this particular discharge have been the subject of considerable research in Eastern Europe. It has been demonstrated that the material from which the counter electrode or anode is made has no effect upon the emission spectrum observed from the cathode. A common practical arrangement is to set the anode of copper or carbon about 2 mm above the sample which is made the cathode. The far ultraviolet transparency is ensured by blowing the spark chamber with argon and the atomic spectra are excited by an overdamped discharge. Carbon is usually determined with a vacuum quantometer at its 31 atomic lines at either 193.0 nm or 165.7 nm, phosphorus at 178.3 nm or 177.5 nm, sulphur at 182.0 nm or 180.7 nm and the iron ion line at 171.3 nm is often used as the internal standard. Spark sources have been used for the analysis of cobalt-and nickel-based alloys for carbon, chromium, niobium and tantalum [28] over the range 6Q-156 nm. The analysis of pig iron has been described for traces of manganese, silicon and phosphorus at 192.1 nm, 198.8 nm and 178.8 nm respectively while powders have been analysed for sulphur, selenium and tellurium at 166.7 nm, 160.6 nm and 167.8 nm respectively. Goto et al. [30] have investigated the carbon.lines at 156.1 nm, 165.7 nm, 172.0-nm and 193.0 nm obtained by using a 900 V oscillating discharge overdamped with a 30 µf capacitor, across electrodes whose inductance was 50 1111. They also applied this atom cell to the determination of arsenic at 197.2 nm and tin at 181.1 nm in iron and steel using the iron line at 210.9 nm as the internal standard. The conventional D.C. are is not a suitable source for far ultraviolet spectroscopy as the atmosphere takes part•1n sustaining the discharge between the electrodes and a strong oxygen spectrum is to be expected. The D.C. plasma, however, where the electrodes are swept' free of oxygen with argon 32 would appear to be a suitable source, although its use has not yet been reported in the literature. The demountable hollow cathode lamp has been used to determine sulphur, chlorine and fluorine in the vacuum ultraviolet [31]. A stainless steel hollow cathode fitted with an internal molybdenum sleeve was set about 10 mm from the entrance slit of a vacuum monochromator. Copper sulphate, copper chloride, sodium fluoride or sodium sulphide was powdered with carbon and place& on the cathode to provide atoms of the element under investigation. The chamber and spectrograph were then evacuated and the chamber filled with helium to a pressure of 15 torr. The discharge was initiated with a 10 kV potential and the lamp operated at a current of about 300 mA. The atomic line at 95.5 rim was used for the detection of fluorine while chlorine was detected at the 134.7 nm atomic line. Malamand [32] used a high energy plasma for the detection of carbon, chromium, molybdenum, nickel, silicon, tantalum, etc. in iron, cobalt and nickel based alloys in the range 50-150 nm, using lines of ionization states III, IV and V. In this system, concentrations in the range 0.5-50 X of these elements in the alloys were determined with the detection limits in the range 0.0005-0.05 X. Carbon 33 was also determined in the concentration range 0.2-5 in cobalt-based alloys which contained refractory carbides. B Atomic absorption spectroscopy The main disadvantage of an emission technique is spectral interference from nearby lines or bands and the problem in very hot sources of isolating the line from the background continuum. As the production of a monochromator capable of isolating a spectral line is not a practical proposition because of the high resolution required, emission spectroscOpy has some limitations. In atomic absorption spectroscopy, a line source, usually a hollow cathode lamp or electrodeless discharge lamp, is used and a monochromator of moderate resolution is set to accept a small bandpass around the resonance line of the atom. The atoms in the atom cell will only, in general, absorb resonance lines and thus characteristic radiation is only absorbed so that spectral interferences are greatly reduced. Table 1.2 lists the principal resonance lines of atoms that lie in the far ultraviolet. Walsh [33] has expressed the absorption coefficient at the line centre in terms of the Doppler half-width, AXD, which can be calculated for any atomic line. • 34

Table 1.2 Elements with principal resonance lines in the far ultraviolet.

Element Wavelength, nm. Argon 107, 105 Arsenic 197, 194, 189 Bromine 158, 149 Carbon 166, 156, 94 Chlorine 138, 135 Fluorine 96, 95 Helium 53 Hydrogen 122 Iodine 183, 178 Krypton 124, 116 Mercury 185 Neon 74, 73 Phosphorus 179, 178, 177, 168, 167 Radon N- 179, 145 Selenium 196 Sulphur 183, 182, 181, 143, 130 Xenon 147, 130 Nitrogen 120, 120. Oxygen 131, 130 35 2 2 -2 TTe 2X r1n2 Nf4 710-9 '1" T L---1 -ac [1.9] Kv T 4neomc 2 This implies that the absorption coefficient is proportional to the concentration of atoms, I1, and this is, in fact, true provided that the atomic concentration is low and other line broadening factors are neglible. In practice, the absorbance is measured as in solution spectrophotometry and this can be shown to be directly proportional to the absorption coefficient. L'Vov [34] has described the construction of a graphite cuvette for the determination of iodine, mercury, phosphorus and sulphur in the far ultraviolet. The sample is nebulized from an electrode into an argon filled heated cell which is mounted on the entrance slit of a vacuum monochromator. Using a high electrodeless discharge lamp, • absolute sensitivities in the range 3-80 pg were obtained. Nassmann [35] described a similar type of furnace for the determination of arsenic at 189.0 nm and 197.2 nm and selenium at 196.0 nm. The major difference between this furnace and that of LIV6v is that the sample is introduced directly into this furnace. The furnace is then heated to atomize the sample after oxygen- has:been swept' from the cell by 36 flushing with argon. Using a temperature of about 2600 °K, detection limits of 0.6 ng and 2 ng were reported for arsenic and selenium respectively, but memory effects are likely to occur with this system. The carbon filament atom reservoir, fig.l.8, as modified by Alder and West [36] is also a suitable atom cell in the far ultraviolet. A 2-5 mm3 sample is spotted onto the filament, 3, with a micropipette. The solvent is then vaporized at a low temperature and then the solute is rapidly vaporized at a much higher temperature with argon or nitrogen flowing over the filament. The absorption peak is then measured on a fast response recorder. The disadvantage of this method is that the- vapour is not kept heated once it leaves the filament surface so that thermally

stable molecules such as S8, S2' P41 SO2' etc. are not readily atomized. Donega and Burgess [37] constructed a similar filament device. 50-100 mm3 samples are placed in a sample boat construced from tantalum, tungsten or graphite. The chamber can be purged with inert gases at pressures between atmospheric and 1 torr. The sample is vaporizdd from the boat into the light path at about 2000 °K. In this cell as with the carbon filament, memory effects are greatly reduced , compared to the Massman furnace. As yet, neither of 37 •

Fig:1.8 The Filament Atom Reservoir.

J

O

A Base B Water-Cooled Electrodes C Water Link Between Electrodes D Laminar Flow Box E Inlet for Shield Gas Y Support Stem for Reservoir G Terminals H Water Inlet and Outlet. J FiThment

• 38 these filament devices have been used successfully in the far ultraviolet. Robinson et al. [38] described the routine determination of mercury•in air using the resonance line at 184.96 nm in a non-flame cell. In their system, fig.1.9, the atmosphere is drawn over a charcoal bed which absorbs the free mercury in the air. The cell and Monochromator are then pumped with argon and the mercury driven into the absorption cell by heating the charcoal with radiofrequency induction. Using a Penlite mercury lamp as the source, a sensitivity of 0.1 pg of mercury per cubic metre of air was claimed. They found that other sources such as the electrodeless discharge lamp or hollow cathode lamp gave very much poorer sensitivity which they attributed to self-reversal of the mercury line in these sources. Conventional flames are the most common atom cell in atomic absorption, but these absorb strongly below 200 nm. The determination of arsenic and selenium at 193.7 nm and 196.0 nm respectively has been described in air-acetylene flames by several workers and even the oxy-acetylene flame [39] has been used but with this flame, the fuel to oxidant ratio had to be strictly controlled so that the flame was neither reducing nor oxidizing.. The flame SAMPLE BLANK

CHARCOAL SCRUBBER GAS TIGHT • SEAL

• O 0 - 0 0 ; f 0 INDUCTION COIL 0 O 0 CHARCOAL 0 O 0 er • • • r t 4 • a • L. EXHAUST MONOCHROMATO SOURCE U PORT o

LENS C HOPPE R RESISTANCE HEATING WIRE

Piga. 9 Mercury' Vapotir Absorption Cell of Robinson et al. 40 transmission is greatly increased by use of the nitrogen-hydrogen or argon-hydrogen diffusion flames. Although the detection limit is improved in these flames, they are not hot enough to overcome chemical interferences and so almost every anion and cation interferes with the determination of arsenic and selenium. The use of the nitrogen separated air- acetylene or nitrous oxide-acetylene flames overcome these chemical interferences while retaining the improved flame transmission of the diffusion flame. Although the sensitivity obtained in these flames is somewhat reduced because of the dilution of the atomic concentration by the separating gas, the detection limit is approximately the same because of the improved flame transmission [40]. Kirkbright et al. have investigated the nitrogen separated nitrous oxide-acetylene flame as an atom cell in the far ultraviolet and have shown that it will transmit down to 175 nm. They have used this flame for the determination of iodine at 183.04 nm [41], mercury at 184.96 nm [42], sulphur at 180.73 nm [43] and phosphorus at 177.50 nm, 178.29 nm and 178.77 nm [15] in aqueous solution. In a typical system, fig.1.10, the radiation from an electrodeless discharge tube used as the source is focussed onto the burner and also the 'entrance slit of the . .Zig.1.10 Modifications to the Optics of a Perkin-Elmer Model 290E Atomic Absorption Spectrometer to Detect Far UV Radiation. R

B Nitrogen Shielded Nitrous Oxide-Acetylene Burner EDT Electrodeless Diicharge Tube in 3/4-Wave Cavity J Nitrogen Inlet to Purge Optical System L Fused Silica Lens M Monochromator Q Fused Silica Windows R Rotating Sector 42 monochromator.using fused silica or calcium fluoride lenses. The whole of the optical path is purged with oxygen free nitrogen to eliminate atmospheric absorption. Atomic Fluorescence Spectroscopy. The sensitivity of atomic absorption is limited by the theory which depends upon the measurement of a ratio. Atomic fluoi.escence has the same advantage as emission spectroscopy in that a sophisticated amplification stage can be used to improve the slope of the calibration curve, but because there is an • absorption process involved, fluorescence does not suffer from spectral interference to the same extent that emission spectroscopy does. The observed fluorescence from an atomic line is a combination of the emission and absorption processes and an instrumental factor, e/4n, which is the solid angle over which the fluorescence is viewed.

F =',1To47;-941—exp—K v1116,v [1.10 In equation 1.10, is the quantum efficiency of the fluorescence process and in the ideal case is unity but is always less than unity in practice. Atomic fluorescence has riot been widely used in the far ultraviolet although arsenic and selenium have been determined in the nitrogen—hydrogen diffusion flame and the air—acetylene flame. The 43 determination of mercury at the 184.96 nm line has recently been reported [44] in a flame. It was found that the sensitivity at this line was not as great as would be expected from a simple theoretical consideration because of an energy loss by the excited state. This resulted in an improvement in the sensitivity at 253.65 nm when mercury atoms were excited by the 184.96 nm line as well as the 253.65 nm line. Many of the non-flame cells described earlier can be used as atom cells for atomic fluorescence. The carbon filament atom reservoir and the Massman furnace have both been successfully used.for the determination of metals in middle and near ultraviolet by atomic fluorescence and should, presumeably, be used in the far ultraviolet in the not too distant future. CHAPTER 2

The Induction Coupled Radiofrequency Plasma. 44 2.1 General The developement of radiofrequency induction coupled plasmas has recently been reviewed by Fassel [45]; only a brief account will be given here. Most of the credit for the developement of the technology of these plasmas should be given to Reed [46] whose apparatus for.the growth of refractory crystals is shown in fig:2.1. The exploitation of these plasmas as excitation sources for atomic emission and as atom cells in atomic absorption was - pioneered by Greenfield in England [47,48,49] and by Fassel in the U.S.A. [50,51,52,53,54]. Later, Pforr et al. [55,56], Britske et al. [57], Hoare and Mostyn [58], Veillon and Margoshes [59], Bordonali and Biancifiori [60], Mermet and Robin [61], Goldfarb and Goikham [62], Kleinmann and Svoboda [63], Morrison and Talmi [64], Truitt and Robinson [65,66] and Boumans and De Boer [67] investigated their analytical potential. The numerous advantages afforded by induction coupled plasmas as excitation sources have been adequately documented in these publications.

The most important .of these.advantazes are :7 a effective injection of the sample into the hot portion of the plasma. b relatively long residence time of the sample in the plasma. •

45

POWDER AND GAS 'INLET

GAS INLET

A

QUARTZ TUBES

1W COIL • C)) (C)) c

PLASMA

PLASMA Fi AME POWDER STREAM

CRYSTAL

Fig.2.1 Schematic drawing' of an induction-coupled plasr a. employed for crystal growth studies. 46 higher gas temperature than combustion flames or arcs. d continuous temperature gradient from about 9000 °K down to room temperature, which allows greater lattitude in selection of the optimum temperature. e free atoms may be generated in the hottest zone of the plasma and then observed in lower temperature zones where the background emission is lower. , chemical environment may be controlled to some extent. g no electrode contamination. There are, however, some limitations to the use of induction coupled plasmas. They have a limited tolerance to sample loading, especially at low power levels, the formation of significant concentrations of molecular gases in the plasma causes an impedence mismatch between the plasma and.radiofrequency field. This in turn to a reduction of the energy available to sustain the plasma so that the sample introduction rate must be kept low. Another problem often encountered at low sample flow rates is the coalescence of aerosol droplets at the sample injection orifice, which usually causes instability in the injector process and leads to noisy analytical 47 signals. The analytical studies of Greenfield [47] and Fassel [51] have shown that chemical interferences are reduced to virtually.neglible proportions in these plasmas. Later observations by Veillon and Margoshes [59] revealed a suprising enhancement effect for the calcium-phosphate system which is difficult to explain, but is probably due to their own system, fig.2.2, in which the sample is allowed to drift up the tube compared to the systems employed by Fassel, fig.2.3, and Greenfield which force the sample into the hottest parts of the discharge. Also Veillon and Margoshes observed the radiation at a much greater height above the work coil than either Greenfield or Fassel so they observed the emission from a much cooler zone. 2.2 The plasma discharge 2.2.1 Formation and Stabilization As plasmas are good conductors of electricity, having free electrons, they will readily interact with a magnetic field. Once formed, therefore, it is easy to couple the plasma to a high frequency magnetic field. This system is employed in the induction coupled radiofrequency plasma. When the radiofrequency power is applied through the work coil with argon flowing through the 48

-1. 4-18.5mm

<-25mm

1 IOOmm

00 00

QUARTZ 0 0-RING 0 BRASS

•■•■•••■••

<--COOLANT GAS

•■•••■■•

SAMPLE AND CARRIER GAS

Fig.2.2 Plasma Assembly as used by Veillon and Margoshes. • 49

FLO rnm I. D,°•''' 2mm 0' 13 20 mro 43" 2mm

1.5 171 M 20 nun

9mr 100 tern 1. D.

COOLANT GAS INLET (TAN GV:NTIAL)

••■•••••■■

WPM...WM=8 r 4mrn •

4 0 mm •

4mm IF. ‘'"-PLASNIA GAS INLET (TANGENTIAL)

M/38

4ntrei I.D. 7rnm 0. D. .

13/D CALL JOINT Ai:CIESOL INLET Fig.2.3 Plasma Torch Assembly as used.by Fassel and associates. 50 fubed silica tubing of the plasma torch no discharge occurs as argon gas is a non—conductor. If the argon is seeded with electrons in the coil space, however, the plasma can be formed. The easiest method of forming the plasma is to lower an isolated carbon rod into the magnetic field [45,46,47,50] as shown in fig.2.4. Once inside.the field, the carbon is quickly heated by radiofrequency induction to such - a high temperature that thermal emission of electrons takes place and some of the gas atoms in the vicinity of the rod also reach ionizing energies. The electrons and ions so formed are accelerated by the high frequency oscillating magnetic Yield induced in the axial direction by- the radiofrequency current .flowing through the coil. These accelerated electrons and ions gain energy and upon collision cause more ionization and as soon as the plasma is adequately ionized, an eddy current which flows in azimuthal circular closed paths around the periphery of the plasma is induced by the axial magnetic- field. This current meets resistance to its flow and Joule heating results. These steps to the almost instantaneous formation of the plasma.. The plasma is thermally very hot and has electron temperatures in the core reported to be in the range 9000-12000 °K. [45]. The Plasma must,. therefore,, 51

:•■■ t r. 7,* arum I P

11 MEM 5 1.8c.

• COMB Sawa.. giro.*

4•1•21•••• 1111•13/•11 OIN=Sib IMMI IMMO.

1110411•VV.11., *NCI. .1.10.111 r4 1 t.,Pair." E.

....„, ta„. , II- ...2...... 4:—...... _ ._ . .. ; s.

■•■••••••■•••....0

r),s--..e0-1.,5,,t e.

CLIIL.\‘•Q 'Ignition' of an induction-coupled p 52 be thermally isolated from the tube walls otherwise the walls collapse during ststained operation. Two schemes of achieving this thermal isolation are shown in fig.2.5. The vortex flow stabilization as devised by Reed [46] and used most frequently in commercial plasma torch designs, is shown on the left of this figure. A high velocity flow of argon is introduced tangentially so that it moves up the tube walls by physically removing the plasma from the walls. The co-axial system as devised by Fassel [50,51,53,54] is shown on the right. This scheme uses laminar flow with the coolant flow being of such a high velocity that the plasma is separated from the walls. 2.2.2 Sample introduction into the discharge In the radiofrequency induction coupled plasma, the gases are heated internally, expanding and accelerating in a direction perpendicular to the exterior surface of.the plasma. This physical gas motion and the high temperature gradients near the plasma surface produce an aerodynamic barrier to the injection of the sample aerosol into the plasma and for many plasma systems, a gas velocity sufficient to overcome this barrier may cause instability or even collapsing of the plasma. The skin-depth effect of indUction heating, however, is ( A)VORTEX: (B) CO-AXIAL

INDUCTION COIL

HIGH VELOCITY COOLANT AND STABILIZING GAS. PLASMA SUSTAINING GAS- SAMPLE. INLET

Fig.2.5 Schemes for the thermal isolation and stabilization: of. induction-coupled plasmas:- 54 utilized to assVre effective entry of the sample into this type of plasma. If the plasma is generated at frequencies of 4-5 MHz [50,51,59,60] a.tear—drop plasma as shown on the left of fig.2.6 tends to be formed. Sample particles that approach the steep thermal gradient follow a rather disconcerting path around the plasma. The sample partidlea then do not actually experience. the high temperatures of the plasma core at these low frequencies. As shown on the right of fig.2.6, an increase of oscillating frequency to 30-40 MHz [47,53,54,58,68,69] causes the eddy current-to flow more closely to the outer poitions of the plasma. This results in the well known skin—depth effect [45,54,67] i.e. the depth to which the eddy current flows is inversely proportional to the square root of the frequency of the current. Thus DOUGHNUT shaped plasmas are formed at high frequencies with the hole in the DOUGHNUT being somewhat cooler than the DOUGHNUT itself. The cooler doughnut hole can be further developed by optimizing the flow velocity of the carrier gas that injects the sample into the plasma. The cooler doughnut hole entry provided by these higher frequencies presents less resistance to the injection of sample particles into the axial o o © SAMPLE o PAUICLES 0 0 o o

HIGH • LOW FREQUENCY FREQUENCY

Fig.2.6 Aerosol path for several plasma shapes.. . — 56 channel of the plasma th.gn in the low frequency plasma. The sample, however, still does not experience the temperature environment of the core, but the temperature of the hole is.generally higher than the outer surface of the plasma so that this still represents' a significant improvement over the low frequency situation. 2.3 Plasma.s as far ultraviolet sources The core Of an induction coupled plasma is a mass of gas around 10,000 °K. If this were a black body then the spectral radiant flux emitted at a particular wavelength, Wx, could be calculated from the Planck equation :-

7the2 6,2k. Wa = x5 e75173707.7" [2.1]

In the plasma system the total surface area of the core that can be observed is about 80 mm2 for many plasma torch designs so that only about 80 W will be radiated at 200 nm. The plasma can of course in no way approach the black body as emission signals of introduced analyte can be observed and the background emission more closely follows the Bremsstrahlung and cyclotron radiation spectra than the black body. In view of this low spectral output for the system which is far more expensive to operate than many of the continuum sources described previously.with an output which is not greatly superior to these 57 other sources, the plasma does not appear to be an ideal continuum source for far ultraviolet spectroscopy. As an excitation source in the far ultraviolet the argon plasma, however, has an application. From table 1.1 it can be seen that a plasma temperature of about 9000 °K produces a population of excited atoms in the non-metals of the same order as that obtained for sodium and calcium in an air-acetylene flame. Also, in the plasma it is to be anticipated that chemical interferences may be minimized [which is difficult with a low temperature flame], so that emission spectroscopy of the non-metals should be possible with such a plasma system with a minimum of interferences. As the atom cell in atomic absorption spectroscopy, the plasma is not ideally suited. In the case of non-metals it has been demonstrated by Kirkbright et al. [15,40,41,42,43,44] that atom formation without chemical interference is possible with a nitrous oxide-acetylene Ilame and that atomic absorption spectroscopy of these elements in this flame Is-possible with a sensitivity near to the theoretical maximum sensitivity for the atom cell [70]. With the plasma, there will be a significant concentration of excited atoms produced and so an intense analyte emission signal will be observed at' 58 the detector..This results in a depopulation of the ground state at the higher temperature and thus a lowering of the attainable sensitivity owing to the lower atomic concentration.in the plasma compared to the flame. The emission signal from the plasma absorption cell can be electronically screened from the emission signal from the source by modulation of the source, but while this may allow the absorption signal to be measured without interference from the emission signal, the plasma emission.is still incident upon the photocathode and leads to an increase in the electronic noise within the system. This in turn means that detection limits are increased. The analytiCal studies of Greenfield and Fasel showed that atomic absorption using the plasma as the atom cell gave no improvement over flames and was often less sensitive, except for those elements which form refractory oxides such as aluminium and the rare earths. These elements, however, may now be determined in the nitrous oxide-acetylene flame without difficulty. Plasmas may also be used as the source in atomic absorption spectroscopy. There'are, however, some limitations to their application in this way. The plasma is very much hotter than the hollow cathode lamp and is maintained in the presence of 59 much greater electric and magnetic fields. This leads to considerable line broadening due to the Doppler, Stark and Zeeman effects than in more conventional sources. With the wide availabilty of commercial hollow cathode lamps and the relative ease with which electrodeless discharge lamps can be prepared this application appears to be limited to: elements for which it is difficult to prepare stable sources. Such an element appears to be phosphorus [15,70] which does not appear to give a stable discharge in an electrodeless lamp but can readily be detected in a plasma discharge of the radiofrequency induction coupled type [50,54,67]. In this study it was decided -Co limit the investigation of the potentialities of the induction coupled radiofrequency plasma to its application as the excitation source for emission spectroscopy, as all other possible applications are dependent upon this. 2.4 The radiofrequency generator The radiofrequency generator employed in this study was a Radyne model 1150/P [Radyne Ltd., Wokingham, Berkshire]. This particular model requires a three phase power supply to generate the high tension current from the power rectification unit. A simplified circuit foc the power rectification 60 is shown in fig.'2.7. The. rectified high tension current is the used to power the control unit and the oscillator unit. The control and motor unit, fig.2.8, contains the safety switches on the door of the torch unit and the water pressure. switch; the latter switch requires a water pressure of at least 25 p.s.i. to complete the circuit. A delay valve, DLS/15, is incorporated into the mains indicator lamp, SL—W, to prevent activation of the radiofrequency circuit for at least two minutes after the rectifiers have been switched on. This allows the rectifier valves, - RR/250, to stabilize before high tension• current is drawn from them. The oscillator unit, fig.2.9, incorporates a smoothing circuit to reduce the fluctuations in the power supplied to the work coil, IND4, from the rectifiers. This smoothing circuit consists of a 1 H inductance [IND2], a 1 pF capacitor [c9] and a 4000 ohm resistor [R5] and is placed before the oscillator valve, BR1126, and represents the main difference apart from the power increase between this model and the Radyne Model SC15 used by Greenfield [47]. The oscillator valve, produces a radiofreauency of about 36 MHz, the exact frequency depending upon the load matching of the work coil' and the plasma timm..•■=6.mi TI

a 0 I T2

J

RI

Al

H ON Fig.2.7 Power Rectification Unit of Radyne Model H30/P Radiofrequency Generator. Sr S2 ' PB —01

DL5/15 RLA SLAY .

R4

Fig.2.8 Control Circuit of the Radyne Model .1130/P Radiofrequency Generator. Fig.2.9 Oscillation Circuit of the-Radyne-Model—H30/P Radiofrequency Generator. Table 2.1 Key to Figs.2.7,2.8 and 2.9.

Symbol Component Location/Function Symbol Component Location/Function Al Ammeter H.T.Current L1,2,3 Input terminals A2 Ammeter Grid current M Blower motor BR1126 Valve Oscillator OCR1 Overcurrent relay 01 Capacitor Motor start PB Switch On-Off Button 02 Capacitor Feed through' R1 Resistor Relay by-pass 03,4,5 Capacitors R2 Resistor Anode C6 Capacitor Variable R3 Resistor Shunt C7 Capacitor Feed through R4 Resistor Ballast C8 Capacitor Filament R5 Resistor Filter C9 Capacitor Filter R6 Resistor Grid bias CH1 Choke Anode R7 Resistor Filament CH20,4 Chokes Filament RR3/250 Valves Rectifier IND1 Inductor Filter S1 Switch Thermal IND2 Inductor Tank coil S2 Switch Door IND3 Inductor Output coil Ti Transformer H.T. IND4 Inductor Work coil T2 Transformer I.T.+Filament 65 discharge. The pOwer input to the work coil is adjusted via the variable capacitor, C6, which alters the capacitance and hence resonance frequency of the oscillator circuit so that the impedence of the circuit can be matched with the impedence of the plasma. The actual power supplied to the work coil is unfortunately not measured directly on this particular model but is monitored indirectly by the ammeter, A2, which measures the grid current of the oscillator valve. A second ammeter, Al, in the power rectification unit measures the high tension current. 2.4.1 The work coil In the standard,commercial Radyne plasma generators, the work coils are constructed from 6.3 mm outside diameter cylindrical copper tubing. This type of work coil was used, although with the lower power instrument square section copper tubing has been reported to give amore stable plasma [69]. The coil used was constructed of two turns wound anticlockwise when viewed from above, with an inside diameter of 32 mm and an outside diameter of 45 mm. The separation.between the turns of the coil was 1.5 mm giving an overall coil height of 22 mm. The work coil is connected to two flexible armoured hoses which supply the power and cooling water to the coil via 0.25 inch B.S.P. fittings. 66 This allows the 'work coil to be moved on its insulated cradle within the torch compartment. 2.4.2 The plasma torch Two types of plasma torch design were investigated initially, one with radial flow of the plasma gases and the other with tangential flow. The design of these'.is similar to that used by other workers. [50,54,58,59,69] and is an improvement- over the standard Radyne design which is constructed entirely from fused silica. The torch used, fig.2.10, consists of a two part brass body with appropriate 0—ring seals to prevent gas leakage within the torch, and two lengths of fused silica tubing. The coolant gas is fed into the upper_ torch body while plasma .gas flows into the lower torch body. The base of the torch is machined to accept a B14 cone, through which the sample is introduced in the injector flow. • The fused silica tubes used with this torch were of 25 mm and 21 mm internal diameters with a nominal wall thickness of 1,5 mm. The outer tube was 100 mm long and extended 30 mm above the top of the inner tube, The work coil was positioned concentrically with the torch so that the base of the coil was 4 mm above the top of the inner tube and the top of the coil was 4 mm below the top of the outer tube. . 67

Fig.2.10 Demountable Plasma Torch used with the Radyne Model H30/2 Radiofrequency Induction-Coupled Plasma. •

Outer silica tube

Inner • silica tube

locking cap Brass sealing Viton ring 0-ring

Upper Cooling- • body gas inlet

Lower Plasma body gas inlet 68 Radial'and tangential flow of the plasma gases was achieved by altering the angle of incidence of the gas flow with respect to the walls of the fused silica tubing as shown in fig.2.11. In practice it was found that for a given set of operating conditions of the plasma, there was little difference between the results obtained with either torch. The radial flow torch, however, was found to give: a much shorter coolant tube life and with this torch the relative positions of the torch tubes and work coil were extremely critical. If the positioning was not correct, the plasma was very difficult to initiate and once initiated tended to be very unstable and quickly devitrified the coolant tube. In view of this it was decided to use the tangential flow torch with this system. Initially the injector flow carrying the sample was introduced via a B14 air-bleed with the end terminating ca 2 mm below the top of the inner fused silica tube. It was found with this configuration that the discharge tended to arc back onto the injector tip. This effect was overcome by increasing the distance between the top of the injector and the top of plasma tube to ca 20 mm. However, the sample introduction efficiency was decreased somewhat and it was found that upon prolonged operation of the 69

- Fig.2.11 Radial and Tangential Flow Plasma Torches;

TANGENTIAL B INLET I--

A inner silica tube B Outer silica tube C Torch body

D Plasma gas inlet ••' N Cooling gas inlet 70 injector, a quantity of solvent became trapped at the base of the torch so that memory effects began to be experienced. In the plasma torch used by Veillon and Margoshes [59], shown in fig.2.2, this was overcome by terminating the sample input level with the base of the tube holder. This was achieved with this system by machining a B14 cone in brass which terminated level with the base of the brass torch body. It was found that with this configuration that all solvent trapping within the torch was eliminated and that sample introduction was not markedly reduced compared to that obtained with the injector tip 20 mm below the top of the plasma tube. A much more important gain with this system was that the plasma discharge stability was much increased so that much better detection limits were attainable with this design although the atomic concentrations were slightly less than those attainable with the original system. The torch body itself is firmly held in a wooden cradle screwed to the torch holder which allows adjustment of the relative positions of the torch and work boil to be made, so that small adjustments to obtain the concentric arrangement are easily achieved. The torch holder is fitted to 71 a carriage which allows movement of the plasma torch and work coil as a whole along the vertical axis so that various portions of the plasma discharge may be selected for observation. 2.5 Sample introduction systems Solids, liquids and gases can all readily be introduced into the plasma discharge. Gases are easily introduced into the plasma and Truitt and Robinson [65,66] fully investigated the introduction of gases into the radiofrequency induction coupled plasma via the coolant, plasma and injector flows.• They found that the injector flow gave the better detection limits and that use of the coolant flow produced poor sample introduction into the discharge. The introduction of solids into the plasma is, however, more difficult because of the difficulty in obtaining a steady flow without segregation of the powder. The fluidized bed appears to be a very convenient method of ensuring a uniform powder flow provided that the particle size is kept fairly uniform. Varying degrees of sucess have been reported for methods for the introdUction of solids based upon this system [46,50,58,60,69]. Liquid samples are probably the easiest type of sample to handle. In the plasma, however, the solvent has a quenching effect and so most must 72 be'removed with low power plasmas. To this end, ultrasonic nebulization followed by desolvation has been widely employed [50,51,54,59,67]. In the original Radyne assembly used by Greenfield [47], an indirect nebulizer was used and large solvent droplets. removed in a large fog chamber. The latter system is much simpler to - manufacture and operate but does not possess the high.transfer efficiency of ultrasonic nebulization. The conventional Radyne solution introduction system uses a Unicam SP900 indirect nebulization unit. The aqueous solution is nebulized tangentially into a conical expansion chamber. The s=11 droplets are carried to the base of the plasma torch via a length of polythene tubing. It was found impossible in practice to reduce the length of this tubing to below 0.5 m. The sample introduction efficiency with this system was therefore very low and it was found that solvent droplets were depo6ited along the tube. The standard Techtron nebulization chamber used with the fuel inlet port sealed and the injector flow flowing through the oxidant inlet afforded a much better sample uptake rate, about 70 mm3s-1 as compared to about 13 mm3s-1 with the Unicam system. The sample transfer efficiency was increased by reducing the distance between the end of the 73 expansion chamber and base of the torch. It was still found that solvent tended to adhere to the sides of the polythene tubing, but this effect was greatly reduced. The system finally employed is shown in fig.2.12. The Techtron expansion chamber was fitted inside the standard burner holder which had been detached from the clamp used to position the unit on the optical bar of a Techtron AA4 flame spectrometer. This burner holder was then connected directly to a brass B14 cone with appropriate 0—ring seals and the cone sealed into the base of the plasma torch body. This system reduces the dead volume between the end of the expansion chamber and the base of the torch to an absolute minimum and allows efficient transfer of sample into the plasma. In operation it was found that the torch body became warmed by radiofrequency induction from the work coil so that any large droplets which have passed through the expansion chamber were prevented from condensing along the sample introduction system. A second advantage of this system is that the sample introduction system and the plasma torch are combined to form a single compact unit, which greatly improves the flexibility of the system. B Burner Holder C Brass B14 Cone E Expansion Chamber N Techtron Nebulizer P Plasma Torch Base Q Pressure Release Plug S Impact Bead T Drain Tube

41■41 P gip E sarftwommommunam matemonlam mulam )11411 141

T

Fig.2.12 Sample Introduction System used on the Radyne • Model H30/P Radiofrequency Induction Coupled Plasma. 75 2.6 Detection systems Greenfield [47,48,49] has used photographic recording of emission spectra from the plasma tailflame while Fassel [50,51,53,54] has used photoelectric recording. The disadvantage of the latter detection system is that the cables used to supply the photomultiplier with high tension current and the cable carrying the photomultiplier output current must be adequately protected from interference from the strong radiofrequency field produced by the generator. Spectrographic recording does not suffer from this disadvantage, but quantitative analysis with such a system,requires an internal standard and use of a densitometer. The monochromator used was a 0.5 m grating instrument [Varian Techtron, Melbourne, Australia] with a reciprocal linear dispersion of 3.3 nm mm .1 The monochromator was bolted onto the body of the radiofrequency generator and positioned so that the optical axis of the entrance slits passed through the vertical axis of the plasma torch body. The radiation from the tailflame of the plasma was collected by a 62.5 mm focal length fused silica convex lens and focussed onto the entrance slit of the monochromator. The E.H.T. to the photomultiplier, a type R213 [Hamamatsu T.V., Japan] was supplied from 76 the Techtron model AA4 amplifier—E.H.T. unit. This supplies a maximum E.H.T. of 800 V, which can be reduced in 25 V steps down to a minimum of 325 V. The signal from the amplifier was finally displayed on a chart recorder, a Servoscribe model RE511.20 [Smiths Industries Ltd., Wembley, Middlesex]. 2.7 Amplification of the analytical signal Before considering what form of amplification of the analytical signal is required, it is worth considering the form of the photomultiplier output. Marshall [70] investigated the output from a very high gain photomultiplier [2000 A L 1] ill the far ultraviolet using an electrodeless discharge lamp as.the source. He found that the signal consisted of discrete pulses rather than a continuous signal and that each pulse had a rise time which exceeded the bandpass of the amplifier used,. 4 MHz. Thus for very low light levels, a photon counting technique would be the most advantageous system to employ. Unfortunately photon counters are also frequency meters so that in practice if .a photon counter is operated in the vicinity of a radiofrequency field all that is measured is the frequency of the field, so that unless a Faraday cage is used to enclose the counter, all leads and the E.H.T supply to eliminate radiofrequency pickup,, the system cannot be used. 77 With a radiofrequency plasma system, however, it is a fair assumption that the light levels involved produce a pseudo-continuous current from a high gain photomultiplier [200 A1-1 ] as the temperature of the system and the concentration of emitting species is greatly increased compared to that from a microwave discharge. Sensitive D.C. amplifiers have been used for the amplification of photomultiplier outputs obtained when plasma tailflames have been viewed [50,51,54]. The use of a battery operated microammeter [model WV84C, R.C.A.Ltd., U.S.A.] was investigated for the amplification of the analytical signal. The current measured was typically less than ca 0.1 p,A. As the amplifier was battery powered a long term drift was observed in the baseline. This could have been overcome by the use of a transformer- rectifier and the A.C. mains. A second and more important disadvantage of this system was that both high and low frequency noise was detected on the analytical signal. The only method of damping the high frequency noise was to use a large capacitor across the recorder terminals to suppress this noise. Unfortunately this caused no reduction in the low frequency noise which must be screened electronically and increased the response time of the system to such an extent that ca two minutes was required to 78 obtain a full scale deflection. The use of an a.c. amplifier requires that the emission signal from the plasma tailflame be either mechanically or electrically modulated. A chopper operating at 285 Hz was used and the resultant a.c. signal was measured at a small bandwidth around 285 Hz. Veillon and Margoshes [59] and Boumans and De Boer [67] have both successfully used phase sensitive amplification of the emission signal modulated by a chopper. The use of the lock-in amplifier from the Techtron model AA4 spectrophotometer used in conjunction with the 285 Hz chopper was investigated. This system produced a substantial improvement in the reduction of noise on the analytical signal, but the amplifier has a limited gain; the final analytical signal was 10 mV for a full scale deflection. As a result of the low gain of the amplifier at wavelengths below 200 nm it was necessary to employ large slit widths to increase the amplitude of the analytical signal. The use of large slit widths, i.e. large spectral band-pass, however, drastically reduced the signal to background ratio and greatly increased the probability of spectral overlap with lines of similar wavelength. .In view of the' improvement in the noise reduction obtained with a.c. amplification, it was 79 decided to investigate a more sensitive phase sensitive amplification assembly, the AIM System 5 [AIM Electronics Ltd., St.Ives, Hunts.]. This amplifier system consists of five separate components; the power supply unit, low noise amplifier, wide band amplifier, tuned filter oscillator and phase sensitive detector. The power supply unit, PSU101, is essentially a mains stabilizer and rectifier which supplies power to the other modules in the system. 2.7.1 Low noise amplifier LNA133 This amplifier consists of a low noise transistor pre-amplifier of fixed gain followed by a potentiometer and switched attenuator feeding into a further low noise transistor, fig.2.13. This design ensures that at any attenuator setting the overall noise within the amplifier is determined solely by the first transistor. If the source produces a noise which can be expressed entirely as a current fluctuation as in the case of a photomultiplier, then it is desirable to use- the amplifier with a high resistance source which leads to the minimum equivalent noise current into the amplifier terminals. The reason for this is that although the noise figure, i.e. the factor by which the apparent noise at the input exceeds the theoretical noise at' the input, is increasing INPUT O A Operational Amplifier T Low Noise Transistor V Variable Attenuator

OUTPUT C)

Pig.2.13 Low Noise Amplifier Circuit — Basic Principles.

81 withresistance, the numerator expressed as a current is constant while the denominator expressed as a current is decreasing with resistance. To achieve this effect, a source resistance of 106 ohm was used. The low noise amplifier has a wide bandwidth, 1 Hz- 200 kHz, and has a maximum overall gain of 100-fold, 40 dB, so that its function may be described as a pre-amplificat:Lon step with a minimum of noise. 2.7.2 Wide band amplifier ACA123 This module is designed as a high gain wideband amplifier which amplifies signals by a factor of 30-1000 without phase shift over a wide bandwidth, 1 Hz-1 MHz. Each of the amplifiers labelled A in fig.2.14 consists of_a three stage d.c. coupled transistor amplifier. The d.c. conditions are defined by the d.c. feedback path, R2, while the a.c. gain is defined by the a.c.feedback resistors, R1,R2,R3, chosen to give a nominal thirty-fold voltage gain. The gain control of the attenuator is placed between the amplifiers so that noise within the system is minimized. 2.7.3 Tuned filter and oscillator TF0129 This module.when used as a filter accepts an a.c. signal over a very narrow bandwidth around the tuned frequency while rejecting both high and low frequency noise. This unit greatly increases R2 P2

INPUT RI 0

ATTENUATOR •

2Pig.2.14 Wide Band Amplifier Circuit — Basic Principles: 83 the signal to noise ratio of the analytical signal, especially if the analytical signal is buried in noise. In the plasma system employed, high frequency noise is to be expected from radiofrequency pickup; a mains ripple is to be expected on the E.H.T. supply; 100 Hz ripple is to be expected on the emission signal from the plasma because of the half-wave rectification employed within the generator power unit. Low. frequency noise is to be expected from fluctuations in the gas flow rates. All of these noise components are almost eliminated when the filter unit is accurately tuned to the frequency of modulation. The circuit input stage, fig.2.15, provides isolation between the input terminal and the analogue simulation, sets the d.c. conditions for the circuit and causes a phase inversion before the signal is applied to the remainder of the circuit. The positive feedback connection, MI passes a proportion of the output back in phase with the input, and, since the basic circuit has near unity gain, no oscillation occurs, but at the resonant frequency the gain is increased. The resonant frequency of the simulated tuned circuit is adjusted by varying the time constants, t1 and t2' which are nominally equal. The bandwidth of the filter is via the, variable factor, M, controlled INPUT OUTPUT STAGE STAGE

X2

Fig.2.15 Tuned Filter Oscillator — Basic Principles. 85 by the Q control, which sets the overall gain at the resonance frequency and thus the Q of the circuit. Q is defined as the ratio of the centre frequency to the fre.quency spread of the bandwidth. From fig.2.15 and assuming unity gain for the input and output blocks, it can be shown that the input and output voltages are connected by equation 2.2 [71]. Vo s[ti+t2] [2.2] 7 l+s[1-m][t1+t2]+s2t1t2 At a steady state frequency it is possible to write :- S = jW [2.3] and the resonance frequency is- given by :- 2 w t1t2 = 1 [2.4] Combination of equations 2.2-2.4 produces :- V o 1 [2.5] = 17/ The Q of the resonance circuit can be shown to be :- n 1 = 717E7 [2.6] Thus numerically, the circuit Q is equal to half the gain at resonance and is constant for all frequencie, as is the gain which is also independent of the time constants t1 and t2. These two facts mean that theoretically the choice of modulation frequency of the signal is 86 not governed by he filter but by the amplifier. As we have already seen, however, the noise in the photomultiplier signal greatly exceeds the amplifier noise so the choice of modulation frequency is governed only by the noise components on the analytical signal. Using the 285 Hz modulator and a Q of 10, then the bandwidth selected by the unit is 285 + 14 Hz with a gain at resonance of 20. The value of M under these conditions is 0.95 and the time constants, t1 and t2, about 1.8 ms.

- 2.7.4 Phase sensitive detector PSD122A When used in conjunction with the tuned filter, the phase sensitive detector can be operated in either of two modes. In the'total mode, it acts as a rectifier for the tuned filter output and converts the d.c. signal into a continuous d.c. current, which, because of the finite band-pass of the tuned filter, is noisier than the in-phase mode which theoretically has an infinitely small bandwidth. The outputs from the signal phase splitter, fig.2.16, are fed into two gates formed by pairs of high speed diodes. Each pair of diodes is biased alternately on and off by the squared reference signal so that the two pairs act as two-way switches in antiphase. The two switched inputs are then summed to give a full-wave output which is then passed into a low-pass filter, a network of fixed resistor and switched TIME CONSTANT INPUT PHASE ••••■■•••••• SPLITTER GATE SUMMING STAGE FILTER GATE REFERENCE PHASE 0 SPLITTER -I-SQUARER D.C. AMPUFER OUTPUT METER

Fig.2.16 Phase Sensitive Detector — Basic Principles. 88 capacitors. The filtered signal is then amplified by a high gain d.c. linear operational amplifier and displayed on a meter connected across the output. In the total mode the reference pulses are derived directly from the:input signal itself. Mathematically the operation performed by the phase sensitive detector corresponds to multiplying the signal input voltage by a square wave, i.e. by a function F1[t] given by equation 2.7.

_F1[t] = sinwt+1/3sin3wt+1/5sin5wt+.... [2.7] If the voltage of the signal is VsF2[t], the output may be written in the form :- t V1V2 = Vs1F2dt [2.8]

Thus the phase sensitive detector performs a Fourier analysis of F2[t] and extracts the component corresponding to sinwt and-its odd harmonics. The phase sensitive detector can be used without the tuned filter, but it was found that in this mode of operation the analytical signals were much noisier than when the tuned filter was used. 2.7.5 The reference signal The reference signal is obtained from the Techtron chopper unit. The Techtron amplifier takes a reference signal from the photocell within the chopper assembly and uses the square wave so produced 89 as a trigger signal with a peak to peak voltage of 28 V. The phase sensitive detector will only accept a reference signal with peak to peak voltage of less than 5 V and prefers a sine wave as the reference. The trigger signal was therefore converted to a sine wave and attenuated by the circuit shown in fig.2.17. The variable resistor in the output circuit provides further attenuation so that the reference signal may be used to tune the filter. 2.7.6 Tests with the phase sensitive amplifier The AIM amplifier system was found to provide a marked improvement in both sensitivity and signal to noise performance compared to the other amplification systems used. The very much higher gain of this amplifier meant that the slit width could be reduced to 25 pm and so the signal to background ratios were vastly improved. It was found, however, that ocassional spurious peaks were obtained with this system. After consideration of this problem it was felt that these pulses were mains borne and were the result of some electrical appliance on the same circuit being switched on or off. When the mains input to the amplifier and E.H.T. supply was filtered through an R.F. mains filter [Model L1829, Belling and Lee Ltd.,Enfield, Middlesex] these pulses were found to be eliminated.

Fig.2.17 circuit to obtain reference signal for phase •sensitive amplifier from Techtron trigger unit. C RI R2 INPUT 0 I I L I

C Capacitor; 0.141? R1 Resistor, 1000 ohm R2 Resistor, 4700 ohm R3 Resistor, 0-1000 ohm

R3 OUTPUT 0 CHAPTER 3

An Investigation of Some of the Operating Parameters of the Induction Coupled Radiofrequency Plasma.

91 3.1 General In optimizing an analytiCal system for atomic emission spectroscopy the most important practical factors that must be investigated are the - magnitude of the analytical signal, the signal to noise ratio and the signal to background ratio. These factors are dependent upon the stability of the source, the atomic concentration in the source and the temperature of the source. -In the induction coupled radiofrequency plasma there are five main operating parameters which can be varied. These are the flow rates of the coolant, plasma and injector gases, the power supplied to the work coil and the height of observation of the emission signal in the plasma tailflame. Each of these operating parameters was investigated with respect to the effects they produced on the temperature of the plasma, the magnitude of the analytical signal, the noise on the analytical signal and the background emission of the plasma. 3.2 Methods for calculation of plasma temperature Methods for the calculation of flame temperatures are well documentate'd by Gaydon [72]. It is clear that for two lines of the same element that equation 1.8 reduces to the form s— I X g.A. Ik k _2_2 [3.1] = X7 gkAk e"2-1 92 In choosing a suitable line pair care must be taken to minimize the effects of self-absorption, and a list of suitable line pairs in the spectrum of iron and the temperature range over which they may be used has been derived [73]. The temperature range over which these line pairs may be used is, however, very limited and the main application of this method appears - to be for the calculation of flame temperature [73,74]. In the method of Corliss and Bozman [75], which they used for the calculation of the temperature of the D.C. arc, equation 1.8 was used in its logarithmic form.

= ln[Nhc/u] - ln[IkX/gkAk] + 20.72 [3.2]

If the intensities of several lines in the emission spectrum of the same species are measured, then a straight line graph of E—against ln[IkX/gkAk] will be obtained with a slope of -1/kT. The use of several lines only improves the precision of the method compared to the two line method provided the transition probabilities are known with sufficient accuracy. Both of these methods suffer from the same limitation. The rate of change of the intensity of a spectral line with temperature is given by dIk EkIk dT [3.3] 93 Thus, the higher the temperature of the system, then the less accurately the temperature can be measured and so neither of these methods is ideally suited to the determination of plasma temperatures. Greenfield [47] used a method for the determination of the plasma temperature based upon use bf the Saha equation for ionization and using data given by Ahrens [76]. The temperature of the has also been determined by this method [77]. _This method of temperature calculation is very much more accurate than the use of the two-line method at high temperatures when the degree of ionization of elements in the tailflame can be readily calculated or measured. The data used by Greenfield is not very accurate; more accurate data was used in this calculation [78]. The Saha equation may be written in terms of the densities of the atoms, ions and electrons in the system as equation 3.4. N N .z 5040V4 - = '-u"+ 21.49+ log u e [3.4]

The electron density of an arc or flame can be measured using a Langmuir probe [77]. This is essentially a metal electrode which is charged to a high negative potential and placed in the arc to collect cations. The current density of the arc 94 can be'phown to be given by :-

J = eNe[kT/2nmr2exp[AE/kT] [3.5] where AB is the difference in energy between electrons on the probe and in the arc. From a plot of J against electrode potential it is possible to calculate the

electron density, Ne/ from the intercept on the potential axis. In a plasma, however, this technique 'cannot be used as the probe will be immediately vaporized. The electron density may also be calculated from the Stark broadening of the Ha or Ho lines in the Balmer series. This will be discussed in greater detail in a later chapter. With the monochromator used here precise measurements of the Stark broadening were not possible as the line at 486.13 nm has a Stark half-width of ca 0.05 nm, which is comparable to the instrumental band-pass. If it is assumed that the plasma is in local thermodynamic equilibrium, DEB, then the electron density caused by the ionization of argon can be calculated from the Saha equation. The densities of argon ions, atoms and electrons can be expressed in terms of the degree of ionization of argon. N N 2 + e cc [3.6] No 95 Substitution of equation 3.6 into 3.4 and assuming the plasma is at atmospheric pressure gives:- 2 5040Vion la -_ ylogT5 6.19 +logE;+ t.3.7Jr 1- The electron density of an argon plasma can be calculated at any temperature from the value of a and the Loschmidt number, LN.

Ke = 273aLN/T [3.8] A plot of electron density against temperature is shown in fig.3.1 and from this it can be seen that the electron density does not vary greatly at the temperatures of the plasma. If any species is added to the plasma, then the high electron density is likely to suppress its ionization. If equation 1.8 is applied to the atom and ion lines of an added -species, then :- N.+ E0-E+ exp-0-- [3.9] U- -IugAXo o + + o Equations 3.4 and 3.9 can then be combined for the added species and upon rearrangement, equations 3.10 and 3.11 are obtained where F[I] = F[T]. I,X+ . g F[I] = log1' + log-=T=°° °° + logNe -27.19 [3.10] o o g+11+

V'1on5940 E+-E° F[T] = 275E; [3.11] Pig.3,1 Variation of the Electron Density of• an Argon Plasma With Temperature.

2)

20-

TEMPERATURE, x fd°K. • 97 In order to calculate the temperature of the plasma, it is necessary to measure the intensity ratio of an atom and ion line of an added species. A first approximation is made by assuming that the electron density in the plasma is 1022 m-3 and a value of F[I] obtained. An approximate temperature is obtained by computing-the various values of F[T] And the temperatUre at which F[T] = F[I] is thus the' approximate temperature. This temperature is then used to calculate the electron density of argon from. which another value of F[I] is obtained. This proceedure is then repeated until consistent temperature values are obtained. , 3.3 . Effect of the injector_gas flow The injector gas flow- was operated with argon at a pressure of 30 p.s.i.g. The injector flow was varied up to a maximum attainable gas flow of 100 cm3s-1, while the plasma flow was maintained at 70 cm3s-1 and the coolant flow at 300 cm3s-1. The emission intensity was viewed 20 mm and 34 mm above the top of the work coil and the high tension current Bet to 0.9 A when the injector gas flow was 50 cm3s-1 3.3.1 Effect of injector gas flow upon plasma temperature Calcium and zinc were used to investigate the effect of the injector gas flow upon the 98 temperature of the plasma. The constants for the solution of equations 3.10 and 3.11 are known fairly accurately for these elements and there appeared to be no self-absorption effects at metal concentrations of ca 10-20 g.ml The values used for the calculation of the plasma temperature are given in table 3.1, together with their origin. The variation of temperature of the plasma tailflame with the injector gas flow is shown in fig.3.2 for heights above the work coil of 20 mm [A] and 34 mm [B]. The temperatures calculated from the observed intensities of the zinc atom and ion pair are shown as triangles, while those obtained for calcium are shown as circles. At a height of 20 mm above the work coil, the injector flow appears to have little effect upon the plasma temperature, while at 34 mm an increase in the injector gas flow, and hence sample introduction rate, causes a drop in the temperature of the tailflame. This latter effect is explained by the fact that the solvent, water, produces a significant concentration of molecular gases in the plasma causing an impedence mismatch between the plasma and the . This has the effect of reducing the applied power to the plasma and hence causes a reduction of plasma temperature. Table 3.1 Constants for zinc and calciuM used in solving the Saha equation.

Species X, nm. gA x108, s-1 Ek, K. u Vion, V. Zn I 213.86 19 46745 1.0 9.39 Zn II 206.19 92 48481 2.0

Ca I• 422.67 6.54 23652 1.0 6.11 Ca II 393.37 6.00 25414\ 2.0

Values for Zn I and Zn II taken from reference 75. Values for Ca I and Ca II taken from reference 79. Values of partition functions and ionization potentials taken from reference 78. 100 Fig.3.2 Variation of the plasma temperature with the injector gas flow rate at 20 mm [A] and 34 mm [B] above the top of the work coil..

.8000-

0

as 7500 - CO

p r13 GPS F. I CI VI a L. ) LY tilt 40 101 At the lower height, the emission is viewed at the tip of the plasma core and is therefore affected by the close proximity of the doughnut which does not allow sample to enter within it. This means that in effect the doughnut is relatively unaffected by the introduction of molecular species into the discharge ' and so the tailflame in contact with this region will not be cooled to the same extent as the upper regions of the tailflame.

3.3.2 Effect of injector gas flow upon plasma background The background emission of the plasma was viewed at 214 nm at both 20 mm and 34 mm above the top of the work coil. Fig.3.3 shows the variation of background emission observed at both these heights. If the background emission was dependent only upon the temperature, the background might be calculated from the black body equation. The observed backgrounds, however, show no resemblence to this curve and only approach the theoretical Brem$'strahlung and Cyclotron emission curves. This implies that there must be some other effect not taken into account. The surface area of the plasma core is governed by the skin—depth effect. The depth that the induced current flows,8, can be shown to depend 102

Fig.3.3 Variation of the background emission intensity' at 214 nm with the injector gas flow rate at A 20 mm above the top of the work coil and B 34 mm above the top of the work coil. .

25 50 • 75 GA3 FLOW NATE., C11.13 . 103 upon the electron mobility, be the frequency of the plasma and the electron density.[45,53,67].

2 _ 1.6x10-22 —bevNe [3.12]

It is a well documented fact [67] that the frequency of the plasma is altered by the effect of gases in • the discharge. Using a digital frequency meter [Model 7737A, AMF Verner Ltd., Surrey] to monitor the radiofrequency, it was found the frequency varied with injector gas flow as shown in fig.3.4. It was observed that the frequency increased by about 0.3 when water was injected into the plasma rather than air. Thus the surface area of the plasma is altered by the introduction of solvent and if this is taken into account then the theoretical Bremostrahlung emission curve is a very close fit to the observed background. As the electrons are produced primarily by the ionization of argon in the plasma core, then the temperature of the system affects the electron density, which in turn affects the skin depth. Thus both the temperature of the plasma and frequency of the plasma produce a secondary effect upon the background emission. 3.3.3 Effect of injector gas flow upon analytical signal As might be expected, an increase in the 104

Fig.3.4 Variationof the mdiofrequency of the plasma with the injector gas flow rate. 35-56-

25 SO - 75 GAS FLOW RATE, 033:1 105 injector gas flow increases the sample uptake rate and hence the efficiency of the nebulizer until a maximum sample transport occurs [80). The effect of the injector flow upon the analytical signal obtained for zinc at 213.86 nm at a height of 20 mm above the top of the work coil is shown in fig.3.5. At a height ' of 34 mm above the top of the work coil, a similar curve is obtained. The variation of analytical signals obtained from Zn II and Ca II are also similar and all these curves pass through a maxima about an injector flow of 50 cm3- s-1 3.3.4 Effect of injector gas flow upon plasma stability The actual electronic noise within the detection system and the plasma, and the noise caused by fluctuations in the gas flows, are all very much smaller than the noise within the sample introduction system. At very low gas flow rates, i.e. less than 20 cm3s-1, the fluctuations in the sample uptake rate are very large and the system is very noisy. As the flow rate is increased so the noise drops until at very high gas flows, the noise within the system again rises due the large concentration of molecular species within the discharge which tend to quench the plasma. This instability can be seen visually at high gas flows as a flicker in the plasma discharge which is compressed into a very small volume, at flows 0

• 106

Fig.3.5 Variation of the emission intensity observed at 213.86 nm from a solution containing 10 g.m-3 of zinc as aqueous zinc sulphate with the injector. gas flow rate. .

CD Cf.)

w

25 50 75

t.:41 1-0 cir..13 3:4 107 above 120 cm3s-1, the plasma is extinguished. Fig.3.6 shows the peak to peak noise observed on the analtyical signal obtained for a solution containing 10 g.m 3 of zinc at 213.86 nm. 3.3.5 Summary of the effects of the injector gas flow The injector flow rate has a minimal effect on the temperature of the plasma at the tip of the plasma core and even high in the plasma tailflame, the temperature reduction is only about 1000 °K for an increase in the injector gas flow from 25 cm3s-1 to 100 cm3s-1 This variation in injector gas flow, in fact, expresses the limits of the range over which any meaningful analytical signals can be observed. The background of the plasma decreases fairly uniformly with increase in the injector gas flow after a small initial increase. At flows above ca .50 cm3s-1 the variation in background is minimal as is the change in the frequency of the applied current. The analytical signals for all four species; Zn I, Zn II, Ca I and Ca II, investigated are maximized at an injector flow of 50 cm3s-1 which also corresponds to the minimum noise on the analytical signal, i.e. the optimum signals noise ratio. 108

Variation .of Peak to Peak Noise on the Analytical Signal obtained from a solution containing 10 g.m-3 of zinc as aqueous zinc sulphate at 213.86nm with the injector gas flow rate.-

CD

ems c4 w

25 50 75 • 100 GAS RCM RLTE, CI'v13 64 109 The optimum gas flow through the injector was chosen to be 50 cm3s-1 as it represents the gas flow which gives the best detection limit for all four species investigated and also corresponds to a maximum in the temperature of the plasma at a height of 20 mm above the top of the work coil. A further advantage of this gas flow is that it easily reset in practice as it corresponds to the maximum attainable analytical signal. 3.4 _Effect of plasma gas flow The plasma gas flow was operated with argon at a pressure of 15 p.s.i.g. The coolant flow rate was set to 300 cm3s-1 while the injector flow was set to 50 cm3s-1 This injector gas flow gave an aqueous sample uptake rate of 70 mm3 s—1 The height of observation was fixed at 20 mm above the top of the work coil and the plasma gas flow varied continuously up to a maximum of 120 cm3s-1. The high tension current was set to'0.95 A with the plasm'a gas flow switched off. 3.4.1 Effect of plasma gas flow upon plasma temperature ' The variation of plasma temperature with plasma gas flow rate is shown in fig.3.7. The temperature decreases fairly uniformly with increasing plasma gas flow and hence the particle density of argon • 110

Fig.3.7 Variation of Plasma Temperature with the Plasma Gas Flow Rate.

8250-

7750-

50

,r r G ,1 P3 Fn1t. fi 4.3

111 within the discharge. Unlike the situation observed for the injector gas flow, there is no buffering of the temperature change induced in the tailflame by the doughnut core since the plasma gas is responsible for the formation of the plasma core. In the system with no plasma gas flowing, a small variation in the injector gas flow produces a noticeable change in the plasma temperature whereas in the system with a large plasma gas flow, then a small variation in the injector gas flow produces no appreciable temperature change at a height of 20 mm above the work coil. This suggeSts that if the density of argon atoms in the plasma is greatly altered the effect is to alter the temperature gradient above the top of the core. This can be seen visually as a contraction of the plasma core at increased plasma gas flow rates.' These temperature observations support the hypothesis stated earlier that the plasma discharge physically stops polyatomic species from entering the discharge while monatomic gases such as argon can freely enter the discharge where they produce a cooling effect on the plasma temperature. 3.4.2 Effect of plasma gas flow upon plasma background The effect of plasma gas flow upon the background emission of the plasma observed at a height of 20 mm above the top of the work coil is shown in

112 . . . fig.3.8. The background increases slightly with increased plasma gas flow which is in direct opposition to the observed temperature variation. With very low gas flow rates the background again rises to a maximum when the plasma flow is switched off. Fig.3.9 shows the variation in the frequency of the applied radiofrequency current when the plasma gas flow is altered. The actual variation of the background appears to be a combination of the temperature and skin depth effects. 3.4.3 Effect of plasma gas flow upon analytical signal The effect of the plasma gas flow upon the analytical signal obtained for zinc at 213.86 nm is shown in fig.3.10. The general variation in this signal and those obtained for Zn II, Ca I and Ca II is similar to that predicted from the temperature variation and dilution of the atomic concentration produced at higher plasma gas flows. 3.4.4 Effect of plasma gas flow upon plasma stability The effect of the plasma gas flow upon the noise on the analytical signal observed for zinc at 213.86 nm is shown in fig.3.11. The noise is minimal at a flow of about 8 cm3s-1 and rises to a maximum at 100 cm3s-1 above which the plasma can be seen visually to flicker and extinguish upon prolonged running. 113 g. 3 . 8 Variation of BaCkground Emission Intensity at 214 nm with the Plasma Gas Flow Rate.

5

sb Ibo 3 -1 GAG CF.11 S. a 114 Fig:3.9 Variation of the radiofrequency of the plasma generator with the plasma gas flow rate.

50 100 r r3 - CAS FLO V E S. 115

Fig.5.10 Variation of the emission intensity observed

at 213.86 nm from a, solution_ containing 10 g.m-3

of zinc as aqueous zinc sulphate with the

plasma gas flow rate. 2

50 100

rir t13 e 11.0!.7 u 'J. Fig.3.11 Variationofthepeaktonoiseobserved116

PEAKTO PEA( 2 flow rate. from asolutioncontaining10g.m as aqueouszincsulphatewiththeplasmagas on theemission.signalobtainedat213.86.nm GAS FICIV RATE, 50

-3 ofzinc or:1 3 100 3. 4 117 the effedts-of the 'plasma gas:flow The plasma gas flow rate greatly affects the temperature of the plasma and the analytical signal, so that both of these factors are optimized without the plasma gas. Minimum noise and background are obtained at gas flows of 8 cm3s-1 and. 25 cm3s-1 respectively. In operation, the plasma gas was not used, although this did not quite conincide with the. optimum signal: noise ratio, which occurs at ca 10 cm3 s-1 . However, reproducibility of this low gas flow rate is not very good so that the long term stability of the plasma from a day to day basis was inadequate. 3.5 Effect of coolant gas flow The coolant gas flow was operated with argon at 15 p.s.i.g. The injector gas flow was set to 50 cm3s-1 with a sample uptake rate of 70 mm3s-1 while the plasma gas was not used. The high tension current was set to 0.9 A at a coolant gas flow of 250 cm3 s—1 . The coolant gas, as its name implies, is primarily a coolant but it does contribute a very small proportion to the plasma discharge. It was not possible, however, to vary the coolant gas flow. outside the range 200-325 cm3s-1 as a flow rate below 118 this caused severe overheating of the 'silica torch walls and the use of excessively high coolant gas flow rates tended to extinguish the plasma. The coolant gas flow has little effect upon the temperature of the plasma at either 20 mm or 34 mm above the top of the work coil, although h slight temperature decrease with increased coolant gas flow was observed at the higher observation position. The analytical signal and background are both relatively unaffected by the .coolant gas flow .over much of the range although they both slightly decrease with increased gas flow rate. The stability of the analytical signal was found to be relatively unaffected by the -coolant -gas-gas flow rate, although the noise and, to a certain 'extent, the background were found to both increase with a low coolant gas •• flow which could be attributed to the direct attack of the plasma upon the silica torch walls. At very high coolant gas flows, the noise was also found to increase as the stability of the plasma was decreased and could be seen to flicker. Fig.3.12 shows:the effect of the coolant. gas flow rate upon the signal: noise and signal:background ratios obtained for an aqueous solution of zinc at 213.86 nm. The optimum coolant gas flow rate can be seen to be ca 250 cm3s-1 119

Fig.3.l2 Variation of signal to noiEe [A] and signal to background [B] ratios observed at 213.86 nm from a solution containing 10 g.m 3 of zinc as aqueous zinc sulphate with the coolant gas flow rate.

GAc. cr GP! FNA .1" 3 -4 LzhtEt j s. 120 3.6 • Effect of height of observation With the optimum gas flow rates set to 250 cm3s-1 for the coolant and 50 cm3s-1 for the injector with a sample uptake rate of 70 mm3s-1 and a high tension current of 0.9 A, the height of observation above the top of the work coil was varied. 3.6.1 Variation of •lasma temperature with hei •ht above the top of the work coil The variation of plasma temperature with height of.observation above the work coil is shown in fig.3.13. The greater the height of observation, the lower the temperature of the plasma and it is noticeable that near the core, i.e. at heights below 25 mm above the top of the work coil, the temperature is fairly constant.

3.6.2 Variation of plasma background' with height above the top of the work coil The variation of background emission at 214 nm with height of observation above the work coil is shown in fig.3.14. The temperature falls off rapidly with height and over the region 20-40 mm _follows closely the predicted curve from the temperature distribution. Above 40 mm, the fall is not as predicted and this is probably the result of air entrainment by the tailflame well above the constriction caused by the torch walls which- causes - 121

••

Fig. 3.13 Variation of the Plasma Temperature with Height Above the Top of the Work Coil.

30 4:0 50 il,F3f3VE TOP OF 1g702:.:0111., 122

Fig.3.14 Variaticn:=of the background emission intensity observed at 214 nm with height of observation above the top Of the work coil.

Ey, iv r) ciJa s 11113617:. TC,'P C7 Vjgrjal COM, MM.

123 a slight expansion in the volume of the tailflame: 3.6.3 Effect of height of observation upon the analytical signal Fig.3.15 shows the variation of analytical signal obtained for zinc at 213.86 nm. The- analytical signal is maximized low in the tailflame, except for species which are readily ionized-,such as calcium, where the effect of ionization at the high temperature near the coil is observed. Fig.3.16 shows the noise on the signal obtained from a solution containing 10 g.m-3 of zinc at 213.86 inn. 3.6.4 Summary of the effect of height of observation For elements which are difficult to ionize and excite, then the optimum.height of observation to achieve maximum signal: noise ratio appears to be as low as possible in the tailflame without actually looking into the core itself. At a height of 20 mm above the top of the work coil, the temperature of the plasma is maximized as is the magnitude of the analytical signal obtained from high energy states are encountered in the vacuum •ultraviolet. This height is, however, not the optimum . for elements which are readily ionized such as calcium. For these elements, slightly higher heights of observation are required. Both Passel [53] and Boumans [67] have commented on this effect and I .'.

, Fig.3.15 Variation o£ the emission intensity obsezyed at 6 213.86 nm £rom a solution containing lO g.m-; of

~...iJ zinc as aqueous zinc sulphate "lith height of ...... ~.<&II observation above the top of the work coil • 11-;:- ~.,. t.l.:LaI·

~~~ ~" /..;..:.-.... a~ C~ E·· ... r:J:.lq l"'''..... t:::~ I .. c.:.;! ~ \ ~"" I·.., on,"" 3- \. 0) .... "'.~ ,~ ... ~ U..3 ~.","," ...... ;v • IOll:.I-

-:-T" It.._.. CL:·.~.- ..... ~ C/J O'} ...... 1%'-'" _I ~.-..... t..!J

30 40 50 125

Fig.5.16 Variation of the peak to peak noise observed on the emission signal obtained at 213.86 nm from a solution containing 10 g.m 3 of zinc as aqueous zinc sulphate with height of observation of the signal above the top of the work coil..

30 40 sb Lanva TOP Vi0,11HCC.ML, 126 have investigated the various regions of the plasma tailfiame to obtain the best signal:noise and signal: background ratios for a wide selection of elements of differing spectrochemical properties. 3.7 Effect of radiofrequency power The gas flow rates were set to the optimum values determined and the height of observation fixed at 20 mm above the top of the work coil. Power to the work coil was altered via the tuning capacitor, 06 in fig.2.9, and Honitored by the grid current and high tension current ammeters.

3.7.1 Effect of radiofrequency power upon plasma temperature Fig.3.17 shows the _effect of the high tension current upon the temperature of the plasma. The high tension current increases approximately directly with the power dissipated in the coil and hence the plasma temperature. The grid current decreases with an increase in high tension current, but the variation is only between 0.37 A and 0.43 A for a high tension current range of 1.10-0.70 A. In view of this it was felt that the high tension current was a more useful monitor of the applied power than the grid current. At a high tension current of less than 0.85 A the plasma can be seen visually to contract in volume and at a current of ca 0.66 A, the plasma is extinguished. 127 Fig.3.17 Variation of the plasma temperature with the high tension current. -

8250

8000

775

0:8 1:0 11 C E NT, ft

128 3.7.2 . Effect of radiofrequency power upon plasma background The effect of radiofrequency power upon the observed background emission at 214 nm is shown in fig.3.18. The shape of the background curve resembles that observed for the temperature and the. observed volume of the core, which at a high tension current of 0.7 A is ca 20 j of that observed at full power. 3.7.3 Effect of radiofrequency power upon the analytical signal The variation in the analtyical signals observed for zinc and calcium at 213.86 nm and 422.67 nm respectively with high tension current are shown in fig.3.19. At high power, the temperature is maximized as is the analytical signal from the high energy states of Zn I, but for elements which ionize to a greater extent, such as Ca I, then the analytical signal is maximized at less than full power. 3.7.4 Effect of radiofrequency power upon the plasma stability The effect of the high tension current on the noise observed on the analytical signal obtained for zinc at 213.86 nm is shown in fig.3.20.. It can be seen from this that the noise passes through a minimum between high tension currents of 0.80 A and 0.95 A. At low power levels, there is insufficient pic.3.18 Variation of background emission intensity observed' at 224 nm with high tension current.

3

1-L.T. CURRENT, - A. Fig.3.19 Variation of the emission intensities observed at 213.86 nm and 422.67 nm. from solutions containing 10 g.m.-3 of zinc and calcium. respectively with the high tension current. • ZnI

Cad'

2

7 0.8 1.0 ELT. CUR[rilT/ A • 131 Fig.3.2P Variation of the peak to peak noise observed on the emi2gion signal obtained at 213.86 nm from a solution containing 10 g.m.13 of zinc with high tension current.

ti

•

•

0.8 1.0 H IT., t.;27.EI:T. 132 power supplied to the plasma to overcome the mismatch in impedence caused by the solvent. Once this threshold is passed, the noise decreases rapidly to a minimum and remains fairly constant until at high power the plasma expands to such a volume' that attack upon the silica walls begins to occur; the noise then increases as silica becomes volatilized. 3.7.5 Summary of the effects of radiofrequency power -The optimum signal: noise ratio is observed for Zn I at a high tension current of ca 0.95 A and a grid current of 0.40 A. For the other species considered, Ca I, Ca II, and Zn II, the same power was also found to be optimum, although the maximum attainable signal: noise ratios for all four species could be distinguished by careful tuning of the power capacitor. This power setting, however, does not correspond to the maximum signal:background ratio, temperature or analytical signal but does yield the best detection limit. 3.8 Comparison of temperature calculation methods As stated earlier, the two—line method of temperature calculation is not very accurate unless the energy separation between the excited states and the energy of the excited states is fairly large. However, if these requirements are fulfilled, then the two lines are usually separated by such a large wavelength that the photomultiplier response is 133 sufficiently different at each line to require consideration in the precise measurement of the intensity ratio of the lines. The two line method was applied to a selection of elements which have reasonably high energies of the excited state and the results obtained are shown in table 3.2. These observations were made under the optimum conditions determined for Zia I, i.e. an injector gas flow of 50 cm3s-1 with a sample uptake rate of 70 mm3s-1, a coolant gas flow of 250 cm3s-1, a height of observation of 20 mm above the top of the work coil and a high tension current of 0.95 A. These operating conditions gave a plasma temperature of 8250 °X using the Saha equation to- calculate the temperature from the measured ratio of the intensity of the ion and atom lines of zinc and calcium with a preciSion of ca + 30 °K. As can be seen from this table, reasonable agreement is obtained where the separation of the upper states is large, the precision also being reasonable. With low excitation energies and a small energy separation, however, the precision is very poor and the temperature uncertain. This is best illustrated by the two temperatures calculated from the Or T lines where the ratio measurement only differs by less than 2 X, yet the temperatures differ Table 3.2 Plasma temperature calculated from the two-line method. 8 -1 . Species X, nm. gA x10 18 Ek, K. Intensity Temperature, °K. 204.38 9.0 70842 46 Cu II 8000 + 200 213.60 14 68731 100 202.55 • 21 49354 20 Zn II 8350 + 300 . 206.19 92 48481 100 393.37 6.00 ' 25414 99 Ca II 7150 + 1650 396.85 . 2.92 25192 50 , 365.02 64 . \ 71431 41 Hg I 8250 + 50 404.66 36 62350 100 357.87 8.3 27935 100 Cr I 8250 + 4450 359.35 7.0 27820 85 357.87 8.3 27935 100 Cr I 5650 + 2000 359.35 7.0 27820 86 •135 by ca 3600 °K. The temperature of the plasma has been calculated and measured experimentally by several workers and the results obtained are consistent with these measured values. Greenfield [47] reported a temperature of ca 8000 °K for the plasma tailflame based upon the observed' degree of ionization of calcium. Goldfarb et al'. 181] measured the plasma temperature as 8550 °K using a similar method. Pridmore7 Brown [82] calculated the temperature of a plasma formed by an electromagnetic field of 25 A m 1 for various radiofrequencies and size of tube and found that the temperature was almost independent of both these factors; at ca 8500 °K. He later measured the plasma - temperatures [83] and found they were in the range 8000-8600 °K which agrees with the similar measurements of Rovinskii et al. [84] who found the temperature to be 8700°K. Stokes [85] measured the plasma temperature from the continuum, the ratio of the intensities of several argon lines and from a theoretical treatment of the electric field distribution. He found that the temperature of the tear-drop plasma was about 8250-9000 °K while that of the doughnut shaped plasma was 8000-8800 °K. The argon and continuum methods both gave a mean temperature of 8500 °K throughout the whole section of the tailflame. CHAPTER 4

An Investigation of the Emission Spectroscopic Properties of Sulphur in an Induction Coupled Radiofrequency Plasma.

•

136 4.1 Introduction Table 4.1 shows the theoretical intensities and spectroscopic data for some of the principal spectral lines of sulphur at a plasma temperature of 8250 °K. The 180.73 nm and 182.04 nm lines are the strongest spectral lines of sulphur and are the most frequently used lines for the determination of sulphur in steel, etc. by the vacuum spark emission'technique. The 216.89 nm line has been used [86] for the determination of sulphur as gaseous sulphur dioxide or hydrogen sulphide introduced into a helium plasma operating at 2450 Hz. Atomic emission of sulphur at the resonance lines in the far ultraviolet has not been reported in a plasma, although a high frequency electrodeless discharge lamp has been used as the source for the determination of sulphur by atomic absorption in the graphite cuvette [34] and the separated nitrous oxide-acetylene flame [43,70]. 4.2 Experimental The main problem in detecting far ultraviolet radiation is that the atmosphere absorbs strongly in this region. Fig.4.1 shows the oxygen absorption spectrum over the wavelength range 175-200 nm. A schimatic diagram of the optical and 137

TA3LE 4.1 THEOUTICAL INTENSITY AT 8250 DEGREES K.

FOR VARIOUS LINES OF SULPHUR

WAVELENGTH G A E(I) - E(K) RELATIVE NM 10**8/SEC K INTENSITY 129.59 8 3.90 0 77166 .0778 130.28 "9 1.62 397 - 77156 .0363 • * 130.59 3 1.70 574 - 77151 .01:(7 131.66 15 .53 0 - 75935 .0243 132.35 8 .57 397 - 75953 .0139 132.66 3 .45 574 - 75952 .0041 140.15 3 .91 ii - 71353 .3174 140.94 3 .50 ' 397 - 71353 40095 141.29 3 .16 574 - 71353 .J030 142.52 15 1.68 0 - 70168 .1946 143.33 8. 2.29 . 397 - 70167 .1403 143.70 3 2.00 574 - 70166 .0459 144.82 3 6.90 9239 - 70290 ..0380 147.42 15 .69 0 - 67829 ..1156 148.31 8 1.03_ 397 - 67821 .0920 148.72 3 .89- 574 - 67817 .0297 166.67 5 5.80 9239 - 69239 .2249 1b8.75 3 .94 22181 - 3144/ .0026 178.23 3 1.50 22181 - 78290 .0067 180.73 3 4.10 . 0 - 55331 1.0000 182.04 3 2.26 - 397 - 55331 .5327 182.63 3 .73 574 - 55331 .1762 190.03 5 .00 0 - 52624 .0004 191.47 5 .00 397 - 52624 .0001 . 216.89 3 .00 9239 - 5331 .0001 ' 868.42 25 .12 634. 63'- 74975 .0016 . . 888.01 35 .10 678-85 - 79143 .0009

' INDICATES A MULTIPLET

VALUES TAKEN FROM REFERENCE 79 Fig.4.1 Variation of the absorption coefficient of oxygen from 175 nm to 200 nm. •10

.0.02

• 175 180 185 •190 Wav6IenE,rth , 139 electrical detection system is shown in fig.4.2. The plasma emission is modulated by the rotating sector and focussed onto the monochromator entrance slit by a 62.5 mm focal length fused silica lens. Oxygen was excluded from the system by provision of glass tubing of 25 mm outside diameter which could be purged via 8 mm outside diameter side-arms. A 25 mm diameter fused silica window was attached to the glass tubing nearest to the plasma torch so that the window was 30 mm from the central axis of the plasma torch. This still leaves an air-path of ca 16 mm between the plasma and purged detection optics, but it was found that this was the nearest to the plasma that the window could be placed without damage from the intense heat of the plasma cure. Although this air-path undoubtedly causes some attenuation of the detected light intensity below 200 nm, it was in practice quite easy to detect radiation in this region. The monochromator was purged with nitrogen via a 6.3 mm outside diameter copper tube sealed into the orifice provided for wavelength re-setting, and the copper tube bent through an angle of 120° to eliminate any stray radiation entering through this hole. The whole optical system was purged with ,oxygen-free nitrogen at a flow rate of ca 30-50 cm3 s -1 Fig.4.2 Schematic. diagram of the modifications to the optical system employed to allow detection of • far ultraviolet radiation.

RECORDER AMPLIFIER

CHOPPER

LENS MONOCHROMATOR

GLASS TUBING

101 NITROGEN 141 The photomultiplier used previously has a window made of Corning glass and was not able to detect radiation below 184 nm. Application of a thih film of sodium salicylate to the window did not produce any real improvement. A photomuitiplier with a fused silica window [type 9783A, E.M.I.Electronics Ltd., Ruislip, Middlesex] selected for maximum overall gain was employed. The signal obtained from this photomultiplier was amplified by the phase sensitive amplifier and displayed on a chart recorder.. The particular photomultiplier used was known to have a response of 200 A.L-1 at an E.H.T. of 850 V; this is above the maximum voltage attainable from the Techtron E.H.T. unit. In order to utilize the photomultiplier at its maximum sensitivity, a Brandenburg model 470 E.H.T. unit [Brandenburg, Thornton Heath, Surrey] was employed. This gave a working voltage range of 100-1250 V and it was found that the maximum signal to noise ratio was obtained at an E.H.T. of 900 V, which was subsequently used throughout this work. 4.2.1 Reagents A stock solution containing 40 kg m 3 of sulphur was prepared by dissolving ammonium sulphate [82.44g] in distilled water and diluting to volume [500 cm3]. 142 Stock solutions containing 1 kg m-3 of sulphur as different anionic forms were prepared by dissolving analytical grade sodium sulphite [7.864 g], sodium thiosulphate [3.876 g], potassium thiocyanate [3.037 g], thiourea [2.374 g] and sulphuric acid [1.67 cm3]in distilled water and diluting to volume [1dm3]. Stock solutions of diverse ions were prepared from analytical grade salts of the diverse ions in question. 4.2.2 Plasma operating conditions The effect of the injector gas and plasma gas flow rates upon the signal:noise and signal:background ratios for-the sulphur 182.04 nm line are shown in figs.4.3 and 4.4 respectively; The optimum flow rates for both these gases are similar to thoe determined for zinc and calcium. The other• operating parameters were also found to be similar. 4.3 Determination of sulphur in the plasma The observed emission spectrum for sulphur in the region 180-183 nm is shown in fig.4.5. The observed intensities of the three lines were found to be different from the theoretical intensities shown in table 4.1. There are four factors which account for this apparent deviation. Firstly, the photomultiplier sensitivity decreases fairly rapidly in the far ultraviolet.

Pig:4.3 Variation of signal to noise [A] and signal to backgrounA [B] ratios obtained at 182.04 nm. from . a solution containing 100 g.m-3 of sulphur as 20 aqueous ammonium sulphate with plasma gas flow rate,

[00 200 GA3 FLOW RATE, era Variation.o± signal to noise [A] and signal to background [B] ratios observed from a'solution containing 100 g.m 3 of sulphur as aqueous ammonium sulphate at 182.04 nm. with the injector gas flow rate.

20-

0

10w a

50 AC)C) • hilp FLOW RATE, 145

Fig.4.5 Emission spectrum observed for sulphur over the wavelength range 180-183 nm. when a solution containing 1 kg.m 3 of sulphur as aqueous ammonium sulphate was nebulized into the plasma.

crq

Em Erg

ry w

NC=

180 • 163

P4.11 LENGTH , . -146 Secondly, the silica windows and lens absorb increasingly more of the incident light in the far ultraviolet, so that the transmission of the optical system decreases at lower wavelengths. Thirdly, oxygen and, to a lesser extent, water increasingly absorb at lower wavelengths, and the small air-path has an effect on the far ultraviolet transmission. Finally, the refractive index of the lens, and hence the focal length of the optical system, is very dependent upon the wavelength of the incident radiation, particularly at short wavelengths. Attempts to detect the 166.67 nm line were unsuccessful. This is not suprising as the optical system used has a poor response below 175 nm, and the theoretical limit to the transmission, assuming ultra-pure materials and total exclusion of oxygen, is ca 160 nm. The sulphur lines at 190.03 nm, 191.47 nm, and 216.89 nm could be detected, but the intensity at these lines was such that it would have been impractical to attempt any quantitative measurements using these lines. 4.3.1 Detection limits and calibration curves The detection limits and relative intensities obtained for sulphur as aqueous ammonium sulphate are shown in table 4.2. The detection limit is defined as that concentration of analyte which can be '147

Table 4.2 Limits of detection and relative intensities at atomic lines of sulphur investigated.

Wavelength Limit of Relative Detection Intensity nm. g.m-3 arbitrary units

180.73 2.2 78 182.04 1.7 100 182.63 3.7 46

Table 4.3 Effect of 50-fold amounts of diverse ions on signal obtained from a solution containing 0.1 kg.m 3 of sulphur as ammonium sulphate .

Diverse Wavelength, nm. Ion 182.63 182.04 180.73 Co[III] + 35,g + 10,g_ + 25X Cr[III] + 15X 0 0 Cu[II] + 25X 0 + iv Fe[III] + 10X + 10: + 15X Mn[II] 0 0 + 10X Mo[VI] + 15X 0 + 10X Ni[II] + 50X + 15Z + 50g 148 determined with a relative standard deviation of 50 X on a set of at least twenty observations. Calibration curves established for sulphur as aqueous ammonium sulphate at the 180.73 nm, 182.04 nm and 182.63 nm lines are shown in figs.4.6-4.8. respectively. Deviation from linearity did not occur at a significant level at the 182.04 nm line with a concentration of=less than 1.8 kg m-3, while at the 180.73 nm line significant deviation occurred at a concentration of 1.2 kg m3. No deviation was observed from linearity at the 186.23 nm line at a concentration of up to 2.0 kg m3. This order of deviationfollows the order which can be predicted if the deviations are due to self-absorption rather than a physical effect in the nebulizer system. 4.3.2 Effect of extraneous ions on sulphur emission As shown in fig.4.9, the introduction of sulphur into the plasma as aqueous solutions of different compounds gives the same detection limit and signal per unit weight of elemental sulphur. No significant physical or chemical interference effects were observed in the emission signals produced at the 180.73 nm, 182.04 nm or 182.63 nm lines from an aqueous solution containing 100 g m-3 of sulphur as aqueous ammonium sulphate or thiourea in the presence of a fifty-fold weight excess of 10- Fig.4.6. Calibration curve obtained at 180.73 nm for sulphur introduced into the plasma as aqueous ammonium sulphate.

5- \

1

• CONG•ENTRATMN 3 Pig.4'.7 Calibration curve obtained.at 182.04 nm. for 8' sulphur introduced into the plasma as aqueous ammonium.sulphate.

rs• 4- 4:at

11..ge le wail t wsny C in seta

2 P . CONCENTRIMON, Fig.4.8 Calibration curve obtained. at 182.63 nm. for sulphur introduced into the plasma as aqueous ammon:tum sulphate. •

3-

1 CONGEMTRAT CM, KG.M:3 Fig.4.9 Emission signals obtained at 182.04 nm. for aqueous solutions containing 100 g.m-3 of sulphur as different sulphur compounds.

(1...... 00.1••■•••••■

sodium potassi urn sulphuric 9mmoum sodium thi ourea sulphate triosaphate sulphite t hiocyanate acid

TIME,s. 153 3+ + + 2+ + + the following ions : Al $ K Li , Mg , Na $ VO2' Zn2+, ammonium, acetate, borate, chloride, fluoride, iodide, nitrate and phosphate. Freshly prepared solutions containing a fifty-fold weight excess of calcium on ammonium sulphate showed no interference) but upon prolonged standing of the solution, the signal began to decrease as calcium sulphate was precipitated from solution. Looyenga and Haber [87] reported an inhibition of the magnesium atomic absorption signal by sulphate in a flame, but no evidence of this effect was observed in the plasma. Interferences were observed when fifty-fold weight excesses of the following ions:- Co2+1 Cr3+, Cu2+, Fe3+1 Mn2+, Mo022+ , and Ni2+ were present in a solution containing 100 g m-3 of sulphur as aqueous ammonium sulphate or thiourea at one or more of the sulphur lines investigated. The magnitude of these effects are shown in table 4.3 [p.147]. The reason for these interferences is not clear but is probably a combination of spectral overlap with lines of these elements from high ionization states and slight changes induced in the plasma geometry and characteristics induced by the metal. The magnitude of these interference effects, however, is quite small and should not present many analytical problems, at the 182.04 nm 154 4.3.3 Choice of analytical line for sulphur The 182.04 nm line is obviously the best sulphur line to use for the determination of sulphur in the induction coupled radiofrequency plasma used. This line was the most intense of the th'ree lines investigated, although in theory better sensitivity should be attainable at the 180.73 nm line, and it gave the lowest detection limit. The maximum linear range was obtained at the 182.63 nm line, but this was the least sensitive of the three lines. The effect of interfering ions was least at the 182.04 nm line. The sensitivity obtained for sulphur in the plasma is such that the direct determination of this element in sea-water, rocks and'soils [88] is possible, although it might be necessary to pass the solution over an ion-exchange column to remove interfering metal ions. The determination of sulphur. in organic matrices such as oils, petroluem products - and pharmaceutical products might also be possible. A problem with organics in this plasma is that the solvent does tend to burn spectacularly and causes great instability of the discharge. This is particularly so of the common oil-products solvent, 4-methylpent-2-one, and alcohols, but solvents such as xylene or carbon tetrachloride are more controllable in the discharge. CHAPTER 5

An Investigation of the Emission Spectroscopic Properties of Phosphorus in an Induction Coupled Radiofrequency Plasma

• 155 5.1 Introduction The spectroscopic data and theoretical intensities at a plasma temperature of 8250 °K of some of the strongest lines of phosphorus are shown in table 5.1. Although the 167.97 nm line is the strongest emission line of phosphorus it is seldom used in its determination by vacuum spark emission spectroscopy because it has a very much poorer transmission through the detection optics; the 177.50 nm and 178.29 nm lines are most frequently used. In routine arc and spark emission determinations, phosphorus is normally determined at the 214.91 nm line. Kopp and Kroner [89] described the determination of trace elements in natural water by spark emission and determined phosphorus at the 253.56 nm line. Wendt and Fassel [50] in early work with an induction coupled plasma reported a detection limit of 10 p.p.m. at the 253.56 nm line. Using more refined instrumentation and operating conditions, Dickinson and Fassel [54] obtained a detection limit of 0.1 p.p.m. at the 213.62 nm line, while Boumans and De Boer [67] recorded a detection limit of 0.07 p.p.m. at the 253.56 nm line. Navrodineanu and Hughes [68] have recorded the emission spectrum of phosphorus in a plasma over the range 228-700 nm. None of these lines previously investigated in plasmas, 156 ••

• TABLE 501 THEORETICAL INTENSITY AT 8250 DEGREES K.

FOR VARIOUS LINES OF PHOSPHORUS

WAVELENGTH G A E(I) - E(K) RELATIVE NM . 104 *8/SEC K INTENSITY 167.17 2 11.00 0 - 59620 .3187 167.46 4 11.00 0 - 59716 .6479 167.97 6 11.00 0 - 59535 1.3000 177.50 6 2.17 0 - 56340 .3263 178.29 4 2.14 0 - 56090 .2231 178.77 2 2.13 0 - 55939 .113 7 185.92 10 2.81 11370 - 65157 .1440 4. 213.55 4 .21 11362 - 58174 .0128 213.62 4 2.83 11376 - 58174 .1711 214.91 2 3.18 11362 - 57877 .1006 215.37 10 .61 16739 - 6,157 .0270 4 253.4O 4 .20 18722 - 55174 .0102 253056 4 .95 18748 - '58174 .03484 255.32 2 .71 18722 * 57877 .0189 255.49 2 .30- 13748 - 57877 .0080

* INDICATES A MULTIPLET

VALUES. TAKEN FROM REFERENCE 79 157 however, are ground state lines and they are not as intense as these lines, so that an improvement in sensitivity should be possible using the resonance lines. 5.2 Experimental The same experimental system as employed for sulphur was again used. Purging of the optical system was, however, only employed for wavelengths. below 200 nm. The optimum operating conditions for phosphorus were found to be identical to those obtained for sulphur, which is not suprising considering their similar physico—chemical properties. The effect of height of observation of the emission above the top of the work coil upon the magnitude of the analytical signal obtained for phosphorus at 185.92 nm is shown in fig.5.1. This effect was also observed for the other lines of phosphorus and the resonance lines of sulphur and is very much more marked than that observed for zinc, fig.3.15. This is the result of the higher energy of the excited states of phosphorus and sulphur which are more affected by temperature than those of zinc. 5.2.1 Reagents Stock solutions containing 1 kg m 3 of phosphorus as phosphate were prepared by the dissolution of analytical reagent grade ammonium 158 Fig.5.1 Variation of emission signal obtained from a solution containing 1 kg.m-3 of phosphorus at 185.92 nm. with height of observation of the signal.

• •

45 ilt:-.0;lag 159 di-hydrogen orthophosphate [3.711 g] in distilled water and diluting to volume [1 dm3]. Stock solutions containing 1 kg m 3 of phosphorus as different anionic forms were prepared by dissolving analytical grade bariun hydrogen orthophosphate [7.534 g], disodium dihydrogen pyrophosphate [3.583 g] and hypophosphorus acid [4.261 g] in distilled water and diluting to volume [1 dm3]. 5.3 Determination of phosphorus The observed emission spectrum of phosphorus introduced into the plasma as aqueous ammonium dihydrogen orthophosphate is shown in fig.5.2 over the range 175-180 nm. As for sulphur, the observed intensity ratios are different from those predicted by theory. The 177.50 nm-line is very much reduced in intensity and this is probably due to the fact that it lies on an oxygen absorption bandhead [fig.4.1] while the 178.77 nm line lies in a trough between adjacent bandheads. The lines around 167 nm could not be detected from a .solution containing 1 kg m-3 of phosphorus. The observed emission spectra of the non- resonance lines of phosphorus are shown in fig.5.3. The 213.63 nm line intensity was found to be about sixty-fold greater than that at 178.29 nm instead of about the same order of magnitude as predicted 160

• Fig.5.2 Emission spectrum observed over the range 177-179 nm. from a solution containing 1 kg.m73 of phosphorus as ammonium dihydrogon orthophosphate.

177 179

p tr-7. 1 ;CI 161

Fig.5.3 Emission spectrum observed over the range 185-256 nm. A from a solution containing 1 kg.m 3 of phosphorus B from a solution containing 70 g.m 3 of phosphorus.

185 137 213 215 253 2(56 kVA t!7 LEi 3 7' N nI .

162 from table 5.1. This effect can be attributed to the attenuation of the intensity at short wavelengths by absorption in the detection system by oxygen and silica. 5.3.1 Detection limits and calibration curves The detection limits and relative intensities obtained for phosphorus are shown in table 5.2. Calibration curves were established for phosphorus at 177.50 nm [fig.5.4], 178.29 nm [fig.5.5], and 178.77 nm. At each of these lines, no significant deviation from linearity was observed with concentrations up to 1 kg m 3. Calibration curves were also established at 185.92-nm [fig.5.6] and 213.62 nm [fig.5.7]. The former line was found to be linear up to a phosphorus concentration of 1 kg m-3 while at the latter line, even with minimum electronic gain, the amplifier was overloaded by the signal produced from a solution containing 150 g m 3 of phosphorus, although by reduction of the photomultiplier E.H.T and the slitwidth [from 25 pm to 10 gm], the linear range could be greatly extended. 5.3.2 Effect of extraneous ions upon phosphorus emission • As shown in fig.5.8, the introduction of phosphorus into the plasma as different anionic forms gave the same detection limit and signal per 163

Table 5.2 Limits of detection and relative intensities at atomic lines of phosphorus investigated.

Wavelength Limit of Detection Relative Intensity rtm g. m-3 arbitrary units

177.50 - 10 .1 a 178.29 3.0 3.5 3.5a 178.78 3.0

Om. b 185.92 0.4 7 213.62 0.08 100. p b 214.91 0.15 65 253.56 2 38b b 255.49 4 16 a relative to intensity of the 177.50 nm line taken as unity. b relative to intensity of the 213.62 nm. line taken as 100 units. Pig.5.4 Calibration curve obtained at 177.50 nm. from an

4- aqueous solution of ammonium dihydrogen orthophosphate.

2-

0.5 1.0 CONCENTRATION, KG.N173 Fig.5 5 Calibration curve obtained at 178.29 nm for aqueous solutions of ammonium dihydrogen orthophosphate.

05 10 V1 CONCE.NITATE,011 a. i.-3 Fig.5.6 Calibration curve obtained at 185.92 nm for phosphorus as.aqueous ammonium dihydrogen orthophosphate solutions.

Jr p„."

Gov wao

C

6JM

CAM mon,.

c •_ LO H (3, 0.5 1JC) cr■ CONGENTaTiall, Fig.5 7 Calibration curve obtained at 213.62 nm for phosphorus as aqueous.ammonium dihydrogen orthophosphate solutions.

• 50 CONCENTRATiON,G.M 'Fig.5.8 Emission signals obtained at 185.92 nm from solutions • contaiAing 100 g.m-3 of phosphorus as different compounds.

di-sodium ammonium barium hypo-p di-hydrogen hydrogen , di-hydrcg en acid .ortho-phosphate ortho-phosphate pyro-phosphate

TIME,s. •1 69 unit weight of phosphorus. The effect of interfering ions was investigated at the 185.92 nm, 213.62 nm and 214.91 nm lines. The 178.29 nm line was not investigated as the iodine 178.28 nm line could not be separated from this line using the Techtran monochromator and the emission characteristics of iodine in the plasma were also to be investigated. No significant chemical or physical interferences were observed at the 185.92 nm, 213.62 nm and 214.91 nm lines when fifty-fold weight excesses of the ions K+, Mg2+, Na+, Zn2+, ammonium, acetate, borate, chloride, fluoride, iodide, nitrate and sulphate were present. in a solution containing 100 g m3 of phosphorus as ammonium dihydrogen orthophosphate. Freshly prepared solutions containing a fifty-fold weight excess of molybdenum [VI] or : -vanadium [V] gave no interference, but upon prolonged standing the signal was found to decrease, pcissibly due to the precipitation of ammonium phosphomolybdate or phosphovanadate respectively. Interferences were observed at one or more phosphorus lines for the cations shown in table' 5.3. Spectral interferences were observed at all three phosphorus lines for aluminium due to overlap with atom lines at 213.5 nm and 215.0 nm and the ion line at 185.8 nm. Calcium has no lines reported near enough to the phosphorus lines to give rise to any 170

Table 5.3 Effect of 50-fold amounts of diverse ions on signal intensity observed for a solution containing 0.1 kg.m-3 of phosphorus as aqueous ammonium dihydrogen orthophosphate.

Wavelength, nm. Diverse ion 214.91 213.62 185.92 Al[III] + 10: + 35X + 25X Ca[II] ' + 10X + 20,E + 15Z Co[II] + 15,E + 40X +125X Cr[III] + 20Z + 35Z 0 Ou[II] 0 +150: 0 Fe[III] ' + 20,E + 15X + 35,E Ni[II] + 35,E + 25X + 10,$ Zn[II] + 15% + 10% + 40% 171 spectral overlap and as the enhancement is almost identical at each line regardless of the manner in which the calcium interference solution was prepared, a form of chemical interference similar to that observed by Veillon and Nargoshes [59] must be postulated. As the two sample introduction systems were similar, it would appear that this interference is a functioh o2 the sample introduction system rather than the plasma. 5.3.3 Choice of analytical line for phosphorus The 213.62 nm line was found to be the most sensitive line for the detection of phosphorus, but the strong interference from copper due to overlap with the copper ion line-at 213.6 nm, reduces the applicability of this line, With a monochromator of higher resolution, the interference at the 214.91 nm line could easily be eliminated as it is in conventional quantometers. The 185.92 nm line appears to coincide with an unlisted cobalt line so that this line may not be analytically useful in some inorganic matricies. The choice of analytical line lies between the 213.62 nm line for sensitivity and the 214.91 nm line for selectivity. 5.4 Application to determination of phosphorus in soils • Soils are usually analysed for phosphorus 172 using a spark technique or by x-ray fluorescence [90]. The use of a plasma jet has been described for the determination of phosphorus in soil extracts [91] and, although it was claimed that calcium did not interfere with the determination, the results obtained by plasma emission and classical gravimetric analysis were significantly different. Sty [92] described the determination of water soluble phosphate in soils. The powdefed dried rock or soil was leached with water and shaken with a cation ion-exchange resin and the phosphate in the extract determined by cool flame emission using the HPO band system. Soil samples were chosen which had a phosphorus content in the,solid of ca 10-80 mg per kg of soil. With a detection limit of ca 100 mg m 3 at the 213.62 nm line, it was necessary to take a 2.5 g sample, which after extraction and dilution - to 100 cm3 gave a phosphorus concentration in the - solution of ca 250-2000 mg 3. The interference studies showed that calcium produced a slight enhancement in the phosphorus signal when present as a fifty-fold weight excess, and in some of the soils the excess was as much as five hundred-fold. Cobalt, copper, iron and zinc were also present in the soils in appreciable quantities and were likely to interfere. These cationic interferences could, however, readily 173 be removed by a simple batch ion-exchange procedure. It was found, in practice, that iron [III] was not fully exchanged. This may be due to the high stability of the iron [III] - phosphate complex. Emission of iron in the final solution could be detected at the 371.99 nm line and corresponded to a concentration of iron of ca 1 g m-3; this supports the above hypothesis. 5.4.1 Method Three 2.5 g samples of each air-dried soil were shaken with 75 cm3 of 2.5 j v/v acetic acid for three hours. 1 cm3 and 2 cm3 aliquoits of an aqueous solution containing 50 g m-3 of phosphorus • as ammonium dihydrogen orthophosphate were added to two of the samples before shaking. The solutions were then filtered through Whatman No. 42 filter paper into 100 cm3 volumetric flasks. The residue was then washed with four 5 cm3 aliquoits of 2.5 ,g'' v/v acetic acid and made up to volume with acid. The solutions were then shaken with ca 1 g of Amberlite IR-120[H] ion-exchange resin for ten minutes and left to settle for a few minutes. The supernatant liquid was then decanted off.ekd sprayed' into the plasma. The phosphorus emission was ' measured at 213.62 nm against a blank of 2.5 v/v acetic acid.

174 5.4.2 Results The phosphorus content of the soil extracts, calculated from the ratios of the intensities of the emission signals obtained from the soil extract and spiked extracts, are shown in table 5.4. The results obtained were in good agreement with those obtained by an independent analysis and the precision, about 3-4 ,g for the majority of the analyses, is at least comparable with, if not better than, that obtained using spark emission or x-ray fluorescence. 5.4.3 Other applications Phosphorus is an essential nutrient in foodstuffs and is generally present as a minor constituent. The usual method of determining phosphorus in milk powders, meat extracts, yeasts, etc. makes use of the formation of reduced phosphomolybdic acid blue complex. The complex formed is then determined colorimetrically. The method is slow and tedious and requires skilled manipulation to achieve reasonable precision. A rapid method for the determination of phosphorus in these materials has recently been reported [15]. In this method, the sample was dissolved in water and sprayed into a nitrogen separated nitrous oxide-acetylene flame where the phosphorus was determined by atomic absorption at the 178.29 nm line. The plasma system is capable 175

Table 5.4 Determination of extractable phosphorus in soil extracts

Soil P205 Phosphorus P205 found Sr sample mg/100g7 content of by plasma Extract, g.m-?

D51023 ' 2.7 0.29 2.66+0.18 +6.8 D46402 5.1 0.58 5.31+0.19 +3.4 D44839 8.0 0.88 8.05+0.32 +3.6 D44189 13 1.44 13.2 +0.50 +3.8 D46755 18 1.99 18.2 +0.69 +3.7

x Results obtained by the Macaulay InstitUe for Soil Research, Aberdeen, Scotland. 176 of analysing these materials for phosphorus with a much higher sensitivity. This can further be increased since very viscous, high carbon containing compounds can be sprayed into the plasma without problems of clogging of the atom cell which occur with the burners used in the flame method. CHAPTER 6

An Investigation of the Emission Spectroscopic Properties of the Halogens- in an Induction Coupled Radiofrequency Plasma.

177 6.1 Introduction With the optical system used, it was possible to detect the most intense iodine resonance lines, but not those of the other halogens. Spectroscopic data and the theoretical line intensities for the strongest lines of iodine are shown in table 6.1. The determination of iodine by emission spectroscopy at the non-resonance lines has been reported in a 2450 MHz plasma [93,94] and a 3 MHz plasma [64]. The determination of iodine by atomic absorption spectroscopy at the 183.04 nm line has been reported using the graphite cuvette [34] and separated nitrous oxide-acetylene flame cells [41]. In both of these systems, a high frequency electrodeless discharge was employed as the source. 6.2 Experimental The plasma was operated using the same conditions as described previously. A stock solution containing 10 kg m-3 of iodine was prepared by the dissolution of analytical reagent grade ammonium iodide [11.43 g] in distilled water and diluting to volume [1 dm3]. Stock solutions containing 10 kg m-3 of chlorine, bromine and fluorine were prepared by dissolving analytical grade ammonium chloride [1.538 g], 178

TABLE 6.1 THLORETICAL INTENSITY AT 8250 DEGREES K.

FOR VARIOUS LINES OF IODINE -

WAVELENGTH G A E(I) LAK) RELATIVE NM 1044 8/SEC K INTENSITY 150.70 4 .18 0 -• 66355 .0129 151.41 6 2.05 0 -• 66021 .2355 158.26 - 2 2.07 - 63187 .1245 161.76 4 1.34 0 •- 61819 .2003 164.21 2 .37 0 60895 .0320 170.21 4 2.05 7600 •• 66355 .1318 178.28 4 2.71 0 •• 56093 1.0000 179.91 . 2 2.11 7600 •• 63137 .1116 183.04 6 .16 0 •'• 55171 .1013 184.44 4 .00 7 7600 •• 618/9 .0090 187.64 2 .03 7600 60895 .0022. 206.16 4 .03 • 7600 56093 .0094 .

•

4 INDICATES A MULTIPLET

VALUES TAKEN FROM REFERENCE 95 179 ammonium bromide [1.218 g] and ammonium fluoride [1.946 g] respectively in distilled water and diluting to volume [100 cm3].1 6.3 Determination of iodine Fig.6.1 shows the iodine emission spectrum observed over the wavelength range 175-210 nm from the plasma. The observed signal intensities deviate from those predicted from theory in the same fashion as previously described. Although the signal intensities decrease in the far ultraviolet, the observed background emission also decreases quite rapidly so that the loss of signal intensity is partly compensated. 6.3.1 Detection limits and calibration curves The detection limits and relative intensities obtained for iodine introduced into the plasma as aqueous ammonium iodide solution are shown in table 6.2. Calibration curves were established for iodine at the 178.28 nm, 183.04 nm and 206.16 nm lines shown in figs.6.2, 6.3 and 6.4 respectively. The non-resonance line at 206.16 nm was found to be . linear up to a concentration of greater than 10 kg m 3 Attempts to observe atomic absorption of iodine at this line [96,97] have yielded poor sensitivity as the lower state of this line is 0.94 eV above the electronic ground state. This means that the effect 180 '

Fig.6.1 Emission spectra obtained over the wavelength range 177-208 nra. for a solution containing 0.6 kg.m-3 of iodine as aqueous ammonium iodide introduced into the plasma. Intensity in B is 1/4 intensity of A.

. 177 188 207 "P. n ViAtiLL L:kiG F.1 I F ri 181

Table 6.2 Limits of detection and relative intensities at atomic lines of iodine investigated.

Wavelength Limit of'detection 'Relative Intensity am, g.m-3. arbitrary units' 178.28 1.0 12 179.91 .-1.5 8 183.04 .0.8 50 184.44 2.5 16 . 187.64 15 . 3 206.16 0.5 100

Table 6.3 Effect of diverse ions on signal obtained from a solution containing 0.1 kg.m-3 of iodine at the 50-fold weight excess level.

Diverse Ion Wavelength, nm. 178.28 183.04 206.16 Co[II] 0 0 + 500,5 Cr[VI] 0 0 +>1000$ Cu[II] + 50 . + 45/5 + 270X Fe[III] + 80,0 + 80,,0 + 75% Mo[VI] 0 0. + 200,5 . Ni[II] 0 0 + 75j . Zn[II] 0 0 - +>1000Z 3- +244 0 0 P04 Fig.6.2 Calibration curve obtained at 178.29 nm. for iodine as aqueous solutions of ammonium iodide.

+°.■ 4r f

Cd

c, A

VMS '3- LR

awed

171*

V.JM Co)

t ZZA

vJ

2 00 CONGENTRATICII, KG.11/7.73 rv• . Pig.6.3 Calibration curve obtained at 183.04 nm. for iodine as aqueous ammonium iodide solutions.

53

5 10 CONCENTRATION, KG.F17:73 Pig.6.4 Calibration cutve obtained at 206.16 nm. for iodine as aqueous ammonium iodide solutions.

6011GENTRATEEMI , ttG.K173 185 of self-absorption of emitted radiation at this line would be predicted to be neglible. The 178.28 nm line and the 183.04 nm line show significant deviation from linearity at concentrations of 2 kg m 3 and 7 kg m 3 respectively; this may be attributed to self-absorption and follows the predicted pattern. 6.3.2 Effect of foreign ions on iodine emission No significant chemical or physical interferences were observed at the 178.28 nm, 183.04 nm or 206.16 nm lines when fifty-fold weight excesses 2+ + of the ions : Al3+, Ca2+, K+ Mg , Mn2+ , Na an2+ ammonium, acetate, chloride, fluoride, nitrate and sulphate were present in a solution containing 100 g m 3 of iodine as aqueous ammonium iodide. Interferences were observed from the ions shown in table 6.3 [p.181]. The interference observed at 206.16 nm from cobalt, chromium, molybdenum and • nickel and for phosphate at 178.28 nm were found to be caused by spectral overlap with lines of these elements. An apparent enhancement of 65 % was observed from a solution containing a fifty-fold weight excess of chromium as chromium [III] chlo:dde at both the 178.28 nm and 183.04 nm lines, but this was not observed when chromium was present as potassium chromate or dichromate. When a solution of chromium [III] chloride was nebulized into a nitrogen 186 separated nitrous oxide—acetylene flame, absorption was observed at the 183.04 nm iodine line. This suggests that the apparent enhancement was caused by an iodide impurity in this reagent. 'Iron [III] produces an enhancement at each of the three iodine lines investigated. In acid solution, iron [III] oxidizes iodide to iodine.

_2Fe3+ + 21 -* 2Fe2+ + 12

When sprayed through the nebulizer, the iodine having a much greater vapour pressure than ammonium iodide, tends to be transferred to the vapour phase to a greater extent than the iodide in the aqueous phase so that the vapour entering the plasma is enriched in iodine. When other oxidizing agents such as nitric acid were added to an iodide solution, the same degree of enhancement was observed. This effect has also been observed for organic iodide [41] and organic sulphur [98] introduced into a flame via an indirect nebulizer. Copper [II] undergoes a reaction with iodide in which free iodine is formed and copper [I] iodide precipitated. 2+ 2Cu + 41— -* Cu2I2 + 12 The suspension formed from a solution containing 100 g m-3 of iodine could still be nebulized, but 187 upon prolonged nebulization the nebulizer eventually blocked. The free iodine in solution produces the enhancement in the signal, and, in fact, the filtered solution gives only an apparent signal depression of 20 „SI' compared to the signal obtained from a • solution containing only 100 g m-3 of iodide as ammonium iodide. 6.3.3 Choice. of analytical line for iodine The 206.16 nm line was found to be the most sensitive analytical line for the determination of iodine, but the spectral interferences observed from many cations, zinc and chromium in particular, reduces the usefulness of this line. This line would be ideal, however, for the determinatioi of iodine in organic matrices and has the added advantage that purging of the monochromator is not required. The 183.04 nm line does not suffer from any spectral interferences and although the magnitude of the observed signal at this line is very much half of that observed at the 206.16 nm line, the detection limit is not substantially poorer. The interference from phosphorus at the 178.3 nm line limits the use of this line, althcughjt has • theoretically the maximum attainable signal:background ratio. The detection limit obtained for this system of about 500 mg m-3, although much better than any 16b previously achieved detection limit, does not provide sufficient sensitivity to permit the detection of iodine in rocks, soils, sea water, river water, etc. [88] without a prior amplification step, such as the Liepert amplification. Such a procedure, however, lengthens the time required to perform the analysis and is liable to give poor piecision. The use of micro-coulbmetrY or potentiometry can readily be used to detect these low levels of iodine so* that any such procedure is unjustified. 6.4 Bromine,Chlorine and Fluorine The resonance lines of these halogens lie well below the transmission limit of the detection optics used. Some of the strongest emission lines of bromine and Chlorine are shown in tables 6.4 and 6.5 respectively together with the spectroscopic details of the transitions and their theoretical intensities at a temperature of. 8250 °K. 'v/hen solutions containing 10 kg m-3 of chlorine as ammonium chloride and 10 kg m 3 of bromide as ammonium. bromide were introduced into the plasma, it was not possible to detect any lines of these elements in the near ultraviolet part of the spectrum. It also proved impossible to detect iodine or fluorine lines in this part of the spectrum, although it was quite obvious that these atoms were formed in the plasma. 189

TABLE 6.4 THEORETICAL INTENSITY AT 3250 JEGRLES Ka

FJR VARIOUS LINES OF 3ROMINE

WAVELENGTH G A E(I) - E(K) RELATIVE NM 10"8/SEC . K INTENSITY 129.30 4 .05 0 - 77330 ou049 129.40 "6 1.74 0 - 77260 ,2799 135.80 4 1.56 3865 - 77330 .1515 144.99 2 1,34 -0 - 68970 .2731 143.36 4 1.66 0 - 67190 .3997 149.50 .2 .02 0 - 66877 .a060 153.24 2 1,95 3685 - 68971 .3760 - 154.03 4 1.28 0 -. 64901 1.0000 157.50 4 .20 3685 - 67191 .1019 157.65 6 .02 0 - 63430 .0283 158.30 2 .08 3685 - .66877 .0221 163.40 4 .10 3685 - 64901 .0737

INDICATES A MULTIPLET

VALULS TAKEN FROHREFERENCE 95 190

TABLE 6.5 THEORETICAL INTENSITY AT 8250 JEdRELS K. FOR VALIOUS LINES OF CHLORINE

WAVELENGTH G A E(I) .(K) RELATIVE NH 1U44 8/SEC K INTENSITY 118.88 10 1.50 0 - .17'--)9 120.14 4 2.39 881 - .1134 133.57 2 1,74 0 74861 .1873 134.72 4 4.19 0 - 74221 1.0000 135.17 2 3.23 881 - 74861 .3435 136.35 4 .75 881 74221 .1769 428.48 4 .02 72276 - 95608 .0000 44-0.85 20 ..01 72276 - 94953 .0001 743. L}1 4 .38 72276 - 85731 .3022 808.58 10 .42 -84116 - .96480 .0009 841.31 20 .27 72276 - 8414 .0091 927.05 12 .23 72276 - 83060 ..0051 959022 6 74221 - 84644 .24 ,.3020

* INDICATES A 1ULTIPLET

VALUES TAKEN FROM REFERENCE 79 191 The introduction of chlorine or bromine into the plasma in gaseous form would almost certainly have produced an appreciable concentration of these atoms in the plasma and made the detection of lines of these elements possible. It is most likely that the reason that the lines of these elements were not observed is that their intensity is very low and that the partial pressure of chlorine or bromine atoms in the plasma is also low, rather than there is insufficient energy available to excite these lines. The energy needed to excite these lines is only about 10 eV and the plasma is sustained by a reaction producing 15 eV, so that allowing for inefficient energy transfer, these lines should be visible. The use of a vacuum monochromator and vacuum optical path coupled with lithium fluoride optics should allow the detection of the resonance lines of bromine and chlorine. It was found that upon prolonged nebulization'of fluoride into the plasma, the silica walls were severely attacked and it appears that routine determination of fluorine is not feasible with this system even if the line could be detected. CHAPTER 7

An Investigation of the Emission Spectroscopic Properties of Carbon in an Induction Coupled Radiofrequency Plasma.

192 7.1 Introduction Spectroscopic data and the theoretical line intensities at a temperature of 8250 °K for some of the strongest lines of carbon are shown in table 7.1. The six resonance lines of carbon at 94.54 nm, 126.13 nm, 127.75 nm; 132.93 nm, 156.10 nm and 165.72 nm are in fact complex• multiplet systems which should be partially resolved"using a spectral bandpass of 0.08 nm. The multiplets at 156.10 nm and 165.72 nm have been used for the determination of carbon using a vacuum spark technique [30], while- the non- resonance line at 193.,04 nm has also been used in spark emission. - Truitt and Robinson [65,66] have investigated the emission characteristics of carbon species introduced into a plasma discharge as gases and found that apart from strong carbon emission at the 247.86 nm, the emission spectra of molecular fragments of the origihal species such as CN, CO

and C2 were strongly developed. 7.2 Experimental The plasma was operated using the same conditions developed previously. Stock solutions containing 1 kg m-3 of carbon were prepared by dissolving dextrose [2.5 g] 193

TABLE 7.1 THcIORETICAL INTENSITY AT 8250 OEGRa..S K.

FOR VAK1OUS LINES OF CARBON

WAV-....LENGTH G A E(I) ..• E(K) RELATIVE NA 10*'..8/SEO K INTENSITY 94054 3 • 61,0- 0 30 - 105801 .0031 126.13 .9 1.20 30 •• 79315 .0140. - 127.75 15 1.60 30 - 78310 .0367 128.04 9 .82 30 ..• 78133 .0116 132.93 9 1.40 30 •.. 75255 .0316 4 143.19 15 1.40 33735 - 103570 .0003 145.90 3 .37 10194 •• 73728 .0014 146.33 7 2.10 10194 •• 78531 .0189 146.74 3 .46 13194 - 78338 .OGi8 148.13 5 .33 10194 77681 .0024 156.10 15 1.50 30 ..• .64091 .3380 4 165.72 9 4.10 30 - 60374 1.0000 * 175.19 3 .87 21648 73728 ....1G27 193.04 3 2.40 10194 61982 .1265 247.86 3 .34_ 21668 - 61982 .0140

* INDIGATESA MULTIPLET

VALUES TAKEN FROM REFERENCE 99 194 and sodium acetate [3.142 g] in distilled water and diluting to volume [1 dm3]. 7.3 Determination of carbon The carbon line at 156.10 nm was not detected in the plasma discharge as it lies below the limit of transmission of the optical system. When a solution containing 10 kg m 3 of carbon as acetic aoid was introduced into the plasma, a weak signal was obtained at 165.72 nm which could be attributable to carbon. This line was, however, not useful analytically. Two strong carbon non-resonance lines were detected at 193.04 nm and 247.86 nm. Fig.7.1 shows the emission spectrum observed. around these lines for carbon upon nebulization of an aqueous solution containing 100 g m-3 of Carbon as dextrose into the plasma. The relative intensity of these two lines was observed to be ca 3:1 rather than the predicted value of 7:1. It was found that purging of the monochromator produced only a slight improvement in the signal and signal:noise ratio obtained at 193.04 nm. 7.3.1 Detection limits and calibration curves The detection limits and relative intensities obtained for carbon introduced into the plasma as either aqueous dextrose or sodium acetate solutions are shown in table 7.2. 195

Fig.7.1 Emission spectrum observed over the wavelength range 192-249 nm. for carbon introduced into the plasma as a ij aqueous acetic acid, solution.

192 195 246 249

• 196

Table 7.2 Limits of detection and relative intensities at atomic lines of .carbon investigated.

Wavelength Limit of Detection Relative Intensity nm. g.m- 3 . arbitrary units.

165.72 . >1000

193.04 0.10 100

247.86 0.96 35 197 Calibration curves established at the 193.04 nm and 247.86 nm lines are shown in figs.7.2 and 7.3 respectively. At both of these lines the linear range was found to extend beyond a concentration of 1 kg m 3 of carbon. The 193.04 nm line is more sensitive than the 247.86 nm line and has better signal noise

••• and signalsbackground ratios. The use of the 247.86 nm line for analytical .purposes is limited in this plasma system as 'the continuum background from the plasma is maximized about 350 nm. As the photomultiplier sensitivity peaks about 250 nm, the very high background which is observed limits the signal amplification attainable at this line. 7.3.2 Effect of foreign ions upon carbon emission As shown in table 7.3, the introduction of carbon into the plasma as aqueous solutions of different carbon compounds gave the same detection limit and signal per unit concentration of carbon. This is true whether carbon is introduced as a water soluble organic compound such as an organic amine, carboxylic acid, phenol, sulphonic acid, etc. or as an inorganic compound such as an acetate, oxalate, carbonate or cyanate. The effect of fifty-fold weight excess of the following ions : Ca2+1 Co2+, Cr3+, Cu2+ Pe3+,

mg2+, mn2+, moo K+, 2 , ammonium 2+' Ni2+, 11 4. 8- Pig.7.2 Calibration curve obtained at 193.04 rim. for. carbon as aqueous solutions of dextrose.

05. ) 0 .CONGENTEIV.MON, KG.M.-3 .Fig.7.3 Calibration curve obtained at 247.86 nm. for carbon 6° as aqueous solutions oi dextrose.

05 • (0 6.1mENTur , EturiT3 200

Table 7.3 Emission signals obtained from solutions containing 0.1 kg.m 3 of carbon as different aqueous solutions of carbon compounds at 193.04 nm.

Compound Signal Sis,X Difference, Dextrose 100 - ONO Ammonium oxalate 99 5 - 1 Ascorbic acid 98 4 - 2 Citric acid 101 7 + 1 Glycine 100 2 0 Lead acetate 99 3 - 1 Phenol 101' 4 + 1 Salicylic acid 99 44 - 1 Semicarbazide hydrochloride98 5 - 2 Sucrose 128 6 +28 Tartaric acid 99 2 - I Thiourea 99 3 - 1 p-Toluene-sulphonic acid 99 5 - 1 Urea 97 3 - 3 201 borate, chloride, fluoride, iodide, nitrate, phosphate and sulphate did not produce any significant chemical or physical interference upon a solution containing 100 g m-3 of carbon as aqueous sodium acetate at the 193.04 nm line. Aluminium produced an enhancement of ca 10 at the 193.04 nm line when present as a fifty-fold weight exces's over carbon. This effect could be attributed to spectral overlap with the aluminium ion line at 193.15 nm. 7.3.3 Purification of argon In operation it was found that there was a high background emission from the plasma discharge at the 193.04 nm and 247.86 nm lineS which was found to be caused by.the presence of unsaturated hydrocarbons and carbon dioxide in the cylinder argon employed. The reported concentrations of both of these gases is ca 0.5 cm3 of gas in 1 m3 of argon. This led to a somewhat noisy signal at the carbon lines. It was found that these carbon containing species could be totally removed from the argon gas flows by passing the argon through a column packed with chromatographic grade alumina. The interesting fact that was discovered in the course of this treatment is that if only the injector gas flow were purified, then the impurity carbon signal was virtually undetectable at the 193.04 nm line. This suggests that the function of the 202 coolant flow is to physically shield the plasma from the tube walls and that the contribution of the coolant flow to the plasma discharge is small. Use of this absorbant provided a slight improvement in the signal:noise ratio obtained from aqueous carbon solutions and considerably improved the signal:background ratio. These hydrocarbons and carbon dioxide could also be removed from the argon by passing through a cold trap of liquid nitrogen, although this system is not so easy to operate in practice'and requires constant replenishing of-the liquid nitrogen. 7.4 Other carbon species Atomization of carbon compounds in the plasma was found to be almost complete, although some C2 band system could be detected when high concentrations were introduced into the plasma. No CN band system was well developed when amines were introduced into ' the plasma and no CH band system could be detected at all. Truitt and Robinson [65,66] did not detect any of this latter band system when they introduced gaseous hydrocarbons into the plasma, but found that the CN and C2 band systems were well developed. This is probably due to the fact that their plasma operated at 8 MHz so that the tear shaped plasma is formed where the majority of the introduced sample does not experience the high'temperature of the plasma 203 core as is the case with the 36 MHz plasma. It seems quite apparent, however, that there must be some C2 and or: molecules formed in the discharge, but that when the carbon is introduced as an aqueous solution via an indirect nebulizer, the concentration of these molecules in the tailflame is very much less than when the carbon compounds are injected as gases. CHAPTER 8

An Investigation of the Emission Spectroscopic Properties of Mercury in an Induction Cou,ple& Radiofrequency Plasma. 204 8.1 Introduction The determination of mercury by atomic spectroscopy is usually performed using the 253.65 nm line in emission, absorption and fluorescence, although it is well known that the 184.96 nm line should provide greater sensitivity. Table 8.1 shows the theoretical intensities at a plasma temperature of 8250 °K for these lines. mercury has been determined by emission from a microwave plasma [94,100] at the 253.65 nm line. Recently, the absorption of mercury in a graphite furnace [34], cold vapour cell [38] and separated flame [41] has been described using the 184.96 nm line. fl 8.2 Experimental Stock solutions containing 10 kg m 3 of mercury were prepared by dissolving analytical grade mercury [I] nitrate [17.09 g] and mercury [II] nitrate [11.40 g] in a small volume of 1:1 nitric acid [50 cm3] and diluting to volume [1 dm3]. 8.3 Determination of mercury The observed emission spectrum for mercury over the ranges 184-186 nm and 253-256 nm [fig.8.l] shows that the introduction of mercury into the plasma as mercury [I] gives an apparent enhancement compared to mercury [II]. This may be attributed 205

TABLE 8.1 THEORETICAL INTENSITY AT 3250 DEGREES K.

FOt-< VARIOUS LINE-S OF MERCURY

WAVELENGTH G A E(I) - E(K) RELATIVE NM 10 1- 8/SEC K INTENSITY 134.96 3 7.55 0 - 5L065 1.0000 253.65 -3 .09 0 - 39412 .1163

INDICT TES A 1ULTIPLET

VALUES TAKEN FROM R.7.FERLNCE 101 •. . Fig.8.1' Emission spec,- ium observed over the wavelength range 184-255 nm. for mercury introduced into the plasma as aqueous solutions containing 20 g.m-3 of mercury as mercury [I] nitrate and mercury [II] nitrate. Gain in B is 1/2 gain in A.

Hg[II]

1,1-1,1w •

VJ L C..17 3 .

1 a5rAS 253 253 207 to the disproportionation of mercury [I] into mercury [II] and mercury [0], the latter having a much greater vapour pressure than the mercury cations. This results in an enrichment of the vapour leaving the nebulizer and entering the plasma. This effect has also been observed in flame systems [42,102]. Although the signal obtained at the 253.65 nm line is greater than that obtained at the 184.96 nm line, the latter line shows the better signal:noise and signal:background ratios. The higher signal:noise ratio is a natural result of the greater intensity emitted at the 184.96 nm line while the better signal:background ratio is caused by the high plasma background around 250 nm. 8.3.1 Detection limits and calibration curves The detection limits and observed relative intensities obtained for mercury introduced into the plasma as mercury [I] and mercury [II] are shown - in table 8.2. Calibration curves established for both mercury [I] and mercury [II] at the 184.96 nm and 253.65 nm lines are shown in figs.8.3 and 8.4 respectively. Significant deviation from linearity occur's at the 184.96 nm line at a lower mercury concentration than at the 253.65 nm line. Also at both lines, mercury [I] has a smaller linear range 208

Table 8.2 Limits of detection and relative intensities at atomic lines of mercury investigated.

Species Wavelength Limit of Detection Intensity nm. g.m-3. arb.units. 184.96 0.001 36 Hg[I] 253.65 0.030 100

184.96 0.002 22 Hg[II] 253.65 0.050 61

Table 8.3 Effect of 50-fold amounts on emission signal obtained from solutions containing 10 g.m-3 of mercury.

Diverse Ion 184.96 nm. 253.65 nm. Hg[I] Hg[II] Hg[I] Hg[II] Co[II] _ 0 0 + 30,E + 60X Fe[III] 0 0 + 80j +130X Mn[II] o o + 10X + 20j mo[a] 0 0 + loj + 15j Ni[II] 0 0 + 4o% + 70% Sn[II] 0 + 80% 0 + 70% Cl[-I] - 80: 0 - ao% 0 PO [-III] 0 0 + 25% + 40% 4 Fig.8.2 Calibration curves obtained at 184.96 nm. for 4, mercury as aqueous mercury [I] nitrate and mercury [II] nitrate solutions.

2'

1 • =CENTRUM, M.Pgr3 Pig.8.-3 Calibration curve obtained at 253.65 nm. for mercury as aqueous mercury [I] nitrate and mercury [II] nitrate solutions.

3

2 CONCENTRATMN ; 211 than mercury [II.] caused by the disproportionation of the former ion. 8.3.2 Effect of foreign ions No significant chemical or physical interferences were observed when fifty-fold weight excesses of A13+, Ca2+, Cr3+, Cu2+, Mg2+, Zn2+, ammonium, acetate, fluoride, nitrate or sulphate were present in a solution containing 10 g m-3 of mercury as either mercury [I] or mercury [II]. Table 8.3 [p.208] shows the magnitude of the interferences that were observed and with the exception of those caused by tin [II] and chloride were all due to spectral overlap. Tin [II] produces an enhancement in the signal obtained from mercury [II]. The signal obtained from this solution is the same as that obtained from a similar concentration of mercury [I]. This suggests that the reduction of mercury [II] to mercury [I] ' had taken place but that the reduction of mercury [I] to metallic mercury had not. Upon leaving the solution to stand for several days, it was found that a greyish-black precipitate was formed and that the mercury concentration of the clear solution was much reduced. Chloride reduces the signal obtained from a solution of mercury [I] since mercury [I] chloride is immediately precipitated upon Mixing the two 212 solutions. If the precipitate was taken up in excess of disodium tetraethylenediamine-tera-acetic acid before the solution was made up to volume, then no interference was observed. 8.3.3 Analytical line of mercury The 184.96 nm line is the most sensitive mercury line investigated and has no significant spectral overlap with lines of foreign elements, whereas the 253.65 nm line overlaps with the spectral lines of several metals and also lies in a region of high plasma background emission. 8.4 Possible applications The detection limit of about 1 mg m-3 should allow the direct determination of mercury in urine. The mercury content of soils is ca 80 mg kg-1 and of shales ca 400 mg kg 1 [88] in the solid so that an extraction procedure similar to that used for phosphorus should enable the determination of mercury in these materials. Other types of rock and coal [103] are usually analysed for mercury by heating the matrix and absorbing the mercury on gold foil or wire. The wire is then heated after the matrix has been decomposed and the liberated mercury driven into a cold vapour cell where it is determined by atomic absorption spectrometry,at the 253.65 nm line. By 213 using the plasma discharge it should be possible to vaporize the mercury directly into the plasma and monitor the emission signal at 184.96 nm. An even more attractive method than this would be to introduce the powdered rock into the plasma directly and to determine all trace constituents simultaneously, but the absence of a suitable powder introduction system means that, at present, this is not possible. CHAPTER 9

An Investigation of the Emission Spectroscopic Properties of Arsenic and Selenium in an . Induction Coupled Radiofrequency Plasma. :214 9.1 Introduction The strongest emission lines of arsenic and selenium are shown in tables 9.1 and 9.2 respectively, together with their theoretical intensities. It can be seen from these tables that the strongest lines of both elements lie in the range 185-230 ma, where there -is*only a slight attenuation of the signals by atmospheric absorption. Both arsenic and selenium have been determined by atomic absorption spectrometry in argon-hydrogen [104,105], air-acetylene [106,107,108,109] and separated nitrous oxide-acetylene [40] flames using the arsenic 193.70 nm and selenium 196.03 nm lines. These elements have alsobeen determined by atomic absorption in a graphite cuvette [34] and I•iassman furnace [35]. The use of emission techniques for the . determination of these elements has also been reported.' Goto et al. [30] determined arsenic in steels at the 197.20 nm line using a low voltage overdamped spark discharge. Selenium has been determined in geological samples [110] and rare earth products [ill] using similar types of discharg6. Arsenic and selenium have been determined in a microwave plasma [94] while Murayama et al. [100] reported the determination of arsenic in a 215

TABLE 3.1 THEORETICAL INTENSITY AT 8250 DEGRES K. FOR VARIOUS LINES OF ARSENIC

WAVELENGTH A E(I) - 2(K) RELATIVE NM 10". 8/SEC K INTENSITY 189.00 6 1,20 0 52910 1,0000 193.70 ". 4 1.15 0 51610 .7S24 197.20 2 1,08 0 50694 .423C 198.97 6 .22 10592 - 60831 .0436 199,05 4 1.31 12592 6u815 .1736 200.34 10 .69 10915 - 60815 .2929 223.81 4 1.63 10915 54605 .5562 23-t.40 4 .19 18186 60835 .0213 236,97 6 .25 18648 - 60835 .6416 237,08 IF .35 13645 - 60815 .0389 278.02 .4 .44.. 18648 .• 54605 91236 .

* INDICATES A MULTIPLET VALUES. TAKEN FROM REFERLNCE 95 216 '

TABLE 942 THEORETICAL INTENSITY AT 8250 DEGR1ES K,

FOR VARIOUS LINES OF SELLNIUM

WAVELENGTH A E(I) - E(K) RELATIVE NM 1C44 8/SEC K INTENSITY 140.50 3 .90 0 - 71290 .0121 143450 5 1433 1959 - 71720 40271 144490 3619 1989 - 70700 .0154 145460 3 1.57 2534 - 71290 .0204 196.03 3 2,99 0 - 50997 1.0000 203.93 3 1.40 1939 50997 44500 2066 23 3 .48 2534 - 50997 .1526

* INDICATES A MULTIPLET

VALUES TAKEN FROM REFERENCE 95 217 2450 MHz plasma and fouid that the arsenic emission was confined almost entirely to the core of the plasma. Dickinson and Fassel [54] also determined arsenic in a radiofrequency plasma using the non— resonance line at 228.81 nm. 9.2 - Experimental. A stock solution containing 1 kg m3 of arsenic was prep.ared by dissolving analytical grade arsenic [III] oxide [1.320 g] in 2M hydrochloric acid and diluting to volume.[1 dm3] with hydrochloric acid. A second arsenic stock solution was prepared by dissolving arsenic [III] oxide in 2M sodium hydroxide [ca 50 cm3] and diluting to volume [1 dm3] with di6tilled water. A stock solution containing 1 kg m 3 'of selenium was prepared by dissolving analytical grade sodium selenite [2.190 g] in distilled water and diluting to volume [1 dm3]. 9.3 Determination of arsenic and selenium Fig.9.1 shows the emission spectrum observed over the range 187-201 nm with a purged optical system when a solution containing 20 g m-3 of arsenic was introduced into the plasma as aqueous arsenous acid. Fig.9.2 shows the same emission spectrum obtained with an air filled optical path. It is quite apparent that purging of the optical path improves 218 •

Fig.9.1 Emission spectrum obtained over the wavelength range 187-201 nm. from arsenic' introduced as a solution containing 20 g.m-3 of arsenic as aqueous sodium arsenite with a purged optical path.

187. 201

3 11, • •219

Pig.9.2 Emission spectrum observed over the wavelength range 187-201 nm. from a.solution containing 20 g.m-3 of arsenic as aqueous sodium arsenite with an unpurged optical path.

187 201 n'tolr7 1-2 5;41;1. 220 the signal obtained for arsenic at 189.0 nm but this effect is much less at 193.70 nm and neglible at 200.34 nm. Fig.9.2 quite clearly shows the effect of the attenuation of the far ultraviolet emission from the plasma by atmospheric absorptioh. Fig.9.3 shows the emission spectrum obtained from a solution containing 20 g m 3 of selenium over the range 194-198 nm with and without purging of the optical path. In this case it can be seen that the effect of purging is almost neglible at the 196.03 nm line. Fig.9.4 shows the emission signals obtained for arsenic around the 228.81 nm lin3 and for selenium over the range 203-207 nm. 9.3.1 Detection limits and calibration curves Tables 9.3 and 9.4 show the relative intensities and detection limits obtained at the principle arsenic and selenium lines respectively. The use of nitrogen purging of the optical system produced a marked improvement in the intensity and detection limit observed at the 189.0 nm line and marginal improvements at the other lines below 200 nm for both arsenic and selenium. Calibration curves were established for arsenic at the 189.0 nm [fig.9.5], 193.70 nm [fig.96], 197.20 nm and 228.81 nm lines and were found to be

221

•Fig.9.3 Emission spectrum observed over the wavelength range 194-198 nm. from a solution containing 20 g.m-3 of selenium as aqueoue sodium selenite. A with a purged optical system, B with unpurged optical system.

A B

LaSa c2-4 ca

saw 7'

g5 (96 Trit1171

Pig.9.4 A Emission .spectrum observed over the wavelength range 228-230 nm. from a solution containing 20 g.rn-3 of arsenic. B Emission spectrum observed over the wavelength range 203-207 nm. from a solution containing 20 g.m-3 of selenium.

A E1

229 2b3 WAVELEUGTH,Nal. 223

Table 9.3 Limits of detection and relative intensities for atomic lines of arsenic investigated.

Wavelength Limit of Detection Relative Intensity nm. g.m 3. .arbitrary units. 189.0 0.11 46 189.0° ' 0.27 22 193.70 0.10 50 197.20 0.16 36 198.97 0.30 15 200.33 0.25 24 228.81 0.13 100

with unpurged optical system.

Table 9.4 Limits of deteCtion and relative intensities for atomic lines of selenium investigated.

Wavelength Limit of Detection Relative Intensity nm. g.m-3. arbitrary units. 196.03 0.11 100 203.44 0.16 72 206.28 0.37 28

•

Pig.9.5 Calibration curve obtained at 189.0 rim. for aqueous , 6 solutions .of arsenous acid.

3

25 5O7 CONCENTRATI ON, G.M. • Pig.9.6 Calibration curve obtained. at 193.70 rim. for arsenic ' 6- as aqueous solutions of. arsenous acid.

. 3

10 CONCENTRATION,G.M -3 •

226 linear to above an arsenic concentration of 50 g_ml The selenium calibration curve obtained at the 196.03 nm line, shown in fig.9.7, was also found to be linear over the same concentration range. 9.3.2 Effect of foreign ions The presence of a fifty-fold weight excess of Al3+, Ca2+ Cu2+ Cr.3+ Fe3+ K+ Mg2+ Mn2+ 1 2+ Na+ , 'ammonium, acetate, chloride, fluoride, Mo02 1 iodide, nitrate, phosphate and sulphate ions, produced no significant interference on_a solution containing 10 g m-3 of arsenic at the 189.0 nm, 193.70 nm or 228.81 nm lines. The interference effects observed for cobalt, nickel and-zinc are shown in table 9.5. The presence of fifty-fold weight excesses 3+ 2+ 2+ 2+ 3+ 3+ 2+ of Al , Ca , Co , Cu -Cr , Fe , KK+, Mg 1 Mn2+, Na+ Ni2+, Zn2+, ammonium, acetate, chloride, fluoride, iodide, nitrate, phosphate and sulphate ions produced no significant interference on a solution • containing 10 g m-3 of selenium at 196.03 nm. Molybdenum produced a 10 signal enhancement at this line when present in a selenium solution as a fifty-fold weight excess, but did not interfere at 203.98 nm. 9.3.3 Analytical lines for arsenic- and selenium The 189.0 nm arsenic line appears to be the most useful line of this element for analytical purposes observed in a plasma as no significant Fig.9.7 Calibration curve obtained at 196.03 rim. for aqueous solutions of sodium selenite.

.41

50 C ONCENTRATION,G.M- Table 9.5 Effect of 50-fold amounts of diverse ions upon the emission signal obtained from a solution containing 10 g.m 3 of arsenic.

Diverse ion Wavelength, nm. and interference effect, X 189:0, 193.70 288.81

Co[II] 0 + 65

Ni[II] 0 +100

Zn[II] 0 + 10 229 interferences were observed at this line and, when the optical system is purged, the detection limit obtained at this line is similar to that obtained at the other lines. The lines at 193.70 nm and 228.81 nm do not require purging of the monochr.omator to obtain satisfactory detection limits. The 228.81 nm line, however, suffers severe spectral interference from the strong lines of cadmiun, cobalt and nickel around 228.8 nm which restricts the analytical usefulness of this line; the 193.70 nm line had only one observed interference which was only slight so that this line represents an adequate substitute for the determination of arsenic to the 189.0 nm line should purging of the optical system not be possible. 9.4 Applications Because of the interference of molybdenum upon the selenium signal at 196.03, the direct determination of selenium in molybdenum sulphide, which is often used as a lubricant additive, would not be possible without a prior chemical treatment, although the 203.98 nm line might be useful for this purpose. Both of these elements could readily be determined in foodstuffs, pharmaceuticals, weed-killers, etc. as well as in steels, alloys,etc. CHAPTER 10

Line Profiles in the Radiofrequency Plasma.

230 10.1 Introduction Detailed accounts of the types of broadening of spectral lines and their origins are given by Narr [112], Kuhn [113], Condon and Shortley [114] and Chandler [115]. In the induction coupled radiofrequency plasma it would be expected that apart from the natural, Doppler and collisional broadening observed in high temperature systems, Stark and Zeeman broadening caused by the high electric-and magnetic fields required to sustain the plasma might be observable. 10.2 Natural Half-width The fundamental broadening of a spectral line is that which occurs for an atom which is completely isolated from its neighbours, is motionless and not subjected to electric or magnetic fields. Under such conditions, a transition has a natural line width as the atom remains in the excited state for a lifetime, tki, which depends upon the transition probability [112,113,114,115].

t 1 [10,1] ki = Aki According to the Heisenberg uncertainty principle, this leads to an energy uncertainty given by -

• tki AE N 27c [10.2] In wavelength terms, if the lower state is assumed to have an infinite lifetime compared to the

231 excited state, this becomes :- Aki x2 -9 N = 2n c x10 [10.3] 10.3 Doppler line width If the light source is moving with a velocity whose component in the line of sight is v, then the frequency appears to be shifted by an amount vov/c compared to the frequency of the source at rest, vo. If the natural line width is assumed to be neglible, then the distribution of the: spectral line is determined by the Maxwellian distribution of the velocities [112, 113] so that if P[v]dv-is the probability that the velocity component lies between v and v+dv, then :- M P[v] =[----2n RT]2 exp[-MA v2/2RT] [10.4] The shape factor of the spectral line due to all Doppler shifted components can be shown [112] to be given by :- M M C2r- Lv7V 12j F[v—V c rA ° 1 0]=— v 2uRT eXP L 2RT 2 J [10.5] 0 The half-width of the Doppler broadened line is obtained by computing the frequency separation where the shape factor falls to half F[0] and is given by :-

2vo r2RT1n21* M [10.6] D = c L A Converting this expression to wavelength units and 232 substituting in the known constants gives equation 10.7 where all wavelengths are in nm.

GAD = 7.16x10-7XPuA [10.7]

10.4 Collisional broadening The classical treatment of collisional broadening as developed by Lorentz [117] makes use of the theory of damped oscillators and the Lorentz electron theory. According to classical kinetic theory, the mean time between collisions of two unlike species is given by 2 rlrl A 1 = 3.33x10 aAaFPLmlu +7 JJ 110.8] ' " "F 2 2 where aA and c are the optical collisional cross- section areas and P is the total pressure of the system. At the pressure of the plasma and with a low particle density of analyte, then collisions between analyte atoms and argon atoms will predominate over collisions between analyte atoms and other analyte atoms so that this latter may be effectively ignored. When equation 10.8 is transformed into wavelength units using the approach described for natural line width in equations 10.1-10.3, then the collisional half width can be shown [112,113,116] to be given by :- Ti AX -62 l-rATLErlrl A 0.9] = 3.50x10 0 aA -FTI [1 c F 233 10.5 Zeeman broadening As early as 1897, Zeeman [118] was able to show that a strong magnetic field produced a broadening of atomic lines and that the broadening was dependent upon the magnetic field strength. In 1898, Preston [119] showed that the lines were, in fact, split into symmetrical multiplets and not broadened. It can be shown [112,113,114,115] that the interaction of the magnetic moment of an eleCtron with a magnetic field would introduce an energy change represented by :-

AE = TTEeh MLB = pBMIB [10.10]

In a weak magnetic field, equation 10.10 can be generalized by writing :-

AE =Mgp,J J B ['ma] where gj is the Lands factor for each particular energy level of the atom. Equation 10.10 represents the case where the normal Zeeman effect is observed. This normal Zeeman effect is only observed in the special case of transitions involving singlet states [S=0]. Where S has a non-zero value, then the spin of the electron means that L-S must be taken into account. The so-called anomalous Zeeman effect is given by equation 10.11. In a very strong magnetic field, the L-S 234 coupling may break down and the L and S vectors may then be considered to precess rapidly and independently about the magnetic field direction forming quantized components ML and Ms. The magnetic interaction terms can then be written as :- AE = IBB[ML+2Ms] [10.12]

10.5.1 Intensities of the Zeeman components For weak field conditions, the splitting is small and the states can be regarded as being equally populated provided that the magnetic energies are small compared to kT. All the statistical weight factors are then unity and thus it is possible to derive general rules concerning the intensities of the Zeeman components in a multiplet. . The sum of the intensities of all the n components [where AML=0] of a given line in the transverse Zeeman effect can be defined as ZEm m I,. i k " Similarly, the sum of all the a components [where

AML .+1] in the transverse Zeeman effect can be defihed as T,1 1\1k l a` The a components appear circularly polarized in the longtitudinal direction of observation, i.e. the z-direction and plane polarized with the electric vector in the x-y plane in the transverse direction of observation. The t component is absent in the

235 longtitudinal direction and plane polarized with the electric vector in the z-direction when viewed in the transverse Zeeman effect. Since light must be regarded as being unpolarized in the limit of B-40, it must be expected that :-

c1.14 [10.13] M.14k I a k In A definite value of H for the atom in a given excited state corresponds to a definite orientation of the total angular momentum in space. Since the orientation cannot affect the rate at which an atom radiates, it must be assumed that the total energy of one given excited state of a given M is the same for any state. The rate of emission of any it component, averaged over all directions is found as the sum of the intensities radiated in the two transverse directions and the longtitudinal direction. 21 + 0 Tc = 7C [10.141 By considering the o components, it is found that components of a given frequency correspond to one oscillation when viewed in any of the two transverse directions and to two oscillations when viewed longtitudinally. Thus :- [10.15] I. 41a In comparing total emission rates of energy, therefore, all a components have twice the weight of all it components when viewed transversely. This . 236 leads to the SUM RULE for the total emission from any two levels represented by [Ml] and [M2]. *5.14.[I,[H1] + 2I01M1]] = 11 [I,[H2]+2Icy[H2]] 1 " " [10.16] where the summation is carried out over all T.ii of the common lower levels and I0. and In are the intensities viewed in the transverse direction. The intensities of the components are independent of the values of L and S but are a function of J only. The values of L and SI however, determine the positions of these components relative to each other in a given multiplet system. 10.5.2 Zeemansylingofsome spectral lines Although it is in theory possible to calculate the shift of the Zeeman components from equation 10.11 and the relative intensities of these components from equations 10.13 and 10.16, such calculation, in practice requires a rigorous mathematical treatment for all but the simplest cases. The mercury 184.96 nm line is an example of the normal Zeeman effect where the states involved in the transition, 1S0-41P11 have a zero electron spin. Fig.10.1 shows the transition involved in the absence and presence of a magnetic field. The lower state is unaffected by the magnetic field since L=S=J=0, while the energy change of the upper state

P1

Tr

•Pig.10„1 Zeeman pattern of the mercury 134.96 nm, line in the presence of a weak magneio. field. 238 is equal to 11)3B. From equation 10.16, it is clear that the u component is twice as intense as the a component. . The 253.65 nm mercury line corresponds to the transition, 1S0-43P1, which is forbidden since there is a change in electron spin, AS=1-1. The appearence of electron'spin in the upper state means that the energir-chango is now equal to gjp..BB where' the value of gj taken from tables [113,114] is 3/2. Although the energy separation at this line is greater than at the 184.96 nm line, the relative intensities of a and u components within each multiplet system is the same. The iodine 206.16 run line corresponds to .41) the transition, 2P 1/2' -3/2, Both the states involved have an electron spin, so the anomalous Zeeman effect is observed. Once again, however, the electron spin changes in the transition so the transition is spin forbidden. Fig.10.2 shows the Zeeman splitting observed for this multiplet. The separation of energies in the 2P state is equal to • 2/3 11.BB and that in-the 4P state 26/15 IIBB. Denoting the state Mj=+-1- as M1 and the state lij=-11- as M2 and considering only the components shifted to low frequency, as those shifted to high frequency are a mirror image, equations 10.13 and 10.16 can be written as :- +1/2

—

" I/2

mIlimm. OM= OM. ••■■•• ••• •■•••••■

0- cr "rr Tr 0""

M2 MI MI M2

Fig.10.2. Zeeman pattern of the iodine 206.16 nm. line in the presence of a weak magnetic field. I,n[N2] = I¢[Ml] + Ia[M2]

[M2] = 2I [Ml] 2Ia[M2] + In a Solving equations 10.17 and 10.18 simultaneously gives :-

1 [M2]:Ia[1,12]:Ia[Ml] = 4:1:3 The other Zeeman component intensities of more complex multiplet systems can be obtained from tables [115]. Fig.10.3 shows Zeeman patterns obtained for some common far ultraviolet multiplet transitions in which the normal practice of showing it components above the line and a components below has been followed. 10.6 Stark broadening In contrast to the Zeeman effect, the Stark effect, which is due to the influence of an electric field,is asymmetrical about the original line [120]. The electric field cannot act upon the magnetic moment of the atom associated with an angular momentum J, but, instead, causes a polarization of the atom. This produces an electric dipole whose moment is proportional to the electric field strength, F, and is a function of J. The interaction of the electric field and dipole produces a precession of J about the electric field direction such that the component Mo. in the field direction is constant.

Multiplet. Shift in units

I (0)1 So

(0) 3 1S0 3P I 2

3 3 (0)2 S1 P0

(1)34. 2

(0)02 3 4 2

4 4 (05 79 II • 542— 5/2P 5

(0)1 P—D2

Fig.10.3 Zeeman Patterns of some common maltiplets in the presence of a weak,magnetic field.

242

Shift Muliplet in units utBB 2p (V 6 U19 21 IS

5P (0) P2 - 2 —2—

4 (2)(6124 28 32 S L 3/2 P3/2 H 15

2r) jirj I I (2)(6)18 22 26 30 15 3/2 512' • 1 1 • .

• 2 4 P p r7 23 29 212 3/2 I 1 1 I 15

(61834 15

iii (I) 3 5 25112 3/2 3

Fig.10.3 cont. Zeeman patterns of some common multiplets.

243 In a highly ionized plasma, the charged electrons and ions predominate as perturbers since radiating atoms are much more strongly perturbed by charged particles than neutral ones'. In its simplest form, the emitter is assumed to be perturbed by the nearest neighbouring ions only [112,113,114]. The probability of finding an ion at a distance r from the emitter if interactions between charged species are neglected is given by equation 10.19 in which ro is the average distance between ions.

dP[r] = exp-4-Pd[p3 [10.19]

The field producing the Stark effect is then given by equation 10.20 for the nearest ion. Ze F - ----2.6 "beim ,2/3 [10.20] o 2 - 41-cso e 41te oro Thus the probability of the radiating atom being subjected to an electric field F is given by equation 10.21 where dP[F] is normalized.. F dP[F] = ,15[-i-]`"exp-[4F 1]3 /2 dF [10.21]

More generally this can *be rewritten as equation 10.22 if we put pH = Vro.

I il dP[pH] - 4c5/2exp-pT3/2dp [10.22] or dP[13H] = wil[pH]apH [10.23] 244 In this equation WHPH is defined as the Holtsmark ion field distribution function. From a simple quantum mechanical treatment and assuming a linear Stark effect it is possible to write the Stark shift as equation 10.24 where Ck is the appropriate constant coefficient for the Stark line component of's_ given atomic line [112,113, 114] and is a function of the principle quantum numbers of the states involved in the transition.

3he oC k 67s 2007cmecZ [10.24] The line profile can be obtained by substitution of equation 10.24 into 10.23. F[O]d[0] = wH[pH]dpii [10.25]

This can be reduced to equation 10.26 where Avo is defined by equation 10.27.

F[0] = VITT[g [1 n nV]o AV 0 1 0.26]

7.8hC A7 k N2/3 [10.27] o 800n2mc e If the half intensity line width is computed by a system similar to that employed for the Doppler half-width and it is assumed that the half-width is small and almost symmetrical, then the half-width can be shown to be dependent upon the Stark shift by equation 10.28. 0 245

st = 22/5A7 [10.28] In terms of the electron density, this expression becomes :- 9.75hCip o kz AV - [10.29] st 800n 2mc e or in wavelength units :-

1.22hX2C AX - k 2/310 -9 [10.30] st n 2me Ne For the hydrogen 486.13 nm line [Hp line], this expression becomes upon substitution of the known constants ;- 2 3 st = 2.84x10-15 Ne/ The measurement of the half-width of the Hu or H lines in an electric field,therefore, represents an accurate method of measurement of the electron density of the system. Hydrogen and hydrogen-like atoms are the only atoms known to give a linear Stark effect, and all other atoms exhibit the quadratic Stark effect. For this latter effect, eauation 10.24 becomes :-

h5e6o Ck A - E2 [10.32] 2511Pe6cV Equation 10.26 and 10.27 can then be rewritten as eauations 10.33 and 10.34 respectively for the quadratic Stark effect. 246 AV 1[A-,7] = -1-wliT[7.p][AV0AV] [10.33] vo

A _ 1.69h5s4Ck N4/3 [10.34] 100m5e4n2cZ e 10.7 Line profiles in the plasma The profiles of some spectral lines of some elements introduced into the radiofrequency plasma are calculated using the optimum plasma operating conditions. This assumes a plasma temperature of 8250 °K and that the emission is viewed at a point 20 mm above the top of the work coil. It is further assumed that the current flowing through the work coil is 1 A and the potential is 5 kV. The natural line width, calculated from equation 10.3 is shown in column 4 of table 10.1. This line width is fundamental to the transition involved and is the same no matter what the environment of the atom. The Doppler half-width of the spectral lines calculated from equation 10.7 would be expected to be the major broadening effect observed in a plasma. As shown in column 5 of table 10.1, this is in fact the case and for the calcium 422.67 nm line, the Doppler half-width is greater than the total measured half-width in a nitrous oxide-acetylene flame [121]. The calculation of the collisional half-width Table 10.1 Half-widths of spectral lines calculated for a temperature of 8250 °K.

AXIS, nm. AX nm t1 , nm. nz, nm. Species Transition X, nm. DI • 0.0000071 • 0.0021 0.00030 0.000018 s 3P0 -3S1 180.73 182.04 0.0000038 0.0021 0.00030 0.000019 3P1 _3S1 182.63 0.0000013 0.0021 0.00031 0.000019 P2 - S1 4p 0.0000165 0.0020 0.00026 0.000017 P 5/2 _4s3/2 167.97 4, _4s , 178.29 0.0000036 0.0021 0.00029 0.000019 '3/2 3/ 2 2p 2n 213.60 0.0000073 0.0025 0.00042 0.000026 '3/2- '5/2 - 1) m 165.72 0.0000060 0.0031 0.00035 0.000012 3P2 3 2 1p 193.04 0.0000049 0.0036 0.00047 0.000010 -1 1 - 2 247.86 0.0000011 0.00078 0.000026 3'1 0.0047 1 0.0000138 0.0008 0.00023 0.000010 Hg 1 P1 So 184.96 3p1 253.65 0.0000006 0.0012 0.00043 0.000027

As 189.00 0.0000023 0.0014 0.00027 0.000022 4'5/2 453''3/2- /2 193.70 0.0000023 0.0015 0.00028 0.000022 4'3/2-3/2 453/2 228.81 0.0000045 0.0016 0.00040 0.000021 2p 3/2 2'5'5/2/2 4•-• Se 196.03 0.0000060 0.0014 0.00029 0.000020 3P1 -3S1 0.0000031 0.0015 0.00031 0.000023 3P2 -3S1 203.98 Table 10.1 Continued.

, Species Transition X, nm. ANN, nm. AND, nm. ANc' nm. AXzt nm. 4p 2 5/2- P3/2 178.28 0.0000046 0.0010 0.00022 0.000018 4p 183.04 0.0000003 0.0011 0.00023 0.000018 3/2 '3/2 206.16 0.0000001 0.0012 0.00030 0.000027 4'3/2 2P1/2 H '3/2 486.13 0.0000011 0.0316 0.00922 • 0.000093 2'5/2-2'3/2 Ca 1P1 -1SO 422.67 0.0000021 0.0043 0.00154 0.000050 } 393.37 0.0000012 0.0040 0.00133 0.000130 Ca ?L3/2- 2 ''"1/21" /2

Transition shown is the transition with greatest transition probability in a'complex multiplet system not resolvable with the band-pass used, i.e. 0.08 nm.

249 depends upon knowledge of the collisional cross-sectional areas of the plasma species. Winefordner [108] has discussed this problem for various flames and has suggested that values of 0.2 nm2 and 1.0 nm2 represent the limits to the range within which most collisional cross-sectional areas are to be found. The values shown in table 10.1 are those calculated from equation 10.9 using the maximum collisional cross- sectional area of 1 nm2. The outstanding feature of the plasma discharge is that the collisional half-width is almoSt neglible compared to the Doppler half-width whereas in a flame the two are similar. The total line width can be calculated using equation 10.31 if resonance broadening and the natural line width are neglible. AV AV , 2 rr AT; 1- AT; -= 2 LL 2 .1 DJ [10.35]

The total half-widths calculated from this eouation are less than 10 j greater than the Doppler half-width, so that, to all extents and purposes, a Doppler distribution can be assumed for a spectral line in a plasma tailflame, and the line profile can readily be calculated from equation 10.5. 10.7.1 Zeeman splitting In order to calculate Zeeman splitting of atomic lines in the plasma tailflame, it is. first 250 necessary to determine the magnetic field strength. This can be conveniently be achieved if the assumption is made that the work coil acts as a short solenoid [fig.10.4]. From simple electromagnetic theory, it can be shown [122,123] that the field at a point P situated a distance x above Q, the centre of a coil of radius a and consisting of n turns per metre is given by :- H = ni[cospi-cosp2] [10.36] where cosP, s + x [[s+x]+a21 s - x and cosp2 = f[s_x]2+a2]?1.

The magnetic flux density is related to the magnetic field strength via equation 10.37.

B = 110[1TH [10.37]

For the coil used on the plasma, the magnetic flux density is typically ca 0.03 mT at a point 20 mm above the top of the coil. The separation of the components in the normal transverse Zeeman effect given by equation 10.10 is ca 0.014 mK. For the calcium 422.67 nm line, this corresponds to a wavelength shift of ca 10-8 nm for the a components. In the coil centre, equation 10.36 reduces to

H = ni2a [10.38] s 2ig.10.4 Schiematic of the magnetic field in the work coil. 252 Thus the magnetic flux density at the coil centre is ca 3 mT which gives a separation of Zeeman components of ca 1.4 mK. The values for ANz shown in table 10.1 are the total separations between the two Zeeman components shifted by the greatest amount to high and low frequency for an atom in the coil centre. 10.7.2 Stark splitting The electron density calculated from the observed degrees of ionization of zinc and ,:alcium 3 using the Saha equation is ca 1021 m for a plasma at 8250 °K. Johnson [124] has reported the electron -3 density in a plasma core to be ca 1-3x1021 m . Electron densities of plasmas quoted by various other workers [67,81,84,85] are also of this order. It is apparent from equation 16.20, that the microscopic field producing the Stark effect will be about 4x10 5 V m-1 in the plasma if the -3 electron density is 1021 m . A field of this strength will produce a small but significant shift in the position of most spectral lines and will shift the H line by 0.06 nm which should just be observeable with the monochromator used at a 10 p.m slit width. The exact magnitude of the Stark shift cannot be calculated for the majority of elements as Stark component constants are unknown except for a few lines of some elements with a simple 253 electronic configuration. Table 10.2 shows the Stark shift for some common elements in the presence of 7 an electric field of 10 V m-1 obtained from tables [77,125,126]. For a it polarization, the electric vector of the radiation is parallel to the applied electric field and for a a polarization it is normal to the applied electric field. When the polarizations are unknown or not separated, the values are placed in the centre of the columns. It can be clearly seem from this table that the shift for most spectral lines, with the notable exception of the hydrogen Balmer series, is small. Taking a typical shift of ca 10 mK for this field strength for a line at 200 nm, then the corresponding shift in wavelength is 0.00004 nm, which for most spectral lines is to higher wavelength. This shift is, however, small when compared to the Doppler broadening and so for most elements will not be observed unless an interferometer is used. The Stark splitting of carbon [127] and iodine [128] has been observed in fields of 107 V m-1 In both of these cases, no observed splitting was reported for the carbon line at 247.8-nm and the iodine line at 206.16 nm. Table 10.2 Stark shifts of some spectral lines in an electric field of 107 V.m 1. Shifts are shown in nm.

Species X, nm. it components a components H 121.6 +0.0189 0 486.1 +1.5, +1.21 +0.909, +0.607 656.3 +1.11, +0.827 ±0.28 Li 460.3 +0.51 +0.48 Na 330.2 -0.0020 330.3 - -0.0020 589.0 +0.000382 ' +0.000382,.+0.000142 589.6 +0.00026 ' +0.00026 404.4 +0.0067, +0.0034 Mg 383.8 -0.027 -0.037 518.4 +0.0013 +0.0013 Ca 422.7 +0.000036 Sr 460.7 +0.00016 Hg 253.7 +0.000018 +0.000054 Fe 506.5 -0.0549 -0.0454 516.2 +0.23 +0.164 536.7 -0.0550 -0.0337 545..5 -0.0893 -0.0851 CHAPTER 11

A Comparison of the Induction Coupled Radiofrequency Plasma and the Nitrogen Shielded Nitrous Oxide - Acetylene Flame as Spectroscopic Emission Sources. 11.1 Introduction In this section the induction coupled radiofrequency plasma will be compared with a premixed nitrous oxide-acetylene flame as an emission source and some of the claims for the plasma as'an excitation source outlined in chapter 2 will be critically examined. In order to facilitate the calculations involved, a simple model of both cells may be constructed in which the following assumptions have been made.

1 The injector gas of the plasma can be equated in function to the combined fuel and oxidant gas in the flame.

2 The coolant gas flow in the pldsma has the same function as the shield gas in the flame, i.e. it does not play a significant part in sustaining the discharge or carrying the sample.

3 All gases enter the atom cell at constant velocity and with laminar flow.

4 The velocity of the gas stream and analyte particles is the same.

5 All energy is supplied in the plasma core and the flame primary reaction zone. Desolvation and vaporization occur principally in the core or primary reaction zone. 256 7 In the plasma, argon accelerates instantaneously upon entering the core and then travels with constant velocity. 8 In the flame, gas molecules accelerate uniformly through the primary reaction zone and then move with uniform velocity through the viewing [inter-conal] zone. 9 Temperature and flame molar expansion on combustion are the only factors affecting changes in flame gas velocity once the gas has entered the atom cell. Most of these assumptions are self-evident, except for the last three. The last assumption is valid if the inter-molecular reactions within the flame or plasma are small and decceleration due to the effect of gravity is neglible, which is the case for both these cells. If this assumption is valid, then the velocity profile of species in the plasma or flame is controlled by the temperature profile of these cells. Fig.11.l shows the observed and assumed temperature profiles of the plasMa system. The observed temperature is that measured for the axial channel of the plasma by Johnson [124]. The observed and assumed shapes of the plasma core are also shown in the lower half of the diagram. From this figure it can be seen that thq assumption made is fairly • 257

Fig.11.1 Observed and assumed temperature profiles in the induction-coupled radiofrequency plasma.

1 1 I VIEW I 1 PLASMA CORE I ZONE 258 near to the actual case in the viewing zone, but in the plasma core gives a lower temperature than the actual temperature. Fig.11.2 shows the temperature profile in the nitrous oxide-acetylene flame. The temperature profile for the inter-tonal zone is after the measurements of Kirkbright and Vetter [74] and that of the primary reaction zone after Jenkins and Sugden [129]. Once again, the estimation of the velocity through the primary reaction zone obtained by the assumption is slightly low, but is reasonable for the secondary zone. Typical operating conditions for the plasma and flame are shown in table 11.1 and these conditions are used throughout the following compatison. 11.2 Self-absorption and calibration curves The atom concentration of an element, NA, entering the atom cell can be calculated from equation 11.1 where QGT is the volume of flame gas at a flame temperature T flowing through the cell per unit time Qs is the volume of sample taken up per unit time by the nebulizer of efficiency (P,

CA is the concentration of analyte in the sample solution, MA the atomic weight of the element and

AN is Avagadro's number. Q 4) ANN NA QGT m CA 259

PRZ VIEW ZONE

Fig.11.2 Observed and assumed temperature profiles in the - nitrogen shielded nitrous oxide—acetylene flame. - 260 Table 11.1 Typical operating conditions for the RF plasma and the nitrogen shielded nitrous oxide-acetylene flame.

Operating parameter Plasma Flame Fuel gas flow rate 55 cm3 s -1. Injector gas flow rate 50 cm3s-1 Oxidant gas flow rate 110 cm3s -1. Coolant gas flow rate 250 cm3s-1 Shield gas flow rate 150 cm3 s-1 Mean gas density, p 1.784 kg.m-? 0.96 kg.mi Mean specific heat, Op 524 Jkg 1° K-1 . 1350 Jkg-1 ° K-1

Flame expansion factor, Y 1.0 1.6667

Sample uptake rate, Qs 70 mm3s-1 70 mm3s-1

Nebulizer efficiency, (P 0.05 0.1 Path length of cell, 20 mm. 50 mm. Width of burner, bs 0.4 mm. Height of reaction zone, ht 25 mm. 3 mm. Surface area of core, S 22 cm. 340

Volume of core, Vo 7.85 60 mEa Maximum flame temperature 10000 °K. 3000 °K. Mean flame temperature, T 8250 °K. 2800 °K. Ambient gas temperature, To 300 '°K, 300 °K. 261. The volume of flame gases at a temperature T is related to the volume of gas entering the cell at room temperature To by the expression :- n T QGT = Y'G [11.21

Substitution of this expression into equation 11.1 gives :- Qs4, To Ali [11.3] HA = QGY T MA CA For the purposes of comparison of the two atom cells, the calcium 422.67 nm atom line, the calcium 393.37 ion line and the mercury atom lines at 253.65 nm and 184.96 nm will be considered. Table 11.2 gives the relative spectroscopic data needed for these lines to compute their absorption coefficients. 11.2.1 Particle densities The particle densities of all calcium and all mercury species in the plasma calculated from equation 11.3 and using the typical operating conditions of table 11.1 are 3.8x1016 m 3 and 7.6x1015 m 3 respectively. In the flames, these densities are 4.0x1016 m3 and 8.0x1015 m 3 for calcium and mercury species respectively. To determine the particle density of each calcium and mercury species it is necessary to allow for the ionization of these elements. Table 11.2 Spectroscopic data for some emission lines of calcium and mercury.

A% nm ' Particle Density, m3

Species %, run. MA ik Plasma Flame Plasma Flame 16 Ca I 422.67 40.08 .1.75 0.0043 0.0025 0.9x1016 2.4x10 16 16 Ca II 393.37 40.08 0.69 0.0040 0.0024 2.9x10 1.6x10

Hg I 253.65 200.6 0.03 %0.0012 0.0007 7.6x1015 8.0x1015

Hg I 184.96 200.6 1.19 0.0008 0.0005 7.6x1015 8.0x1015 263 In the plasma, the degree of ionization of calcium was found from the relative intensities of the atom and ion pair to be ca 0.75. The calculated degree of ionization of calcium ion was less than 0.01 so this may be neglected. For mercury, the calculated degree of ionization using the Saha eauation is less than 0.01 and may also be neglected. In the nitrous oxide-acetylene flame the degree of ionization of calcium has been calculated to be 0.43 [ 130] using the Saha equation and assuming a calcium pressure of 0.1 N m2. The measured degree of ionization of calcium in this flame has been reported to be 0.43 [131] and 0.38 [132]. The calculated degree of ionization of mercury in this flame [130] is less than 0.01 so can be neglected. The particle densities shown in table 11.2 for calcium were obtained assuming the degree of ionization in the plasma to be 0.75 and in the flame 0.40. It can be seen from these figures that the particle densities of elements which are difficult to ionize such as mercury are about the same in both atom cells. Elements which are easily ionized, however, have a lower particle density in the plasma than the flame but their ionization product, e.g. Ca II, have a much higher particle density in the plasma than the flame. 264 11.2.2 Absorption coefficients The absorption coefficients of Ca I at 422.67 nm, Ca II at 393.37 nm, Hg I at 253.65 nm and Hg I at 184.96 nm calculated on the basis of the particle densities of the individual species computed above are shown in table 11.3. As the optics of the plasma system used approach the case of an infinite object focussed by a lens, the values of KVL and A are calculated for the case where the emission is observed primarily from the edge of the plasma furthest from the detector. For the flame, the values are calculated assuming that the detector is positioned to receive all the emission from the centre of the burner. In the plasma system, this assumed situation does not necessarily arise. If there is a doughnut shaped plasma generated then the particles of analyte are forced either through the axial channel or around the edges of the discharge and, since the axial channel is much hotter than the edge of the plasma, the greatest emission will occur from the centre of the plasma. Thus the emission from the edge nearest to the detector cannot be self-absorbed if the skin of emitting species is thin. The emission from the central channel will be absorbed by the skin nearest the detector to a small extent and the radiation from the edge furthest from the detector will be Table 11.3 Absorption coefficients in the plasma and flame atom cells. Plasma Flame _ -1 A K mi K I, A Species X, nm. Kv'm • Kv11 v' v

Ca I 422.67 5.43 0.109 0.0474 25.0 0.624 0.271

'Ca II 393.37 6.42 0.128 0.0558 5.88 0.147 0.0638

Hg I 253.65 0.10 0.002 0.0009 0.18 0.005 0.0020

Hg I 184.96 3.39 0.068 0.0301 5.87 0.147 0.0637 266 greatly absorbed by the central channel and the nearest edge. , • 7, 1.oe possible to eliminate the effects of self--absorption by positioning the monochromator so that a grazing angle is obtained by the optical system upon the Plasma and only the skin emission is observed. There are, however, two disadvantages with this technique. Firstlyl - the plasma edge is much cooler than.the central channel and secondly, the solid angle of incident radiation upon the monochromator entrance slits is much reduced so that it is only possible to increase linearity of the calibration curve in this manner at the expense of sensitivity. 11.2.3 Theoretical calibration curves' From the values Kv L obtained for these lines in table 11.3, it is possible to construct theoretical calibration curves on the basis of equation 11.4.

Iobs = Iem[1- exp-K v v [11.4]

Iem is defined by equation 1.8. Curves obtained for the four lines considered are shown in figs.11.3-11.6. It is quite apparent from these curves that the plasma has an extended linear range at high concentrations compared to the flame. Also, as the absolute emitted intensity at any line is very much

Pig.11.3 Theoretical calibration curves for calcium emission •in the plasma and flame at the 422.67 nm. Ca I line. 2250 • TY SI SITY N TEN TE IN

IN

ON ON SSI SSI I EMI EM A E SM M LA F PLA

CONCENTRATION, Theoretical emission .calibration curves for calcium at. the Ca II 393.37 nm. line in the plasma and flame. •

35000 ON SSI MI E SMA PLA

5 CONCENTIIIITIEM, EtaStfi:3 Fig.11.5 Theoretical emission calibration curves for mercury emission in the plasma and flame at the mercury' 253.65 nm. line.

5 CCNCENTRATION, KG.Nr3 FLAME EM ISSI ON , Fig.11.6Theoreticalemissioncalibrationcurvesformercury in theplasmaandflameat.mercury184.96nm.line. • CONCENTRATION, EIELM: .5 3 - -4)40 7 271 greater than in a flame, the potential sensitivity of the technique is much higher and an extended range is obtained at low optical densities, i.e. concentrations, for the plasma compared to the flame. A second advantage of the plasma system is that as the absolute emitted intensity is very much higher than that obtained for a flame, it is possible to drastically reduce the sample uptake rate of the nebulizer system. This reduces the particle density obtained in the plasma and hence reduces the value of K. This, therefore, offers a very simple way of extending the linear ranges obtained in the plasma with only a slight sacrifice to sensitivity. 11.2.4 Observed calibration curves In order to test this theoretical observation an adjustable nebulizer [Perkin Elmer Ltd., Beaconsfield, Bucks.] was used in place of the fixed Techtron nebulizer used previously and its effect upon the temperature of the plasma and the linearity of calibration curves of calcium and zinc investigated. Fig.11.7 shows the variation of plasma temperature with sample uptake rates at a position 20 mm above the top of the work coil and coolant gas flow of 250 cm3s-1 and injector gas flow of 50 cm3s-1. It is apparent from this figure that the temperature increases as the volume of water Fig.11.7 Variation of 'the plasma temperature at a height of . 20 mm. above. the top of the work coil with the sample uptake rate.. 8500

op

75001

50 rob SAMPLE UPTAKE RATE, MMaS • 273 introduced into the plasma decreases. This suggests that water has a quenching effect upon the plasma- and this can be seen visually; at high sample uptake rates the plasma core contracts and becOmes very unstable unless the power input to the plasma is greatly increased. At low sample uptake rates, the converse is true and the plasma shape expands appreciably so that the silica walls are attacked unless the coolant flow is greatly increased or the power input level reduced. It is noticeable that if the coolant flow is greatly increased the magnitude of the analytical signal is only slightly affected, e.g. increasing the coolant flow from 250 cm3s-1 to 500 cm3s-1 produces a signal reduction of only ca 5 X. This increase in temperature at low sample uptake rate means that as well as the reduction of the effect of self-absorption by reducing the particle density, the emitted intensity increases to partially compensate the loss of sensitivity caused by the lowering of the particle density. Calibration curves obtained for calcium at the atom line at 422.67 nm, the ion line at 393.37 nm and the zinc atom line at 213.86 nm are shown in figs. 11.8-11.10 respectively-for different sample uptake rates. From these curves it is obvious that at low Fig.11.8 Observed emission calibration curves for Calcium at 422.67 nm. at various sample uptake rates.

6

c3)

w

1.) 20 CONCENTRATION, [tGA1:3 Fig.11.9 Observed emission calibration curves for calcium at 393.37 nm. at various.sample uptake rates.

10 20 • ND

CONCENTRATION SKG M-3 • \-31 • Pig.11.10 Observed emission calibration curves for zinc at 213.86 nm. at various sample uptake rates.

10Ornm5S-

• 0

10 N. CONCENTRATION) KG Nri. 277 sample uptake rates very large linear ranges are obtained even from elements which absorb strongly at their principal resonance lines. The detection limit, however, gets poorer at low sample uptake rate caused by the lower total emitted intensity and an increase in nebulizer instablilty at low sample uptake rates. This is unimportant, however, where a simultaneous multi-element analysis is required as precision is often more important in this case than the absolute detection limit, and it is essential that the calibration curves for all the investigated elements should be linear over very wide concentration ranges. 11.3 Residence times_in the atom cells One of the great advantages of a plasma system is that it is claimed that the residence time of an analyte particle in the discharge and tailflame is very long compared to flame or arc excitation cells. As it can be assumed that the linear velocity of analyte particles will be the same as that for the gas atoms or molecules of the carrier gas, then the residence time of analyte particles will be the same as that for gas atoms. Measurements of the velocity of solid particles in a moving gas stream [133] have shown that at atmospheric pressure, the velocity of gas molecules is less than 1 j greater than that of solid particles in the gas stream. 278 The linear velocity of gas through the entrance port into the plasma is given by equation 11.5 and into the flame by equation 11.6 where L is the path length of the cell and bs is the burner slot width.

4QG vo2 = [11.5]

= QG v0 LD7- [11.6] s Thus the linear velocity of argon gas entering the plasma is 0.16 m s-1 while the linear velocity of gas into the flame is 8.25 m s-1. As the plasma carrier gas has a low linear velocity and must travel ca 10 cm within the plasma tube against the force of gravity, it can only carry very small droplets of water [133] so that the nebulizer efficiency may be considerably less than that of the flame even when the same nebulizer is used. After passing into the plasma core or primary reaction zone of the flame, a volume expansion takes place given by equation 11.2 where Y is the flame expansion factor. Thus the linear velocity of gases at the end of the reaction zone is given by equation 11.7 where it assumed that no lateral expansion of the flame or plasma occurs.

vT = voY To [11.7] 279 Solving this equation for the plasma and flame gives linear velocities of 4.4 m s-1 and 1 130 m s - respectively. The residence time of analyte particles in the reaction zone or core can be calculated from equation 11.8, where ht is the vertical height of the reaction zone and it-av is the average velocity of particles thi'ough the zone. h t = t [11.8] vav For the plasma it is assumed that the average velocity through the reaction zone is 4.4 m s-1 and for the flame it is assumed that the average velocity through the primary reaction zone is 70 m s-1. The calculated residence times are 5.7 ms and 44 µs for the plasma and flame respectively. If it is assumed that the emission intensity is viewed over a length 1 cm above the top of the plasma core or primary reaction zone, then the residence time of analyte particles in the viewing zone can be calculated from equation 11.9. tv = [100-72]-1 .[11.9]

The residence time for an analyte particle in the plasma is therefore 2.3 ms compared to 77 µs in a flame. 280 The plasma system can readily be seen to retain the analyte particle for a much longer length of time than the flame so that more energy can be supplied to the analyte in. the core than in the primary reaction zone of the flame. The added bonus is that the excited particle is retained in the viewing zone of the plasma for a length of time about thirty-fold that. in the flame so there is a greater probability of it emitting several more times in the viewing zone of the plasma than in the flame. 11.4 Energy of the atom cells The rate of supply of energy to the plasma can be calculated from- equation 11.10 assuming an efficiency of 1.'

Winn d. [11.1o] For the plasma operated under normal conditions, the potential difference across the ends of the work coil is 5 kV and the current flowing through the coil is ca 0.9 A so that the applied power is 4,5 kW. The rate of supply of energy to the flame can be calculated from the heat of reaction of the gas oxidation process,,AHf, and the total flow of gas, Qf, and assuming an efficiency of 1 from

equation 11.11. where VM is• the molar volume. QfAHf ZJin = T 281 For the nitrous oxide-acetylene flame reaction :- 2N20 + C H -* 2N + 200 + H 2 2 2 2 • the heat of reaction is 179 kcal mol-1, so that the power dissipated in the primary reaction zone is 1.8 kW. 11.4.1 Thermal energy The thermal energy available in the atom cell can be calculated from equation 11.12 where Op is the specific heat of the gas, and mg is the mass of gas in the flame defined by equation, 11.13 where p is the mean gas density. E = mgCp[T- To] [11.12]

d[mg] dt QGP Combination of these two expressions gives :-

= Q O p[T-T WT G p o For the plasma, the calculation is simple as only argon is involved, but for the flame an average density and specific heat have to be assumed. The average density of flame gases in table 11.3 was calculated for the average molecular weight of flame gases of 21.67 reported by Taylor [134]. The specific heat of flame gases was calculated from this molecular weight assuming a diatomic flame gas and ideality of the flame species. 282 The actual thermal energy which can be transferred to an analyte particle in the primary reaction zone or plasma core can be calculated from equation 11.15.

JT = WTtr [11.15] Thus the plasma has a greater energy available in the core than the flame [2.6 J as compared to 0.03 J] in the primary reaction zone although the thermal power dissipated is lower [450 W as compared to 630 W]. These thermal energies are calculated using.the maximum plasma core temperature experienced by the bulk of the plasma gas, i.e. 10000 °K [124] and the maximum flame temperature of 3000 °K. 11.4.2 Bremsstrahlun: continuum radiation Unsold [135] has derived an expression for the intensity of the radiated continuum due to the Bremsstrahlung and recombination radiations. If a single electron is accelerated under the influence of ions in its vicinity then, according to classical electrodynamics, the electron moving in a non-dissipative medium of refractive index p. radiates electromagnetic energy at the rate

dEe2 e 3, Iv! = o ne-.0c where -0- is the acceleration. The total energy emitted as the electron passes an ion is then :- 283 +00 e2p t 2at 6ne c3 o -04 To find the frequency spectrum, it is necessary to carry out a Fourier analysis of the acceleration. The total radiation in the frequency range w to w+dw can then be shown [112] to be :- 00 2 E[w]dw = e p t[t]exp[-jwt]dt 2 [11.18] 6ne c3 o CP where E[w]dw is the total energy emitted per dw into 47t steradians. The emission of electromagnetic from a weakly ionized unmagnitized plasma is due to interactions of electrons with atoms or molecules. In a more highly ionized system, ion-electron collisions are dominant. The exact form of the Bremsstrahlung spectrum depends on the form of the collision, the frequency range considered and the collective effects of the plasma which cause self-absorption, etc. As a simple case, the classical picture of an electron in the Coulomb field of an ion of charge Ze is considered. The acceleration term t[t] can then be written as s-

2 t[t1 = EL1 _ Ze rrr.1 1]-2 J m 4ne 0m L Lu which for distant encounters with small deflections reduces to the forM of equations 11.20 and 11.21. 284

t Ze2 [11.20] . n[t] 47teom [b2+[vt]2-1572-

t [t] Ze2 vt a 4neom [b2+[vt]2]3/2 which gives the dominant contribution to low frequency radiation. The impact parameter is b and the time of closest approach is taken as t = 0. Substituting into equation 11.19 gives :— 2 2 2 E[W,2.:2r b v] e 3 Z 4w 2 wb 2 b [11.22] 31-4116o ci m nv— T [K 1[]+K 77 o[141]] where the K's are modified Bessel functions of the second kind [136]. As a given electron passes through a plasma, the number of ions per second lying with impact parameters between b and b+ db is

dN1 = N1v2nbdb [11.22]

where N. is the ion concentration so that the total power emitted per electron per second at an angular frequency w [assumed to be small] is :— 2 N.Z2 1, 1, - 16Tcr e ]3 1 [0!an 122JS] [wvjaw= --TLAne c 2 u . [11.23] . 2r- ' m v pun The total number of electrons per unit volume having a speed v is :— 2 dNe[v] = Nef[v]4nv dv [11.24] • where f[v] is the velocity distribution function normalized so that :-

285

4n f[v]v2dv = 1 [11.25] o Thus -

2 3 N N.Z2 b . 2 W [widw = 16Tc [ 4ne e2 7 [ vt 4nv dw.dv [11.26] 3 eo c] c, min

For a Naxwellian distribution of velocities

similar to equation 10.4, then equation 10.26 can

be reduced to the form :-

2 2 3 N N Z g b- 16r21 e 1 e i W T]dw.= lne:Edw [11.27] B n j L 4nE C j 3 L 0 [In kt]-2-: min

From quantum mechanical principles it can

be seen that the Bremsstrahlung power spectrum is

essentially independent of frequency up to :-

2nkT w = [11.28]

so that it must be reduced to zero beyond this region.

In practice it is found that the spectrum is only

independent of frequency at low frequency which

implies that the power distribution might be improved

by changing the velocity integration term and integrating

•from a minimum velocity such that s-

2 hw = nmvMn [11 i .29]

instead of from v=0 since particles with velocities

min cannot radiate at w. Thus equation 11.27 can

be rewritten as :-

2 3 N_N,Z g b 16r212 e2 hw WBNITidw = TLF.1 c] lnEMEexIA- 2nkV" [11.30] o [m-ktP min 286. The classical treatment of free-free Bremsstrahlung was originally due to the work of. Kramers [137] but Unsold [135] found that a correlation factor called the Gaunt factor was required. If equation 11.30 is integrated over all frequencies, then :- •2 3 N N.Z2 W rml = 32n 1 2n 1 ehl HBL-.; L [".T]2 Geff 33m 4neoc where Geff is the Gaunt factor averaged over both electron velocity and frequency. For the induction coupledradiofrequency plasma considered here, the electron and ion densities in the plasma core can both be taken to be 4x1021 m-32 the core temperature 9000 °K and the Gaunt factor • to be unity. Using this data, the total Bremsstrahlung radiation is ca 90 MW mm- 3 which for this plasma • shape approximates to 700 W. For the flame, Bremsstrahlung radiation may be ignored as the natural electron density is so low. 11.4.3 Cyclotron radiation A plasma immersed in a steady magnetic field radiates as a consequence of the acceleration of charges in the orbital motions of the atoms in the plasma around the magnetic lines of induction. The emission is primarily from the free electrons and 287 for a sufficiently dilute plasma, dispersion effects and corrections for the motion of neighbouring ions can be disregarded. The relativistic equivalent to 'equation 11.16 for the power radiated by an accelerated electric charge [138] is ;- 2 2 W - p.e t [11.32] 6ne oc3[i _v2/c2]2

If the acceleration of the electron due to the magnetic Lorentz force is given by :- eV B = [11.33]

where V is the velocity component perpendicular to the field, then the cyclotron frequency can be written as equation 11.34. eB W = m [11.34] The cyclotron radiated power can be expressed in the form of.equation 11.35 if a Maxwellian distribution is assumed, where W is the total radiated power per unit volume. 2 2 r ,T1 = N e e wcy kT r + 5_ h.T. ] [11.35] cyL"cy 37ce0 c mc 2 ' 2 me2 The power radiated by virtue of the radial acceleration of an electron about the magnetic field lines of force can be significant for magnetic field 288 conditions such that the cyclotron angular frequency wcy is comparable to the plasma angular frequency. It can readily be seen that the ratio of the Bremsstrahlung to cyclotron radiation is given by :- 2 2 [711— ) {—LT [11.36] cy [37c .j kT eff Under conditions, of high magnetic field and high - temperature, the cyclotron radiation competes with the Brems6trahlung radiation, but the forms of the radiation are, however, different. Bremsstrahlung radiation is continuous and at low frequencies is essentially independent of frequency changes. Cyclotron radiation, on the other hand, is emitted and propogated anisotropically so that the intensity is dependent on the direction of observation with respect to the magnetic field direction.- For a magnetic field of 3 mT at.the coil centre, the cyclotron angular frequency is 5.27x108 -1 rad s while the plasma frequency is 2.26x108 rad s-1. Thus the cyclotron radiated power for the plasma is • ca 95 W. At the normal height of observation, the temperature is 8250 °K and the electron density.ca 2X 1021 m-3 so that the Bremsstrahlung radiation is reduced to ca 20 MW m-3 compared to 90 m-3 at the coil centre. The Cyclotron radiation at this point is, however, almost negible as the magnetic flux is only 0.03 mT. The value of cyclotron radiation is 289 reduced to CA 3 kW m-3 compared to 12 MW m-3 at the coil centre. Fig.11.11 shows the observed background emission spectrum corrected for the photomultiplier response curve at a position 20 mm above the top of the work coil compared to the calculated Bremsstrahlung spectrum. It is quite. apparent that these two curves are virtually identical's° that black body radiation from the plasma must be'a minor factor in determing the background. 11.4.5 Energy loss processes The main loss of energy from the atom cells is probably that lost by conduction of the shielding gas. From classical heat conduction theory, it can be shown that the rate of transfer of heat from a body whose temperature is T and whose area of contact with a conductor of thickness y,is S is given by equation 11.37, kTS[T-To] [11.37] y is the thermal conductivity of the conductor. where kT IA the atom cells considered, loss of heat . by conduction within the reaction zone can be neglected so that only .the effect of 'the coolant or shielding gas will be considered. In the model of the plasma system, the surface area of the curved surface of the plasma core in contact with the shield gas is the surface are of the Core cylinder which Fig.11.11 Comparison of the actual bacground from the plasma [A] with the theoretical Bremsstrahlung radiation [B] at a point 20 mm above the top of the work coil.. A

30 obo . Wavelength, 291 approximates to 15.7 cm2 and the thickness of the coolant stream can be taken to be ca 3 mm around the plasma core. The thermal conductivity of a gas can be calculated from equation 11.38 where Y is the ratio of Cp/Cv. kT =0.25[9Y-5]90v [11.38] The viscosity of a gas at a temperature T21) can be calculated from equation 11.39 where I) is the viscosity of the gas at S.T.P and C is Sutherland's constant.

r273+c] r T 3/2 I/T - loi T+C L273J [11.39]

The variation of Cv with-temperature can be calculated from equation [11.40 where al b and c are constants. 2 v = a + bT + cT [11.40] Thus the thermal conductivity of a gas is given by I- 2 0[272+34-0][273T ]3/2 a+TcT kT = 0.25[91- 5b In the plasma taking the average temperature of the coolant gas to be 4000 °K, the calculated thermal conductivity is 0.09 W m 1 °K-1 assuming Sutherlands constant to be 142 for argon. Thus the conducted heat loss given by equation 11.37 is 400 W. In the flame, the heat is mainly conducted 292 from the flame gas via the nitrogen along the burner length. This gives a surface area of contact of 150 mm2 on each side of the flame. The thickness of the nitrogen shield is typically about 2 cm, but it is likely that all conductive heat loss takes-place in the first 5 mm from the flame. Assuming Sutherlands constant for nitrogen to be 104 and a mean gas temperture of 1500 bK, then the thermal conductivity of nitrogen is 0.07 Wm-1 'K-1 so the calculated heat loss is ca 120 W. A small amount of heat is lost in both cells by black-body radiation, conductive heat loss at the burner slot, etc., but all these heat losses are very small compared to the energy dissipated in the cell and consumed by the processes discussed above. 11.5 Discussion of the emission properties of the plasma and the flame One of the most striking things to be noticed from the comparison of the plasma with a flame, table 11.4 is that the plasma is a very inefficient excitation cell. Of the 4.5 kW supplied to the plasma, only about 10 X is actually used for heating the argon and exciting the analyte species while in a flame about 30 X of the energy is put to useful purposes. This is about the only detrimental property of the plasma compared to the flame as an Table 11.4 Comparison of the Plasma and Flame as Excitation Cells

Parameter Plasma. Flame Linear velocity into cell, v 0.16 m s 1 8.25 m s-1 Linear velocity in viewing zone, v2 4.4 m s-1 130 m s-1 Residence time in reaction zone, tr 5.7 ms 44 ps

Residence time in viewing zone, tv 2.3 ms 77 Ps Power supplied to cell, Win 4.5 kW .1.8 kW TheYnal power of cell, W2 \ 450 W 630 W Thermal energy in cell, J2 2.6 J 0.03 J. Bremsstrahlung radiation, WB _700 VT Cyclotron radiation, Wcy 95 W Conducted heat loss, We 400 VT 120 W Power accounted for in cell 1.7 kW 750 W 294 excitation cell. The great advantage of the plasma compared to the flame is that the residence time of analyte

particles in the core is ca one hundred•-.fold greater than that of an analyte particle in the primary reaction zone of a flame. As a result of this, much more energy is available to an analyte particle and almost complete.atomization of all introduced molecular species is to be expected. This fact is indeed observed experimentally and band systems due to molecular refractory oxides such as A10, CaO, CuO, etc. are only detected high in the plasma tailflame where the gas has cooled appreciably and entrainment of air has occurred. The solvent, water, is also atomized to a very large extent in the plasma and the Ha and Ho lines are very strong while the band system due to OH bands is virtually neglible. This means that apart from a few argon lines, the spectrum obtained from the introduction of a species into the plasma is virtually pure and only a small background emission is obtained. In the flame there are very extensive band systems caused by ON, NH, OH, etc. which makes large parts of the spectrum difficult to use for emission spectroscopy. The long residence time of particles is due to the fact that the sample carrier gas flow 295 rate is often low in plasmas and wide tubes are used to carry the gas resulting in very low linear velocities of particles entering the discharge. Although this low velocity means that only very small droplets can be carried, the preheating these droplets experience in passing up the plasma tube from conducted and radiated heat from the plasma core means that it is fairly certain that volatilization of the droplets is almost complete before the analyte particles begin to enter the discharge. This means that to a certain extent some of the energy from the plasma that is'wasted'is put to some useful function. A great experimental advantage of a plasma as an excitation source is that the analyte particle remains in the zone of observation for a much longer period of time than in any other type of excitation cell which greatly increases the probability of the detector seeing the emission from the analyte particle. This, coupled with the much higher excitation energy transferred to the analyte particles means that the detection limits from the plasma are likely to be much higher than those obtained with flames as shown in figs.11.3-11.6. In the far ultraviolet, the flame produces no detectable emission spectra of analyte species [68] so that as far as this region of the spectrum is concerned, only a plasma system can be used as an excitation cell. 296 Although simultaneous multi-element analysis in the plasma requires some compromises to be made as to the optimum choice of observation height and carrier gas flow rate so that the detection limit attainable for each element is higher in this instance than when each element is determined separately [45,53,67], the much greater linear range obtained in the plasma compared to the flame for most elements [45] means that the plasma can determine high concentrations of sensitive elements such as zinc or calcium at the same time as low concentrations of insensitive elements such as cerium or tungsten without much difficulty. In the plasma, also, the effects of ionization suppression and compound formation in the discharge are often not observed and even when observed are generally minor compared to the flame where several matrices such as aluminium, blood serum, sea-water, silicates, etc. present severe difficulty. Greenfield and Smith [139] have recently pointed out the importance of distinguishing between general properties of plasmas and individual instrument operating parameters. Not only does this mean that the optimum operating conditions are different from one unit to another but that the calculations' for each plasma differ. The general temperature 297 distribution of all plasmas are fairly constant [45, 67,82,83,85,124] as are the linear velocities of the plasma gases in the system [47,50,54,67] so that the residence time of analyte particles in the plasma core and viewing zones are of the same order as those calculated for this plasma system. Dickinson and Fassel [54] operated their plasma with a gas flow rate of 40 cm s-1 in plasma tubes of 13 mm internal diameter. The ultrasonic nebulizer operated with a sample uptake rate of 10 mm3 s-1 was claimed to have an efficiency of 0.8 so that the particle density in their plasma is ca three times greater than in the plasma used here. The residence time of analyte particles in the discharge is ca 4-5 ms. Boumans and De Boer [67] operated their plasma with a gas flow rate of 30 cm3 s-1 in a 12 mm diameter tube. The ultrasonid nebulizer gave a sample uptake rate of 20 mm3 s-1 so that the particle density is about eight times greater than that in the plasma used here, while the residence time is ca 5 ms. It is noticeable that these workers have claimed detection limits which are generally about one order of magnitude bettter than those obtained by Fassel [54] which is in part due to the higher particle density in their system. It can be seen that although the particle 298 densities are different for all three plasma systems considered, the residence time of analyte particles is not vastly different so that one can say, in general, that plasmas have broadly similar spectrochemical excitation times although the individual syatems differ somewhat. CHAPTER 12

Conclusions and Suggestions for Further Work. 299 One of the main limitations to the sensitivity of the present system is that there is a small air path between the plasma and the purged optical system. The use of a shield box similar to that used on the carbon filament atom reservoir could perhaps eliminate this; a design for such a box is shown in fig.12.1 If the box is constructed from metal it must be placed so that it is insulated from the coil and should have minimal penetration of the magnetic field of the coil to prevent radiofrequency induction heating. This system should enable the system to be used down to 165nm; this limit being due to the limit of transmission of fused silica. There is no reason, in theory, why the plasma cannot be used as an excitation cell down to ca 100 nm. To achieve this, an evacuated system must be used with lithium fluoride windows and lenses. The evacuation of the plasma torch unit presents a slight engineering problem because of the very long hot tailflame, i.e. up to 50 cm long, but it should be possible to overcome this. Recent trends in plasma technology have been towards improvement in the power transfer efficiency of the electronic circuitry and to minutarization of the plasma assembly so the technique can be commercially competitive with atomic absorption spectrometry.

300

O 0 O 0 O 0

I !: II

•

Fig.12.1 Proposed construction and positioning of shield box to purge air path of optical system with nitrogen. , 301 Boumans [67] has redesigned the oscillation circuit of the radiofrequency generator so that the work coil takes part in defining the frequency of the generator so that the resonance condition for maximum power transfer to the plasma is automatically fulfilled. Thus a change in impedence of the plasma only affects the frequency of the generator and has a second order effect upon the power transfer. In common generators, such as the Radyne model H30/P, the oscillation circuit must be tuned with the aid of a variable capacitor to obtain resonance with the primary frequency of the oscillator. Any variation in the circuit parameters, e.g. a change in capacitance of a condenser as a result of heating or a change in impedence of the plasma reduces the power transfer efficiency to the plasma appreciably and causes instability in the plasma discharge. If the system is to be minutarized while the high energy of the plasma is to be retained, then certain criteria must be adhered to. The mean temperature of the plasma core must be kept at ca 9000 °K and the electron density ca 2x1021 m-3. If the plasma is to be operated at 36 MHz, then the skin depth, computed from equation 3.12, is 1.8 mm, so that the minimum tube diameter which will allow the doughnut to be developed is 12 mm. This is approximately half the size of the tube used with 302 this particular model, so that to achieve the same plasma shape with this smaller tube, the injector gas flow rate must be reduced. If the plasma height is also reduced by half, then the surface area is reduced by one-quarter and the core volume by one- eiM. If the injector flow rate, is reduced by one- ej4th also, then the linear velocity of gas through the plasma is halvied and the residence time remains constant. The power which must be supplied to thermally energize the argon is ca 60 W with this smaller plasma. The Bremsstrahlung radiation asscociated with this plasma temperature and.electron density will still be ca 90 MW m-3, but as the surface area is reduced the total Bremsstrahlung radiation will be ca 90 W. As the power input to the plasma can be substantially reduced, the current flowing in the work coil can be reduced as can the size of the coil. The magnetic flux density of the work coil, however, will not be greatly reduced from the larger work coil and thus the cyclotron radiation will be reduced to ca 1.20 W. Thus the total power required to sustain a plasma of this size is less- than 500 W compared to 3 kW for the larger version. Having computed this power input, it must be remembered that no two plasmas are alike [139] and thus this power calculation gives only a guide to 303 the actual power requirements. It should be possible, however, to operate a plasma of these smaller dimensions with a radiofrequency generator of 750 W without any difficulty. Desk-top solid state gerierators capable of supplying radiofrequency power of this magnitude are now currently becoming available commercially, e.g. the model 131202 [Philips Electronics Ltd., Eindhoven, Netherlands] is capable of giving up to 2 kW of radiofrequency power at 52 1411z. One important analytical application of plasmas would appear to be in the field of multi-element analysis. Fassel [45] has reported linear calibration curves for a number of common elements extending over 4-5 orders of magnitude using ultrasonic nebulization of aqueous solutions in conjunction with a desolvation technique. In some studies made with the Radyne H30/P using a variable uptake nebulizer, linear calibration curves covering concentration ranges of over four orders of magnitude were observed for aqueous solutions of zinc and calcium, figs.11.8-11.10, and similar linear ranges were observed for aluminum, chromium, cobalt, manganese, nickel, tin and uranium. It would seem that by manipulation of the operating parameters of the plasma, i.e. the sample uptake rate, plasma gas flow rates and power input, a great deal of control is possible over the emission of analyte elements obtained. Thus it is, in. 304 theory, possible to choose operating conditions for either maximum sensitivity of a single element or maximum linear range of the calibration curves of several elements, or, indeed, any desired intermediate position may be obtained. The great handicap of plasma emission spectroscopy as an analytical technique is that, in common with all emission techniques, selectivity is dependent upon the ability of the monochromator to separate the spectral line of the element required from all other lines in the spectrum. Various methods of trying to overcome this difficulty have been suggested and tried. Walsh [140] has suggested the use of resonance detectors to eliminate the need for monochromators. In these detectors, an atomic vapour of the element under investigation is maintained usually in a hollow cathode lamp. The detector is then ple.ced in the light path to absorb radiation characteristic of that element. A solar blind photomultiplier is placed normally to the light path to collect the fluorescent radiation from this detector. With suitable stacking of these detectors around a plasma it should be possible to detect several elements simultaneously. In the far ultraviolet another method of obtaining the atomic vapour is possible as all the elements possess high vapour pressures. Mercury vapour 305 can be readily formed by placing nanogram quantities of mercury in a flask and evacuating the flask. Vapours of iodine, sulphur and phosphorus can also be readily formed by a similar process, but atomic vapour is not formed from these elements unless extra energy is supplied thermally or electromagnetically. The advent of the echelle monochromator [141,142443,144] opens new horizons for multi-element analysis.- These monochromators not only have a very much higher resolution than conventional monochromators but also have high luminosity, wide spectral coverage and compactness of design. The resolution of these monochromators is so good that it is often possible to observe some fine structure on spectral.lines [144], so as an analytical instrument for emission spectroscopy they should enable the effects of spectral overlap to be minimized. It would not be expected, of course, to totally remove all spectral overlap as even resonance detectors can suffer from this, e.g. iodine overlaps with bismuth at 206.16 nm. The echelle monochromator obtains its high resolution by dispersing the incident radiation with a diffraction grating and selecting the higher orders which are then further dispersed by a prism assembly. Thus the linear dispersion is not constant over the whole wavelength range. In the far ultraviolet, the use of high diffraction orders is often a disadvantage 306 as the luminous flux from sources such as the electrodeless discharge tube is low in this part of the spectrum. 307 APPENDIX

Principal symbols and abbreviations used.

Symbol Units.

A Absorbance IWO -1 Aki Transition probability 14.-k s. • AN Avagadro numbet- 6.023x1023 moll a Radius df circle m. B - Magnetic flux density T. bs Burner slot width m. - 2 -1 -1 be Electron mobility m V s _3 C Concentration kg.m .

Ck Stark constant-for S. spectral line C 'Specific heat of a gas at constant J.kg-1°K-1 P pressure 0 Specific heat of a gas at constant J.kg-1 ° K-1 . v volume c velocity of light ' 2.998x108 m.s-1 . D Linear dispersion of spectrometer • nm.m-1 . d Grating spacing - m. . E Energy J.

Ek 'Energy of state k K. e Electronic charge 1.602x10-19 C. F Electric field strength v,... m- •1 , 3 -1 FX Fluorescence intensity W.m sr-1s . F[X] Function of X - f Focal length of spectrometer m.

fik Oscillator strength 1.---)k -

em• gJ Lande factor 308 Symbol Units

Geff Gaunt factor - gk statistical weighting factor of k 4 H Magnetic field strength .....A m * h Plancks constant 6.625x10-34 Js. ht height m. I Intensity of light li.ml i Current A. J Current density .....A ,-2 . j complex root of -1 - K Constant - -1 Kv Absorption coefficient - m k Boltzmanns constant 1.381x10-23 J.°K-1 0.695 iciorl w.m-10K-1 kT Thermal conductivity L Length m. LIT Loschmidt number 2.867x1025 m-3 M Molecular weight of A - A MJ Magnetic angular momentum quantum number - ML Magnetic orbital quantum number - MS Magnetic spin quantum number - m Mass of electron 9.109x10-31 kg. Ni Particle density of i m-? n Integer P Pressure ....m1.1. „-2 • P Density kg.m-?

Q Flow rate m 3s -1. 309 Symbol Units R Universal gas constant 8.314 J°K-1 mol- .1

R0 Radius of curvature m. Rm Rydberg constant 109737 K. ✓ Distance between charge's m. S Surface area m. s Semi-length of inductor coil m. T Temperature °K. t Time s. - ui Partition function of species i ✓ Potential difference V. V10 Ionization potential V. VM Molar volume at S.T.P. 0.0224 m? -1 ✓ velocity m.s . m cf.-2 t Acceleration iiie■-, do W Power W. W Work function of photocathode J. e , w Angular frequency rad.s-.1 X Displacement from a point m. y Thickness of gas stream m.

Z Atomic number ••• a Angle rad. a Degree of ionization Angle rad.

Flame expansion factor ONO AX Change in X AXD Doppler half-width of spectral line nm 310 Symbol Units 8 Specific electrical conductivity A.V lm-1 c Extinction coefficient m2 kg .

0 Electrical constant 8.854x10-12 F.m-1 -2 4 Viscosity Nsm . 0 Solid angle sr. X Wavelength nm. Refractive index -1 PB Zeeman shift per unit field 9.273x10-24 J.T . -6 PO Magnetic constant 1.257x10 H.m-1 111. Releative Permeability v :Frequency Hz. Wavenumber K. Pythagorean number 3.142 a2 Optical cross-sectional area nm.2 a Stefen-Boltzman constant 5.670x10-8 Wm 2°K Quantum efficiency

Nebulizer efficiency MIS 311 REFERENCES

1 J.C.Boyce, Rev.Mod.Phys., 13 1 [1941]

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