Basic Principles of Atomic Absorption and Atomic Emission Spectroscopy

1 Flame Atomic Emission Spectrometer

P Signal Processor Source Wavelength Selector Detector Readout

Sample

2 Flame Atomic Emission Spectrometer

3 Emission Techniques

Type Method of Atomization Radiation Source Arc sample heated in an sample electric arc (4000-5000oC) Spark sample excited in a sample high voltage spark

Flame sample solution sample aspirated into a flame (1700 – 3200 oC) Argon sample heated in an sample plasma argon plasma (4000-6000oC)

4 Emission Spectroscopy Flame and Plasma Emission Spectroscopy are based upon those particles that are electronically excited in the medium. The Functions of Flame and Plasma 1. To convert the constituents of liquid sample into the vapor state. 2. To decompose the constituents into atoms or simple molecules: M+ + e- (from flame) -> M + hn 3. To electronically excite a fraction of the resulting atomic or molecular species M -> M*

5 Emission Spectroscopy

Measure the intensity of emitted radiation

Exc ited State

Emits Special Electromagnetic Radiation

6 Ground State 7 Argon Inductively Coupled Plasma as an atomization source for emission spectroscopy

8 9 Why plasma source or other high energetic sources?

• Flame techniques are limited to alkali (Li, Na, K, Cs, Rb) and some alkaline earth metals (Ca and Mg) • More energetic sources are used for – more elements specially transition elements – simultaneous multielement analysis since spectra od dozen of elements can be recorded simultaneously – Interelement interference is eliminated – Liquids and solids

10 Advantages and disadvantages of emission spectrometry

Advantages • rapid • Multielement (limited for alkali and some alkaline earth metals

• ICP-AES has become the technique of choice for metals analysis.

Disadvantages • initial cost of ICP instrumentation • continuing cost of operation (Ar required)

11 Flame Atomic Absorption Spectrometer

P P o Signal Processor Source Wavelength Selector Detector Readout

Chopper

Sample

12 Flame Atomic Absorption Spectrometer

13 Atomization Techniques Type Method of Atomization Radiation Source Atomic sample solution aspirated HCL (flame) into a flame atomic sample solution evaporated HCL (nonflame) & ignited (2000 -3000 oC) (Electrothermal)

Hydride Vapor hydride generated HCL generation

Cold vapor Cold vapor generated (Hg) HCL

14 Advantages and disadvantages of atomic absorption Advantages • sensitive (GFAA) • selective

Disadvantages • intended for metallic/metalloid atomic species, not nonmetals or intact molecular species • lamps - one element at a time • not easy for solids • calibration curves nonlinear above A = 0.5 • more costly and less widely applicable than UV vis • matrix effects - easy for some metals to be masked 15 16 Flame Fluorescence Spectrometer

P P o Signal Processor Wavelength Selector Detector Readout

90o

Source

Sample

17 Fluorescence Techniques

Type Method of Atomization Radiation Source atomic sample soln. aspirated sample (flame) into a flame atomic sample soln. evaporated sample (nonflame) & ignited x-ray none required sample fluoresence

18 Processes that take place in flame or plasma

19 T 1 Flames

Regions in Flame Temperature Profile

20 21 22 23 24 25 26 Flameless atomization Electrothermal atomization Graphite furnace atomization

27 28 29 30 Why do we use a temperature program in flameless AA?

31 Why do we use an inert gas with Flameless atomization?

32 Light source used for AA

• What light source do we use with AA? • Would it be a continuous light source or a line light source?

• A line light source is used for AA

33 T 2

34 35 How does the line source provide a single wavelenght?

38 AA spectrophotometer

39 40 41 42 43 Relationship Between Atomic Absorption and Flame Emission Spectroscopy

• Flame Emission -> it measures the radiation emitted by the excited atoms that is related to concentration. • Atomic Absorption -> it measures the radiation absorbed by the unexcited atoms that are determined. •Atomic absorption depends only upon the number of unexcited atoms, the absorption intensity is not directly affected by the temperature of the flame. •The flame emission intensity in contrast, being dependent upon the number of excited atoms, is greatly influenced by temperature variations. 44 Measured signal and analytical concentration 1. Atomic Emission

Signal = Intensity of emission = KNf = K’Na =K’’C Nf = number of free atoms in flame Na = number of absorbing atoms in flame C = concentration of analyte in the sample K, K` and K’’ depend upon: • Rate of aspiration (nebulizer) • Efficiency of aspiration (evaporation efficiency) – Flow rate of solution – Solution concentration – Flow rate of unburnt gas into flame • Efficiency of atomization (effect of chemical environment). This depends upon – Droplet size – Sample flow rate – Refractory oxide formation – Ratio of fuel/oxygen in flame • Temperature effect (choice of flame temperature) 45

2. Atomic absorption

Signal = I absorbed = Absorbance = A = k l C

• For the measurement to be reliable k must be constant; k should not change when a change in matrix or flame type takes place. • K depends upon same factors as those

for the atomic emission spectroscopy

46 Background and Background Correction

47 • Definition • Sources of Background • Background correction: – Blank correction method – Two-line correction method – Continuous source correction method – Zeeman effect correction method

48 What is a background?

• It is the signal observed when the element sought is absent

• The light at a specific wavelength (Analyte wavelength) is attenuated by the effect of flame components or matrix components in the sample

• Thus measured absorbance and analyte concentrations are too high • If problem from flame, blank aspiration will correct for it

• It is more serious at short wavelengths (< 430 nm) and with graphite furnace

49 Sources of Background in atomic absorption 1. Absorption by flame itself (Serious at  below 220 nm; e.g., As, Se, Zn ) • Water blank can compensate for this flame absorption • In flame absorption, background interference is insignificant at  > 230 nm

50 2. Absorption by concomitant molecular species originating from the matrix like NaX or solvents containing X (halogen) like CCl4. • Halides absorb at  < 300 nm

3. Scattering of radiation from the particulate material in the flame

• Particulate material: Unevaporated droplets; unevaporated refractory salt particles • Scattering is more serious at short  • 2 and 3 are most common with electrothermal atomizer

51 Specific Applications Require background correction 1. Graphite furnace

2. Flame determination of low concentrations of an element in the presence of high concentrations of dissolved salts

3. Flame analysis where sample matrix may show molecular absorption at  of the resonance line

4. Flame determination of an element at  where flame absorption is high

52 Background correction methods

1. Using a blank • Measure the absorbance of the metal resonance line by both flame & blank (flame system) • Measure the absorbance of the metal resonance line by sample and flame system (flame + blank) • A is the difference

53 The main two methods used for background correction:

1. Continuous source correction method 2. Zeeman effect correction method • These two methods are based on the fact that • Atomic bands are very narrow • Background bands are molecular in nature thus they are broad bands

54 2. The continuous-source correction method • This method is an available option with most instruments • Deuterium lamp is used in conjunction with the HCL lamp • The two lamps are observed by detector alternatively in time

• Background usually absorb radiation from D2 lamp and HCL

• Absorption of the analyte from D2 lamp is negligible • The lamps may be pulsed at different frequencies, thus the signal processing electronics can distinguish and process separate absorption signals • Thus

Acorrected = A HCL - AD2

55 The continuous-source correction method

Chopper

56 57 Background correction by continuous source correction method

HCL

HCLHCL

58 59 Disadvantages of the continuous source correction method • Correction may degrade detection limit. The current of HCL is reduced to match the D2 lamp • The correction may be in error if the background s (sample and standards) are not the same • Alignment of the two sources to pass through the same area of atomizer is tedious • Beam splitter reduces the intensity of radiation which may affect the limit of detection • Additional light source and electronics are needed 60 Zeeman effect correction method

61 Zeeman effect correction method

62 • Under the magnetic field, atomic spectral line (emitted or absorbed) splits into three or more polarized components • Two components will be displaced at equal wavelength intervals higher and lower than the original line. • The original line is polarized in plane parallel to the magnetic field and the other lines are polarized perpendicular to the magnetic field • The original line is absorbed by both background and analyte. However, the other lines are absorbed by background only • The signal corresponding to the analyte is the difference

63 Advantages of Zeeman method

• Only a single lamp is required • Problems of alignment of two beams is eliminated • It provides more accurate correction for background than other methods because the background absorbance is measured at or very near the wavelength that the analyte absorbance is measured

64 Disadvantages of Zeeman method • The implementation is rather complex and expensive • The original and the split bands may overlap causing an increase in the background signal • This causes a curvature in the calibration curve • The instrument is bulky because of the magnet • Spectral lines for some elements may undergo more complex splitting. Thus sensitivity will be reduced (It gave good results for 44 elements) • It is more difficult to engineer with flames

65 Source self reversal background correction method

• It is based on self-reversal of the HCL spectral lines at high current • At high current, many free atoms are generated causing an increase in the number of unexcited atoms that absorb the center of the emitted line causing the self reversal • Specially good for correcting for high molecular absorption for that by phosphate on Se and As • A total is measure

66 • Hollow cathode lamps normally operate at currents of 3-15 mA. • If the applied power is raised to several hundred mA, they exhibit a phenomenon called self- reversal. • This giant pulse of current changes the nature of the analyte absorption line so it will only measure the background absorbance.

• Atotal is measured during the low current period. Abackground is measured during the high current period • The low current (6-20 mA) is pulsed at 100-500 mA Corrected signal = Signal before the pulse –Signal After the pulse

67 Advantages of Smith-Hieftje technique

• Only a single source is used • It offers all the advantages of other methods at low cost • It does not lose radiation like in Zeeman but this mehtod with the Zeeman one are less sensitive than the D2 one

• Drawbacks – less sensitivity – lamp life decreased

68 Background in atomic emission

• Sources 1. Emission from flame 2. Emission from the sample matrix

69 Electrothermal Atomizer (Graphite Furnace)

• Main components • Atomization steps • Interferences and matrix modification

70 Graphite Furnace Atomic Absorption Spectrometry, GFAAS

 A technique to minimize dilution during atomization of the analyte prior to its determination with atomic absorption spectrometry  A technique with more interferences than the more reliable flame atomization  A technique with high sensitivity and very good detectability but not so good throughput and precision

71 Advantages of GFAS

• Increase in the residence time thus more sensitivity will be obtained • Possibility of analyzing samples of various matrices • Analyzing small size samples even on the microliter scale • Avoiding the formation of refractory oxides that cause serious interference sourcse

72 • Minimize the surface adsorption

73 Why do we use an inert gas with GFAAS?

74 Atomization Process

75 Steps in graphite furnace atomization

• The steps of a GF atomization • 5-100µl sample – drying – ashing – atomization – burnout – cool

76 Steps in GF atomization

• The sample introduction – The sample is injected, usually between 10-30µl but sometimes the range could be extended to 100µl. – The solution is preferably made up of dilute nitric acid as matrix – sometimes solid samples are introduced

77 • The drying step – to volatilize the sample solvent – use some degrees above the solvents boiling point (for water use 110°C) and about the same time in seconds as the injected volume in µl – check that the solvent only volatilizes and not boil, otherwise bad reproducibility may occur

78 • Ashing of the sample – Maybe the most important step – Remove sample matrix without losing the analyte – Highly dependent on both the analyte and sample matrix – Conditions for every new sample type should be optimized – Inorganic matrix more difficult, because higher temperatures are needed

79 • The atomization step – This step is to vaporize and atomize the analyte – the vaporization pressure of the analyte dominate the supply of analyte atoms inside the tube – The convection of expanding gas and diffusion dominate the transport of analytes out of the tube – rapid heating is necessary for good sensitivity

80 • The burnout step – it is used for taking away parts of the sample not completely volatilized during atomization – temperatures in the range 2700-3000°C is most often used – if vaporization of the sample is achieved, completely, the sensitivity will slowly decrease and the imprecision increase

• Cooling step – its purpose is to cool down the furnace before next sample is added

81 Steps in GF atomization

temperature progamme 3000

2500

2000 Atomization 1500

Temp Temp (°C) 1000 Ashing 500 Drying time (sec) 0 0 20 40 60 80 82 Interferences with graphite furnace and its control

• In the early days this technique was accepted as a highly interference technique • Later, new instrumental and analytical procedures were developed which avoided the conditions found to be the source of interference problems

83 Interferences in GFAAS • The graphite furnace atomization technique is much more affected by interferences than the flame which depends on higher concentrations of non-analytes –physical • change in surface properties of the graphite tube influences the atomization rate –Spectral • molecular absorption by molecules or atoms other than the analyte atoms • light scattering –non spectral or chemical • molecular binding between analyte and matrix • formation of less volatile compound

84 • reactions with the graphite tube

Interference removal

Spectral interferences

• Emission interference • Non-analyte absorption and scattering

85 1. Emission interference • Radiation emitted by the hot graphite tube or platform that covers a wide range of waqvelengths (250-800 nm) • Following elements will be obscured: – Zn 213.9 nm; might be ok – Cr 357.9 nm – Ca 422.7 nm – Ba 553.7 nm mostly affected • This effect is eliminated by removing the graphite tube wall or platform from the view of the detector. This is achieved by ine of the 86following arrangements: • Setting a narrow monochromator slit • Furnace alignment • Cleaning graphite tube and windows to prevent light scattering • Atomization temperature should not be higher than required

87 2. Non-analyte absorption and scattering • It is considered as a background absorption • It is the most severe spectral interference problem • It occurs as a non specific attenuation of light at the analyte wavelength due to matrix components in the sample • It leads to having a broad band covering 10- 100’s nm due to molecular absorption caused by “undissociated sample matrix” components in the light path at atomization Remedy? • Matrix modification • Furnace control procedures

• 88Background correction as with flame techniques

Matrix modification • Matrix should be more volatile than the anayte so as to be removed before atomization 1. Formation of easily volatile matrix components, e.g.,

Using NH4NO3 or ammonium acetate to modify NaCl matrix

NaCl + NH4NO3 NaNO3 + NH4Cl nonvolatile matrix More volatile compounds Matrix modifier

• The more volatile components can be driven off at lower pyrolysis temperature

89 2. Converting the analyte element into a less volatile (if it is too volatile) component using

a modifier e.g., Ni (NO3)2 for Se determination. • Se is a highly volatile component • With the modifier Se form nickel selenide. Thus, Se can be heated up to 900 oC without loss. Consequently the matrix would be removed without affecting Se.

90 3. Varying the sample volume • Can be done if concentration allows • Reduce the sample size by injecting smaller samples, thus the mass of background producing matrix components will be reduced.

4. Spectral background correction • A continuous background correction is not very efficient specially for complex matrices. Matrix modification is a must. • Zeeman background correction is very efficient for complex matrices

91 Chemical Interference

• It occurs when other components of the sample matrix inhibit the formation of free analyte atoms

Mo + Xo MX

Free analyte atoms Different matrix component

92 Interference Removal 1. Ashing with the optimal temperature –matrix modification (mentioned above) • Delay of anayte release to furnace allows time for a constant furnace temperature • thermal stabilization of analyte (to hold the analyte on the graphite surface to a higher temperature) and vaporization of interference during the ashing step – Thus, volatility of interfering matrix compound is increased • On the other hand volatility of interfering compound is decreased and selective vaporization during the atomization step takes place 93 2. Use of pyrolytically coated Tubes and Platforms

•Ordinary graphite has a porous surface thus it forms with the analyte a carbide that is nonvolatie •The tubes are more or less always fabricated from pyrolytic carbon or covered with a pyrolytic carbon layer. It’s a more inert form of the graphite. It is more dense surface and less absorption •Tubes could sometimes be coated with tungsten or tantalum for prolonged lifetime or other special applications •The platform technique is used to vaporize the analyte into a more uniform and higher temperature than the release from the wall. The analyte vaporizes after the tube 94 wall and gas phase reach a steady state • L'vov Platform – Thin graphite plate inserted into tube with flat trough – L'vov barely contacts sides of tube – Sample is heated more uniformly by air – Platform delays the appearance of analyte until the furnace has reached atomization temp

95 Functions of the platform

• It resists tndency for sample to saok into surface before or during drying • It tolerates high acid concnetration • It gives bigger signal and less background – It is heated gradually by radiation from walls – When atomization step is reached the temperature of the sample and furnace environment will be similar – No chance for sudden cooling and recombination forming molecular species inhibiting atomization I

96 Disadvantages of Graphite Furnace

• Prone to interferences, poor precision

• Must take Step to control Precision: 1. Autosamplers vs. Manual Injection - more precise 2. Duplicate analyses for all samples (repeat if poor agreement) 3. Standard addition method - making standards up in sample

4. Background correction for Molecular Absorption) 1. D2 2. Zeeman Background Correction 3. Matrix Modifiers to control atomization

97 INDUCTIVELY COUPLED PLASMA • Main components of the instrument • Performance of ICP • ICP-mass spectrometry • interferences

98 Introduction

• The device which produces the ICP is commonly referred to as the ICP torch. It consists of two to four Argon flows depending on the manufacturer: • Nebulizer gas (inner Argon flow), at about 1 L/min, carries the analyte aerosol • Sheath gas for producing a laminar flow to improve low excitation energy elements eg group • I & II elements • Auxiliary gas (if present), lifts the plasma above the injector tube, used when measuring organics • Plasma gas, at about 12-16 L/min, sets the plasma conditions, e.g., excitation temperature

99 Sample Introduction • An ICP-AES instrument consists of a sample delivery system, an IC plasma to generate the signal, one or more optical spectrometers to measure the signal, and a computer for controlling the analysis.

• The most common sample delivery system consists of a peristaltic pump and capillary tube to deliver a constant flow of analyte liquid into a nebulizer.

100 Inductively Coupled Plasma Torch

101 How does plasma generate?

• RF induction coil powered by RF generator will generate a fluctuating magnetic field around the coil • Ionization of Ar is initiated by a spark • Ar+ and e- will interact with the magnetic field thus they will be forced to move in annular paths • Omic heating is the result of their resistance to the forced movement in limited area

102 Inductively Coupled Plasma Emission Source

103 Inductively Coupled Plasma Emission Source and Supply

104 105 106 • An ICP torch can be used to atomize and ionize materials for quantitative trace and ultratrace elemental analysis • The compounds in the samples are broken down into their elemental form and some fraction of each elemental species is simultaneously ionized • The monocationic (+1) form is preferred for the assay, and conditions are set to enhance production of that form • The torch is normally operated at a temperature of ~3000K

107 Schematic diagram of a typical ICP-OES instrument

108 Schematic diagram showing the different regions in the IC Plasma

109 Processes that take place in the plasma

•Aerosol vapor is transported to the plasma

•Vapor desolvates

•Atomization occurs within the plasma

•Atoms get excited to atomic and ionic states

•Rich spectra produced because of presence of both atomic and ionic lines

•Because of their different excitation energies, different emission lines will have maximum intensities at different vertical positions in the plasma 110 Typical detection limits in ICP-OES

111 112 Typical detection limits in ICP-OES

113 Sampling

Liquids • Form an aerosol (<5 µ droplet size) of the liquid sample • Eliminate larger droplets • The use of liquids facilitates automatic sampling: Gases • May use gasses directly or indirectly, e.g., form hydrides Solids • May use slurries (if 95% of particles <5 µ) • May use spark ablation or laser ablation to generate a small metal vapor

114 Viewing position

• The plasma generated in an ICP can be viewed by the spectrometer, side-on or end-on. • These viewing positions are called radial and axial viewing, respectively. • Each has advantages and disadvantages, so each tends to be used for different applications: radial viewing for normal analysis and complex materials, axial viewing for low detection limits in simpler materials.

115 116 Advantages of Radial View

• Should collect signal from the entire normal analytical zone – no torch adjustment for different elements – fewer matrix effects and interferences, especially in organics – less stray light – better detection limits in difficult matrices, especially in alkalis and organics – can run any matrix – less maintenance of the torch – torches last longer especially with dissolved salts – lower consumption of argon

117 Advantages of Axial View: • More intensity 118 • Better detection limits in simple matrices

RF-Generator • ICP's generally require an RF power of 1-2 kW maximum output to maintain the plasma. • High efficiency required - especially for organics (not less than 1.5 Kw)

Frequency • Fixed versus Free Running – Fixed: Crystal controlled, eg at 40 MHz – Free running: floats eg at 40 MHz ± 2 MHz • 40 MHz versus 27 MHz – Because of the skin-effect in RF plasmas, higher frequency gives a thinner plasma with a wider dynamic range (less self-absorption), with lower backgrounds and fewer interferences.

119 120 • > 90% element coverage • 0.1 to 10 ppb sensitivity for most elements • RSD ~ 2% to 4%

121 Typical laser ablation/ICP-MS mass spectrum of a rock sample

122 Comparison of Detection Limits Black = ICP/MS Cross-Hatch = ICP/AES Gray = Furnance AA

123 Comparison of ICP/AES & ICP/MS for the analysis of Cerium solutions

ICP/AES (100 ppm Ce)

Background ions originated from the Ar, O, H and N elements

ICP/MS (10 ppm Ce) • Simple to interpret, especially for rare elements • Less interference from atmospheric contaminants or molecular bands

Nitric acid is commonly used as acidification agent rather than hydrochloric acid, sulfuric acid and 124 phosphoric acid in sample preparation. Why? Limitations for ICP/MS Quantitation • Formation of monocationic ions, doubly-charged ions and stable metal oxides ions in the torch depends on the identity of the element and several experimental conditions, such as the plasma r.f. power and the nebulizer gas flow rate. • Spectroscopic interference: 1. Isobaric Interference - isotopes of different elements (or isobaric interference) e.g. 87Rb+ and 87Sr+; 40Ar+ and 40Ca+; 58Ni+ and 58Fe+

125 – Polyatomic Ion Interference • interactions between species in the plasma and species in the matrix or atmosphere, e.g. molecular ions from the acids of digestion • e.g. ClO+ and 51V+

Which element(s) does not have isobaric interference caused by the presence of other elements?

126 • Matrix effect: (a)Limited tolerance for salt concentration (< 1% is acceptable but < 0.1% is preferred!) – The deposition of salts leads to a decrease in the aperture diameter, so the sensitivity worsens and the signal gradually decreases as a function of time (b)Organic solvents tend to quench the plasma torch

127 Approaches to reduce the occurrence of polyatomic interferences • Adjust the plasma conditions to minimize the formation of polyatomic species

e.g. use 5-10% N2 in Ar, the axial temperature of the plasma increases by ~ 1000 K and the electron density increases by 30%. A marked reduction of MO+ ions • Minimizing the amount of water vapor introduced into the plasma in order to limit the signal intensity of oxygen- containing species e.g. reduce the spray chamber temperature(?) • Use He instead of Ar as plasma gas to avoid interference from polyatomic ions containing argon • Use electrothermal volatilization for sample introduction • 128Use high resolution ICP/MS Approaches to correct for or overcome matrix effect • Dilution • Matrix matching • Use of internal standards – Addition of a known and preferably an equal concentration of an internal standard to all samples (& standards). The analytical signal is taken as the ratio of the signals for the analyte elements in the sample (& standards) and the signal for the internal standard • Standard addition – A known amount of the element(s) of interest is added to the sample and hence suffers the same matrix effect as the analyte 129