Chapter 8

Atomic Absorption SpectroscopyAASYang Yi College of Science, BUCT

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Atomic Absorption SpectroscopyAASIntroductionElementary TheoryInstrumentationInterferencesExperimental preliminariesApplications

What is AAS ?Atomic absorption spectroscopy is a quantitative method of analysis that is applicable to many metals and a few nonmetals.Introduction

What is AAS ?The technique was introduced in 1955 by Walsh in Australia (A.Walsh, Spectrochim. Acta, 1955, 7, 108)Alan Walsh 1916-1998 http://www.science.org.au/academy/memoirs/walsh2.htm#1The application of atomic absorption spectra to chemical analysis

What is AAS ?The technique was introduced in 1955 by Walsh in Australia (A.Walsh, Spectrochim. Acta, 1955, 7, 108)The first commercial atomic absorption spectrometer was introduced in 1959

What is AAS ?An atomic absorption spectrophotometer consists of a light source, a sample compartment and a detector.

Light SourceDetectorSampleCompartment

What is AAS ?A much larger number of the gaseous metal atoms will normally remain in the ground state.These ground state atoms are capable of absorbing radiant energy of their own specific resonance wavelength.If light of the resonance wavelength is passed through a flame containing the atoms in question, then part of the light will be absorbed.The extend of absorption will be proportional to the number of ground state atoms present in the flame.

What is AAS ?the gaseous metal atomsspecific resonance wavelengththe extend of absorption vs the number of ground state atoms present in the flame.extend of absorption

Characteristic wavelengthCharacters of the atomic absorption spectrumE = E1 E0 = hc / E1 - excited stateE0 ground stateh Plancks constantc velocity of light - wavelength

Elementary Theory

K0 - maximal absorption coefficient - half width0 -central wavelengthCharacters of the atomic absorption spectrumProfile of the absorption line

The absorption line is not a Geometrical line

Natural broadening determined by the lifetime of the excited stateand Heisenbergs uncertainty principle10-5 nm Doppler Broadening 10-3 nm results from the rapid motion of atoms as they emit or absorb radiationCollisional Broadening collisions between atoms and molecules in the gas phase lead to deactivation of the excited state and thus broadening the spectral linesCharacters of the atomic absorption spectrum

Doppler Broadening 10-3 nm results from the rapid motion of atoms as they emit or absorb radiationCharacters of the atomic absorption spectrum

It = I0e -KlThe relationship between absorbance and the concentration of atomsA = log I0/ It= 0.4343 K l Beers lawIt - intensity of the transmitted lightIo intensity of the incident light signall the path length through the flame (cm)

K d=(e2/mc)N0Integrated absorptionThe relationship between absorbance and the concentration of atoms

K - the absorption coefficient at the frequency e the electronic chargem the mass of an electronc the velocity of lightf the oscillator strength of the absorbing lineN0 the number of metal atoms per milliliter able to absorb the radiation

The measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysisThe relationship between absorbance and the concentration of atoms K d=(e2/mc)N0

The line width of an atomic spectral line is about 0.002 nm.The relationship between absorbance and the concentration of atomsTo measure the absorption coefficient of a line would require a spectrometer with a resolving power of 500 000.The absolute measurement of the absorption coefficient of an atomic spectral line is extremely difficult.

This difficulty was overcome byThe relationship between absorbance and the concentration of atomswho used a source of sharp emission lines with a much smaller half-width than the absorption line. and the radiation frequency of which is centred on the absorption frequency.Walsh,

The relationship between absorbance and the concentration of atomsIn this way, the absorption coefficient at the centre of the line, K0 , may be measured instead of measuring the integrated absorption.

A = 0.4343 K0 l = K1N0vA = KCThe relationship between absorbance and the concentration of atoms

InstrumentationLine sourceMonochromatorDetectorRead-outNebulizerSchematic diagram of a flame spectrophotomerAtomization

Resonance line sources--- Provide the sharp emission lines with a much smaller half-width than the absorption lineEmit the specific resonance lines of the atoms in question--- Intensity--- Purity--- Background--- Stability--- Life-time

Hollow cathode lamp (HCL)

Cathode--- in the form of a cylinder, made of the element being studied in the flameAnode---tungsten

A hollow cathode lamp for Aluminum (Al)

SpectrAA - AAS motorizedMirrorHCL

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Sample atomization techniques

Flame atomization Electrothermal atomizationHydride atomizationCold-Vapor atomization

Processes occurring during atomizationFlame atomization

Nebulizer - burnerA typical premix burnerFlame atomization

Nebuliser - burnerTo convert the test solution to gaseous atomsNebuliser --- to produce a mist or aerosol of the test solutionBurner head --- The flame path is about 10 12 cmVaporising chamber --- Fine mist is mixed with the fuel gas and the carrier gas Larger droplets of liquid fall out from the gas stream and discharged to waste

Fuel and oxidant flameb Air acetyleneAir- propaneAir- hydrogenb Nitrous oxide acetyleneAuxiliary oxidantFuel

Common fuels and oxidants used in flame spectroscopy

Disadvantages of flame atomizationOnly 5 15 % of the nebulized sample reaches the flameA minimum sample volume of 0.5 1.0 mL is needed to give a reliable readingSamples which are viscous require dilution with a solvent

Graphite furnace techniqueEletrothermal atomization

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Plateau Graphite Tube

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Graphite furnace techniqueprocessdryingashingatomization

Graphite furnace techniqueAdvantagesSmall sample sizes ( as low as 0.5 uL)Very little or no sample preparation is neededSensitivity is enhanced ( 10 -10 10-13 g , 100- 1000 folds)Direct analysis of solid samples

Graphite furnace techniqueDisadvantagesBackground absorption effects Analyte may be lost at the ashing stageThe sample may not be completely atomizedThe precision was poor than the flame method(5%-10% vs 1%)The analytical range is relatively narrow(less than two orders of magnitude)

Cold vapour techniqueHg2+ + Sn2+ = Hg + Sn (IV)

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Hydride generation methodsFor arsenic (As), antimony (Te) and selenium (Se)As (V) AsH3As0(gas) + H2NaBH4(sol)heatin flame[H+]

--- diffraction gratingMonochromator

Detector--- photomultiplier

Read-out system--- meter --- chart recorder--- digital display

Atomic absorption spectrophotometer

Interferences Spectral interferences Chemical interferences Physical interferences

Spectral interferences ----- spectral overlap +, positive analytical error Cu 324.754 nm, Eu 324.753 nmAl 308.215 nm , V 308.211nm,Al 309.27 nmAvoid the interference by observing the aluminum line at 309.27 nm

Spectral interferences ----- non-absorption line ----- molecular absorption + combustion products (the fuel and oxidant mixture)Correct by making absorption measurements while a blank is aspirated into the flame

Spectral interferences ----- light scatter +Metal oxide particles with diameters greater than the wavelength of lightWhen sample contains organic species or when organic solvents are used to dissolve the sample, incomplete combustion of the organic matrix leaves carbonaceous particles that are capable of scattering light

Spectral interferences ----- light scatter +The interference can be avoided by variation in analytical variables, such as flame temperature and fuel-to oxidant ratioStandard addition methodZeeman background correction

Chemical interferences----- Formation of compound of low volatilityIncrease in flame temperaturebbbUse of releasing agents (La 3+ ) Separation Ca 2+ PO43- Mg2+, Al3+bUse of protective agents (EDTA)

Chemical interferences----- IonizationbAdding an excess of an ionization suppressant (K)

Physical interferences----- viscosity ----- density----- surface tension----- volatilitybMatrix matching

Experimental preliminariesPreparation of sample solutionsOptimization of the operating conditions----- resonance line----- slit width----- current of HCL----- atomization conditionCalibration curve procedure

The standard addition technique

Sensitivity and detection limitSensitivity----- the concentration of an aqueous solution of the elements which absorbs 1% of the incident resonance radiation----- the concentration which gives an absorbance of 0.0044

Detection limitSensitivity and detection limit----- the lowest concentration of an analyte that can be distinguished with reasonable confidence from a field blankD = c 3 / A

Sensitivity and detection limit(ng/mL)

Advantages and disadvantagesHigh sensitivity [10-10g (flame), 10-14g (non-flame)]Good accuracy (Relative error 0.1 ~ 0.5 % )High selectivityWidely usedA resonance line source is required for each element to be determined

The end

abbreviationThe absorption line is not a Geometrical line 10101010101515151515