1 Chapter 8 Atomic Absorption Spectroscopy （ AAS ） Yang Yi College of Science, BUCT

2  Introduction  Elementary Theory  Instrumentation  Interferences  Experimental preliminaries  Applications Atomic Absorption Spectroscopy （ AAS ）

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

4 What is AAS ? The technique was introduced in 1955 by Walsh in Australia (A.Walsh, Spectrochim. Acta, 1955, 7, 108) Alan Walsh memoirs/walsh2.htm#1 The application of atomic absorption spectra to chemical analysis

5 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

6 What is AAS ? An atomic absorption spectrophotometer consists of a light source, a sample compartment and a detector. Light Source Detector SampleCompartment

7 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.

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

9 Characteristic wavelength Characters of the atomic absorption spectrum ΔE = E 1 – E 0 = hc / ΔE = E 1 – E 0 = hc / E 1 - excited state E 0 – ground state h – Planck’s constant c – velocity of light - wavelength Elementary Theory

10 K 0 - maximal absorption coefficient Δ - half width 0 - central wavelength Characters of the atomic absorption spectrum Profile of the absorption line

11 Natural broadening determined by the lifetime of the excited state determined by the lifetime of the excited state and Heisenberg’s uncertainty principle and Heisenberg’s uncertainty principle （ nm ） Doppler Broadening Doppler Broadening （ nm ） results from the rapid motion of atoms as they emit or absorb radiation Collisional Broadening collisions between atoms and molecules in the gas phase lead to deactivation of the excited state and thus broadening the spectral lines Characters of the atomic absorption spectrum

12 Doppler Broadening Doppler Broadening （ nm ） results from the rapid motion of atoms as they emit or absorb radiation Characters of the atomic absorption spectrum

13 I t = I 0ν e -Kνl The relationship between absorbance and the concentration of atoms I 0ν / A = log （ I 0ν / I t ） = K l Beer ’ s law I t - intensity of the transmitted light I o – intensity of the incident light signal l – the path length through the flame (cm)

14  K d =(  e 2 /mc)  N 0 Integrated absorption The relationship between absorbance and the concentration of atoms K - the absorption coefficient at the frequency e – the electronic charge m – the mass of an electron c – the velocity of light f – the oscillator strength of the absorbing line N 0 – the number of metal atoms per milliliter able to absorb the radiation

15 The measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis The relationship between absorbance and the concentration of atoms  K d =(  e 2 /mc)  N 0

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

17 This difficulty was overcome by The relationship between absorbance and the concentration of atoms who 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,

18 The relationship between absorbance and the concentration of atoms In this way, the absorption coefficient at the centre of the line, K 0, may be measured instead of measuring the integrated absorption.

19 A = K 0 l = K 1 N 0v A = KC The relationship between absorbance and the concentration of atoms

20 Instrumentation Line source Monochromator Detector Read-outNebulizer Schematic diagram of a flame spectrophotomer Atomization

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

22 Hollow cathode lamp (HCL) Cathode--- in the form of a cylinder, made of the element being studied in the flame Anode---tungsten

23 A hollow cathode lamp for Aluminum (Al)

24 SpectrAA - AAS motorized Mirror HCL

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26 Sample atomization techniques Flame atomization Electrothermal atomization Hydride atomization Cold-Vapor atomization

27 Processes occurring during atomization Flame atomization

28 Nebulizer - burner A typical premix burner Flame atomization

29 Nebuliser - burner To convert the test solution to gaseous atoms Nebuliser --- to produce a mist or aerosol of the test solution Burner head --- The flame path is about 10 –12 cm Vaporising 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

30 Fuel and oxidant flame  Air – acetylene Air- propane Air- hydrogen  Nitrous oxide – acetylene Auxiliary oxidant Fuel

31 Common fuels and oxidants used in flame spectroscopy

32 Disadvantages of flame atomization Only 5 – 15 % of the nebulized sample reaches the flame A minimum sample volume of 0.5 – 1.0 mL is needed to give a reliable reading Samples which are viscous require dilution with a solvent

33 Graphite furnace technique Eletrothermal atomization

34 Plateau Graphite Tube

35 Graphite furnace technique process drying ashingatomization

36 Graphite furnace technique Advantages Small sample sizes ( as low as 0.5 uL) Very little or no sample preparation is needed Sensitivity is enhanced ( – g, folds) Direct analysis of solid samples

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

38 Cold vapour technique Hg 2+ + Sn 2+ = Hg + Sn (IV)

39 Hydride generation methods For arsenic (As), antimony (Te) and selenium (Se) As (V)AsH 3 As 0 (gas) + H 2 NaBH 4 (sol) heat in flame[H+][H+]

40

diffraction grating Monochromator

42 Detector --- photomultiplier

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

44 Atomic absorption spectrophotometer

45 Interferences Spectral interferences Chemical interferences Physical interferences

46 Spectral interferences spectral overlap （ +, positive analytical error ） Cu nm, Eu nm Al nm, V nm, Al nm Avoid the interference by observing the aluminum line at nm

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

48 Spectral interferences light scatter （ + ） Metal oxide particles with diameters greater than the wavelength of light When 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

49 Spectral interferences light scatter （光散射）（ + ） The interference can be avoided by variation in analytical variables, such as flame temperature and fuel-to – oxidant ratio Standard addition method Zeeman background correction

50 Chemical interferences Formation of compound of low volatility Increase in flame temperature    Use of releasing agents (La 3+ ) Separation Ca 2+ ， PO 4 3- Mg 2+, Al 3+  Use of protective agents (EDTA)

51 Chemical interferences Ionization  Adding an excess of an ionization suppressant (K)

52 Physical interferences viscosity density surface tension volatility  Matrix matching

53 Experimental preliminaries Preparation of sample solutions Optimization of the operating conditions resonance line slit width current of HCL atomization condition Calibration curve procedure

54 The standard addition technique

55 Sensitivity and detection limit Sensitivity the concentration of an aqueous solution of the elements which absorbs 1% of the incident resonance radiation the concentration which gives an absorbance of

56 Detection limit Sensitivity and detection limit the lowest concentration of an analyte that can be distinguished with reasonable confidence from a field blank D = c × 3σ / A

57 Sensitivity and detection limit (ng/mL)

58 Advantages and disadvantages High sensitivity [ g (flame), g (non-flame)] Good accuracy (Relative error 0.1 ~ 0.5 % ) High selectivity Widely used A resonance line source is required for each element to be determined

59 The end