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Atomic Absorption spectroscopy

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AAS

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  • abbreviation
  • The absorption line is not a Geometrical line

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  • 1. Atomic Absorption spectroscopy ( AAs ) Yash Rao CUG india 1
  • 2. ATOMIC ABSORPTION SPECTROSCOPY ( AAS ) • • • • • • Introduction Elementary Theory Instrumentation Interferences Experimental preliminaries Applications 2
  • 3. Introduction What is AAS ? Atomic absorption spectroscopy is a quantitative method of analysis that is applicable to many metals and a few nonmetals. 3
  • 4. Conti….. ? The technique was introduced in 1955 by Walsh in Australia (A.Walsh, Spectrochim. Acta, 1955, 7, 108) The application of atomic absorption spectra to chemical analysis Alan Walsh 4
  • 5. Conti….. ? The technique was introduced in 1955 by Walsh in Australia The first commercial atomic absorption spectrometer was introduced in 1959 5
  • 6. Conti….. ? • An atomic absorption spectrophotometer consists of a light source, a sample compartment and a detector. Sample Compartment Light Source Detector 6
  • 7. Conti….. ? 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. 7
  • 8. Conti….. ? the gaseous metal atoms specific resonance wavelength extend of absorption the extend of absorption vs the number of ground state atoms present in the flame. 8
  • 9. Elementary Theory Characters of the atomic absorption spectrum Characteristic wavelength Δ E = E 1 – E 0 = hc / λ E 1 - excited state E 0 – ground state h – Planck’s constant c – velocity of light λ - wavelength 9
  • 10. Characters of the atomic absorption spectrum Profile of the absorption line K0 - maximal absorption coefficient Δ ν - half width ν 0 - central wavelength 10
  • 11. Characters of the atomic absorption spectrum Natural broadening determined by the lifetime of the excited state and Heisenberg’s uncer tainty principle ( 10-5 nm ) Doppler Broadening ( 10-3 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 11
  • 12. Characters of the atomic absorption spectrum Doppler Broadening ( 10-3 nm ) results from the rapid motion of atoms as they emit or absorb radiation 12
  • 13. The relationship between absorbance and the concentration of atoms Beer’s law I t = I 0ν e -Kνl A = log ( I 0ν / I t ) = 0.4343 K ν l I t - intensity of the transmitted light I o – intensity of the incident light signal l – the path length through the flame (cm) 13
  • 14. The relationship between absorbance and the concentration of atoms Integrated absorption ∫ Kν dν=(πe2/mc)ƒN0ν Kν - the absorption coefficient at the frequency ν e – the electronic charge m – the mass of an electron c – the velocity of light f – the oscillator strength of the absorbing line N0 – the number of metal atoms per milliliter able to absorb the radiation 14
  • 15. The relationship between absorbance and the concentration of atoms ∫ Kν dν=(πe2/mc)ƒN0ν The measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis 15
  • 16. The relationship between absorbance and the concentration of atoms The line width of an atomic spectral line is about 0.002 nm. To 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. 16
  • 17. The relationship between absorbance and the concentration of atoms This difficulty was overcome Walsh, by 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. 17
  • 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. 18
  • 19. The relationship between absorbance and the concentration of atoms  2( v − v 0 )  Kv = K 0 log  − ln 2  ∆v 0   2 K0 = ∆ν D 2 ln 2 π .e 2 ⋅ fNov π mc A = 0.4343 K 0 l = K 1 N 0v A = KC 19
  • 20. Instrumentation Line source Atomization Monochromator Nebulizer Detector Read-out Schematic diagram of a flame spectrophotomer 20
  • 21. Resonance line sources Emit the specific resonance lines of the atoms in question --- Provide the sharp emission lines with a much smaller half-width than the absorption line --- Intensity --- Purity --- Background --- Stability --- Life-time 21
  • 22. HOLLOW CATHODE LAMP (HCL) Cathode--- in the form of a cylinder, made of the element being studied in the flame Anode---tungsten 22
  • 23. A hollow cathode lamp for Aluminum (Al) 23
  • 24. HCL motorized Mirror 24
  • 25. 25
  • 26. Flame atomization Electrothermal atomization Hydride atomization Cold-Vapor atomization 26
  • 27. Flame atomization Processes occurring during atomization 27
  • 28. Flame atomization Nebulizer - burner A typical premix burner 28
  • 29. Nebuliser - burner To convert the test solution to gaseous atoms Nebuliser --- to produce a mist or aerosol of the test solution 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 Burner head --- The flame path is about 10 –12 cm 29
  • 30. Fuel and oxidant flame Auxiliary oxidant Air- propane Fuel Air- hydrogen  Air – acetylene  Nitrous oxide – acetylene 30
  • 31. Common fuels and oxidants used in flame spectroscopy 31
  • 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 32
  • 33. Eletrothermal atomization Graphite furnace technique 33
  • 34. PLATEAU GRAPHITE TUBE 34
  • 35. Graphite furnace technique process drying ashing atomization 35
  • 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 ( 10 -10 –10 -13 g , 100- 1000 folds) Direct analysis of solid samples 36
  • 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) 37
  • 38. Cold vapour technique Hg 2+ + Sn 2+ = Hg + Sn (IV) 38
  • 39. Hydride generation methods For arsenic (As), antimony (Te) and selenium (Se) As (V) (sol) NaBH 4 [H + ] AsH 3 heat As 0 (gas) + H 2 in flame 39
  • 40. 40
  • 41. Monochromator --- diffraction grating 41
  • 42. Detector --- photomultiplier 42
  • 43. Read-out system --- meter --- chart recorder --- digital display 43
  • 44. Atomic absorption spectrophotometer 44
  • 45. Interferences Spectral interferences Chemical interferences Physical interferences 45
  • 46. Spectral interferences ----- spectral overlap ( +, positive analytical error ) Cu 324.754 nm, Eu 324.753 nm Al 308.215 nm , V 308.211nm, Al 309.27 nm Avoid the interference by observing the aluminum line at 309.27 nm 46
  • 47. 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 47
  • 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 48
  • 49. Spectral interferences ----- light scatter ( + ) 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 49
  • 50. Chemical interferences ----- Formation of compound of low volatility     Ca 2+ , PO 4 3Mg 2+ , Al 3+ Increase in flame temperature Use of releasing agents (La 3+ ) Use of protective agents (EDTA) Separation 50
  • 51. Chemical interferences ----- Ionization  Adding an excess of an ionization suppressant (K) 51
  • 52. Physical interferences ----- viscosity ----- density ----- surface tension ----- volatility  Matrix matching 52
  • 53. Experimental preliminaries Preparation of sample solutions Optimization of the operating conditions ----- resonance line ----- slit width ----- current of HCL ----- atomization condition Calibration curve procedure 53
  • 54. The standard addition technique 54
  • 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 0.0044 55
  • 56. Sensitivity and detection limit Detection limit ----- the lowest concentration of an analyte that can be distinguished with reasonable confidence from a field blank D = c × 3σ / A 56
  • 57. Sensitivity and detection limit (ng/mL) 57
  • 58. Advantages and disadvantages High sensitivity [10 -10 g (flame), 10 -14 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 58
  • 59. End of presents 59