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Chapter 9

Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9. Chapter 9.

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Chapter 9

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  1. Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Atomic Absorption and Atomic Fluorescence Spectrometry Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9 Chapter 9

  2. Solution of analyte Nebulization Spray Desolvation Solid/gas aerosol Volatilization Gaseous molecules Excited molecules hn molecular Dissociation (reversible) Atoms Excited atoms hn atomic Ionization (reversible) Atomic ions Excited ions hn ionic Atomization Processes

  3. Regions in a Flame Stable molecular oxides Secondary combustion zone Interzonal region Primary combustion zone Rich in free atoms C2, CH and other radicals

  4. Temperature Profile in oC for a Natural Gas /Air Flame 5 1863 1858 1830 1800 4 1750 1700 1600 3 1400 Distance above orifice cm 2 1 0 1.5 1.0 0.5 0 0.5 .0 1.5 Burner tip

  5. Stable products Atoms

  6. Flame Absorbance profile for Some Elements Absorption Cr Ag Mg 0 1.0 2.0 Height cm

  7. The Premixed Burner Fuel Oxidant

  8. Laminar-flow burner

  9. Dry 150 Ash 600 Atomize 3000 Graphite Furnace oC

  10. Graphite Furnace Cooled electrical connections Cooled electrical connections External gas inlet Graphite tube Entrance window Exit window L’vov platform Internal gas inlet

  11. Output Signal 0.8 Ash 0.7 0.6 0.5 0.4 0.2 0.3 2 mL of canned juice 0.1 0.2 Dry Atomize 0.05 0.1 0.0 Standards mg/L Sample

  12. Electrothermal atomizer

  13. Graphite Furnace

  14. Glow Discharge Atomizer Argon Water cooling Sample (cathode) Anode To Vacuum pump

  15. Hydride Generation Atomizer To detector From spectrophototmeter Sample and NaBH4 Quartz absorption tube Inert gas Stirrer

  16. Atomic Absorption Instrumentation • Radiation Sources • Hollow cathode lamps • Electrodless discharge lamps • Source modulation • Spectrophotometers • Single beam • Double beam

  17. Radiation Sources • Atomic lines are narrow (0.002-0.005nm). • In chapter 13: Beer’s law is obeyed when line width of the source is narrower than absorption line width. • When ordinary spectrophotometers with continium source is used nonlinear calibration curves are obtained. • Line sources are used

  18. Absorption of a resonance line by atoms Monochromator bandwidth P0 Emission spectrum of source Sample absorption spectrum Emission spectrum after passage through sample and monochromator A=log P0/P P

  19. Hollow Cathode Lamp Hollow cathode Anode Quartz or pyrex window Ne or Ar at 1-5 torr Glass shield

  20. Sputtering

  21. Electrodeless Discharge Lamp RF Coil Lamp Ceramic holder Quartz window

  22. Abs = 0.762 Lamp Zn Current = 3.0 Source Modulation Phase sensitive amplifier + = Detector Lamp supply Ebert monochromator Burner

  23. Typical Single Beam Flame Atomic Absorption Spectrophotometer Detector Lamp supply Ebert monochromator Burner

  24. Interferences in Atomic Absorption Spectroscopy • Spectral Interferences • Chemical Interferences

  25. Spectral Interferences • Line • Broadband • Scattering nm

  26. Scattering Absorption

  27. Background Correction • Two-line method • Continuum source method • Self-reversal method • Zeeman method Signal = ( Background + Signal ) – ( Backgound )

  28. Two-line method • Monochromator at analytical wavelength: signal + background are measured • Monochromator is scanned to nearby line not absorbed by the analyte: backgound is measured • Signal = The difference of two measurements Monochromator bandwidth

  29. Continuum Source Method • Monochromator at analytical wavelength: signal + background are measured • Slit is opened, source is replaced by deuterium lamp (continuum source): • tiny amount of light is absorbed by sample and the rest is scattered by backgound. • Signal = the difference Monochromator bandwidth

  30. Deuterium lamp Analyte Hollow cathode lamp To monochromator Electrothermal atomizer Chopper Schematic of a continuum source background correction system

  31. Self Absorption Line width also increase As the current increases As the current increses further mA Power Supply Number of unexcited atoms increase and absorb the center of the line.

  32. Self-Reversal Method • Monochromator at analytical wavelength: signal + background are measured. • Current is pulsed high • Background is measured. • Signal = the difference Monochromator bandwidth

  33. Chemical Interferences • Formation of Compounds of Low Volatility • Anion • Cation To eliminate the effect: • Higher temperature • Releasing agents • Protective agents

  34. Chemical Interferences • Dissociation Equilibria • MO M + O • M(OH)2 M + 2 OH • Ionization Equilibria • M M+ + e-

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