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AA and Atomic Fluorescence Spectroscopy Chapter 9

AA and Atomic Fluorescence Spectroscopy Chapter 9. Sample Atomization Atomic Absorption Instrumentation Interference Atomic Absorption Techniques Atomic Fluorescence Sample Atomization For techniques samples need to be atomized Techniques are useful for element identification

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AA and Atomic Fluorescence Spectroscopy Chapter 9

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  1. AA and Atomic Fluorescence Spectroscopy Chapter 9 • Sample Atomization • Atomic Absorption Instrumentation • Interference • Atomic Absorption Techniques • Atomic Fluorescence • Sample Atomization • For techniques samples need to be atomized • Techniques are useful for element identification • Molecular information destroyed by atomization • Flame Atomization • Sample nebulized • Mixed with fuel • Carried to flame for atomization

  2. Technique

  3. Flame Atomization • Evaporation of solvent • Produces molecular aerosol • Molecules dissolution leads to atomic gas • Atoms ionize to product cations and electrons • Property of flame can affect process

  4. Flame ionization • Flame temperature in range of 1700 °C to 2400 °C in air • From 2500 °C to 3100 °C with oxidant • Need to keep flame stable • Flame structure • Different zones are properties of fuel and oxidant • Primary combustion zone • Blue luminescence due to C2 and CH • Thermal equilibrium not reached in primary zone

  5. Flame ionization • Interzonal region • Central part of flame • High concentration of free atoms • Used for spectroscopy • Secondary combustion region • Convert elements to oxides • Disperse sample to air

  6. Flame Structure secondary interzonal Primary zone Maximum temperature

  7. Best location for absorbance? • Variation due to the degree of oxidation for a given element • Mg • Atomizes then oxidizes as Mg approaches secondary combustion area • Formation of MgO reduces absorbance • Ag • Does not readily oxidize • Atomization over flame area • Cr • Forms oxidizes readily so that oxide is main species in flame • Need to consider based on flame sample area • Does instrument sample entire flame or just small area?

  8. Absorbance Profile

  9. Electrothermal Atomization • Atomization of entire sample in short period • Average sample time in optical path is seconds • Evaporation of sample • Microliter volume • Low temperature • Sample ashed at higher temperature • Increase current • Sample temperature goes to 2000-3000 °C • Sample measured above heated surface • High sensitivity for small samples

  10. Electrothermal atomizer Sample concentration

  11. Graphite Furnace

  12. Atomization Techniques • Glow Discharge • Sputtering of samples due to Ar ion acceleration • Mixture of atoms and ions • Hydride generator • Forms volatile species • As, Sb, Sn, Se, Bi, Pb • Cold Vapor (Hg)

  13. Atomic Absorption Instrumentation • Radiation Source • Sample Holder • Wavelength selector • Detector • Radiation sources • AA has narrow lines (0.005 nm) • Most light sources provide light with greater bandwidths • Absorption of source light • Need narrow source

  14. Atomic Absorption Instrumentation • Light source • Use source for element detection • For Na, use Na vapor lamp • 3p to 2s transition at 589.6 nm • Minimize line broadening • Doppler • Pressure • Temperature • Need a separate light source for each element

  15. Atomic Absorption Instrumentation • Hollow Cathode Lamp • Ionization of inert gas by potential • Gas acceleration to cathode • Atoms on cathode into gas state • Some excited • Deexcite with photon emission • Need to excite specific elements for measurement

  16. Atomic Absorption Instrumentation • Electrodeless Discharge Lamps • Inert gas in quartz tube • Excite gas with RF • Similar to cathode expect excitation

  17. Spectrophotometers • Single Beam • Shutter controls beam • Collect blank • Blank provides 100% transmission • Insert sample and measure absorbance

  18. Spectrophotometers • Double Beam • Light source split • Measure light through flame and light reference light • Determine %T • Does not consider light absorption in flame

  19. Interference • Spectral interference • Overlap of sample spectra • Not very common due to narrow line widths • If occurs select different transition • Scattering • Formation of oxides • Correct with different methods • Two line method • Continuum source • Zeeman effect • Polarize and split light with magnetic field

  20. Interference • Chemical Interference • More common than spectral interference • Formation of compounds with low volatility • Additives to remove such compounds • EDTA • Dissociation equilibria • Reaction of oxide species • Ionization equilibria • Formation of ion species, liberation of electron

  21. Interference

  22. Detection Limits

  23. Atomic Fluorescence Spectroscopy • optical emission from gas-phase atoms that have been excited to higher energy levels • Enhancement of sensitivity over AA • Examine electronic structure of atoms • Light source • Hollow Cathode Lamp • Laser • Detection • Similar to AA

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