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AA and Atomic Fluorescence Spectroscopy Chapter 9

AA and Atomic Fluorescence Spectroscopy Chapter 9

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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