1. AA and Atomic Fluorescence Spectroscopy Chapter 9
Sample Atomization
Atomic Absorption Instrumentation
Interference
Atomic Absorption Techniques
Atomic Fluorescence
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
Fuel Gas
Oxygen ºC
Air ºC
Methane
2810
1957
Ethane
1960
Propane
2820
1980
Butane
1970
Hydrogen
2660
2045
Acetylene
3100
2400

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 °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 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