AA and Atomic Fluorescence Spectroscopy
Chapter 9
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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
5-1
Technique
5-2
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
Methane
Oxygen ºC
Air ºC
2810
1957
Ethane
Propane
1960
2820
Butane
1980
1970
Hydrogen
2660
2045
Acetylene
3100
2400
5-3
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
5-4
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
5-5
Flame Structure
secondary
interzonal
Primary
zone
Maximum
temperature
5-6
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?
5-7
Absorbance Profile
5-8
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
5-9
Electrothermal atomizer
Sample concentration
5-10
Graphite Furnace
5-11
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)
5-12
Atomic Absorption Instrumentation
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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
5-13
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
5-14
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
5-15
Atomic Absorption Instrumentation
• Electrodeless Discharge
Lamps
 Inert gas in quartz
tube
 Excite gas with RF
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Similar to cathode
expect excitation
5-16
Spectrophotometers
• Single Beam
 Shutter controls
beam
 Collect blank
Blank provides
100%
transmission
Insert sample
and measure
absorbance
5-17
Spectrophotometers
• Double Beam
 Light source split
 Measure light through
flame and light reference
light

Determine %T
Does not consider
light absorption in
flame
5-18
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
5-19
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
5-20
Interference
5-21
Detection Limits
5-22
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
5-23