Atomic Absorption Spectroscopy AAS

1
Atomic Absorption Spectroscopy AAS

• Comparatively easy to use
• Low maintenance
• Low consumables
• Good for measuring one element at a time.

2
Block Diagram

• hn M ? M
• Sample is vaporized/atomized by
• Flame
• Electrothermal Vaporizer (ETV)

3
AA Sources- HCL
4
Flame AAS sample introduction

• Sample is dissolved into solution (usually
  acidic).
• Sample is pulled through straw into nebulizer.
  Most of samples goes to waste.
• Nebulizer sends droplets/aerosol to flame to be
  de-solvated, resulting in gaseous molecules/atoms.

5
Flame processes

• To excited free atoms, flame must break any
  molecules apart into discrete atoms.
• Potential Problems
• Atoms recombine readily in flame, especially with
  O2.
• If flame is too energetic, ionization of atoms
  can occur - wont be detected.

• MX (g) ? M (g) X(g)
• M O ? MO
• M ? M e-

6
Burner Head

• want a long optical pathlength.

7
Temperature of Some Flames
Fuel Oxidant Temperature (K)
H2 Air (20 O2, 80 N2) 2000-2100
C2H2 Air 2100-2400
H2 O2 2600-2700
C2H2 N2O 2600-2800
8
Electrothermal Vaporization

• First demonstrated in 1961 by L'vov (USSR)
• Use electrically heated carbon furnace
• Excellent LOD

More sensitive than flame Entire sample
atomized at once Residence time of vapor in
optical path gt1 s
Potential problem poor precision due to
sampling variability
LOD for Mg Flame 0.1 ppm ETV 0.00002 ppm - 20
pptr
9
Cold Vapor (CV) Atomization

• Mercury
• Reduced by SnCl2

10
Physical Interferences

• Droplet size from nebulizer depends on surface
  tension of solution.
• Organic solvent (alcohol, ester, ketone) can lead
  to smaller droplets, more intense signal.

11
Chemical Interferences

• Formation of compounds of low volatility, ex.
  CaSO4
• Ionization
• M ? M e-
• Add ionization suppressors - create electron-rich
  environments. Ex, alkali metals

12
Spectral Interferences

• Two or more lines within monochromators spectral
  bandpass - requires appropriate resolution from
  diffraction grating (line spacing)

Mn 403.31 nm K 404.40 nm Ga 403.30 nm

13
Double-Beam Instrument

• A reference beam is used as a blank signal.
• To get T measurement, you have to know what the
  measurement for 100 T is.
• Because the flame is always fluctuating, we need
  the reference beam to give a point of reference
  at any time during experiment - compensates for
  drift.

14
Background Correction

• Flame creates a messy background - scattering,
  absorbance by molecular species (oxides,
  hydroxides)
• Another wavelength is passed through the flame,
  its T measured.
• Any loss of T is due to scattering, losses
  unrelated to absorption by analyte.
• The analyte T is corrected for these losses.

15
Atomic Fluorescence Spectroscopy
Fundamental Process    hn M ? M ? M hn'
Photon emitted is not the same energy as the
photon absorbed it has lower energy fluorescenc
e signal is directly proportional to
concentration Enhanced sensitivity over
AAS Signal collected at 90 angle - avoid having
to filter out source radiation
Fluorescence
16
Quantitative Analysis - Calibration Curve

• Test a series of standards and plot Abs v. conc,
  find LDR
• Run your sample and determine conc with line
  equation.

ppm ?g/mL ppb ng/mL For aqueous solutions
(1 g/mL)
17
Quantitative Analysis - Standard Additions Method

• Spike your standards into your samples - its all
  the same matrix.
• cx bcs/mVx
• S standard
• X unknown
• From graph
• Cx -(x-int)cs/Vx