Atomic Absorption Spectroscopy AAS
Atomic Absorption Spectroscopy AAS
Comparatively easy to use
Low maintenanceLow consumablesGood for measuring
one element at a time.
Comparatively easy to use
Low maintenanceLow consumablesGood for measuring
one element at a time.
Block DiagramBlock Diagram
h + M → M*
Sample is vaporized/atomized by:FlameElectrothermal Vaporizer (ETV)
h + M → M*
Sample is vaporized/atomized by:FlameElectrothermal Vaporizer (ETV)
AA Sources- HCLAA Sources- HCL
Flame AAS: sample introduction 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.
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.
Flame processesFlame processesTo excited free atoms,
flame must break any molecules apart into discrete atoms.
Potential Problems: 1. Atoms recombine
readily in flame, especially with O2.
2. If flame is too energetic, ionization of atoms can occur - won’t be detected.
To excited free atoms, flame must break any molecules apart into discrete atoms.
Potential Problems: 1. Atoms recombine
readily in flame, especially with O2.
2. If flame is too energetic, ionization of atoms can occur - won’t be detected.
MX (g) → M (g) + X(g)
M + O → MO
M → M+ + e-
MX (g) → M (g) + X(g)
M + O → MO
M → M+ + e-
Burner HeadBurner Head
want a long optical pathlength.
want a long optical pathlength.
Temperature of Some Flames 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
Electrothermal VaporizationElectrothermal VaporizationFirst demonstrated in 1961
by L'vov (USSR)
Use electrically heated carbon furnace
Excellent LOD
First demonstrated in 1961 by L'vov (USSR)
Use electrically heated carbon furnace
Excellent LODMore sensitive than flame:
Entire sample atomized at once
Residence time of vapor in optical path >1 s
LOD for Mg
Flame: 0.1 ppm
ETV: 0.00002 ppm - 20 pptr
Potential problem: poor precision due to sampling variability
Cold Vapor (CV) AtomizationCold Vapor (CV) Atomization
Mercury
Reduced by SnCl2
Mercury
Reduced by SnCl2
Physical InterferencesPhysical Interferences
Droplet size from nebulizer depends on surface tension of solution.
Organic solvent (alcohol, ester, ketone) can lead to smaller droplets, more intense signal.
Droplet size from nebulizer depends on surface tension of solution.
Organic solvent (alcohol, ester, ketone) can lead to smaller droplets, more intense signal.
Chemical Interferences Chemical Interferences Formation of compounds of low
volatility, ex. CaSO4
IonizationM M+ + e-
Add ionization suppressors - create electron-rich environments. Ex, alkali metals
Formation of compounds of low volatility, ex. CaSO4
IonizationM M+ + e-
Add ionization suppressors - create electron-rich environments. Ex, alkali metals
Spectral InterferencesSpectral Interferences
Two or more lines within monochromator’s spectral bandpass - requires appropriate resolution from diffraction grating (line spacing)
Two or more lines within monochromator’s spectral bandpass - requires appropriate resolution from diffraction grating (line spacing)
Mn 403.31 nm
K 404.40 nm
Ga 403.30 nm
Double-Beam InstrumentDouble-Beam InstrumentA 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”.
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”.
Background CorrectionBackground 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.
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.
Atomic Fluorescence Spectroscopy Atomic Fluorescence Spectroscopy Fundamental Process
h + M → M* → M + h'
Photon emitted is not the same energy as the photon absorbed – it has lower energy
fluorescence signal is directly proportional to concentration
Enhanced sensitivity over AAS
Signal collected at 90° angle - avoid having to filter out source radiation
Fluorescence
Quantitative Analysis - Calibration Curve
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.
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)
Quantitative Analysis - Standard Additions Method
Quantitative Analysis - Standard Additions MethodSpike your standards into
your samples - it’s all the same matrix.
cx = bcs/mVx
S = standardX = unknown
From graph :
Cx = -(x-int)*cs/Vx
Spike your standards into your samples - it’s all the same matrix.
cx = bcs/mVx
S = standardX = unknown
From graph :
Cx = -(x-int)*cs/Vx