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