Atomic Absorption Spectroscopy, principle, working, parts, sample preparation, pros and cons, applications

1. Atomic Absorption
Spectroscopy
Manikandan R
Megha Rajasekhar
Rakesh M Badami

2. Introduction
• Atomic absorption spectroscopy is
quantitative method of analysis of metals and
some non-metals
• The technique was introduced in 1955 by Sir
Alan Walsh in Australia
• Concentrations are found in g/mL
range (ppm and ppb)

3. Principle
• In gaseous state atoms absorb ultraviolet or
visible light and make transitions to higher
electronic energy levels.
• The wavelength of each transition is specific to
each element – qualitative analysis
• Beer-Lambert’s law can be applied to find
concentrations using a calibration graph
prepared from standards
• Absorbance is directly proportional to path
length and concentration

4. Selection criteria for Sample
preparation
The selection of a preparation method is
dependent upon:
• The analyte
•The analyte concentration level
•The sample matrix
•The required sample size
•The environmental considerations.

5. Sample Preparation
• Dilution – sample is diluted in distilled water, acids
or organic solvent
• Decomposition – isolation of required element from
the sample by heating with/without a reagent
– Wet/acid decomposition (300C)
– Dry ashing (400-500C )- destroying the combustible
portion of the sample. Oxidising agents may be used
– Microwave decomposition (100-200C ) – sample
decomposed at high pressures in a Teflon container
• Calibration curve must be prepared using different
concentrations of the sample

6. Working
• The atoms of the solid are converted to gaseous
state in the atomiser
• Radiation of specific wavelength is emitted by the
hollow cathode lamp onto the gaseous atoms in the
atomiser
• The monochromator focuses the specific
wavelengths onto the detector
• The detector finds the amount of light absorbed
• The concentration of atoms in the sample is directly
proportional to the absorbance

7. Working

8. Instrumentation

9. Hollow Cathode Lamp
• Cathode is in the form of a hollow cylinder made of
the metal which has to be analysed
• Anode is made of tungsten filament
• They are sealed in a tube filled with inert gas like
Neon or Argon
• A large voltage across anode and cathode causes
the inert gas to ionize and form a plasma
• These ions are accelerated towards the cathode
causing atoms to be sputtered off

10. Hollow Cathode Lamp
• The ions and metal atoms are excited due to
collisions
• They give off photons of a certain wavelength
when they reach ground state

11. Nebuliser
• The nebuliser forms a mist or aerosol of the
sample
• This is done by forcing the sample at high
velocities through a narrow tube
• The sample is mixed with a fuel and oxidant
• Commonly used fuel-oxidant mixtures are
acetylene-air and acetylene-nitrous oxide.

12. Nebuliser

13. Atomiser
• In the atomiser the sample solutions is
vaporised and the molecules are atomised
• Atomiser can be of two types
– Flame atomiser – Laminar consumption or total
flow
– Electro-thermal atomiser or graphite furnace

14. Atomiser
• Flame atomiser
– The flame is usually in the form of a sheet to
increase path length and hence increase the
absorbance and sensitivity.
sample mist
Solid/gas
aerosol
Gaseous
molecules Atom

15. Atomiser
• Flame atomiser
– Laminar flow – a mixture of sample, fuel and
oxidant is continuously introduced to the burner
head
– Total consumption – sample and fuel-oxidant
mixture are delivered separately to the burner
head. This is a much safer method and can be
easily constructed.

16. Atomiser
• Electro-thermal atomiser

17. Atomiser
• Electro-thermal atomiser
– Graphite rods heated by passing current
– Sample goes through three phases to get vaporised
– Drying - the solvent is evaporated
– Pyrolysis
– Sample temperature is then increased rapidly to
vaporise it
– Light is then passed through the sample

18. Interferences
• Chemical interference
– Presence of thermally stable compound that is not
totally decomposed by the energy of the flame
– High flame temperature provides energy for
breakdown on interference
– Addition of releasing agent which reacts with the
interference

19. Interferences
• Ionization interferences
– Atoms of the samples are
ionized causing reduction in
number of electrons and
absorbance
– Addition of excess element,
like alkali elements, which
gets ionized easily
– Flame temperature may be
reduced

20. Interferences
• Matrix interference
– Due to viscosity, burning characteristics, surface tension of
solvent
– Due to usage of different solvents in calibration and sample
– Addition of diluents to reduce viscosity
• Background absorption
– Light scattering by particles in flame or absorption by
undissociated molecules
– This must be measured and subtracted from final results
– Absorption of elements occurs as a narrow line whereas
interference occurs over a broad range

21. Applications
• Level of metals could be detected in tissue samples
like Aluminum in blood and Copper in brain tissues
• Presence of metals as an impurity or in alloys could
be found easily
• Determination of elements in the agricultural and
food products
• Determination of lead in petrol
• Determination of calcium and magnesium in
cement

23. Advantages and Disadvantages
• Advantages
– High sensitivity
– Easy to use
– Inexpensive
• Disadvantages
– Different cathode lamp for different elements
– Can detect only metals and some non metals
– Only one element detected

24. References
• Analytical Methods for Atomic Absorption
Spectroscopy – Perkin-Elmer
• Sample Preparation For Flame Atomic Absorption
Spectroscopy: An Overview - Nabil Ramadan Bader
• Atomic absorption spectrometry – Royal Society of
Chemistry
• NMSU web notes