Atomic absorption spectroscopy

Atomic absorption spectroscopy, instruments for detection of trace ions and compounds.

1. Atomic Absorption Spectroscopy Megha Rajasekhar Manikandan 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 I n 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. S election criteria for Sample preparation The selection of a preparation method is dependent upon: (1) the analyte(s), (2) the analyte concentration level(s), (3) the sample matrix, (4) the instrumental measurement technique, (5) the required sample size, (6) instrument operation conditions, costs, and (7) 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 collisons 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.

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

22. 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

23. 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