Atomic absorption spectrophotometry
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Atomic absorption spectrophotometry is a technique for elemental analysis that was introduced in 1950. It uses the absorption of light to detect specific metal elements in liquid or biological samples. The technique works by vaporizing the metallic species in a flame and measuring the absorption of light at the characteristic wavelength of each metal atom. It provides sensitive detection of metals down to concentrations of 1 part per million or lower. The instrument uses a hollow cathode lamp, chopper, atomizer, monochromator, detector, and other components to selectively measure the absorption of light by the metal atoms.
Atomic absorption spectrophotometry
- 2. Introduction • Introduced by Alan Walsh(1950) • It is a method of elemental analysis • Useful for determining trace metals in liquid or biological samples • Highly sensitive-can detect metal in conc. Lower than 1ppm
- 3. Principle • When a solution containing metallic species is introduced into a flame, the vapour of metallic species will be obtained. • Some of the metal atoms may be raised to higher energy level and emit characteristic radiation. But large amount of metal atoms will remain in non emitting ground state. • These ground state atom of particular element are receptive of light radiation of their own specific resonance wavelength. • Thus, when a light of this wavelength passed through a flame having atom of metallic species, part of light will be absorbed and the absorption will be proportional to the density of atom in the flame.
- 4. Instrumentation Components: • Radiation source: are of two types • Hollow cathode lamp- emits spectral line of that metal which is used in the cathod • Electrodeless discharge lamp • Chopper: • It is a rotating wheel interposed between the hollow cathode lamp and flame. • Gives pulsating current in the photocell • Atomiser: • Flame atomiser: total consumption burner and premixed burner • Non flame atomiser
- 5. • Nebulization of liquid sample: • method of formation of small droplet from the liquid sample • Nebulization is done by use of a gas moving at high velocity • Monochromator: • Function of monochromator is to select given absorbing line from spectral line emitted from hollow cathode • Common monochromator are prism and gratings
- 6. • Detctor: • Photomultiplier tube is most suitable • Has good stability if used with stable power supply • Amplifier: • It amplifies electric current • Lock in amplifier are preferred to achieve excellent signal to noise ratio(measure of signal strength relative to background noise) • Read out device: • Digital read out device are used
- 7. Operation • A meter is adjusted to zero absorbance when blank solution is sprayed on the flame, and rest of the light passes on to the photomultiplier tube. • Then the light solution containing absorbing species is introduced a part of light is absorbed, resulting in a decrease in light intensity falling on a photomultiplier tube.
- 8. Types • Single beam AAS: • Light source is placed ahead of flame with mechanical chopper between light source and flame • Has low stability • Double beam AAS: • Beam from hollow cathod lamp is divided into two part one passes through flame while other bypass it
- 9. Difference AAS • Amount of light absorbed by ground state atom is measured • Absorption intensity doesnot depend upon temperature • Beers law is obeyed over wide range of concentration FES • Amount of light emitted by the excited atom is measured • Is greatly influenced by temperature variation • Not obeyed
- 10. Atomic Emission Spectroscopy, AES
- 12. Advantages of AAS • Technique is specific because the atom of particular element can only absorb radiation of their own characteristic wavelength • Is independent of flame temperature
- 13. Disadvantages • Separate lamp for each element to be determined is required • Technique cannot be used sucessfully for estimation of element like Mo,Si etc because these element give rise to oxide in the flame • Predominant anion affect the signal to negotiable degree
- 14. Application • Qualitative and quantitative analysis • Determination of metallic elements in biological system • Determination of metallic element in food industry • Determination of Ca, Mg, Na, K in serum • Determination of lead in petrol