Atomic absorption spectroscopy, Principle, Instrumentation and Processes
Atomic absorption spectroscopy has proved itself to be the most powerful instrumental techniques for the quantitative determination of trace metal in liquid.
Atomic absorption spectroscopy is a techniques of elemental analysis. It is specially useful for determining trace metals in liquids and is almost independent of the molecular from of the metal in the sample. For instance, we can determine the total cadmium content of a water sample, it does not matter whether the cadmium exists as a chloride, nitrate, sulfate or other salt.
The underlying principal of atomic absorption spectroscopy is the absorption of energy exclusively by ground state atoms while they are in the gaseous from.
It may be further explained as follows below :
1. A solution consisting of certain metallic species when aspriated into a flame, it will.produce the corresponding vapours of metallic species.
2. Some metal atom would be raised directly to an energy level to such an extent as to emit the particular radiation.
3. At the critical point, a sufficiently large quantum of the metal atom of a particular element would still remain in the non – emitting ground – state, which in turn shall be receptive of light radiation having their own specific wavelength.
4. Consequently, when a light of this wavelength is passed through a flame along the atom of the metallic species a portion of the same would be absorbed.
5. The resulting absorption has been found to be directly proportional to the density of the atom present in a flame at the material time.
6. In Atomic absorption spectroscopy (AAS ), one logically determines the amount of light absorbed.
Advantages of Atomic Absorption Spectroscopy
1. The atomic absorption techniques is specific because the atom of a particular element can only absorb radiation of their own charactrastic wavelength.
2. Because of the much larger number of metal atom that contribute to an atomic absorption signal.
Disadvantages of atomic Absorption Spectroscopy
1. A separate laml for each element to be determined is required.
2. In aqueous solution, the predominant anion affect the signal to a negotiable degree.
1. Single Beam Atomic Absorption Spectroscopy :
In single beam equipment, a light source is kept ahead of the flame with a mechanical chopper between the light source and the flame. When there is no sample in the flame, the output of the detector amplifier unit is arranged to provide full deflection after, this the sample solution is sprayed into the flame and the output of the detector amplifier is recorded.
Disadvantage – one main disadvantage of single beam equipment is low stability..
2. Double Beam Atomic Absorption Spectroscopy :
1. The chopped beam from the hollow cathod lampn is divided into two parts, one part goes through the flame while, the other part by passes it..
2. After recombination, the two beam pass through a Monochromators to a detector and readout system.
Advantage – in double beam equipment, there is no effect of lamp drift and there is no change in detector sensitivity with time.
1. Radiation source
The radiation source for atomic absorption spectroscopy should emit stable, intense radiation of the element to be determined, useually a resonance line of the element..
A rotating wheel is interposed between the hollow cathod lamp and the flame. This rotating wheel is known as chopper and is interposed to break the steady light from the lamp into an intermittent or pulsating light. This given a pulsating current in the photocell.
In order to achieve absorption of atoms, it becomes necessary to reduce the sample to the atomic state. This is done by
4. Nebulisation of the liquid sample
Before the liquid sample enters the burner it is first of all converted in to small droplets this method of formation of small droplets from the liquid sample is known as neubilsation. A common method of nebulisation is by use of a gas moving at high velocity. Known as pneumatic nebulisation. The Beckman total consumption burner is generally used in atomic absorption measurement.
In atomic absorption measurements, the most common monochromators are prisms and gratings.
Industrially packaged atomic absorption instrumentation commonly includes a Monochromators of about 1/2m focal length with a linear reciprocal dispersion in the range 16-35 A/mm. The function of a monochromator ia to select a provided absorbing line from spectral line emitted from the hollow cathod.
For atomic absorption spectroscopy, the photo multiplier tube is most suitable. It has good stability if used with a stable power supply.
The electric current from the photo multiplier detector is fed to the amplifier which amplifier the electric current many time. Generally “Lock -in ” amplifier arr preferred which provide a very narrow frequency band pass and help to achieve an excellent signal- to – noise ratio.
8. Read out device
In most of the atomic absorption measurement, chart recorder are used as read out device. A chart recorder is a potentiometer using a servometer to move the recording pen.
Application of Atomic Absorption Spectroscopy
1. Qualitative Analysis:
In atomic absorption spectroscopy, a different hollow cathod lamp is to be used for each elements to be tested.
2. Quantitative Analysis
The technique of quantitative analysis is depend upon the Determination of the amount of radiation absorbed by the sample.
3. Determination of metallic elements in Biological material
4. Determination of metallic elements in food industry.
5. Determination of calcium, magnesium, sodium and potassium in blood serum.
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