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Atomic absorption occurs when a ground state atom absorbs light of a specific wavelength.The amount of light absorbed is governed by Beer Lambert’s law and will increase as the number of atoms of the element in the light path increases. The component parts of Atomic Absorption Spectrometer are similar to a UV -Vis spectrophotometer as both operate on same principle with a basic difference that the sample cell of UV-Vis spectrophotometer is replaced by an atomization source (flame or graphite furnace)
Light sources – Hollow Cathode Lamps
The light source commonly used is a hollow cathode lamp. A different element hollow cathode lamp is required for each element determination. Cathode is made of same metal that is to be estimated in the sample
Single element lamps are used commonly though multi-element lamps are also available
Lamps are made of glass with quartz windows and filled with an inert gas such as argon
Light sources – Electrodeless Discharge Lamps
Used for volatile elements such as As,Sb, Sn,Cd,Pb, etc
EDL’s have greater lamp life and high energy throughput
The burner assembly comprises of nebulizer to reduce the liquid sample to a fine aerosol, a spray chamber and a burner head which is used to generate a flame to produce atoms of the same elements that are present in the sample.
A monochromator disperses the incident light beam and permits the selected wavelength to reach the detector.
Detector commonly used is a photomultiplier tube which produces a signal proportional to the amount of light received by it.
In this section you shall be introduced to two working configurations, namely single beam and double beam instruments
Light from the source is modulated electronically or chopped mechanically by rotating chopper. This helps isolate and remove sample cell emissions from light emitted by the source. The specific wavelength isolated by monochromator is led to the detector and the electrical signal generated is proportional to the elemental concentration in the sample
Double Beam Schematic
Light beam from source is split into two teams by the chopper. One beam passes directly through the flame and the other beam passes round the flame. Detector response represents the ratio of sample and reference beams. Fluctuations in light intensity are eliminated electronically to get greater reliability of results.
The sensitivity is lower than single beam instruments but the popular acceptance of double beam configuration is due to advantage of elimination of background changes in the atomizer.
The individual parts are described in more detail in subsequent modules.