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Study Notes: The Application of AAS

In a flame atomic absorption spectrophotometer, the UV-Vis light from the source lamp is passed through a flame which contains the population of free atoms that have been generated by heat-dissociation of the different species present in the sample (the atom population will therefore be a mix of different elements). Any atoms of the particular element that is excited at the wavelength of the source light will selectively absorb the radiation (otherwise the light will pass through the flame unaffected).

Light from a typical AAS lamp is made up of a number of discrete emission lines (emission occurs when an excited species relaxes or returns to its ground state and in so doing releases or emits light of a specific wavelength). One of these emission lines is the light that is selected to pass through the flame thereby enabling absorption to occur.

The hollow cathode lamp contains traces of a particular element to be analysed. Heat from the lamp causes the element to emit light of energies that are characteristic of the element. If the sample contains this element, these energies will be strongly absorbed because the same orbital energy levels are involved. This means that the instrument is very sensitive and it is also selective because other elements, with different energy levels, will not absorb the light. A range of lamps, each containing a different element can be used.

Points to note about the selected emission line:

  • its wavelength exactly corresponds to the atomic absorption wavelength of the test sample
  • its wavelength range is narrower (about 100 times) than the atomic absorption peak of the sample element (an emission line is of the order of 10-5 nm wide).

Let’s examine how these two properties are used for absorbance measurement in AAS

Section a of the diagram illustrates that the AAS lamp, in this case, has 4 emission lines. One of the lines is isolated by the instrument’s monochromator positioned after the flame (monochromators select a specific wavelength, the actual range of which is dependent upon its bandwidth). Section A of AAS principle diagram

Section b shows the flame atomic absorption spectrum of a sample after radiation with the selected emission line - note that the peak here is at exactly the same wavelength as the emission line and is substantially broader. Section B of AAS principle diagram

Section c demonstrates how the intensity of the emission line has been diminished by passage through flame, that is, the emission radiation has been largely absorbed by the sample atoms. Section C of AAS principle diagram

That the width of the emission line from the lamp is significantly less than the bandwidth of the absorption peak supports the proposition that absorbance is linearly related to concentration (i.e. Beer’s Law applies). The greater the number of atoms of the target element in the light path in the flame, the greater the amount of light that will be absorbed.

Note that the wavelength of the selected emission line is referred to as the ‘analytical’ wavelength in quantitative AAS.

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