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Study Notes: The Application of AAS
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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). |
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| 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. |
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| 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. |
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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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