Instrumentation
Sources emitting radiation characteristic of element of interest (hollow-cathode lamp). Flame or
electrically heated furnace or carbon rod. Monochromator, photomultiplier, recorder.
Applications
The most widely used technique for the quantitative determination of metals at trace levels (0.1– 100
ppm) in a wide range of materials. Relative precision 0.5–2%.
Disadvantages
Sample must be in solution or at least volatile. Individual source lamps required for each element.
When electromagnetic radiation characteristic of electronic transitions in the outer orbitals of atoms of a
particular element is passed through an atomic vapour of that element, the radiation at certain
frequencies is attenuated. The absorbed radiation excites electrons from the ground state to various
higher energy levels (excited states) and the degree of absorption is a quantitative measure of the
concentration of ground-state atoms in the vapour. The energy changes involved correspond to radiation
in the UV and visible regions of the spectrum. As only atoms in the ground state will respond in this
way, the conditions used for volatilizing and decomposing the sample to produce an atomic vapour
must induce the minimum of ionization. This can be achieved by flame excitation where temperatures
seldom exceed 3000 K. Reference to the Maxwell-Boltzmann equation p. 275 and Table 8.7 shows that
for most elements, practically 100% of atoms will be in the ground state even in a moderately hot flame
such as air-acetylene (2400 K). The only exceptions are the easily ionized alkali and alkaline earth
metals where the energies of the first excited states lie relatively close to those of the ground states.
Even in these cases, over 90% of such atoms are likely to remain in the ground state if cooler flames,
e.g. air-propane, are used (Table 8.7). The situation should be contrasted with that encountered in flame
photometry which depends on the emission of radiation by the comparatively few excited atoms present
in the flame. However, because of fundamental differences between absorption and emission processes
it does not follow that atomic absorption is necessarily a more sensitive technique than flame emission.
Absorption of Characteristic Radiation
The extent to which radiation of a particular frequency is absorbed by an atomic vapour is related to the
length of the path traversed and to the concentration of absorbing atoms in the vapour. This is
analogous to the Beer-Lambert law relating to samples in solution (p. 357 et seq.). Thus, for a
collimated, monochromatic beam of radiation of incident intensity Io passing through an atomic vapour
of thickness I,