Radiation absorbed by atoms under conditions similar to those used in atomic absorption spectrometry
may be re-emitted as fluorescence. The fluorescent radiation is characteristic of the atoms which have
absorbed the primary radiation. Thus its wavelength and intensity may be used as the basis for an
analytical technique, atomic fluorescence spectrometry (AFS). The radiation is emitted in all directions
and may be monitored from angles other than in a direct line with radiation from the irradiating source.
This ensures that the detector will not respond to the primary absorption process or to unabsorbed
radiation from the lamp. The intensity of fluorescent emission is directly proportional to the
concentration of the absorbing atoms but it is diminished by collisions between excited atoms and other
species within the flame, a process known as quenching. Nitrogen and hydrocarbon vapours enhance
quenching, and flames incorporating either should be avoided or their effect modified by dilution with
argon.
The instrumentation required for atomic fluorescence measurements is simpler than that used for
absorption. As the detector is placed so as to avoid receiving radiation directly from the lamp, it is not
strictly necessary to use a sharp-line source or a monochromator. Furthermore, fluorescence intensities
are directly related to the intensity of the primary radiation so that detection limits can be improved by
employing a high-intensity discharge lamp.
Atomic fluorescence spectrometry has a number of potential advantages when compared to atomic
absorption. The most important is the relative case with which several elements can be determined
simultaneously. This arises from the non-directional nature of fluorescence emission, which enables
separate hollow-cathode lamps or a continuum source providing suitable primary radiation to be
grouped around a circular burner with one or more detectors.
In principle, atomic fluorescence is a simpler and more versatile technique than atomic absorption, but
suffers from a susceptibility to quenching effects and to background noise arising from the scattering of
radiation by particles in the flame. The latter is particularly serious for refractory materials and in high-
temperature flames. Detection limits for some elements are lower than by atomic absorption or flame
emission measurements, e.g. elements with resonance lines around 200 nm or below, such as As, Se,
Te. Instruments based upon the use of a chemical flame as the atom reservoir have not proved to be
generally successful. The introduction of the ICP torch renewed interest in atomic fluorescence and new
instruments based on the ICP torch as a source of free atoms were constructed. However, these seem to
have been only slightly more satisfactory than earlier instruments and have not come into widespread
use. Some detection limits are included in Table 8.6.
It is in the determination of a group of important elements, which are in themselves volatile, e.g. Hg, or
can be made so as hydrides or as organoderivatives that important developments have taken place and
AFS has