Page 153 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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Figure 6.1
Types of atomic fluorescence. The solid lines represent radiational processes
and the dashed lines non-radiational processes. In the latter, a single-headed
arrow represents non-radiational deactivation and a double-headed arrow a thermal
activation process. The term anti-Stokes is used when the radiation emitted
is of shorter wavelength, i.e. greater energy than that absorbed.

an atom to reach an excited state. A simple mechanism is that by which the atom is thermally excited by
the atom cell (flame) into a more excited state. Absorption of radiation by this excited state will excite
the atom to an even higher state. Radiation emitted from this state is termed stepwise excitation
fluorescence. It occurs most frequently when the first excited state is very close to the ground state,
although it can occur elsewhere if an extremely hot atom cell is used. This type of fluorescence is weak
though and has found few analytical applications. It can give rise to anti-Stokes fluorescence, i.e. when
the light emitted from the atom is at a shorter wavelength than that used to excite it.

The intensity of the atomic fluorescence radiation, I , is proportional to the intensity of the absorbed
F
radiation, which when using a line source is directly proportional to the source intensity, I .
s
At low concentrations, I is proportional to the concentration of the analyte; at higher concentrations,
F
self-absorption is observed (cf. AES). This behaviour is reflected in the growth curves of log I versus
F
log N. Figure 6.2 is based on these results. These curves often show linearity of four orders of
magnitude before curvature.

As I F depends on I , a stable, intense sharp-line source greatly enhances AFS sensitivity. Similarly, the
s
geometry of the atom cell is important.