Page 152 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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6
Atomic Fluorescence Spectrometry.

Atomic fluorescence spectrometry is based on the absorption of optical radiation of suitable frequency
(wavelength) by gaseous atoms and the resultant deactivation of the excited atoms with the release of
radiation. The frequencies (wavelengths) emitted are characteristic of the atomic species.

Although AFS combines, for some elements, the advantages of the large linear dynamic range typical
of atomic emission techniques and the high selectivity of atomic absorption, the method has so far not
found widespread use.

6.1 Theory

The six types of flame AFS which have been observed are summarized in Fig. 6.1. Resonance
fluorescence is generally the most useful type because it generates the most intense fluorescence. It
occurs when the atom re-emits a spectral line of the same wavelength as that used for excitation. Direct
line fluorescence has also been exploited analytically. It occurs when transitions between the excited
state of the resonance line and a lower intermediate level are allowed by the selection rules. As a result,
this type of fluorescence occurs at a higher wavelength than the excitation radiation. The advantage of
this technique is that if appropriate filters are used, scatter from the excitation radiation can be
eliminated. Stepwise line fluorescence occurs when an atom is excited to a higher state by radiation
and then undergoes a partial deactivation (by collision or other radiationless process) to a lower state
rather than return directly to the ground state. If a transition then occurs from this level to a still lower
level (often the ground state), stepwise line fluorescence is observed.

Thermally assisted fluorescence is the converse of stepwise line fluorescence. It occurs as a result of a
stepwise absorption of energy by