Page 156 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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The high intensity of lasers makes them obvious, but expensive, candidates as sources for AFS, and
instruments for laser-induced atomic fluorescence spectrometry (LIF) have been developed in recent
years. The advent of the tuneable dye laser led to the possibility of selecting wavelengths from a laser,
and frequency doubling (second harmonic generation) has permitted the excitation of AFS lines in the
ultraviolet region (Ca 220 nm using a N laser pump or Ca 180 nm with an Nd:YAG laser pump). The
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light from such a source can be sufficiently intense to cause saturation fluorescence (i.e. population
inversion), thus nullifying the effects of quenching (e.g. radiationless return to the ground state induced
by flame species at flame temperatures) and self-absorption.

6.4 Atomization

6.4.1 Flames.

The fluorescence power yield is always less than unity. This non-radiative loss of energy is referred to
as 'quenching'. Quenching increases with temperature (the number of collisions) and quenching cross-
section of the colliding particle (argon has a negligible, hydrogen a low, oxygen a high quenching
cross-section). The ideal atom cell for AFS would also exhibit no background, thus allowing the
detection of very small signals.

There has been interest in the low radiative background, low quenching argon-hydrogen diffusion
flame. The temperature of this flame is too low to prevent severe chemical interferences and therefore
the argon-separated air-acetylene flame has been most widely used. The hot nitrous oxide-acetylene
flame (argon separated) has been used where atomization requirements make it essential. In all cases,
circular flames, sometimes with mirrors around them, offer the preferred geometry.

Elements such as As, Se and Te can be determined by AFS with hydride sample introduction into a
flame or heated cell followed by atomization of the hydride. Mercury has been determined by cold-
vapour AFS. A non-dispersive system for the determination of Hg in liquid and gas samples using AFS
has been developed commercially (Fig. 6.4). Mercury ions in an aqueous solution are reduced to
mercury using tin(II) chloride solution. The mercury vapour is continuously swept out of the solution
by a carrier gas and fed to the fluorescence detector, where the fluorescence radiation is measured at
253.7 nm after excitation of the mercury vapour with a high-intensity mercury lamp (detection limit 0.9
ng 1 ). Gaseous mercury in gas samples (e.g. air) can be measured directly or after preconcentration
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on an absorber consisting of, for example, gold-coated sand. By heating the absorber, mercury is
desorbed and transferred to the fluorescence detector.