Non-resonant AFS

As especially in the case of non-resonant AFS the fluorescence intensities are low, special precautions have to be taken for signal acquisition. Indeed, the signal-to-noise ratios can be improved considerably by using pulsed signals and phase-sensitive amplification, as is easily done e.g. in the case of laser sources (see Section 2.1). 

The detection of the fluorescence radiation differs in resonant and in non-resonant AFS. In the first case, the radiation is measured in a direction perpendicular to that of the incident exciting radiation. However the system will suffer from stray radiation and emission of the flame. The latter can be eliminated by using pulsed primary sources and phase-sensitive detection. In the case of non-resonant fluorescence, stray radiation problems are not encountered, although the fluorescence intensities are lower, which necessitates the use of lasers as primary sources and spectral apparatus that will isolate the fluorescence radiation. A set-up for laser excited AFS (Fig. 126) may make use of a pulsed dye laser pumped by an excimer laser. The selection of the excitation line is then done by the choice of the dye and... 

In flame AFS, elements which form thermally stable oxides such as Al, Mg, Nb, Ta, Zr and the rare earths are hampered by insufficient atomization. This is not the case when an ICP is used as the fluorescence volume. Here the detection limits for laser excitation and non-resonant fluorescence are lower than in ICP-AES (Table 18) [663]. ICP-AFS can be performed for both atomic and ionic states [664]. 

Note that as nanoparticle absorption occ X) is completely independent from the fiber loss af(/l) in the absence of nanoparticles, sensitivity (11) of a non-resonant... 

Fig. 16.45 (a) Molecular siruclurc of ihe f liI.H,.anion lb). lc) fronl and back of the lB =Hframework showing one of many canonical forms conlribuling lo Ihc resonance hybrid, [(a) From Mueilenies. E. L. The Chemistry af Boron and its Compounds Wiley New York. 1967 lb) from Jolly, W. L. The Chemistry of the Non Mettiis Prentice Hall Englewood ClilTs. NJ. 1966. Reproduced with permission.)... 

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