Laser excited AFS

Laser-excited AFS with flame atomization or laser-induced plasma as atom source have also benefited from direct USNn. One example is the method used to obtain on-line and size-segregated information on the elemental ohemical oomposition of ultrafine aerosols [12]. In oontrastto plasma-based deteotors, laser-excited AFS instruments afford miniaturization for field applications. 

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... 

Very sensitive determinations of Mo can be performed by dry solution residue analysis with laser-excited AFS in a hollow cathode discharge as the atomizer, as shown by Grazhulene et al. [667]. Bolshov et al. [668] showed that very low levels of lead in Antartic ice samples could be determined by laser-excited AFS using dry solution residue analysis with graphite furnace atomization. 

When performing laser-excited AFS at a laser plume, it would appear to be useful to produce the laser plasma at pressures below atmospheric pressure, as then the ablation depends only slightly on the matrix [226, 669]. 

The application of laser-excited AFS in the analytical laboratory up to now has been seriously hampered by the complexity and cost of tunable lasers, which may change with the availability of less expensive but powerful and also tunable diode lasers, which can be operated in the complete analytically important wavelength range. 

The fluorescence technique combines the advantages of the large dynamic range of emission techniques with the simplicity and high selectivity of absorption techniques. Flame sources have been extensively used, however, for elements with refractory oxides, the ICP source has been found to be more satisfactory for AFS. A system for hollow cathode lamp excited ICP-AFS, as proposed by Demers and Allemand (1981), is commercially available as a modular simultaneous multielement ICP system. Although fluorescence techniques often offer two orders of magnitude sensitivity improvement over absorption, the multielement approach for AFS has not yet been commercially successful. Also promising for the future is the laser-excited furnace AFS where the detection limits for most elements are comparable to those of ICP-AES and for some elements, for eg, As, Cd, Pb, Tl, Lu, even lower (Omenetto and Human, 1984). The future for AFS techniques has been discussed by Stockwell and Corns (1992). 

Whatever the analytical method and the determinand may be, the greatest care should be devoted to the proper selection and use of internal standards, careful preparation of blanks and adequate calibration to avoid serious mistakes. Today the Antarctic investigator has access to a multitude of analytical techniques, the scope, detection power and robustness of which were simply unthinkable only two decades ago. For chemical elements they encompass Atomic Absorption Spectrometry (AAS) [with Flame (F) and Electrothermal Atomization (ETA) and Hydride or Cold Vapor (HG or CV) generation]. Atomic Emission Spectrometry (AES) [with Inductively Coupled Plasma (ICP), Spark (S), Flame (F) and Glow Discharge/Hollow Cathode (HC/GD) emission sources], Atomic Fluorescence Spectrometry (AFS) [with HC/GD, Electrodeless Discharge (ED) and Laser Excitation (LE) sources and with the possibility of resorting to the important Isotope... 

Laser-excited atomic fluorescence spectrometry is capable of extremely low detection limits, particularly when combined with electrothermal atomization. Detection limits in the femtogram (10 g) to attogram (10 g) range have been shown for many elements. Commercial instrumentation has not been developed for laser-based AFS, probably because of its expense and the nonroutine nature of high-powered lasers. Atomic fluorescence has the disadvantage of being a singleelement method unless tunable lasers with their inherent complexities are used. 

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]. 

In a nonresonance fluorescence transition, the photons involved in absorption and fluorescence processes have different wavelengths (Figure IB). The particular transition shown in Figure IB is called Stokes direct-line fluorescence, which is frequently used for AFS with laser excitation. Nonresonance transitions have the advantage that a wavelength selection device can be used to distinguish between fluorescence and scattered source radiation. 

Atomizers The same requirements that exist for AA also exist for AE Consequently, similar sources have been used. However, since the long path length needed with AA to maximize the sensitivity is not needed with AF, ICPs and more circular flames have been used in place of the traditional chemical combustion flame with a slot burner. ETAs have also been used to enhance the sensitivity for AF as has been done for AA, and the resulting LODs are some of the best for the atomic spectroscopic suite of techniques when combining an ETA with laser excitation. 

Radiation sources As in AA, line sources are typically used, although high-intensity sources are much more critical in AF. Boosted HCLs and EDLs have been employed. However, tunable lasers certainly provide the optimal sensitivity, i.e., laser excited atomic fluorescence spectrometry. 

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