AAS with diode laser

Because of the wide analytical range already accessible with second harmonic generation, many elements routinely determined by conventional AAS in analytical flames or furnaces can also be determined by AAS with diode lasers. The availa-blility of laser diodes with lower wavelengths will only make the approach cheaper, as then second harmonic generation will become superfluous. The elements now accessible with X > 630 nm with resonance lines are already manifold Li, Na, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Cr, Ba, La, Hf, Ta, W, Re, Ir, Pt, Tl, Pb, Nd, Sm, Eu, Gd, Ho, Tm, Yb and Lu. Also U and some of the actinides can be determined. Important elements such as Be, Mg, As and Hg with diodes emitting in the blue region will eventually become accessible. 

Because of the wide analytical range already accessible with second harmonic generation, many elements routinely determined by conventional AAS in analytical flames or furnaces can also be determined by AAS with diode lasers. The availability of laser diodes with lower wavelengths will only make the approach cheaper, as then second harmonic generation will become superfluous. The elements now accessible with 2 > 630 nm with resonance lines are already manifold Li, Na, Al, K,... 

Additional laser diode technologies recently reported include a continuous-wave rhodamine 700 dye laser with a maximum wavelength output at 758 nm, powered by two laser diodes each operated with two standard AA batteries (RDT E division of the US Naval Command, Control, and Ocean Surveillance Center, San Diego, California), and a tunable laser diode with output laser wavelengths of 650, 780, 850, and 1320 nm (New Focus, Mountain View, California). 

The modest electrical drive requbements of these diodes, and the resulting option to power the laser with standard penlight (AA) batteries, allow these CnLiSAF lasers to boast an impressive electrical-to-optical efficiency of over 4 %, which until recently" was the highest reported overall system efficiency of any femtosecond laser source. The amplitude stability of the laser output was observed to be very stable with a measured fluctuation of less than 1% for periods in excess of 1 h. These measurements were made on a laser that was not enclosed and located in a lab that was not temperature-controlled. In a more enclosed and conbolled local envbonment we would expect the amplitude fluctuations of this laser to be extremely small. While the output powers achievable from these lasers have been limited by the available power from the AlGalnP red laser pump diodes, there are already sbong indications that commercial access to higher-power suitable diode lasers is imminent. 

By far the most common lamps used in AAS emit narrow-line spectra of the element of interest. They are the hollow-cathode lamp (HCL) and the electrodeless discharge lamp (EDL). The HCL is a bright and stable line emission source commercially available for most elements. However, for some volatile elements such as As, Hg and Se, where low emission intensity and short lamp lifetimes are commonplace, EDLs are used. Boosted HCLs aimed at increasing the output from the HCL are also commercially available. Emerging alternative sources, such as diode lasers [1] or the combination of a high-intensity source emitting a continuum (a xenon short-arc lamp) and a high-resolution spectrometer with a multichannel detector [2], are also of interest. 

For element-specific detection in gas chromatography, diode laser AAS is very powerful. When using several diode lasers simultaneously, signals for several elements can be determined at the same time and the composition of molecular species determined, e.g. Cl at the 837.60 nm line and Br at the 827.24 nm line and this with detection limits of down to 3 ng/mL or with respect to the injection volume 0.1 pg/s or 1 pg absolute. 

In addition, for speciation coupling of flow injection analysis and column chromatography with flame AAS and also a direct coupling of HPLC with flame AAS, as is possible with high-pressure nebulization, are most powerful. Here the Cr line in the visible region can be used, which makes the application of diode laser atomic absorption spectrometry possible [325]. This has been shown recently by the example of the determination of methylcyclopentadienyl manganese tricarbonyl. 

AAS with flames and furnaces is now a mature analytical approach for elemental determinations. However, its development has not yet come to an end. This applies to primary sources, where tunable diode lasers open new possibilities and even eliminates the requirement of using expensive spectrometers. It also applies to atom reservoirs, where new approaches such as further improved isothermal atomizers for ETAAS or the furnace in flame technique (see e.g. Ref. [326]) have now been introduced, but also to spectrometers where CCD based equipment eventually with smaller dimensions will bring innovation. Furthermore, it is dear that on-line coupling both for trace element-matrix separations and speciation will enable many analytical challenges to be more effectively tackeld. 

Fig. 122. Dielectric barrier (db) discharge microstrip plasma in set-up for diode laser AAS. (DL1, DL2) diode lasers, (BS) beam splitter, (M) mirror, (G) grating, (PDI, PD2) photodiodes. (Reprinted with permission from Ref. [608].)...

When using two lasers and applying two-photon spectroscopy, only those atoms that do not have a velocity component in the observation direction will undergo LEI. Then the absorption signals become very narrow (Doppler-free spectroscopy). This enhances the selectivity and the power of detection, however, it also makes isotope detection possible. Uranium isotopic ratios can thus be detected, similarly to with atomic fluorescence [673] or diode laser AAS. Thus for dedicated applications a real alternative to isotope ratio measurements with mass spectrometry is available. 

Constraints for the use of DLs in AAS arise from their limited availability with emission wavelengths in the range 190—315 nm, which is also inaccessible with frequency doubling techniques. This however will most likely not remain a fundamental limitation, as the development of short wavelength diode lasers is driven by the telecommunication and electronics industry. 

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