Elemental diode lasers

Soft, silver white metal that melts in the hand (29.8 °C) and remains liquid up to 2204 °C (difference 2174 °C, suitable for special thermometers). Gallium is quite widespread, but always in small amounts in admixtures. Its "career" took off with the advent of semiconductors. Ga arsenide and Ga phosphide, which are preferential to silicon in some applications, have extensive uses in microchips, diodes, lasers, and microwaves. The element is found in every mobile phone and computer. Ga nitride (GaN) is used in UV LEDs (ultraviolet light-emitting diodes). In this manner, a curiosity was transformed into a high-tech speciality. 

Figure 6.5. Diode laser head. The laser diode output passes successively through a three-element collimating lens, cylindrical lens, and anamorphic prism pair to become a fully collimated, circular, anastigmatic beam. Redrawn from Melles Griot product literature.

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. 

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. 

Non-metals such as H, O, S, noble metals and the halogens cannot be determined by flame or furnace AAS, but have long-life excited states from which strong absorption transitions can be induced by the red and near-IR radiation of diode lasers. Furthermore, also elements such as Se and Hg can be determined by diode laser radiation absorption from metastable states. Metastable states can be produced in low-pressure plasmas such as dc, microwave and high-frequency discharges,... 

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. 

Instrumentation for diode laser based AAS is now commercially available and the method certainly will expand as diode lasers penetrating further into the UV range become available, especially because of their analytical figures of merit that have been discussed and also because of their price. In diode laser AAS the use of monochromators for spectral isolation of the analyte lines becomes completely superfluous and correction for non-element specific absorption no longer requires techniques such as Zeeman-effect background correction atomic absorption or the use of broad band sources such as deuterium lamps. 

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. 

Niemax K., Groll H. and Schnurer-Patschan C. (1993) Element analysis by diode laser spectroscopy, Spectro-chim Acta Rev 15 349-377. 

Zybin A., Schurer-Patschan C. and Niemax K. (1992) Simultaneous multi-element analysis in a commercial graphite furnace by diode laser induced fluorescence, Spectrochim Acta, Part B 47 1519-1524. 

Zybin A. and Niemax K. (1997) GC analysis of chlorinated hydrocarbons in oil and chlorophenols in plant extracts applying element-selective diode laser plasma detection, Anal Chem 69 755-757. 

Zybin A., Schnurer-Patschan Ch. and Niemax K. (1995) Wavelength modulation diode laser atomic absorption spectrometry in modulated low-pressure helium plasmas for element-selective detection in gas chromatography, J Anal At Spectrom, 10 563-567. 

Koch J. and Niemax K. (1998) Characterization of an element selective GC-detector based on diode laser atomic absorption spectrometry, Spectrochim Acta, Part B 53 71—79. 

Several optical signals are multiplexed on the microsecond or millisecond time scale. Multiplexing of signals can be accomplished by switching several diode lasers, either electronically or by fibre switches, or by rotating elements in an optical system. The channel signal indicates the current state of the multiplexing... 

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