Atomic absorption, diode lasers

X 10 cm molecule" s" is obtained, and discrepancies in the previously reported values are probably due to neglect of the reverse processes taking place.For the corresponding state in atomic fluorine, diode laser measurements of the absorption cross-section of the "Pi - transition at 404cm" yields a radiative lifetime of 660s for the upper spin-orbit state. ... 

The glow-discharge also facilitates atomic absorption from non-ground state levels, which is often required in diode laser AAS. 

Accordingly, it was very soon found that using sources for which the physical widths of the emitted analyte lines are low is more attractive. This is necessary so as to obtain high absorbances, as can be understood from Fig. 76. Indeed, when the bandwidth of the primary radiation is low with respect to the absorption profile of the line, a higher absorption results from a specific amount of analyte as compared with that for a broad primary signal. Primary radiation where narrow atomic lines are emitted is obtained with low-pressure discharges as realized in hollow cathode lamps or low-pressure rf discharges. Recently, however, the availability of narrow-band and tunable laser sources, such as the diode lasers, has opened up new per-... 

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. 

Through the availability of tunable diode laser sources the set-up used for atomic absorption (see Section 4.2.2) can be simplified considerably. In this way, several additional advantages can be realized. As through wavelength tuning, measurements in the wings of the absorption profile can also be made, and possible improvements to the linear dynamic range in a number of cases could be expected. 

As discussed earlier, present limitations of diode laser atomic absorption lie in the fact that the lower wavelengths are not yet accessible and that the whole wavelength range cannot be covered continuously as there are wavelength gaps between the wavelength ranges of the diodes presently available. 

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. 

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. 

Krivan V., Barth P. and Schnurer-Patschan C. (1988) An electrothermal atomic absorption spectrometer using semiconductor diode lasers and a tungsten coil atomizer design and first applications, Anal Chem 70 3625-3632. 

Ng K. C., Ali A. H., Barber T. E. and Winefordner J. D. (1988) Multiple mode semiconductor diode laser as a spectral line source for graphite furnace atomic absorption spectroscopy, Anal Chem 62 1893-1895. 

Groll H., Schnurer-Patschan C., Kuritsyn Yu. and Niemax K. (1994) Wavelength modulation diode laser atomic absorption spectrometry in analytical flames, Spectrochim Acta, Part B 49 1463-1472. 

Speciation of methylcyclopentadienyl manganese tricarbonyl by high performance liquid chromatography-diode laser atomic absorption spectrometry, Anal Chem 71 5379-5385. 

Diode laser sources Already in 1980, lasers had been suggested as excitation sources for atomic absorption spectrometry [11]. Tunable dye lasers can provide virtually any atomic hne between 213 and 900 run with a bandwith corresponding to the natural hne width of an atomic hne and with a comparatively high intensity. However, they have not found widespread acceptance for this apphcation so far due to their cost and complex operation compared to hollow cathode or electrodeless discharge lamps. This situation seems to have changed with the advent of inexpensive, mass produced diode lasers (DL) [12, 13]. 

For the experimental realization of optical cooling, which uses a collimated beam of atoms and a counterpropagating cw laser (dye laser or diode laser. Fig. 9.6) the following difficulties have to be overcome during the deceleration time the Doppler-shifted absorption frequency o) t) = k v(t) changes with the decreasing velocity V, and the atoms would come out of resonance with the monochromatic laser. 

Tunable lasers (preferentially dye lasers and diode lasers) are used as primary sources for atomic absorption spectroscopy with various atomizers such as flames, furnaces, or plasmas LAAS laser atomic absorption spectrometry CRS cavity ring-down spectroscopy... 

Butcher DJ, Zybin A, Bolshov MA, and Niemax K (2001) Diode laser atomic absorption spectrometry as a detector for metal speciation. Review of Analytical Chemistry 2 79-100. 

Koch J, Miclea M, and Niemax K (1999) Analysis of chlorine in polymers by laser sampling and diode laser atomic absorption spectrometry. Spectrochimica Acta B 54 1723-1735. 

Wizemann HD and Niemax K (2000) Measurement of Li-7/Li-6 isotope ratios by resonant Doppler-free two-photon diode laser atomic absorption spectroscopy in a low-pressure graphite furnace. Spectrochimica Acta B 55 637-650. 

---