Laser hollow cathode discharge

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. 

LEI has been applied successfully to the trace determination of T1 [674] for certification purposes, and for combinations with laser evaporation and all other atomization techniques represents a powerful approach to detection. Laser photoionization and galvanic detection have been applied to hollow cathode dark space diagnostics [675]. Photoionization is produced to measure the dark space widths of linear field distributions directly. A theoretical model has been developed and its predictions verified with experimental findings for a uranium hollow cathode discharge operated in neon or xenon. Variations in the ground-state densities of sputtered neutrals have also been measured. 

Babin F. J. and Gagne J. M. (1992) Hollow cathode discharge (HCD) dark space diagnostics with laser photoionisation and galvanic detection,... 

The long absorption cells naturally lend themselves to a hollow-cathode discharge configuration for the study of molecular ions. A preliminary experiment revealed the HCO line at 1 THz with a signal-to-noise ratio (100 1 with a 1 s time constant) equivalent to that obtained using the laser sideband technique (16). Possible transitions in H2D and OH have also been observed however they are weak and only tentatively identified, and further work is underway. 

Figure 5.9 Time evolution of an OG signal in a hollow-cathode discharge lamp, associated with an atomic transition (Ar line at 811.369 nm), after excitation with a 10 ns pulse from a Ti sapphire laser...

The narrow spectral line of a DL enables isotope selective analysis. For light and heavy elements (such as Li and U) the isotope shifts in spectral lines are often larger than the Doppler widths of the lines, in this case isotopically selective measurements are possible using simple Doppler-limited spectroseopy - DLAAS or laser induced fluorescence (LIF). For example, and ratios have been measured by Doppler-limited optogalvanic. spectroscopy in a hollow cathode discharge. DLAAS and LIF techniques have been combined with laser ablation for the selective detection of uranium isotopes in solid samples. This approach can be fruitful for development of a compact analytical instrument for rapid monitoring of nuclear wastes. 

Fig.6.25. Opto-galvanic spectra (a) of a neon discharge (1mA, p O.Storr), generated with a broad band CW dye laser [6.77] and (b) of A1 and Fe vapor sputtered in two hollow cathode discharges and simultaneously illuminated with a pulsed dye laser [6.78]...

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. 

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... 

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-... 

The most important radiation sources in atomic absorption spectrometry are the hollow cathode lamps and electrodeless discharge lamps. Other sources which have been used are lasers, flames, analytical plasmas, and normal continuum sources like deuterium and xenon arc lamps. 

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]. 

---