Glow discharge AES

The fact that glow discharges have much lower atom number densities than atmospheric pressure plasmas is responsible for the fact that the measurement of fundamental parameters such as electron number densities, electron temperatures, etc. by techniques such as Thomson scattering is much more difficult than in the case of atmospheric pressure plasma discharges, and that this is only now becoming a field of active research. Moreover, the lower collision frequency, which causes large departures from local thermal equilibrium, is responsible for the fact that many more processes are significantly involved in the excitation mechanisms than in the case of atmospheric pressure plasmas. 

The four 4s levels play a key role in analytical glow discharges (e.g. for Penning ionization of the sputtered atoms) and they cannot easily be depopulated by radiative decay (due to forbidden transitions for the metastable levels, and to radiation trapping for the resonant levels). Therefore some additional loss processes are incorporated for these levels, in order to describe them with more accuracy  

The features of glow discharges can be realized in the hollow cathode and related sources, in glow discharges with flat cathodes with dc or rf power and in special sources such as the so-called gas-sampling glow discharges. 

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Table 4. Deieclion limits (ng/g) for steels in spark and glow discharge AES and glow discharge MS...

Table 8.60 shows the main features of GD-MS. Whereas d.c.-GD-MS is commercial, r.f.-GD-MS lacks commercial instruments, which limits spreading. Glow discharge is much more reliable than spark-source mass spectrometry. GD-MS is particularly valuable for studies of alloys and semiconductors [371], Detection limits at the ppb level have been reported for GD-MS [372], as compared to typical values of 10 ppm for GD-AES. The quantitative performance of GD-MS is uncertain. It appears that 5 % quantitative results are possible, assuming suitable standards are available for direct comparison of ion currents [373], Sources of error that may contribute to quantitative uncertainty include sample inhomogeneity, spectral interferences, matrix differences and changes in discharge conditions. 

GD-OES (glow discharge optical emission spectrometry) are applied. AES (auger electron spectroscopy), AFM (atomic force microscopy) and TRXF (transmission reflection X-ray fluorescence analysis) have been successfully used, especially in the semiconductor industry and in materials research. 

Fig. 3. The dashed area shows the range of variation of

Glow-discharge-atomic emission techniques (GD-AE) for solid analysis are instru-mentally simpler and more inexpensive than GD-MS [200], Major trade-offs, however, include poorer limits of detection and reduced elemental coverage. Although compact commercial GD-AE instruments were popular in Europe during the 1980s, their use in the USA remained modest until the 1990s. 

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... 

This chapter deals with optical atomic, emission spectrometry (AES). Generally, the atomizers listed in Table 8-1 not only convert the component of samples to atoms or elementary ions but, in the process, excite a fraction of these species to higher electronic stales.. 4, the excited species rapidly relax back to lower states, ultraviolet and visible line spectra arise that are useful for qualitative ant quantitative elemental analysis. Plasma sources have become, the most important and most widely used sources for AES. These devices, including the popular inductively coupled plasma source, are discussedfirst in this chapter. Then, emission spectroscopy based on electric arc and electric spark atomization and excitation is described. Historically, arc and spark sources were quite important in emission spectrometry, and they still have important applications for the determination of some metallic elements. Finally several miscellaneous atomic emission source.s, including jlanies, glow discharges, and lasers are presented. 

Figure 8.29. Substitutional doping in glow discharge a-Si H (a) Changes in room temperature d.c. conductivity by doping with P and B. Intrinsic region is marked as defect-controlled (b) d.c. conductivity activation energy AE (= Ej ) is shown for the dopants (After Spear and Le Comber 1976).

Generally, AES systems are calibrated with multi-element standard samples. In the case of sparks, arcs, glow discharges, and laser ablation, solid samples are required, which are rarely available in large enough numbers to provide a satisfactory calibration. Hence, in solid sample analysis secondary standards are usually prepared. 

Radio-frequency glow discharges are very useful sources for atomic spectrometry. By means of a bias potential in the vicinity of the sample, insulating samples such as ceramics can be directly ablated and analyzed by AES [295]. 

Glow discharges [1S2] are known from their use as radiation sources for AES, and have been recognized as powerful ion sources for mass spectrometry. This development started from spark source mass spectrometry, where there was a continuous search for more stable sources to reduce the matrix dependence of the analyte signals [51]. 

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