DC GD sources

A schematic DC GD source is shown in Fig. 7.43. The gas is present at a pressure of a few torr. The DC GD source can be operated with a DC potential of 800-1200 V applied between the electrodes. The sample is in electrical contact with and serves as the cathode as seen in Fig. 7.43. The applied potential causes spontaneous ionization of the... 

Figure 7.43 A DC GD source. A flat conducting sample serves as part of the cathode. [Courtesy of Jobin Yvon, Inc., Horiba Group, Edison, NJ (www.jyhoriba.com).]...

The first application of GD-OES using a DC source was for direct multielement analysis of solid metals and alloys, much like a spark source. The bulk composition of the sample was determined. The DC GD source for analysis of conductive solids has several advantages... 

Applications now include the analysis of nonconductive materials such as polymers, ceramics, and glasses using the RF Marcus-type source. The advantages are similar to those just discussed, with a major improvement in detection limits and a decrease in sources of error. With a DC GD source, a nonconductive material had to be diluted with a conductive powder this decreased the amount of analyte that could be detected in the sample. Use of an excess of conductive powder and the process of blending and pressing always introduced the possibility of contamination of the sample. This source of error has been eliminated by direct analysis of nonconductive materials. 

The quantification algorithm most commonly used in dc GD-OES depth profiling is based on the concept of emission yield [4.184], Ri] , according to the observation that the emitted light per sputtered mass unit (i. e. emission yield) is an almost matrix-independent constant for each element, if the source is operated under constant excitation conditions. In this approach the observed line intensity, /ijt, is described by the concentration, Ci, of element, i, in the sample, j, and by the sputtering rate g, ... 

Figure 2.25 Experimental arrangement of modified Grimm-type dc GD ion source design with flow tube. (Reproduced by permission of N. lakubowski, Nachrichten aus der Chemie, 2003.)...

A GD mass spectrometer from Thermo Fisher Scientific with a direct current (dc) glow discharge ion source based on the mass spectrometric arrangement of the Element (Element GD) has been available on the analytical market since 2005 for sensitive multi-element analysis of trace impurities in conducting samples. The experimental arrangement of the dc GD ion source (Grimm type) and... 

The major driving force for the development of rf-powered GD-MS sources is of course the broad diversity of possible analytical samples to which the devices may be applied. It should be noted at the outset that a number of the cited works have shown that the performance characteristics of the sources are equal to or better than that of dc GD-MS for metallic, conductive samples. In the discussion that follows, the use of rf GD-MS is highlighted for the analysis of bulk insulators, oxide powders, and polymeric materials. 

Glow Discharge Sources. Glow discharge (GD) ion sources and some of their applications using different mass analyzers have been discussed in earlier chapters of this volume. Virtually all work that couples these sources to FT-ICR mass analyzers has involved dc discharges (see Chapter 2 for further discussion of the types of GD sources). 

These factors clearly influence plasma formation, the requirements for which depend on whether the GD source is of the rf or dc type and also on the detection technique to which the GD is coupled. As early as 1976, Gough [177] reported improved direct solid atomic absorption analysis achieved by using a flowing gas instead of a static cell and... 

Another popular approach to glow discharge spectroscopy is to use rf power instead of traditional dc power sources. The main advantage of rf-GD is its ability to sputter nonconductive samples, hence elemental analysis for polymers and ceramics becomes a matter of simple solids analysis. 

Besides the conventional Grimm-type dc source, which has dominated the GD-OES scene for approximately 30 years, other discharge sources are well known. Among those are various boosted sources which use either an additional electrode to achieve a secondary discharge, or a magnetic field or microwave power to enhance the efficiency of excitation, and thus analytical capability none of these sources has, however, yet been applied to surface or depth-profile analysis. 

In contrast with the dc source, more variables are needed to describe the rf source, and most of these cannot be measured as accurately as necessary for analytical application. It has, however, been demonstrated that the concept of matrix-independent emission yields can continue to be used for quantitative depth-profile analysis with rf GD-OES, if the measurements are performed at constant discharge current and voltage and proper correction for variation of these two conditions are included in the quantification algorithm [4.186]. 

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