RF 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 

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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]. 

Figure 7 Radio frequency glow discharge mass spectrometry (rf GD-MS) source designs employed on a VG GloQuad instrument for the analysis of a, Pin-type samples. (From Ref. 28.) b, Flat samples. (From Ref. 29.)...

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. 

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... 

In speciation, glow discharges are excellent detectors for GC work as shown earlier. In addition to the low power and pressure ICPs they can be used successfully for element-specific detection for gas chromatography. An rf-GD-MS system has been used as a detector for GC by Olson et al. [661], The set-up should consist of a temperature-controlled transfer line of stainless steel from the exit of the GC to the inlet of the GD source. The system has been tested with tetraethyl-Pb, tetraethyl-Sn and tetrabutyl-Sn and provided useful structural information for the identification of these compounds through the observation of fragment peaks the detection limits were down to 1 pg. 

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. 

As for SNMS, low pressure GD sources separate erosion and excitation/ionization but this is the same plasma that assures both functions. Analytical GD instruments are mainly characterized by their ease of use and rapid sputtering capability [30,31]. When the source is powered with RF, both conductive and nonconductive layers can be measured readily. 

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. 

The introduction of rfpowered sources has extended the capability of GD-OES to non-conductors, and several rf sources of different design have become commercially available. This is of the greatest importance for surface and depth-profile analysis, because there exists a multitude of technically and industrially important non-conductive coating materials (e. g. painted coatings and glasses) which are extremely difficult to analyze by any other technique. 

In conclusion, GD-OE S is a very versatile analytical technique which is still in a state of rapid technical development. In particular, the introduction of rf sources for non-conductive materials has opened up new areas of application. Further development of more advanced techniques, e. g. pulsed glow discharge operation combined with time-gated detection [4.217], is likely to improve the analytical capabilities of GD-OE S in the near future. 

An rf-planar-magnetron GD has also been coupled with TOF-MS. For rapid sample changing without venting the mass spectrometer, a sliding PTFE seal was placed at the interface. The seal in turn holds a Macor ring, shields it from the plasma and supports the sample [601]. Detection limits for conducting and insulating materials were of the order of 0.1 and 10 gg/g for B and Mg in Macor, and Bi, Cr, Mn, Ni, and Pb in aluminum, respectively. The source-spectrometer combination still needs improvements at the interface with respect to the extraction location for analyte ions, the scattered ion noise and the extraction repetition rate. 

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