Nonconductive samples glass

If the rf source is applied to the analysis of conducting bulk samples its figures of merit are very similar to those of the dc source [4.208]. This is also shown by comparative depth-profile analyses of commercial coatings an steel [4.209, 4.210]. The capability of the rf source is, however, unsurpassed in the analysis of poorly or nonconducting materials, e.g. anodic alumina films [4.211], chemical vapor deposition (CVD)-coated tool steels [4.212], composite materials such as ceramic coated steel [4.213], coated glass surfaces [4.214], and polymer coatings [4.209, 4.215, 4.216]. These coatings are used for automotive body parts and consist of a number of distinct polymer layers on a metallic substrate. The total thickness of the paint layers is typically more than 100 pm. An example of a quantitative depth profile on prepainted metal-coated steel is shown as in Fig. 4.39. 

These are simple in basic structure, consisting typically of two metal plates or eleclrodes spaced within an insulating chamber. Examples are show n in Figs. 6 and 7. This arrangement permits isolation and measurement of a portion of lhe solution and serves lo make the measured resistance independent of sample volume and proximity to conductive and nonconductive surfaces. In laboratory cells, platinum eleclrodes mounted in a glass are commonly employed for their excellent chemical resistance. 

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. 

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