Page 289 - Inorganic Mass Spectrometry - Fundamentals and Applications

~nalysis of Non~onductive Sample Types 215

1 E*07

for
and PVC, stable plasmas could be maintained extended periods, with the spec-
tra dominated by atomic ions rather than molecular fragments. Figure depicts
7.5
the ion signal stability achieved for a PTFE sample. The raw signal intensities were
observed to vary by 3% to 20% RSD after the 15-min sputtering time. The 12C+
signal was the base peak for each of the three polymers investigated in the study,
with an average current value of 1-6 x A. In general, small molecular clus-
ters were present at levels of much less than 1% of the 12C+ kensity; the vast ma-
jority of the expected species appeared at far lower levels. Therefore, polyatornic
ions do not provide substantive molecular info~ation, but rather pose problems
as isobaric interferences for trace elemental species. In the absence of interferents,
the carbon matrix ion signal level would suggest that high part per billion limits
of
detection may be possible. As with the previous discussion, though, blank levels
due to the mask material are a limiting factor. In any case, trace element analysis
by direct current (dc) CD-MS with a secondary cathode appears to offer capabil-
ities for polymer analyses that are not available from traditional atomic'spectro-
scopic methods.

The use of the sample compaction and secondary cathode sampling methodolo-
gies has certain positive attributes based on the initial sample form, analysis time
constraints, and analytical performance criteria of a given application. A common
shortcoming of both approaches is that a material other than the analytical speci-
men is subjected to the CD atomizatio~ionization processes, inflicting additional