Page 281 - Inorganic Mass Spectrometry - Fundamentals and Applications

Sample Types
Analysis o~~oncond~ctive 267
ion beam ratios ranged from 1.2% to 3.6% RSD over an additional 20-min analy-
sis period. The sample-to-sample precision, which is affected by the reproducibil-
as the
ity of the sample drying, mixing, weighing, and pressing procedures, as well
plasm~ins~ment operation, was found to range from 8.9% to 40% RSD for the
5 1 analytes. By looking at the variations across the range of elements, it was sug-
gested that inhomogeneities in the sample pins were the dominant factor in the ir-
reproducibility, The most important aspect of this work was the realization that
the
level of quantification for compacted soils was very similar to that of bulk solid
GD-MS on the VG 9000. Raw ion signals produced qualitative analysis with ac-
curacies within four times the certified values, and the use of “book-value” rela-
tive sensitivity factors (RSFs) produced values to within a factor of 2. Finally, the ,
use of the NIST Peruvian Soil to produce RSFs to analyze NIST 2704 Buffalo
River Sediment produced better accuracy, with only As (having a mass interfer-
ence) producing an error of greater than 75%.
Stuewer and coworkers [45] described the use of a GD ion source that was
similar in design to the Grim-type lamps used in GD-AES for the analysis of alu-
minum oxide powders on a quadrupole analyzer. Copper was chosen as the host
matrix and sample-to-host ratios of 1 : 1-1 : 10 were investigated to determine the
role of composition in plasma stability. A 1 :5 mixture was chosen for the analyti-
of
cal characterization on the basis its high temporal stability and minimal analyte
dilution. Both neon and argon were used as discharge gases, as the former gener-
ally provides a background mass spectrum that poses fewer interferences ele-
for
ments commonly found in oxide samples [i.e., low-mass (<40 amu) elements].
Temporal profiles of both matrix and analyte species, as well as total ion current,
give strong indication of two phases of plasma stabilization, as shown in Fig. 7.2.
Early in the plasma lifetime (40 min), a high-current situation exists, in which
the ion beam is dominated by molecular species as the sample is suggested to be
more “oxide” in nature. A definitive second phase occurs abruptly as the total ion
current drops but is now composed mainly of atomic (analyte) ions. Spectral com-
parisons between compacted oxide and pure metal powder samples indicated that
the plasma and ion beam characteristics were not appreciably different once the
plasmas had reached the stable second phase. For example, limits of detection for
the oxide samples (as determined through calibration curves) were generally in the
single-microgram-per-gra~ range, in the absence of isobaric interferences. As a
result of the nature of the compacted samples, of course, greater amounts of mo-
lecular species are present in the mass spectra obtained,
Another example of the analytical utility of the sample compaction method-
ology for CD-MS is work by Wayne [44] that used a Kratos double-focusing an-
alyzer for the analysis of precious metals deposited on cordierite supports (i.e., au-
tom’otive catalysts). Spectral interferences from TaO species on the target
palladium, rhodium, and platinum analytes made aluminum a better choice as the
matrix element. Because the Kratos instrument does not have cryogenic cooling