Page 298 - Inorganic Mass Spectrometry - Fundamentals and Applications

Harrison and coworkers have described in a pair publications the evalua-
of
tion of operating characteristics of the rf GD-MS of oxide powders [31] and the
comparison of this “direct” method to the use of metal binders for conventional
CD-MS analysis [32]. As mentioned, direct compaction geological materials in
of
this case produces very complex mass spectra consisting a large range of mo-
of
lecular species related to water and trapped gases. This situation was remedied to
a very large extent with the use of a liquid nitrogen cold finger in the plasma re-
to
gion for periods of greater than 30 min prior mass analysis [3 l]. The same pro-
found improvement (70%-100% reduction in gaseous ions) was found as well for
A. detailed evaluation of the roles of discharge
bulk nonconductors such as Macor.
power and pressure and ion sampling distance on the signal responses of analyte,
residual gas, and argon-related species provided very interesting and useful in-
sights into plasma processes and methods of optimizing spectral responses [31].
Although sets of discharge conditions that provide very good sensitivity and high
SN ratios are readily identified, the sensitivity to variations in conditions is quite
for
profound and can cause difficulties. Relative sensitivity factors were compared
a number of analytes and oxide materials, with the span of values for most ele-
ments fairly well defined and not out of line with literature values. In fact, semi-
of
quantitative analysis, based on simple ion beam ratios, was within a factor 2 for
a firebrick standard. The rf GD-MS of directly compacted oxides produced very
stable plasmas (<5% RSD for 1 hr), which are advantageous for quantitative de-
te~nations. It should be pointed out that these results are quite comparable to
those of Pan and Marcus [68], who evaluated the use of rf CD-AElS in the analy-
sis of glass powder samples.
The comparison of rf and dc powering by Hamison and coworkers E321 was
of
to
carried out at a number different levels. An NIST iron standard was first used
evaluate the relative characteristics for conducting samples. Discharge conditions
that produced similar levels of analyte ion signals were employed for rf and dc,
to
though admittedly these conditions were a compromise relative the optimum for
rf signal intensities. Even so, very few analytical differences existed for the two
powering schemes, including relative sensitivity factors, stability
(45% RSD), and
sample-to-sample reproducibility (~20%). Interestingly, the rf plasma produced
much higher levels residual water signals than the dc plasma in case in which
the
of
cryogenic cooling was not utilized. (This
is of no analytical consequence, as cool-
ing is now the nom in all GD-MS analyses.) Trade-offs between conditions yield-
ing high R values (R = M+/(&€+ + MO+)) and ion signal intensities were seen for
the pure La,O, model oxide, with high La+ signals and R values of 40% produced
for a finite set of compromise conditions. As seen in the earlier study [31], these
values were sensitive to changes in plasma and sampling conditions. Comparison
of the rf and dc discharges was made for nonconductor analysis using Ag as a very
weak getter matrix and again discharge conditions that yielded similar signals.
La+
A. relatively low rf power of 8 W was employed as higher powers produced sput-
tering conditions that tended to clog the sampling orifice. Under such conditions,