Page 292 - Inorganic Mass Spectrometry - Fundamentals and Applications

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The first real indication of the possible utility of the rf powering mode for “ana-
in
lytical” chemical applications was presented a review article by Coburn and Har-
rison in 1981 [ 1 l]. In a way, the authors provided a bridge between two sides of a
river: Coburn viewed mass spectrometry as a tool to understand funda~ental
processes occurring in plasma deposition sources, whereas Harrison, whose back-
it
ground was in spark and plasma source mass spectrometry, viewed as a tool for
spectroche~cal analysis. At that time, rf CD-MS was simply a tool of physicists
and engineers, but the ability to derive composition- and depth-resolved informa-
tion about an insulating target was clearly presented. Soon after that publication,
a single report describing the use of rf powering for a hollow cathode GD-MS
ion
source [termed an ~ c ~ v i ~source (CIS)] was presented by Donohue and Har-
rison in 1975 [62]; in it mass spectra derived from metallic, glass, and solution
of
residue samples illustrated the possible utility the approach, The analytical per-
formance (i.e., spectral characteristics, sensitivity, and stability) of the source was
found to be equivalent to or better than that of the much more fully developed dc
powered hollow cathode sources, suggesting a great deal of promise.
Despite the possible utility demonstrated through the previously cited works,
the field of rf CD-MS lay dormant for almost 15 years until a paper by Duckworth
and Marcus reintroduced the concept in 1989 [26].
In that paper, the operating prin-
ciples and basic design criteria for “analytical” rf GD-MS sources were presented
for a simple diode source used for the analysis of 0.5-inch-diameter sample disks,
including metals, solid glass, and compacted (without binder) metal oxide pow-
ders. In addition to illustrating the flexibility of the rf CD ion source, data were
presented that indicated a more complex inte~elationship between discharge con-
for
ditions and ion extraction (sampling) position than had been seen dc powered
sources. The requirement to optimize ion sampling position and discharge condi-
tions, along with a need for efficient sample interchange, has led to the imple-
mentation of more user-friendly designs based on 4.5-inch-diameter direct in-
sertion probes (DIPs) [27]. Figure 7.7 illustrates the general approaches used in rf
by
GD-MS source design as implemented the Clemson University laboratory. Use
of DIPs provides a means of mounting the sample, providing electrical contact,
position optimi~ation, and introduction through a vacuum interlock. All subsequent
of DIP approach. On the basis of the
rf GD-MS sources have employed some sort
diameter of the probe, implement in^ sample holders for pin-shape and small disk
samples is relatively straightfo~ard (Fig. 7.7a). A number of groups have de-
of
to
scribed such designs. All these designs are somewhat different in respect the
ion volume and spectrometer geometries (as well as investigators’ preferences);
hovyever, the opti~zation of discharge and sampling positions are remarkably sim-
ilk.
Given the wide diversity of solid sample forms and the particular difficulty
of machining oxide (e.g., glass and ceramic) samples to a fixed form, rf GD ion