Page 53 - Inorganic Mass Spectrometry - Fundamentals and Applications

Glow Discharge Mass Spectrometry 43

Seconary Ionization ~ech~isms in the Glow Discharge
Nonsymmetric charge transfer x+ + MO-+M+ + x0
Symmetric (resonance) charge transfer A&, + A:?,, + Aiow + A!ast
Pissociative charge transfer X+ + MA +M+A +Xo
Associative ionization X~+M+XA4++e-
Photoionization MO + hu -+ M+ + e-
Cumulative ionization MO + e- -+ M* + e- -+ M+ + 2e-
Source: Ref. 19.

elin

During the last few years, several groups (primarily led by Professor R. Gijbels at
the University of Antwerp) have been trying to model the interactions of atoms,
electrons, ions, and excited state species in the glow discharge. Using sophisti-
cated mathematical relationships in concert with an abundance of previously
obtained experimental information, these investigators have, among other things,
of
attempted to predict the role metastable argon atoms [SS], simulated the motion
of species in the cathode dark space [59], and described the thermalization process
of sputtered atoms [60]. In most instances these calculations have agreed well with
of
experimental data. The real success these efforts, however, will be the extension
of the work to predict some as yet undiscovered glow discharge phenomenon, or
to explain data for which no answer has been previously hypothesized (e.g.,
relative sensitivity factors for exotic matrices). In the next several years, the bene-
fits of this approach may be realized in improved analytical performance in
GDNIS.

The inst~mental components of a glow discharge mass spectrometer are hodge-
a
podge of ionization sources in combination with virtually every type of mass
spectrometer that exists today. Several commonalities, however, can be found
among the wide number of possible combinations. First, all of the sources consid-
ered here operate at reduced pressures (0.01-10 torr for the examples given).
Typically, this pressure is for an ambient rare gas; however, examples can be
found in which other fill gases have been employed [61,62]. Because most mass
analyzers operate optimally at a lower pressure than the ion source, differential
pumping is required to obtain pressures torr. These pressures facilitate a
collision-free ion flight path but cause problems in interfacing GD sources to
certain analyzers (e.g., Fourier transform mass spectrometers). Often the operat-
ing pressure, cathode/anode arrangement, and power supply output dictate what