Page 255 - Inorganic Mass Spectrometry - Fundamentals and Applications

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Surface ionization constitutes a wide range processes whereby ions are emitted
from a hot surface. The physics of Saha-L~gmuir (S-L) ionization [l], whereby
is
a neutral vapor-phase atom strikes a high-temperature solid surface and re-emit-
ted as either a positively or a negatively charged atom, has been understood for
many years. The derivation of these formulas is specific for the removal or addi-
is
tion of an electron to a neutral vapor-phase atom striking a hot surface and not
valid for molecules or molecular transformations, The S-L equation can be fit rea-
sonably well to experimental data (to at least a first approximation) for situations
in which an element in the zero oxidation state sublimes from a hot surface-for
example, sublimation of positive and negative rhenium ions from a pure rhenium
filament at 2100°C [2]. These situations are referred to here as psedo-4-l;
processes (as distinct from a true S-L process) and specifically situations in which
an element in the zero oxidation state is volatilizing from the surface as a mix of
positive ions, negative ions, and neutrals. Saba-Langmuir ionization is explained
in detail in Chapter l.
In practice there are few systems in which the S-L model actually describes
the ion formation process, since molecular transformations are usually involved in
this process. A more typical thermal ion source has a multiple-filament mange-
ment wherein a neutral molecule volatilizes from a sample filament at a modest
temperature, undergoes fragmentation on a much hotter ionizing filament, and
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