Page 258 - Inorganic Mass Spectrometry - Fundamentals and Applications

244 ~el~ore

plemented with three new types of instruments constructed or modified from more
of
standard designs to accomplish specific types measurements:
1. An ion imaging instrument [S] for imaging the regions on an emitter
from which ions are originating
2. An iodneutral mass spectrometer [7] that allows the ions and the neu-
trals (via electron bombardment ionization) volatilizing from the emit-
ter to be measured in sequence
3. A secondary ion mass spectrometer (SIMS) able to analyze the chemi-
cal composition and speciation of the surface of ion emitters at full op-
erating temperature
Several new types of source mounts have also been developed to allow op-
timal placement and operation of the ion emitters in these instruments. These in-
struments provide complement^ information on the properties of ion emitters
that are helping to elucidate the mechanisms by which ion emitters function. This
knowledge has allowed the development of better ion emitters.

Before describing sample mounting techniques is important to discuss the issue
it
of voltage spread across an ion emitting surface and the way it interacts with the
of
extraction and focusing of the emitted ions, This is a very important aspect both
the experimental apparatus and the implementation in useful ion sources. In ther-
mal ionization (and in SMS), the single most important element in the ion lens
system is the sample itself. This is because the ions are at their lowest velocity just
as they are emitted from the sample surface, and thus their flight paths are more
readily altered by either electric or magnetic fields. Another aspect of this issue is
that it is physically more difficult to design and build a useful ion emitter/ion lens
with a high level of symmetry around the emitter than for the rest of the lens. Any
minor irregularities in the electrostatic fields in the region near the emitter have a
far greater effect than in regions after the initial acceleration of the ions. This is
why the NBS style mass spectrometer is operated with a nearly field-free region
is
su~ounding the filaments when a triple-filament source used, and it is operated
with a substantial draw-out voltage when a single-fila~ent source is used. The
t~ple-filament assembly adds sufficient electrostatic asymmetry to the source re-
be
gion that only a weak draw-out voltage can tolerated.
In general, it is important to maintain electrostatic symmetry around the axis
of a lens that has cylindrical geometry, and to maintain electrostatic symmetry
along the y and z axes of a lens with planar geometry. Deflection electrodes vio-
late this rule, but they are generally downstream in the lens, where the ions have
much greater velocity and hence they have less effect on the focusing. Also, the
deflection offset is quite small when the source properly aligned, mini~zing de-
is