Page 263 - Inorganic Mass Spectrometry - Fundamentals and Applications

The mission of Ions 249

face of the emitter. An interesting feature is the enhanced ion emission from the
cracks in the face of the emitter in Fig. 6.4a. These appear to be due to enhanced
migration of ions along the surfaces within the cracks. This information by itself
to emission. Another
provides the insight that ions can migrate along surfaces prior
possible inte~retation of the enhanced ion emission from the cracks is that the
edges may provide an enhancement eEect, possibly due to electrostatic field gra-
at
dients that are stronger an edge. The fourth image is Bi+ from a molten glass ion
emitter. Although the image does not show any surface topographical features, the
majority of the ions are originating from the face of the bulk of the deposit. The
ions forming the ring around the main area of ion emission are thought to origi-
of
nate from sample residing on the outer rim the tube source. The intensity was
su~ciently low that an exposure of several minutes would have been required to
obtain a good photograph. The features changed with time rapidly enough that this
was not practical, hence the image in Fig, 6.4d is from the video camera. An in-
teresting topographical feature of the molten ion emitter, which unfortunately can-
not be demonstrated to the reader, was the fashion in which bubbles could be ob-
served forming and bursting on the surface. If such a bubble were to occur in a
sample for which an isotope ratio were being measured it would be impossible to
get accurate results since the beam intensity would fluctuate excessively. This may
be the reason why many analytical procedures that use silica gel require the use of
very small loadings.
It is recognized that there may be other ion emitting condensed phase mate-
rials for which ions originate from a region other than from the bulk of the mate-
to
rial, but since these have not been identified date, the remainder of this chapter
deals with those systems emitting ions from bulk of the deposit.
the

Material volatilizing from the surface of most condensed phases at high tempera-
a
tures consists predominantly of neutral species, with only few materials emitting
in
ions to an appreciable extent. Ion emitters are unique this regard. In an eEort to
understand the mechanisms responsible for this phenomenon better an instrument
was constructed to measure the ions and the neutrals volatilizing from a single
specimen of an emitter sequentially, The purpose is to gain better understanding
of the types and quantities of neutrals volatilizing from the materials along with
the ions so as to gain new insights into the processes governing ion emission.
"his instrument [7] measures three types of ions in a sequential mode: the
positive and negative ions emitted from the surface of the ion emitter, and the neu-
tral species volatilizing from the surface and ionized by electron impact (EI). A
commercially available quadrupole mass spectrometer equipped with an source
E1
was modified to allot a specially designed thermal emitter to be just barely inserted
into the ionization chamber. The chamber is much cooler than. the emitter; there-