Page 266 - Inorganic Mass Spectrometry - Fundamentals and Applications

If
both silver neutral atoms and silver atomic cations are emitted from the surface.
it can be determined that atomic silver in the condensed phase the species that
is
is responsible for the formation of both the neutral and the ionic silver species, then
the conclusion can be drawn that the ion emission process is a pseudo-S-l-type
process. ~relimin~ data are consistent with the conclusion that silver on the sur-
face is in the zero oxidation state.

There are dozens of analytical methods published in both the open literature and
in government and other reports that describe the preparation of various ion emit-
ters for the purpose of measu~ng isotope ratios. There are also many methods that
various laboratories have developed that have never been published but are referred
to in oral presentations. Thus there are probably many groups of ion emitters that
have been developed and used but whose mechanistics properties are not under-
stood. If the mechanism were understood, however, it is likely that the method
could be improved. At the beginning of this chapter ion emitters were divided into
two categories, presynthesized ion emitters and those with ongoing chemical and
physical reactions that produce the ions in situ. Examples of each of these are pre-
sented here, and their mechanistic characteristics are described to the extent they
are understood.

In the class of ion emitters in which the ions are presynthesized in the solid state,
It
the ion of interest is presynthesized and embedded in a suitable matrix. thought
is
state
that ions are subliming directly from the solid into the gas phase, as supported
by the images in Fig. 6.4a+. These images clearly demonstrate that ion emission
is from the face of the bulk of the deposit. The enhanced ion emission from the
cracks in Fig. 6.4a indicates that ions migrate from bulk to a surface, and then
the
migrate across the surface until reaching a location where they can sublime. The
actual ion migration and sublimation mechanisms are not understood, and the role
the electrostatic field may play in these processes has not been studied. Two broad
classes of ion emitters of this type have been identified and studied:
1. Anion emitters: This category of emitters is based on the rare earth ox-
ides [3,9], with the best characterized ones based on the Eu203 matrix,
the
and have been shown to be efficient emitters of perrhenate (Reo4-)
and the halide (Cl-, Br-, and I-) anions. It is probable that the analyt-
ical method for producing ions from technetium [l41 also is in this cat-
egory.