Page 270 - Inorganic Mass Spectrometry - Fundamentals and Applications

256 Delmore

is always the question, though, of whether the method could be improved signifi-
is
it
cantly if the mechanism were understood. For this reason, useful to attempt to
gain an understanding of these processes. Although the primary application the
of
SIMS instru-
prefo~ed ion emitters tends to be as the primary ion guns in static
ments, the significant application of these emitters is the measurement of isotope
ratios.
There are two main subdivisions of ion emitters in this group of which we
have some understanding. The first of these subdivisions is an emitter in which an
ongoing chemical reaction synthesizes an ion that is then sublimed. An example
of this is the production of perrhenate anions [l51 from the rare earth oxide cat-
alyzed oxidation of rhenium by water vapor. The second subdivision is the pro-
duction of neutral atoms within a matrix and the vaporization of fluxes of neutral
and ionic species. This can be thought of as a type of pseudo-S-L process since
of
there is an atom on the surface that has a finite probability volatilizing as either
an ion or the atom. The trick is to be able to produce these atoms in the zero charge
state in the matrix. There are a number of ion emitters thought to be in this cate-
gory, but none proven. There are many other “recipes” that have been used over
the years; among these there are probably other types of ion emitters that have been
In
used but whose mechanistics properties have never been identified. truth, there
is very little that is known with certainty about the mechanistics of ion emitters in
of
these categories; hence the rest this chapter primarily outlines areas that require
research.

Several refractory metals emit the respective atomic cation and, in cases, the
some
atomic anion at temperatures approaching the burnout point of the material. The
work of Scheer and Fine [2] demonstrates that tungsten and rhenium emit both
atomic cations and anions of the respective metal, and that the ratio of cations to
anions as a function of temperature is consistent with the S-L equations. Such be-
~ the characterization
havior can be termed ~ ~ e behavior, as ~ ~ ~ - ~ ~ g ~ ~ ~ ~
~ ~ indicates that this Q
~situation is not that of an atomic beam impinging on a
e
~
hot metal surface. This does not conclusively prove that these are S-L processes,
however, since no attempt was made to measure the neutral atom flux. This raises
of
the question, If the S-L equation applies to the sublimation atoms of the major
(or only) element making up a high temperature metal, will it also be applicable
to the emission of minor impurity metal ions? This has never been proved experi-
mentally, but it seems a logical extension of the preceding conclusions. There are
situations in which such processes may be occurring. First, there are analyses for