Page 256 - Inorganic Mass Spectrometry - Fundamentals and Applications

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less
revolatilizes as an atomic ion. These processes are well understood since they
involve a chemical change from a molecule to an atom in addition to the change
of
in charge state. Experimentally the temperature and work function (WF) the ion-
(IP) or electron affinity (EA) the ion
ization surface, and the ionization potential of
of
species, continue to be important parameters. This chapter deals with the types
thermal ionization processes in which an inorganic solid heated to a high tem-
is
perature and ions are emitted directly from this deposit. These are referred to as
ion emitters and are conveniently divided into two categories: ion emitters with the
ion of interest presynthesized and embedded into a suitable matrix from which this
preformed ion sublimes, and emitters in which the ion of interest is produced via
a chemical or physical process either within the matrix or during volatilization from
the matrix, ~nderstanding of the basic principles of both types of ion emitters is
still in an early state of development, and some of the concepts presented in this
chapter are preliminary and in some cases are just now being submitted for jour-
nal publication. Still other topics are presented here as research opportunities.
The vast majority of materials vaporize only neutral species elevated tem-
at
peratures, and only a few materials emit ions to an appreciable extent. As an in-
troduction to the topic of ion emitters it is useful to ask the question, What causes
a thermally hot condensed phase material to vaporize atoms and/or molecules as
ions rather than as neutral species? This is a nontrivial question and only recently
have some models that address this question begun to emerge for a few ion emit-
ters, These models are still in the conceptual state, and although they are consis-
tent with limited sets of experimental data, ideally they will achieve considerable
refinement in the coming years.
The IP for cation emission and the EA for anion emission continue to be im-
portant experimental parameters for ion emitters, as would expected since these
be
parameters define the thermodynamic stability of the particular ion. The WFs of
the surfaces of the mixtures that constitute emitters are largely unknown and are a
it
gap in our understanding of these materials, although would be expected that this
parameter would also be important. The work functions of the pure material that
is blended into the emitters as the major constituent are known, however. Anion
emitters tend to have a major constituent with a low work function, such as a rare
earth oxide or an alkaline earth oxide. The situation for cation emitters is less clear,
although they tend to be mounted on filaments made of a metal with a high work
function such as metallic rhenium or platinum. Experimental measurement of the
work functions of some of these surfaces is a good research opportunity. Since ion
emission is from the deposit itself, one would think that the filament material would
not be important as long as it could tolerate the required temperatures. Experi-
mental results suggest this is not always true. In two instances [3,4] solubility of
the base metal in the ion emitter has the effect of poisoning emission. On the other
hand, in the case of the alkali metal zeolite cation emitters it has been shown that
[5], as there does not seem to be a
various supporting materials work equally well
solubility problem for the metals in the zeolite at the experimental temperatures.