Page 272 - Inorganic Mass Spectrometry - Fundamentals and Applications

258 Delmore

A recent study [4] conducted on large deposits of bismuth ion emitters pre-
pared by a method of this type identified the material as a molten glass at operat-
of
ing temperatures, Figure 6.4d identifies the surface the bulk of the material as
It
su-
the ion emitting surface. has been known for many years that rhenium was a
the
perior filament material to tantalum these analyses, and this study identified
for
reason, Tantalum dissolves in the molten glass and poisons the emitter. Thus we
have reason to believe that the molten glasses are highly corrosive, especially to-
ward Ta, and are especially so if strong mineral acids are used.
gel
A more recent study has been conducted on silver/silica ion emitters. Sil-
ver was chosen because it has only 0 and +l oxidation states, greatly simplifying
the determination of the oxidation state responsible for ion emission. The question
is, Is it 0 or +l? If it is + 1, it would be a preformed ion, whereas if it is 0 it would
be a pseudo-S-L-type process with a mix neutral and cationic species volatiliz-
of
ing from the surface. Silver oxides are known to decompose to the metal and
gaseous oxygen at temperatures well below the operating temperatures of these
ion emitters; hence there is a high probability that silver is reduced to the metal
from the temperature alone. Indeed, oxygen is evolved from these emitters at ap-
proximately this temperature range. There is evidence that silver is in the 0 oxida-
tion state at operating temperatures, spite of the fact that it is in an oxide matrix.
in
The reasons for this claim are as follows:
1. In the ionlneutral mass spectrometer the silver species identified were
the atomic neutral and cationic species.
2. The HT SIMS data indicate that the only silver species on the surface
is
of the emitter is metallic. Hence, if metallic silver on the surface sub-
~iming to give a mix of atomic neutral and cationic species, this looks
like a pseudo-S-L process. Again, the data are not quantitative enough
to compare to the S-L equation.

If this model, where the metal is reduced to the zero oxidation state, applies
to silver, it is conceivable that it applies to the other elements that can be readily
reduced to the metal. The question that remains to be answered is, Does the model
for silver apply to other elements that can be reduced to the metal in an aqueous
solution?
Another concept that warrants mention is “desolvation.” When silver metal
in bulk is heated, it tends to sublime as neutral species. When both neutral and
cationic silver species volatilize from these silica gel matrices tbey are exclusively
monatomic. This indicates that silver atoms in the zero oxidation state are not “sol-
is
vated” by each other or by a component in the matrix. This concept admittedly
speculative but does offer a concept as how this ion emitter matrix may operate.
to
the
Thus, for those elements that are readily reduced to zero oxidation state,
a model that is consistent with experimental evidence can be offered. This model
is the following: