Page 273 - Inorganic Mass Spectrometry - Fundamentals and Applications

The Emission of Ions

l. The element to be ionized is reduced to the zero oxidation state.
2. The element is "desolvated" in the high-temperature molten glass ma-
trix; therefore, the atoms of the element are not tightly bonded to each
other or to other species.
3. The element volatilizes as a mix of neutral and singly charged positive
atoms and perhaps is a pseudo-S-L process.
This model is offered somewhat tentatively, but the features are consistent
with a body of experimental evidence. At a minimum, it offers a target against
which to conduct additional experiments that will, it is hoped, shed light on this
subject.
Ion formation mechanisms for silica gel matrices have never been studied
for those elements that are not readily reducible to the metal. The solvatio~desol-
role
vation mechanism hypothesized previously may have a in enhancing ion emis-
sion from these materials, but it would not be expected that an alkaline earth ele-
ment could exist in the zero oxidation state in these glass matrices, which are oxide
to
based. The species in the molten glass would be expected be in the stand~d 4-2
oxidation state, but the experimentally observed species is + l. Indeed, there has
never been a +2 species reported from thermal ionization, so there is the question
of how the +2 species in the molten glass is converted to and emitted as a +l ion.

In concluding this chapter, we point out that there are far more research opportu-
nities than hard answers in this field ion emitters. This field is dominated by sys-
of
is
tems in which the element from which ions are emitted embedded in a matrix
that enhances ion emission. Indeed, with the exception small number of emit-
the
of
ters in which ions are emitted from pure refractory metals at the temperature lim-
its of the material, pure materials predominantly volatilize neutral atoms and/or
molecules when heated to temperatures sufficiently high to force volatilization to
the gas phase. Thus, the key to the development of superior ion emitters seems to
force
be to develop better understanding of the processes that cause the matrices to
the element to volatilize as ions rather than neutrals. With this better understand-
ing perhaps new and better ion emitters can be developed.

1. I. Langmuir and K. H. Kingdom, Proc Royal SOC (London) 1925,12107,61.
2. M. D. Scheer and J. Fine, J. Chem. Phys., vol. 46, No. 10, (1967) 3998-4003.
3. J. E. Delmore, A. D. Appelhans and E. S. Peterson, Int, J. Mass Spectro. Ion Processes,
146/147 (1995) 15-20.