Page 267 - Inorganic Mass Spectrometry - Fundamentals and Applications
P. 267
The ~~ission Ions 253
of
2. Cation emitters: The alkali metal zeolites, and other alkali metal alumi-
of
nosilicates, are efficient emitters alkali metal cations. The cation emit-
ters have been known for a much longer time than the anion emitters,
but the anion emitters are better understood from a chemical perspec-
tive; hence they are discussed here. Both types emitters, however, can
of
be scaled up in intensity readily to be used for the primary ion guns in
static SMS inst~ments. Ion beams of 50 pA to 1 nA focused to a l-m
spot size are routinely produced by using these emitters. These emitters
to
are primarily used in SIMS guns, as opposed being used for isotope
ratio analyses.
of
The importance of the work function and temperature the surface, the ion-
ization potential for positive ion emission, and the electron affinity for negative ion
emission are well established for conditions in which the S-L equations are valid.
Expe~mentally, the IP and EA are also important for thermal emitters. For exam-
ple, the alkali metals all have low IPS and are emitted in good yields from the ze-
olites impregnated with the corresponding alkali metal. The halide and perrhenate
anions all have high EAs and are emitted in good yield from certain of the rare
earth oxides, The temperature is also quite important, but possibly not for the same
reasons as for the S-L conditions. Under S-L conditions a higher temperature is
more likely to strip an electron or to add an electron to an atom.
Sublimation of preformed ions from the surface is a process much different
from S-L ioni~ation since these ions already have their oxidation and charge states.
It may be surmised that a higher temperature increases the migration rates of the
preformed ions and increases the sublimation rate, until the temperature gets so
high that some undesirable process occurs in the matrix. As an example, the oxi-
dation state of the ion of interest could change with excessive temperature, de-
stroying the ion. emission properties, or the matrix could undergo an undesirable
phase ans sf or mat ion. The importance of the IP and EA may be more related to the
degree of polarization between the preformed ion and their counter ions. The WF
of the surfaces of these emitters has never been measured, not even at low tem-
peratures. The better anion emitters are in rare earth oxide matrices, and these
ma-
terials are known to have relatively low WFs at low te~peratures, but the effects
to
of blending these components and elevating the temperature have the potential
alter these values. This area needs more study.
Experimentally, the compound consisting of the preformed ion and its
counter ion (such as barium perrhenate for perrhenate emission) does not produce
ions when heated-instead, only neutral species sublime. It must be embedded in
a suitable matrix and then heated. The limited experimental evidence collected to
date indicates that the ion to be emitted must have significantly greater mobility
andlor vapor pressure in this matrix than its counter ion, allowing diffusion of the
ion of interest. When the tempera~re gets sufficiently hot the ion migrates to the