Page 31 - Inorganic Mass Spectrometry : Fundamentals and Applications

The~al Ionization Mass Spectrometry 21

layer of platinum over the sample. The sample atoms are thus forced to migrate
through the platinum layer before leaving the filament. In this manner, the ad-
vantages of using rhenium as a filament material (high melting point, ductility)
were retained while the higher work function of platinum was exploited to
enhance ionization efficiency. A mean ionization efficiency of 0.34% for plu-
tonium was reported; that efficiency is a substantial improvement when compared
to analyzing samples loaded as solutions, which is generally an order of magni-
tude lower.
Similar reasoning led Smith and Carter to develop a method using an
overlayer of pure rhenium powder slurried with a source of carbon, such as starch
solution [19]. Rhenium and carbon do not form a stoichiometric compound, but
carbon dissolves in the metal to form a composite surface that has higher work
function than pure polycrystalline rhenium; this has been measured as 5.8 eV,
about the same as the work function of platinum [Xi]. Rhenium powder, when
heated to the operating temperatures required for many elements (> 1SOO0C),
sinters to form a barrier to evaporation that forces analyte atoms to migrate
through it before leaving the filament. Thus, in a manner similar to the platinum
overcoat electrolytically deposited, the advantages of rhenium as a filament mate-
rial and an emitting surface of high work function are both exploited. The higher
work function explains in part the benefits derived from loading samples on resin
beads; the carbon skeleton of the bead that remains after its thermal decomposition
dissolves in the rhenium substrate, forming a local surface with a higher work
function than that of the surrounding metal [ 171.
It is not clear which of the two overcoating methods provides greater
enhancement in ionization efficiency. Smith et al. report ion collection efficiencies
of 4%-9% for plutonium [76], but this was for an experiment specifically de-
signed to evaluate this ability; Perrin et al. [18] were more concerned with
improving the stability of their ion beam (and hence precision) than they were
with determining ionization efficiency. Substaritially improved ion beam stability
and reduced isotopic fractionation were noted by both sets of authors as a signifi-
cant benefit of both methods of overcoating.

Thermal ionization has widespread application in areas where measurement of
isotope ratios is the goal of the analysis. Each area has its unique problems and
challenges, and there is considerable cro~s-fe~lization among disciplines. Ex-
haustive treatment would require a book of its own; no attempt has been made here
to cover all areas addressed by thermal ionization, Rather, a few areas of current
and historical interest have been selected; these should give the reader a good idea
of the versatility of the technique.