Page 18 - Inorganic Mass Spectrometry : Fundamentals and Applications

8 Smith

The Saha-Langmuir equation also does not apply to situations in which
chemical as well as physical processes occur at the ionizing filament. An instance
of this is the double- or ~ple-filament arrangement common in thermal ionization,
in which procedures that ensure stable ion emission cause species other than gas-
phase atoms to impinge on the ionizer. An example of this is uranium; the loading
to
procedure includes heating the fil~ent dull red heat in air for a few seconds.
The uranium is almost always loaded in a weak nitric acid solution, which means
uranium is present in oxide form. The heating step is an oxidative one, presumably
taking uranium to its highest oxidation state. Because ura~um oxides are more
volatile than the metal, this procedure ensures that almost all the uranium evapo-
rates as either UO or UO,, which in turn means interaction with the ionizer must
break the U-0 bond as well as produce U+. This is not the situation described by
the Saha-Langmuir equation [ 151.
Despite these caveats, the Saha-Langmuir equation is useful in predicting
order-of-magnitude estimates of ionization efficiency and in comparing ionization
efficiencies of two or more elements. For example, the ionization efficiencies of
Pu, U, and Th calculated from the equation are relatively correct, approximately
corresponding to experimentally observed behavior, which is roughly an order of
magnitude decrease in efficiency with each step in going from Pu to U to Th.

1.4 IN~TR~~ENTATI~N

Most isotope ratio measurements have been performed using sector mass spec-
trometers. Some work has been reported, notably by Heumann [35], in which a
qua~pole-based system was used. Instruments used for measurement of isotope
ratios are most often dedicated to that purpose, In most instances only a relatively
small mass range needs to be monitored, just enough to encompass the isotopes of
the analyte element. Without the ability to scan the entire elemental mass range
[usually from mlz = 6 (Li) through mlz = 238 (U) for elemental analysis], mass
spectrometers designed to measure isotope ratios cannot readily be adapted for
other purposes. See Chapter 2 for a discussion of inst~mentation required for
elemental analysis of solid materials and Chapter 3 for a treatment of the in-
st~menation needed for elemental analysis of solutions.

1.4.1 Filament Considerations
The filament material most commonly used in thermal ionization is rhenium.
There are several properties that dictate its choice. It has a high enough melting
point (3 18OOC) that it can withstand the temperatures required for efficient ioniza-
tion (up to about 2200°C). It has the highest work function of any metal with a
high enough melting point; like all metals, its work function varies with the crystal