Page 257 - Inorganic Mass Spectrometry - Fundamentals and Applications

of
The ~~~ssion 243
Ions
of
In general, pure materials do not emit appreciable quantities ions; the one
[2]. In the ma-
exception is self-ionization from high-temperature refractory metals
to
jority of useful ion emitters the material from which ions are be produced is em-
bedded in a matrix that is usually more refractory than the material itself and al-
lows migration of the species of interest. For many ion emitters the more refractory
matrix is thought to have the effect of increasing the temperature of volatilization
of the species from which ions are to be produced, and the higher operating tem-
perature increases ionization efficiency for most emitters. There are other matrix
effects that have been identified or hypothesized for various ion emitters such as
the
migration of the species of interest, and these are mentioned as emitters are dis-
cussed.
The importance of the chemical composition of these inorganic deposits in
regard to ion emission has been understood many years and is illustrated by the
for
work of Studier et al. [6]. A deposit of uranium on a hot filament emitted a variety
of ions, depending on the oxidizing and reducing agents added to the material.
They presented their data in the following format:
Oxidizing agents (e.g., oxygen)
uc +, U+, uo+, UO,C, UO,+
Reducing agents (e.g., carbon)
These results illustrate the importance of the chemical species of the element
present in the deposit with regard to ion emission (and gives insight into the effect
of the oxidizing/reducing nature of the ion emitter) but tell little about the actual
mechanisms active in the ion emitting process. As an example, the ions could be
emitted either from the deposit itself or from an intermediate material that formed
as a consequence of the chemical properties, or it could be entirely an interface
phenomenon in which the deposit only served as a repository for the uranium
species and the supporting filament served as the ionization surface.
The study of the mechanistics of ion formation from hot ion emitters has be-
come a topic of research in the author's laboratory in recent years, As with all re-
search, the deeper one probes the more there is to study. menever a new level of
understand in^ was achieved, new questions arose that required new approaches
and in some instances new inst~ments custom designed and built to accomplish
measurements needed to achieve the next level of understanding. These instru-
ments are described with examples of how they are applied.

This work was initiated with a standard National Bureau Standards (NBS) style
of
single magnetic sector mass spectrometer (see Chapter
1). This capability was sup-