Page 269 - Inorganic Mass Spectrometry - Fundamentals and Applications

The Emission of Ions 255

remaining after cation emission. For an anion emitter a reduction reac-
tion within the matrix is required to generate a new cation to satisfy the
bonding requirements of the remaining cation after anion emission.
5. It is probable that the oxidation or reduction occurs nearly simultane-
ously with ion migration, since any appreciable electrical charge buildup
is the~odynamically improbable.
This model is based on studies of anion emitters in rare earth oxide matrices
in the +3 oxidation state extending over several years. These studies can be sum-
marized as follows:
1. It is an experimental observation that only the rare earths starting out in
the +3 oxidation state are efficient ion emitters.
2. The rare earth must have a reasonably stable
+2 oxidation state, although
the majority of the material must be in the +3 state. The elements eu-
ropium (Eu) and ytterbium (Yb) have by far the most stable +2 oxida-
tion states of the rare earths, and the oxides these elements make the
of
most effective matrices for anion emission. Eu is approximately two or-
ders of magnitude more effective as a matrix that Nd when penhenate
emission is not pushed to high levels.
3. The included anion to be emitted must have a high electron affinity. The
species demonstrated to be emitted from these matrices are the halides
(Cl-, Br", I") and perrhenate (Reo,-). The electron affinities are 3.7
eV, 3.5 eV, 3.2 eV, and 4.5 eV, respectively. The borate anion (EA of 3.0
eV) can also be readily observed as an impurity in these emitters.
4. These anions appear to be quite stable in these matrices. Studies con-
ducted with the ionheutral mass spectrometer could not detect any gas-
phase reduction products of perrhenate or any oxidized species of the
halides, leading to the conclusion that the Eu,O, matrix is neutral or
nearly so in regard to oxidatio~reduction potential.
5. Studies on the io~neutral mass spectrometer have verified that the bar-
ium counter ion is nonvolatile in these matrices, and possibly nonmi-
gratory.

by
Emitters that have ions produced in situ are far the largest group of known ion
emitters. They are much more difficult to scale in intensity than preformed ion
emitters. In general, methods have been developed that give stable and reproducible
ion beams with sufficient intensity to provide an isotope analysis for the par-
ratio
ticular element to be analyzed. As stated earlier, it is not necessary to understand
the mechanistics of ion emission to use these ion emitters for isotope ratio analy-
sis as long as the ion beams are sufficiently stable with adequate intensity, There