Page 268 - Inorganic Mass Spectrometry - Fundamentals and Applications
P. 268
surface and sublimes, leaving the counter ion behind. This process continues until
something happens to limit further diEusion andor sublimation. This limiting step
The
is thought to be the buildup of charge (or color centers) within the matrix. sec-
for
ond major requirement seems be some mechanism to compensate the buildup
to
of these charge centers.
The most successful of the anion emitters are based on rare earth oxide ma-
trices, with the rare earth in the +3 oxidation state. Europium oxide, Eu,O,, is the
most successful of these anion emitters and also has the most stable 4-2 oxidation
state of all the rare earths. This feature of Eu,O,, the stability of the +2 oxidation
state, is thought to be responsible for this compound’s being the best matrix.
Whereas the matrix in the -1-3 oxidation state allows the migration of the anions
away from the counter ion, the stability of the +2 oxidation states allows reactions
of the following type to take place:
Eu20, + Ba(ReO,), + 2e- * 2ReO,- (gas phase) + 2EuO + BaO (6.1)
This reaction involves the reduction of Eu in the +3 oxidation state to Eu in
the +2 oxidation state, freeing an oxygen anion to combine with the Ba counter
ion left after the perrhenate anion migrates. There are a variety of other chemical
reactions of a similar type that can be written, such as the follo~ing:
Eu203 + 2Ba(ReO,), + 2e- * 2ReO,- (gas phase)
+ 2EuO + Ba2~(Re04), (6.2)
This equation has the advantage over the previous one that the barium con-
taining molecule on the right of the equation retains a partial ionic character that
permits it to serve as a “transfer station” additional migration of perrhenate an-
for
ions.
A model is proposed to account for the capability of these materials to be ef-
ficient emitters of ions:
1. The singly charged ion needs to be presynthesized, paired with a suit-
able counter ion (all successful systems studied to date have the ion of
interest singly charged and the counter ion doubly charged), and em-
bedded in a matrix that allows the singly charged ion to migrate away
the
from the doubly charged counter ion when heated. Otherwise, entire
neutral molecule can sublime, bypassing the channel for ion emission.
2. The ion of interest needs to be stable in the matrix. This places various
requirements on redox properties of the matrix for each type of ion.
3. There needs to be some mechanism available for ~aintaining charge
neutrality in the lattice after ion migration. Electron migration between
the filament and the power supply can maintain charge neutrality, but
this alone cannot satisfy the bonding requirements of the counter ion.
4. For a cation emitter, an oxidation reaction within the matrix is required
to generate a new cation to satisfy bonding requirements of the anion
the