Page 264 - Inorganic Mass Spectrometry - Fundamentals and Applications

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fore, condensable species must be ionized prior to striking the wall of the ioniza-
tion chamber as they condense onto the walls. This is Langmuir vaporization, in
contrast to Knudsen vaporization, in which the chamber walls are at the same tem-
perature as the sample, which measures the vapors in thermodynamic equilibrium
. with the surface. Both the miniaturized filament source and the tube source have
worked well in this instrument. On heating, neutral atoms and molecules are va-
porized from the surface and ionized by electron ionization, allowing the determi-
nation of the chemical species in the neutral vapor. This requires a specific set of
focusing voltages, which conveniently do not allow ions emitted from the surface
of the emitter to pass into the mass spectrometer. Additional discrimination against
any surface ions is obtained by biasing the emitter.
Both ions and neutrals volatilized into the chamber. Positive ions emitted
from the surface of the emitter are focused into the mass spectrometer with a to-
tally different set of focusing voltages. Negative surface ions are also focused into
_ the mass spectrometer, again with a unique set of focusing voltages, and of course
with the quadrupole voltages reversed and the detector configured for negative ions.
Computer control of the voltages greatly simplifies rotating among the three
modes. The filament producing electrons for El is turned off when surface ions are
being measured, further ensuring that ions of different origins are differentiated.
Two to four minutes is allowed for spectral scanning and data storage prior to se-
quencing on to the next mode. :
This instrument has allowed several studies that provide information not ob-
tainable by other means to be conducted. Four examples are presented as follows:
The first example concerns the question of the mechanism of emission of potas-
sium ions from potassium zeolite [7]. Earlier studies had made the assumption that
this was an S-L type of ion formation mechanism [8], implying that there was a
neutral potassium atom flux accompanying the flux of atomic potassium cations.
Experiments performed on this instrument clearly showed that this is not the case;
there was no detectable neutral atomic potassium flux accompanying the cation
flux. Thus this instrument was used to answer a long-standing question with an ex-
periment conducted in one afternoon and allowed the conclusion to.be reached that
the mechanism is potassium ions in the solid state subliming into the gas.phase.
The second example is the analysis of silver zeolite [7], in which it was shown
that there is a substantial silver atom flux accompanying the relatively weak silver
cation flux. There were no molecules or clusters containing silver in the gas phase.
Pure silver metal heated to the sublimation point gives primarily neutral atoms and
metallic clusters with no ions. Thus, sublimation of atomic silver ions from zeo-
lite may be a pseudo—S-L type of process, although additional evidence concern-
ing the species of silver in the solid state would be required prior to making this
assertion.
The third:example is the analysis of perrhenate emitters [3,9] discussed later
in this chapter. Pure Ba(ReO 43 volatilizes as neutral molecules, whereas the blend