Page 279 - Inorganic Mass Spectrometry - Fundamentals and Applications

~nalysis Noncond~ct~ve Sample Types 265
of
lyte ion beam intensities. ~~o~unately, the background spectra for both materials
of
are quite complex, with many metal dimers present at high levels. Each the first
three matrices tends to yield mass spectra that have a large amount metal oxide
of
MOH+) because of the presence of residual gases.
and hydroxide ions (MO+ and
To counter the negative effects of water vapor on the observed spectra, alu-
minum, iron, and tantalum (metals with high affinities for water and air species)
act
were evaluated. These “getter” metals tend to as water scavengers as volatile
the
molecules evolve into the gas phase during the early sputtering times as the sam-
ple temperature rises. The removal of residual vapor produces mass spectra that
are more atomic in nature and also enhances the atomi~atio~excita~on processes
for the desired analyte species. Iron, being multi-isotopic, produces a more com-
plex mass spectrum than aluminum and tantalum and also yields greater amounts
of dimer and argide ions. Other getter elements such as zirconium and titanium
have too many isotopes in themselves; therefore they were not seen as viable can-
didates. Of the two remaining candidates, tantalum has a high mass and higher
sputtering rates that make it the most effective matrix. Across the range of matri-
ces and analytes studied by Tong and Harrison [39], data were tabulated to depict
the direct relationship between the relative amount of the oxides of a given ele-
ment (specifically the MO+/(M+ + MO+> ratio) and their respective metal-oxide
bond energies. As would be expected, the rare earth elements are most affected by
the presence of water vapor and air, and some of these elements show as much as
a 10% oxide ion fraction [39]. Even so, as shown in Fig. 7.1, high-quality spectra
were obtained with the use tantalum as the host matrix for a mixture of rare earth
of
oxides.

Just as the previously cited work: illustrated the role of matrix selection in the qual-
ity of the mass spectra, Mei and Harrison E401 studied the underlying equilibrium
involved in the formation and removal of metal oxides from the spectra of com-
pacted oxide samples. The analyte for these studies was La20,, chosen because of
the high affinity of lanthanum for atomic oxygen, a product of the dissociation of
both residual air and water vapor in compacted samples. Because La-0 bond
the
strength is very high (8.30 eV), this oxide represents a case in which the dissoci-
ation of the original “analyte” would be difficult as well. On the basis of previous
50% of the lanthanum species sput-
studies [41], it was assumed that approximately
tered from the surface would be free La atoms, and the other half would exist as
oxides of one form or another.
The GD plasma enviro~ent is an interesting vessel to study gas-phase
chemical properties. The gettering qualities of carbon, silver, tantalum, titanium,
and tungsten were compared by Mei and Harrison E401 on the basis of the atomic
ion fraction of all of the observed lanthanum species; R = La+/(La+ + Lao+) X