Page 226 - Inorganic Mass Spectrometry - Fundamentals and Applications

Secondary Ion Mass Spectrometry 21 1

S to measure isotopic ratios is one of its great strengths in sur-
face analysis. This capability may be utilized in two ways: first, to determine iso-
topic ratios in dating (geological chronology), cosmology, nuclear physics, chem-
istry, monitoring of nuclear enrichment activities, etc., and second, to study or
monitor diffusion, corrosion, or reaction mechanisms in solid materials through
stable isotope substitution.
easure~ent of isotopic composition in a mass spectromete
not as simple a process as might be perceived at first glance. In S
urements are complicated by isotopic fractionation, which is gene
and characterized by enrichment of the lighter ions with respect to the heavier ions.
art [ 1361 found that secondary ions were always enriched in the
lighter ions and the fractionation per atomic mass unit (amu) followed a depend-
ence proportional to high mass/low mass (M~/M~). E~ichment factors ranged
from 6.5%/amu for boron to 0.6%/amu for lead. They also found that fractiona-
tion was dependent on the spatial location and kinetic energy of the extracted ions
relative to the sputtered site. Thus, reproducing instrument parameters is impor-
tant. Schwarz [ 13-73, however, found negative fractionation coefficients for M~+,
M", and M2" ions in certain energy ranges. Gnaser and Hutcheon [l381 found that
of
isotope fractionation depended linearly on the inverse velocity the ejected ions.
They showed that this correlation followed from an exponential dependence of the
ionization probability, P, on the inverse velocity, v-l, such that
exp (q)

P (4.11)

In experiments on B, Si, and Ca, they found a value of -2 X lo6 cm/sec for vg.
Lyon and Saxton [ 3391 found fractionation due to sample chemical characteristics
and c~stallograp~c orientation in some cases but stated that with extensive cali-
bration and careful control parameters, reproducible isotopic ratios could be de-
of
tennined to one to two parts per thousand.
An illustration of the power of using isotopic substitution with SIMS to study
reactions on the surface, and in the solid near the surface, involves isotopic sub-
stitution on the solid surface. The differences in the oxidation and fluorination of
nickel were illustrated by depositing a thin layer (-10 nm) of 62Ni-enriched nickel
to
on the surface of a nickel substrate, exposing the surface the reactant, and depth
profiling the 62Ni/58Ni isotopic ratio [140,141]. In the oxidation case, the profile
at
showed a low ratio of 62Ni/58Ni the surface, then a higher ratio of 62Ni/58Ni as
the oxide was sputtered through, and finally the normal substrate ratio. This showed
that Ni atoms diffuse through the oxide to react at the surface with the oxygen. In
the fluorination case, however, the 62Ni layer was not displaced as fluorine diffused
through the fluoride layer to react at the metal-fluoride interface.