Page 229 - Inorganic Mass Spectrometry - Fundamentals and Applications

2 14 Cristy

by
a measurable degree. Thus, the inhibition oxygen occurs in the reaction process
at the oxide-metal interface, not by blocking reaction at the oxide surface.

Surface diffusion is an area in which SIMS is unmatched because of its good res-
An illustration of this was an experiment to determine
olution and high sensitivity.
of
the diffusion of molecular oxides over the surface salt to ascertain whether sur-
face diffusion would be a serious pathway for migration of plutonium oxides or
daughter products from storage locations in salt formations [145]. In the experi-
ment, La,O, was substituted for plutonium oxides and SrO for daughter products.
L%O, and SrQ (600 and 1400 nm thick, respectively) were deposited on a pol-
ished NaCl substrate by using an electron beam. Each sample was heated 693°K
to
for 473-920 hr. The samples were carbon coated to provide conductivity. An 0-
beam was used to sputter areas X 38 pm starting at a point distant from the edge
30
of the deposit and moving toward the deposit. The resultant concentration data
were superposed on plots generated from the usual one-dimensional diffusion
equation for a se~-infinite source. The fit of the data for SrQ annealed for 4’73 hr
is shown in Fig. 4.46. At the low end of the plot, SIMS is detecting ~l0-l~
g
of
SrQ. The diffusion rate of 2.0 X 10-lo cm2/sec indicated that surface diffusion
would not be a serious loss mode for salt formation storage.

The closely allied topics of secondary neutral mass spectrometry (S~S), fast
atom bombardment (FAB), and laser ablation SMS are important, but are beyond
the scope of this chapter. SNMS is a technique in which neutral atoms or mole-
cules, sputtered by an ion beam, are ionized in an effort to improve sensitivity and
to decouple ion formation from matrix chemical properties, making quanti~cation
easier. This ionization is commonly effected by electron beams or lasers. FAI3 uses
a neutral atom beam to create ions on the surface. It is often useful for insulator
analysis. Laser ablation creates ions in either resonant or no~esonant modes and
can be quite sensitive and complex.
Computer software and hardware are also important to the modern SIMS
laboratory for automated instrument control, data acquisition, data display, inter-
pretation, and quantitation.
S has reached the status of a mature technique, ins~menta-
ftware improvements continue at a rapid pace as range and
applications continue to grow. High detection sensitivity, high spatial resolu
three dimensions, detection of all elements, and isotopic capability give S
strong and unique position among surface analysis techniques.