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Overview
The most common questions we get from visitors concern sample
preparation. These questions are generally in three areas: sample mounting, polishing, and coating.
Sample Mounting
The UCLA Cameca IMS 1270 accommodates up to three sample holders that are
designed to accept 1" diameter cylindrical samples with a maximum
thickness of ~1/2". The sample is held in place against a machined lip
by nonmagnetic
springs and backing plates (see below). Samples generally
consist either of 1" round, glass-mounted, polished petrographic
sections, polished grain mounts potted in epoxy, or composite mounts
composed of these materials. Thick petrographic sections are preferred
over the standard 30µm variety, especially when the samples are
porous and where the mounting medium (epoxy) may interfere with the
analysis (e.g. C-isotopes in carbonates). Although preservation issues
may dictate that entire 1" round thin sections must be employed,
we find that (when possible) it is more efficient to produce a composite mount
in which relevant portions of polished thin sections (i.e., those
containing planned analysis points) are trimmed down via diamond coring
or a wafering saw and juxtaposed together with pre-polished standards. Grain mounts are
produced by positioning grains upon double-sided tape and potting in
epoxy. Finally, more irregular shaped polished samples can
also be accommodated using special sample holders
with Ta masks.
Although the OD of the sample mounts are 1"
round, it is important to emphasize that the entire upper surface of
the sample mount is not accessible for analysis. The
sample holder have a 0.1" wide lip machined onto their upper surface to
facilitate uniform positioning of the sample. This blocks the
outer 0.1" of the mount. In addition, the field inhomogeneity
caused by the finite thickness of the sample holder lip effectively
requires analysis points to reside in the inner ~0.7" of the sample
mount.
In producing epoxy mounts, the resin should be of
low viscosity to reduce risk of air bubbles being trapped next to the
grains, and should be hard when cured to facilitate polishing. We use
3M® 701DL double-sided tape for mounting and produce air-cured
mounts by mixing 5 parts (by weight) Buehler®
20-8130 epoxide resin with 1 part (by weight) Buehler®
20-8132 epoxide hardener and allowing 6-8 hours to cure. A thin
coat of Buehler®
20-8185 release agent applied to the confining 1" ID ring prior to
pouring the epoxy mixture facilitates later removal. No release
agent should applied to the mounting surface. Epoxy out-gasses a
significant volume of water vapor in the vacuum of the analysis
chamber. For work where hydride interferences are a concern (e.g. 17O
measurements) the following steps should be taken to minimize
out-gassing. Store samples in a warm oven (50°C), a desiccator or,
if possible, a vacuum oven. If samples can be left in the instrument
overnight before analysis begins, hydride background will be further
reduced.
Sample
Polishing
The primary objective of polishing in producing SIMS mounts is to
produce a flat analysis surface. Avoid excessive polishing!
Prolonged polishing with sub-micron polishing compound may yield a fine
polish but will also tend to "round" grains in grain mounts and
excavate trenches around them, hindering analysis near grain boundaries
or the analysis of small (<50µm) grains. Diamond, alumina, or
silica polishing media may be used. Grain mounts can be effectively
sectioned through the use of grinding paper. Typically, a brief,
final polish with 1µm diamond yields an adequate polish without
compromising flatness. Do not use a polishing medium which will
contaminate the sample with the elements to be studied. After polishing, ultrasonically clean the mount with soapy
water in clean glassware. Handle the cleaned sample with gloves and
rinse with DI water. In the case of zircon U-Pb analysis,
ultrasonically clean the sample in 1N HCl for 1 minute to reduce the
common Pb contamination (omit this step for monazite). Final cleaning
consists of 5 minutes ultrasonic treatment with DI water in clean
glassware. Loosely cover the mount with Al foil and place it in a
drying oven at 50-75ºC for a least one hour.
Sample Coating
For almost all work in the geological sciences, samples must have an
electrically conductive coating which makes contact with the lip on the
sample holder. While either gold or carbon may be employed, gold is
preferred since it is more conductive and more easily sputtered than
carbon. Gold coats should be 20-40nm. The thickness of carbon
coats should be much thinner than that used for electron probe
microanalysis (indigo blue on polished brass surface) since C sputters
very slowly. Carbon should be avoided for mounts prepared for
U-Th dating because of an isobaric interference on mass 246 (the
230Th-oxide peak). Clearly, carbon coating is also not suitable if
C-isotopes are to be measured.
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