Sample Requirements
Basic Requirements
There are only a few
requirements that samples must meet for imaging or analysis:
- They (and their mounting media) must be stable under
high vacuum.
This restricts samples to solid materials, and prohibits some (primarily
organic) solids with high vapor pressures.
- They must be stable under the beam. Irradiation by a finely focused
electron beam can cause local heating of samples in the range of 100°
-200° C. For example, if you consider that the current density for a
focused (0.2 m m
diameter), 2 nA beam is on the order of 50 A/cm2,
it is easy to see why heating occurs. This makes the analysis very
difficult for some materials, especially organic compounds with large proportions
of water or other volatile components in their structures. For this
reason, many such materials must be well desiccated (dehydrated) or
de-volatilized prior to analysis.
- They must be electrically conductive (and grounded
to the stage).
This prevents the build-up of an electrical charge on the sample surface,
which will deflect the beam off of the sample. If your samples are not
electrically conductive, don’t worry. Most of the materials that we
analyze are very good electrical insulators. They can be made conductive
by the deposition of a thin (~200 Å) layer of amorphous carbon on their
surfaces by vapor precipitation in our prep lab.
- For quantitative analysis by WDS or quantitative
analysis of backscattered electron images (BSEI), the samples must be flat
and well polished (final grit size of 0.25-0.05 mm). For WDS analysis, this is
because the x-ray take-off geometry must be fixed (note that pits or holes
will cause x-ray emergence from the sample at angles other than the proper
geometry). For BSEI, this is because surface topography causes variations
in signal intensity due to both thickness and edge effects. For
determination of pore volume in porous media by BSEI, impregnation of the
samples with epoxy is recommended. Because of these requirements, the sample
holder system of our microprobe uses a "top-referenced" method
in which the sample surface seats flat against the underside of the sample
holder. This limits the sample
mounts to few specific types that can mate with our sample holders: perfect cylinders (3/32”, 1/4”, 1",
1.25”, 1.5”, or 2” in diameter, with analytical surface normal to the
cylinder axis), 27 x 46 mm (petrographic) polished thin sections, and 1"
(25-26 mm) polished circular microprobe thin sections.
Sample Size
Samples need be no larger
than is necessary to be representative of a material, and are constrained in
size for our instrument by the physical dimensions of the stage and sample
holders. Samples can be no thicker than about 13 mm (~1/2"), but a variety
of sample footprints can be accommodated. We have holders that can fit samples
up to about 2 inches in diameter. Among these we currently have holders for up
to three 27x 46 mm polished petrographic thin sections, up to six 1" round
samples (thick sections or cut cylinders), two cylinders with 1.25” or 1.5”diameter, one 2”cylinder,
and several devices that will hold small samples which we commonly mount (by
casting in epoxy resin) in ¼" (up to six per device) or 3/32" (up to
43 per device) brass tubes. Small loose samples for surface analysis or imaging
can be adhered to 1” diameter stubs (with or without a central well for
particles more than 1 mm tall).
Samples submitted by private individuals
for the purpose of material identification must be no larger than 5 mm x 5mm x
2 mm; we cannot cut samples for identification from larger bulk specimens.
Examples of
Sample Holders and Appropriate Mounts
Click on the following links
to see more about:
1-inch diameter samples
Larger samples
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