Sample Requirements
We
have on-site facilities for preparing samples of most types of materials. To assist you in evaluating the
appropriateness of your materials or prepared samples for analysis in our facility,
we offer the following summary of basic sample requirements.
Basic Sample 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 bream
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 under side of the
sample holder. This limits the
sample mounts to few specific types that can mate with our sample holders: perfect cylinders (1", 1.25”, 1.5”,
2”, 1/4", or 3/32" in diameter, with analytical surface normal
to the cylinder axis), 1" x 2" (petrographic) polished thin
sections, 2" x 3" polished thin sections, 1" polished
circular microprobe thin sections.
Sample Size
Sample size is the last principal
consideration, being constrained for our instrument by the physical dimensions
of the stage and sample holders. Samples can be no thicker than about 19 mm
(3/4"). We have a variety of holders that can accommodate samples up to
about 2 inches in diameter. Among these, we currently have holders for up to
three 1"x 2" polished petrographic thin sections, up to six 1"
round samples (thick sections or cut cylinders), 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. If you
have a large number of samples for which we do not have an appropriate holder,
one can be fabricated. Finally, consider that analytical time on the instrument
will be optimized by loading as many samples as possible into the machine at
one time (reducing time spent on sample changes). Thus, your time in the lab
will be made more efficient by using the smallest sample size that you feel is
representative of the materials that you wish to examine.
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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