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Sample Requirements

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Sample Requirements

Basic Requirements

There are only a few requirements that samples must meet for imaging or analysis:

  1. 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.
  2. 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.
  3. 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.
  4. 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

Top referenced samples for analysis by WDS or BSEI

Sample sizes

1 - 1" round polished microprobe thin section

2 - holder for six 1/4" cylinders (usually epoxy cast in brass tubes)

3 - holder for forty 3/32" cylinders (usually epoxy cast in brass tubes)

4 - 1" thermal plastic cast cylinder

5 - 1" epoxy or acrylic cast cylinder

6 - 1" epoxy cast in phenolic plastic ring form

Mounts for loose grains or fragments; Samples cannot protrude more than 1-2 mm above mount surface; Samples can be affixed with adhesive (double-sided carbon tape, carbon putty, or glue)

Sample sizes

7 - 1" carbon planchette

8 - 1" x 1/4" tall aluminum stub

9 - 1" x 1/2" tall aluminum stub

10 - 1" x 3/4" tall aluminum stub

11 - 1" x  3/4" brass stub with 1/16" deep recess (~1/2" across) for medium grains

12 - 1" x 3/4" aluminum stub with 3/16" deep recess (~3/4" across) for tall grains