3.3.1. Samples

Most solids, both natural and synthetic, can be analyzed with the microprobe. These include rocks, ores, metals, alloys, ceramics, teeth, bone, and archaeological artifacts. However, because the electron optics require high vacuum (about 2 x 10^-6 torr), wet or sticky samples cannot be analyzed in the microprobe. Microprobe analysis is non-destructive, but some material is lost during sample preparation. Samples may be standard petrographic thin sections or materials embedded in epoxy resin mounts. The sample stage takes two sample holders. Holders accommodate either two 1" circular sections or a standard thin-section (Figure 3.3.1).

Figure 3.3.1. Sample holders.

All specimens have to be highly polished, or quantitative analysis will not be possible, and artifacts may be introduced into X-ray maps and electron images. Sample surfaces must also be cleaned so that they are free of dust, oils, polishing material, and then coated with a thin layer of graphite. The carbon coat is applied by evaporation of carbon under vacuum. The thickness of the resulting carbon coat is monitored by observing the interference colors it produces on a piece of polished brass. The resulting colors correspond to thickness; a coat of 250 Å is standard for microprobe analysis.

 

There is evidence that variations in the thickness of the carbon coat can affect the count rates of the lightest elements (probably up to Na). Thus care must be taken to ensure that the standards and unknowns have identical carbon-coat thicknesses.