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 holds up to three 1" diameter samples with maximum thickness of ~1/2". Samples are typically polished grain mounts, thick sections, or thin sections although other samples can be accommodated. Note: only a 0.8" diameter circular area can be analyzed (see figure). In addition, the field inhomogeneity caused by the finite thickness of the tantalum cover plate prevents analyzing within ~0.05" of the plate, reducing the useful sample area to ~0.7" in diameter.

Grain assemblages should be mounted in epoxy resin. 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. Thick sections are preferred over thin sections, especially when the samples are porous and where the mounting medium (epoxy) may interfere with the analysis (e.g. C-isotopes in carbonates). 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. For grain mounts, thin the 1" epoxy disc using a lathe or cut-off saw, to between 3/16" - 1/4" thick. 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 is to produce a flat analysis surface. 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 (<50mm) grains. Diamond, alumina, or silica polishing media may be used. Typically, a brief, final polish with 1mm 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.

Sample Coating For almost all work in the geological sciences, samples must have an electrically conductive coating which makes contact with the tantalum cover plate on the sample holder. Either gold or carbon coats are used. Gold is preferred, being more conductive and more easily sputtered than carbon. Clearly, carbon should be avoided if C-isotopes are to be measured. The carbon coat thickness should be similar to that used for electron probe microanalysis; gold coats should be 20-40nm.