Analytical spectroscopic methods such as XRF, OES, and IR often require samples in the form of round, flat-surfaced discs. Although sample discs can be cast from a fusion melt, (refer to section on Fusion Products), they often begin as a powder and are pressed to shape in a pellet die.

The following “how-to” suggestions apply chiefly to XRF sample preparation, but the basic principles of forming sample discs are the same for OES, spark ablation, IR, etc. The major differences are the diameters of the discs, and the nature of the binders or additives. IR discs are 13 mm across and consist largely of pure potassium bromide (KBr) with the pulverized sample blended in; no Spec-Cap-type jacket is used, as the sample disc must be able to transmit infrared light. OES and spark ablation systems generally use 31 mm discs. As the disc must be electrically conductive for these techniques, most samples are blended with 50% pelletizing-grade graphite, and often pressed in an aluminum Spec-Cap. The diameter of XRF sample discs is dependent on the size of the spectrometer sample holder, which may be 31 mm, 35 mm, 40 mm, or even larger. XRF discs do not require a binder or Spec-Cap if the sample coheres under pressure, but most analysts use a binder or a Spec-Cap or both. The prime requirement for an XRF binder is that it does not contribute impurities; XRF binders include
cellulose, paraffin, graphite, orthoboric acid, polyvinyl alcohol, and proprietary products with special properties, e.g. Ultrabind.

For preparation of pellet samples, SPEX SamplePrep offers a full range of laboratory presses, pellet die sets, and die accessories. Presses include a 12-ton and 25-ton manual press, the 25-ton air-actuated Bench-Press, and motorized and automated models of the 35-ton
X-Press. Pellet dies are available in the standard sizes of 13 mm, 31 mm, 35 mm, and 40 mm. Accessories include: aluminum Spec-Caps to form and protect 31 mm, 35 mm, 40 mm pellets;
cellulose and paraffin Prep-Aid binders, as well as SPEX SamplePrep’s own UltraBind; and Sleeve-and-Plunger sets to form sample pellets with a thin layer of sample on a binder matrix.

While methods and mills for pulverizing are as varied as the samples themselves, pellet pressing requires simpler tools. SPEX SamplePrep presses, dies, and pelletizing accessories are sophisticated in design, but functionally simple and very effective.

Reproducibility is a cardinal virtue in sample preparation. SPEX SamplePrep dies and presses ensure production of uniform sample discs, whatever the sample or analytical technique.