To make a sample disc, you need a sufficient quantity of homogeneous powder, often a mixture of thoroughly pulverized sample material and an appropriate binder; a pellet die set, including two pellets of polished steel or other hard, smooth material for sandwiching the sample powder; and a machine for exerting the necessary pressure on the powder. An optional aluminum cup such as the Spec-Cap contains the powder, protects the die bore against abrasive samples during pressing, and subsequently protects the pellet against chipping or breaking. An alternate technique, which yields a durable pellet of solid binder with a thin layer of sample on one side, involves the use of a sleeve-and-plunger set with a pellet die.

The powder to be pressed must be fine-grained and homogeneous, in any case. If the pulverized sample material is not readily compressed into a stable, cohesive disc, it may be necessary to add a binding/ briquetting agent such as cellulose, paraffin, or UltraBind. A binder can be uniformly blended with the sample or used as a backing for a thin layer of sample. Binders when properly used enhance analytical accuracy by stabilizing the sample disc and keeping it from distorting or crumbling during analysis and/or storage. See “Binders,” for tips about the selection and use of these important pelletizing aids.

In an ideal sample disc pressed for XRF, the analytical surface of the disc is not only flat but also uniform, and truly representative of the original sample. This is true whether the sample was pressed without binder in a Spec-Cap, blended with a binder before pressing, or pressed in a thin layer on a binder matrix. Compact, flat, stable, and uniform sample discs can be ensured by the use of reliable SPEX SamplePrep dies, presses, accessories, and PrepAid binders.

While the pressing of powders into discs for XRF analysis is, in all its details, a fairly intricate exercise, it can be routinized to yield consistent, reliable analytical results.