In pressing samples for XRF, the loaded pellet die is placed in a hydraulic press, and the pressure raised to a level that will cause the sample or sample/binder mixture to cohere into a stable sample disc. Manual and motorized hydraulic presses are available for this task: SPEX SamplePrep offers 12-ton and 25-ton manual SPEX SamplePrep-Carver presses, the 3628 air-actuated Bench-Press, and the 3630 Automated 35-ton X-Press. Any SPEX SamplePrep pellet die will fit these presses.

A basic pressing sequence consists of raising the pressure to a specific level, holding it there for a certain length of time, and then releasing it, preferably slowly.

The maximum pressure for a given task varies considerably, depending on the size of the die and the nature of the sample. Obviously maximum pressures should not exceed the load limits of the
die; the 13 mm die has a 10-ton limit, so some care must be exercised, but 31 mm and larger dies usually have load limits higher than either the capacity of the press or the pressure required to form a sample disc. Typical pressures for a 31 mm disc are from 20 to 25 tons; for a 35 mm disc, from 22 to 30 tons; and for a 40 mm disc, from 25 to 35 tons. Some samples cohere adequately at low pressures, but uniform high pressure is recommended. As “infinite depth” is very shallow for XRF analysis of most elements, a matter of microns, compaction of the sample decreases pore space and increases analytical accuracy.

Holding time and bleed time are both important. If a sample is simply brought to maximum pressure, and the pressure is abruptly released, the sample disc often does not hold together. This may be due to elastic rebound of gases trapped in the sample, or because a binder may take time to completely penetrate the sample, or for other reasons. A holding time at maximum pressure of at least 30 seconds is recommended; some analysts hold pressure for 5 minutes or more. During holding time the pressure should be maintained as well as possible; an advantage of the 3630 Automated X-Press is that it turns on the pump if the pressure drops more than 1 ton during the holding period.

A gradual release of pressure after the hold period is perhaps even more important than pro-longed holding time. A minimum bleed time of 15 seconds is recommended; for samples that do not bind well, several minutes may be appropriate. A slow, careful bleed period can be difficult to accomplish with manual presses, as the pressure release control is often not sensitive, but it is still never a good idea to dump pressure abruptly in any pelletizing procedure. One significant
advantage of the 3630 X-Press is that lengthy hold times and precisely controlled pressure release can be programmed in, and will remain the same, sample after sample. The overall uniformity of the sample discs (and hence of the analytical results) will inevitably be greater.