The Use of the Vacuum in Pellet Pressing All SPEX SamplePrep evacuable pellet dies have a vacuum hose attachment, which enables a vacuum pump to be hooked up to the die before and during a pressing operation. Most samples contain gases, moisture, and pore space, and removal or reduction of these can in fact affect the stability and uniformity of the sample disc. In the case of 13 mm dies, which are primarily used to produce KBr discs for infrared spectr-ometry, the use of a vacuum is necessary to draw moisture out of the KBr. Most XRF analysts do not bother to evacuate their 31 mm, 35 mm or 40 mm dies during pressing, but in fact the withdrawal of air and moisture from the sample can improve disc compaction and quality. Troublesome samples will often benefit from this technique. When evacuating a die, it is advisable to make sure both the upper and lower O-rings are in place and in good condition.