For maximum precision in XRF analysis, the surface of the sample disc should be very flat and very clean. It follows that these discs should be handled carefully, to avoid both surface con-tamination and damage. Sample discs should be handled only by their edges, and preferably not with bare skin; sweat, skin oils, and incidental contaminants are easily transferred to the anal-ytical surface of the sample disc.

Physical damage to the disc surface also affects accuracy: scratches, chips, pits, and swelling or curving should all be watched for and guarded against. As a general rule, it is best to analyze a sample disc as soon as possible after it is pressed; if it must be kept as a reference sample or standard, it should be preserved in a desiccator. Most pressed powders will absorb moisture with time, and swell; some samples exhibit a kind of elastic rebound shortly after pressing.(Flatness can be checked by holding a sample disc against a polished pellet.) Sample discs can also be degraded by X-rays. Even the heat produced by the X-ray beam can cause small flakes or particles to break loose from the face of the disc, leaving the disc with an irregular and possibly non-representative sample surface, and contaminating the spectrometer.

There can be sample discs which, after the use of binders and the most careful pressing procedures, will still degrade in the spectrometer. The most common remedy is to place a piece of XRF window film (e.g. SPEX SamplePrep 4 micron Ultralene) where it will protect the spectrometer; one ingenious solution has been to place a pressed 31 mm sample disc inside a SPEX SamplePrep 3527 40 mm X-Cell.

If a pressed sample disc is to be used as a standard, it is advisable to have that standard made in duplicate, with the spare disc carefully preserved. The standard in daily use should be periodically compared with the standard kept in reserve; often the first symptom of sample
disc degradation is a decline in analytical accuracy.