It is impossible to say a priori how fine to pulverize a given sample. It has been known for many years that the X-ray fluorescence intensity from a sample will increase as the particle size of the sample is decreased. This is due to the reduction in the size and extent of the voids in the sample. By the same reasoning, as the particle size of one of two sample components is decreased, it will yield a higher intensity relative to the component of fixed particle size. Further, if the particle size of both components is decreased, their respective intensities may increase or decrease depending on their relative absorption coefficients. Fortunately, when the particle size becomes small enough, the intensities stabilize.

As an X-ray enters a pelletized sample disc the exciting radiation is absorbed by the matrix. When a particle within the matrix absorbs radiation, the resulting fluorescence is also absorbed, in whole or in part, by the matrix. Hence there is a limit to the depth to which the existing radiation can penetrate and result in fluorescence emitted from the sample. This depth is usually called the critical or infinite thickness.

As particle size becomes small relative to the critical thickness, fluorescence intensities emitted from different sample components approach stable values. This is shown graphically below. In case A, the average penetration of the X-ray is of the same order of magnitude as the large particles, and a change in the size of these particles would have a substantial effect on the fluorescent intensity. This results from the filling of the voids by components with smaller particle sizes. In case B, since the average penetration depth covers many particles of many components, changing the particle size should have little or no effect.

In practice, the limiting particle size is often determined empirically by grinding the sample for a given length of time, measuring the particle size, analyzing the sample, then repeating at longer grinding times until the intensities reach a plateau. SPEX SamplePrep offers laboratory mills, grinding vials, and sieve sets that are ideal for this procedure.