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HOW TO PREPARE POWDER SAMPLES FOR XRF ANALYSIS

X-ray fluorescence spectrometry often requires the sample to be in a homogeneous powdered form with a planar surface. Although this can be accomplished by spreading the powder on the window film of a cell designed for liquid samples (such as a SPEX SamplePrep X-CELL®), compression of the powder in a pellet die yields a denser, flatter surface that provides greater analytical accuracy and sensitivity, especially for wave length XRF.

There are many different procedures for preparing powdered samples for XRF analysis. Typically, a representative quantity of the sample is first pulverized, and then split to obtain enough powder for an XRF sample disk, usually 6 to 10 grams. That powder is blended with a binder and placed in a pellet die (with or without a Spec-Cap®). The sample is then pressed into a sample disk which will hold together and has a flat, compositionally uniform surface. This disk is then placed in the sample holder of the XRF spectrometer.

An alternate technique, particularly useful when only a small amount of sample is available, incorporates a thin layer of sample on a disk of binder. The SPEX SamplePrep Sleeve-and- Plunger set, used with the appropriate pellet die, makes this procedure easy.

In the end, an exact procedure must be developed for each type of sample. The sampling method and the amount of sample to be ground, the type of mill and the grinding time, what size of die and what pressure to use, whether to include a binder or press the disk in a Spec-Cap®, all these details may vary and must be worked out by the analyst to suit his or her samples, equipment, and analytical requirements. Running a series of standards or known samples will help to confirm that your chosen procedure results in accurate, reproducible data.  An alternative to pressing pellets is fusion, for information on this technique please see our borate fusion page.

PARTICLE SIZE EFFECTS

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 disk 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 on the next page. 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.


 

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