Highlights
- Pulverizing powder samples to a consistent fine particle size—typically less than 50 μm—reduces grain-size and segregation effects and can significantly improve XRF accuracy and reproducibility.
- Pulverization conditions must be carefully controlled because grinding time, vessel material, pulverization aids, moisture, and contamination can all affect measured X-ray intensities and quantitative results.
- Vessel selection is particularly important for trace analysis: material worn from the vessel can contaminate the sample and may introduce spectral overlaps that interfere with analyte determination.
Summary
Sample preparation is a major potential source of error in X-ray fluorescence analysis of powders. Differences in particle size, mineralogy, and segregation can alter X-ray fluorescence intensity, making pulverization an important step when high accuracy and reproducibility are required. A particle size below approximately 50 μm is generally desirable. Pulverization is also useful before fusion because a finely ground starting material facilitates preparation of homogeneous fusion beads.
Vibration-type disk, rod, and ball mills are commonly used, while very small samples can be ground manually with an agate mortar. Pulverizing vessels may be made from alumina, tungsten carbide, chrome or special steel, or zirconia. The choice affects grinding efficiency as well as the possibility of sample contamination. Tungsten carbide, for example, provides high grinding capability but can introduce tungsten, cobalt, and carbon into the sample.
Pulverizing time generally ranges from tens of seconds to about five minutes, with harder materials requiring longer treatment. For quantitative XRF, standards and unknowns should be prepared using consistent sample amounts, grinding times, and other pulverization conditions. Ideally, pulverization time is established experimentally by determining when the measured X-ray intensity becomes essentially constant.
Pulverization aids can improve grinding when samples agglomerate or adhere to vessel surfaces. Liquid aids such as n-hexane enable wet pulverization and can be removed by drying, while solid aids remain in the sample and therefore require a constant sample-to-aid ratio. Binders used for pressed-pellet preparation can also be incorporated during pulverization.
Contamination must be controlled both between samples and from wear of the grinding vessel and rod. Cleaning between samples and pre-washing the vessel with a small amount of the next sample can reduce carryover. Vessel composition should also be considered when selecting equipment, especially for trace-element measurements. Tungsten contamination from a tungsten carbide vessel, for example, can interfere with determination of Hg, Se, and As through spectral overlap.
Pulverized samples also require appropriate handling after grinding. Their increased surface area makes them more susceptible to moisture absorption, so drying at 105–115°C for about two hours followed by storage in a desiccator is recommended. Materials susceptible to ignition require additional precautions, while samples containing components that resist uniform grinding, such as mica, may be better prepared using fusion rather than pressed-powder methods. Ultimately, the preparation procedure should balance the required analytical accuracy and precision against preparation time and effort.
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