The size of the sample needed for XRF analysis depends on the type of sample and the specific requirements of the analysis. For solid and powdered samples, a flat and clean surface of at least 32 mm or 40 mm in diameter is typically required. For powdered samples, the optimal grain size should be less than 75 µm to ensure a homogeneous mixture. Liquid samples can be measured directly without specific size requirements.
Solid and Powdered Samples: For solid samples, the primary requirement is a flat and clean surface for measurement, typically with a diameter of either 32 mm or 40 mm. This ensures that the XRF instrument can accurately analyze the elemental composition across the entire sample area.
Powdered samples require additional preparation to ensure homogeneity and accurate results. The sample must be ground to a fine powder, with an optimal grain size of less than 75 µm. This fine grinding helps in achieving a uniform distribution of elements within the sample, which is crucial for accurate XRF analysis. After grinding, the powder is poured into a cuvette to form a flat, even surface without voids between the grains. The pressing procedure involves applying a load to compress the powder into a solid pellet. The required load varies depending on the type of sample, ranging from as low as 2 tons for foodstuffs to up to 40 tons for mineral ores.
Liquid Samples: Liquid samples can be analyzed directly using XRF without specific size requirements. The method is not sensitive to the state of aggregation, allowing for straightforward measurement of liquid samples.
Special Considerations: In some cases, such as when analyzing iron in a sample, using Tungsten Carbide die pellets can help avoid contamination. Additionally, for samples requiring better homogenization, a fused bead technique can be used, where the sample is mixed with a flux and heated to high temperatures. However, this method may dilute trace elements, affecting their detection.
In summary, the size and preparation of the sample for XRF analysis depend on whether the sample is solid, powdered, or liquid. Solid and powdered samples require a specific surface area and preparation to ensure accurate analysis, while liquid samples can be measured directly. Proper preparation, including grinding and pressing, is essential for obtaining reliable results from XRF analysis.
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