The size of the sample for XRF (X-ray Fluorescence) analysis typically requires a larger sample surface, usually either 32 mm or 40 mm, depending on the type of die used. The choice of sample size and preparation method depends on the specific material being analyzed and the desired level of accuracy.
Sample Size and Preparation for Different Materials:
- Food Samples: These may only require 2-4 tons of pressure and can be prepared by grinding to ensure homogeneity.
- Pharmaceutical Products: These may require up to 20 tons of pressure, making them ideal for manual XRF presses. Preparation typically involves grinding and ensuring a flat, polished surface.
- Mineral Ores: These may require up to 40 tons of pressure. Preparation often includes grinding the sample to a fine particle size (<75 µm) and sometimes using fused bead techniques for better homogenization, although this method may dilute trace elements.
General Sample Preparation Techniques:
- Grinding: This is crucial for achieving a homogeneous mixture, ensuring that the analysis represents the entire sample rather than individual grains. The optimal grain size is <75 µm.
- Surface Preparation: For solid samples, a perfectly flat surface is ideal. Irregular surfaces can introduce errors by changing the distance from the sample to the x-ray source. Surface finish is also critical, especially for lighter elements, as rough surfaces can cause scattering and re-absorption of longer wavelength elements.
- Fused Bead Technique: This method involves mixing the sample with a flux (like lithium tetraborate) in specific ratios and heating to high temperatures. It is used when better homogenization is required but may not be suitable for detecting trace elements due to dilution.
Considerations for Sample Preparation:
- Sample to Source Distance: All XRF systems are calibrated based on a fixed sample to source distance. Any deviation can affect the intensity of the elements being measured.
- Energy Dependence: The effect of surface roughness on analysis is energy-dependent. For example, lighter elements like carbon or sulfur may be more affected by rough surfaces compared to heavier elements.
In summary, the size and preparation of the sample for XRF analysis depend significantly on the material being analyzed and the specific analytical requirements. Proper preparation techniques, including grinding, surface finishing, and sometimes specialized methods like fused bead preparation, are essential for accurate and representative results.
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