Fused beads for XRF analysis are prepared by mixing a finely powdered sample with a flux at a specific ratio, heating the mixture to a high temperature, and then casting it into a mold to form a homogeneous glass disc. This method is particularly effective for reducing mineralogical or matrix effects, leading to more accurate analyses.
Summary of the Preparation Process:
- Sample Preparation: The sample is ground to a particle size of less than 75 µm to ensure uniformity and to facilitate dissolution in the flux.
- Mixing with Flux: The finely ground sample is mixed with a flux, typically a lithium tetraborate or a tetraborate/metaborate mixture, at a ratio of 5:1 to 10:1. This high ratio helps in dissolving the sample completely and creating a homogeneous mixture.
- Heating: The mixture is heated in a platinum crucible to temperatures between 900°C and 1000°C. This high temperature is necessary to melt the flux and dissolve the sample, ensuring a uniform distribution of elements.
- Casting: The molten mixture is then poured into a mold with a flat bottom, where it solidifies into a glass disc or fused bead. This bead is a homogeneous representation of the sample, free from any mineral structures.
Detailed Explanation:
- Sample Preparation: Grinding the sample to a fine powder is crucial as it ensures that the sample is evenly distributed in the flux. This step is essential for achieving a homogeneous final product, which is necessary for accurate XRF analysis.
- Mixing with Flux: The choice of flux and the ratio of flux to sample are critical. Lithium tetraborate is commonly used due to its ability to dissolve a wide range of sample types and its low melting point, which aids in the fusion process. The high flux-to-sample ratio ensures that the sample is thoroughly dissolved and mixed, minimizing the risk of analytical errors due to inhomogeneities.
- Heating: The high temperatures used in this step are necessary to melt the flux and dissolve the sample. Platinum crucibles are used due to their high melting point and resistance to chemical reactions, ensuring that the crucible does not contaminate the sample.
- Casting: After heating, the molten mixture is poured into a mold. The shape and size of the mold can vary, but a flat bottom is typically used to facilitate analysis. The cooling process must be controlled to prevent cracking or other defects in the bead.
Review and Correction: The reference mentions that fused beads can be susceptible to infinite thickness issues for heavier elements and that the technique has a higher cost associated with it. These points are accurate and should be considered drawbacks of the fused bead method, particularly for samples requiring analysis of trace elements or for laboratories with budget constraints.
The reference also mentions that a food sample may require 2-4 tons, a pharmaceutical product may require 20 tons, and a mineral ore may require up to 40 tons. These figures seem to be a typographical error as they are unusually high for sample preparation in XRF analysis. It is more likely that these figures refer to the pressure required to prepare pressed pellets, not fused beads. In the context of fused beads, the sample amount is typically much smaller, usually just a few grams.
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