The main difference between fused beads and pressed pellets lies in their preparation methods, homogeneity, and the specific advantages and disadvantages associated with each technique.
Fused Beads: Fused beads are created by mixing a finely powdered sample with a flux in a specific ratio and heating it to a high temperature in a platinum crucible. The sample dissolves in the flux, typically a lithium tetraborate or a mixture of tetraborate and metaborate, and is then cast into a mold. The resulting glass disc, or fused bead, is a homogeneous representation of the sample, free from mineral structures. This method reduces mineralogical or matrix effects, leading to more accurate analyses and the ability to include various matrix types in the same calibration curve. However, it involves higher initial costs due to the need for fusion equipment, platinum crucibles, and consumables. Additionally, fused beads are typically thin (about 3mm), which can lead to issues with the analysis of heavier elements due to infinite thickness problems.
Pressed Pellets: Pressed pellets are prepared by pressing loose powders into a ring or cup using a press machine. The choice of die type depends on the sample characteristics. Pelletization can be improved by pulverizing the sample or by adding a forming agent (binder) if the sample is difficult to pelletize. Pressed pellets are considered convenient, cost-effective, and reliable, making them an industry standard for sample preparation. They offer greater flexibility and lower costs compared to fused beads, but they may not deliver the same level of homogeneity and accuracy, especially in reducing matrix effects.
Comparison: While fused beads provide a near-perfect homogeneity and accuracy by eliminating mineralogical and matrix effects, they come with higher costs and potential issues with the analysis of heavier elements due to their thinness. Pressed pellets, on the other hand, are more cost-effective and flexible but may not offer the same level of homogeneity and accuracy, especially in terms of reducing matrix effects. The choice between fused beads and pressed pellets depends on the specific requirements of the analysis, including the need for accuracy, homogeneity, and budget considerations.
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