Fusion beads are prepared by mixing a finely powdered sample with a flux in a specific ratio and then heating the mixture to a high temperature in a platinum crucible. The process involves several key steps:
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Sample Preparation: The sample must be finely powdered, typically less than 75 micrometers in size. This ensures a more uniform mixing with the flux.
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Mixing with Flux: The powdered sample is mixed with a flux, usually a lithium tetraborate or a mixture of tetraborate and metaborate. The flux-to-sample ratio ranges from 5:1 to 10:1. This ratio is critical as it determines the homogeneity of the final bead and the efficiency of the fusion process.
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Heating: The mixture is heated to temperatures between 900°C and 1000°C in a platinum crucible. This high temperature is necessary to dissolve the sample completely in the flux, creating a homogeneous liquid mixture.
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Casting: The molten mixture is then poured into a mold with a flat bottom. The mold is typically made of platinum to withstand the high temperatures and corrosive nature of the molten mixture.
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Cooling and Solidification: After casting, the mixture cools and solidifies into a glass disc or fused bead. This bead is a homogeneous representation of the sample, free from any mineral structures.
The benefits of this method include the reduction of mineralogical or matrix effects, leading to more accurate analyses. Additionally, it allows for the combination of several different matrix types into the same calibration curve. However, the method also has downsides, such as relatively high sample dilution, which can affect the analysis of trace elements, and higher costs associated with the equipment and materials needed.
The typical thickness of fused beads, around 3mm, can lead to issues with infinite thickness for heavier elements. The initial costs for equipment and platinumware are higher, but the cost per sample to prepare is similar to that of pressed pellets.
In summary, fusion beads are prepared through a meticulous process of mixing, heating, and casting, which results in a homogeneous sample suitable for accurate analysis, albeit with some trade-offs in terms of cost and complexity.
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