The most common way of preparing a powder sample is the borate fusion process. It includes fusion of the sample specimen with an excess of lithium tetraborate and casting into a glass bead with a flat surface. During the fusion process the phases of the sample are converted into glass-like borates leading to a homogeneous bead perfectly dimensioned for XRF analysis.

In a first step, the fine ground sample is mixed with a borate flux (usually lithium) in a 95% platinum, 5% gold crucible. Then the crucible is heated to temperatures above 1000°C until the sample is dissolved in the flux. Agitation of the molt is used to support homogenization during fusion. A non-wetting agent (bromide. Iodide or fluorine) may be added to reduce sticking of the melt to the platinum ware.

If the sample is not completely oxidized it is usually mandatory to add an oxidizer and start oxidizing at lower temperatures before fusion. Not oxidized metals may form a eutectic alloy with the platinum leading to lowering of the melting point and subsequently destruction of the crucible.

Improving analysis results:

Sample preparation by fusion provides a very high level of analytical accuracy which is due to several reasons. First, unfused samples of identical composition may differ from each other with respect to mineralogy and particle size. This can cause different elemental count-rates. Fusion of samples usually eliminates these factors and increases accuracy. Secondly, the fusion process requires dilution of the sample. This leads to reduced inter-element interaction and matrix effects. Thirdly, fusion significantly facilitates calibration. On the one hand it is possible to produce perfectly matrix-matched standards for a wide range of material. On the other hand, synthetic standards can be applied if reference standards are not available. Accordingly, standards can be synthetically prepared for almost any type of sample avoiding multiple regression analysis in order to establish calibration curves.

Avoiding errors:

Fusion is a crucial step in sample preparation for XRF, AA and ICP analysis. Fusion is an excellent method for avoiding systematic errors that may adversely impact the precision of analysis results. Fusion is the simplest and most reliable approach to eliminate errors due to inhomogeneous grain size distribution, mineralogical effects or surface finish of the sample.

Improving sample dissolution:

Fusion can easily dissolve oxide samples that are usually difficult to prepare by acid digestion. Conventional acid digestion of resistant materials like, e.g., silica, alumina, zirconia and others takes a long time and often leads only to partial dissolution. Complete dissolution of the sample is an important determinant for improving the accuracy and reliability of analytical results.

Perfectly prepared for XRF analysis:

Sample fusion produces a glass bead optimized for XRF analysis instruments. The glass bead is perfectly dimensioned, shows an excellent homogeneity and surface flatness.

Saving time:

A typical fusion procedure rarely lasts longer than 10 minutes. In contrast, acid digestion procedures may take hours before satisfactory results can be achieved.

Safe procedure:

Fusion is a safe sample preparation method free of potentially hazardous acids and reagents requiring special health and safety measures. Fusion is safe especially if it takes place in an instrument with automated handling, fusion and melting.