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Borate Fusion

The basics

Borate fusion is a widespread sample preparation method for XRF, AA and ICP analysis. It outperforms many other procedures for the preparation of a wide range of materials including: carbonates, catalyst, cement, ceramic, ferroalloys, geological samples, glass, pure metals, silica and slag.

For XRF analysis, the production of an homogenous glass disk reduces effects associated with particle size, mineralogy and matrix to obtain better accuracy than pressed powders.

For solutions, borate fusion is a rapid and safe method to eliminate the use of concentrated and dangerous acids (such as HF) in sample preparation. The preparation time is shorter than conventional acid digestions. When using high purity fusion fluxes and appropriate analytical parameters, the fusion can easily become a powerful routine technique to prepare a wide range of samples.

Fusion is a method where an oxidized sample is dissolved in the molten flux at temperatures of around 1050⁰C. For optimal dissolution, the right flux composition must be used. Two basic formulations, lithium tetraborate (LiT or Li₂B₄O₇) and lithium metaborate (LiM or LiBO₂) are commonly used in various proportions.

A Simple Procedure

The Acidity Index

To ease the flux selection, we base the choice of the flux on the Acidity Index (Ai) of the sample. For a given oxide, this Index is the ratio between the number of atoms of oxygen and the atoms of metal. The higher the index, the more acidic is the oxide. For example, Al₂O₃ (aluminum oxide) would have an Ai of 1.5 (3 oxygen / 2 metal).

As a simplified rule, the tetraborate reacts with basic oxides while the metaborate reacts with acidic oxides. It means that if you are dealing with a quicklime sample (CaO, a basic oxide; Ai of 1), tetraborate would be the optimal flux.

It’s not always that simple and even if general guidelines can be derived for the acidity index of your sample, real samples are often a mix of several oxides and some calculations are involved to predict the best match between your sample and the flux. If you have complex sample, contact us and we would be glad to help. Careful selection of the flux yields better and quicker dissolution of the sample and shorter fusion time.

The figure below shows how to use the Ai in the selection of the right flux. In this example, we would use 100%LiT for CaO, 50%LiT/50%LiM for Al₂O₃ and 100%LiM for SiO₂. However, for the preparation of a glass disk, we tend to use 35%LiT/65%LiM instead of 100%LiM to avoid crystalization.

Additives in the Flux

Most fusion methods involve the use of non-wetting agents (nwa). These additives are incorporated to the flux to avoid sticking of the melt to the crucible and promote easier casting in the molds. Their final concentration in the glass disk can influence XRF results and by using flux with integrated nwa, you get the same concentration every time you prepare a sample.

When preparing solutions for wet chemistry, the use of a higher nwa concentration is preferred. The complete quantitative transfer of the melt in the diluted acid is very important to maintain the precision of this procedure.