Borate fusion is a widely used sample preparation method in which an oxidized sample is dissolved in molten flux at temperatures of around 1050°C.
This method can be employed for a large number of samples for analysis by X-ray fluorescence (XRF), inductively coupled plasma (ICP), atomic absorption (AA) and other analytical techniques. Typical samples prepared using this method include glasses, pigments, ceramics, rocks, sediments, ores, and cements. Sub- oxidized samples such as sulfides, polymers, catalysts, pure metals, alloys and ferroalloys can also be processed by borate fusion after an initial oxidation step. The most commonly used fomulations are lithium tetraborate and lithium metaborate.
Borate fusion is a rapid and safe method as it avoids the use of concentrated and dangerous acids in sample preparation and sample preparation time is shorter than that of the conventional acid digestions. The fusion is very effective while using high purity fusion fluxes and appropriate analytical parameters.
Borate fusion involves mixing a solid sample with a solid strong base in a platinum crucible. The crucible containing the sample is then heated at a low temperature for 1-2 mins. The sample mixture is heated to initiate and speed up the oxidation reaction that will quickly follow. Distilled water is gently added to the sample mixture to initiate oxidation between the sample and the base.
Most of the fusion methods employ non-wetting agents that are incorporated into the flux to prevent sticking of the melt to the crucible and improve casting performance of the molds. These non-wetting agents are used in high concentration for preparing solutions for wet chemistry. Once the sample is oxidized, a non-wetting agent and a lithium-borate flux are added to the oxidized mixture.
The fusion is then carried out at high temperature around 1000°C within an automatic fusion fluxer. At this high temperature, the sample is melted and dissolved by the lithium borate flux to form a perfectly homogeneous mass. The molten mixture is then poured into dilute mineral acid and stirred until the glass flux dissolves. In some cases, the melt is poured into a preheated platinum mold to produce the desired sample for XRF analysis or into an unbreakable beaker with an acid solution for AA or ICP analysis.
The Acidity Index
The acidity index of the sample is the ratio between the number of atoms of oxygen and metal. The higher the acidity index, the more acidic is the oxide. For instance, Al2O3 will have an acidity index of 1.5.
Tetraborate reacts with basic oxides, and metaborate reacts with acidic oxides. For example quicklime is used as an oxidized sample, tetraborate would be the optimal flux.
Companies Involved in Borate Fusion
Some of the companies that offer borate fusion sample preparation products are listed below:
SPEX SamplePrep, founded in 1954 is a leading supplier of sample preparation equipment and supplies. It offers the full line of SPEX CertiPrep Fusion Flux and Katanax Automated Electric Fusion Fluxers for preparation of samples for ICP, AA and XRF. Its products produce reproducible samples for every analysis to achieve accurate, consistent results.
Scancia provides the best fusion fluxes for use in high volume industrial laboratory or a research facility. Its products offer an extensive range of formulations to suit all applications.
Socachim offers pre fused borate fluxes with perfect homogeneity, controlled particle size distribution, high purity, reserved production batches, and fused white vitreous powder. It also supplies fluxes integrated with oxidizer or nonwetting agents.
Gammadata Instrument AB, founded in 1987 is a leading supplier of equipment and tailor-made solutions for analytical fusion techniques, instrumentation, and fine chemistry science. Its product range includes all aspects of borate fusion, ranging from flux, platinum crucible to fluxers, and even fully automated solutions for the weighing of flux.
The borate fusion process has several advantages over other preparation methods, as it maintains constant grain size and does not produce orientation effects. The sample thus obtained will be almost perfectly homogenous.
The borate fusion method is simpler and quicker than dissolution with acid in a microwave pressure vessel. The complete procedure, from the heating of samples to decanting of a clear solution takes less than 15 mins using an automated fluxer.
This technique can also minimize matrix effects which are observed in XRF analysis. It avoids the use of diluter hydrochloric or nitric acid to dissolve hazardous reagents such as HF and other concentrated acids, though does require some caution in evacuating heat and fumes.
Borate fusions are widely used in samples that are either difficult to prepare, hard to dissolve in acid, or both. Lithium borate is best suited for the dissolution of samples containing a high concentration of aluminosilicates, carbonate rocks, and non-metallic basic oxides.
Other minerals that are decomposed by this method include zircon, certain rare-earth phosphates, rare-earth fluorides, and sulfides. In recent times, the borate fusion method is also being widely used in the lead-zinc-copper industry.
Sources and Further Reading