Borate fusions are widely used for samples which are either difficult to prepare as homogeneous pressed powders (e.g. cement), hard to dissolve in acid (e.g. zirconia and alumina), or both (e.g. metal ores and silicate rocks). For borate fusions to be successful, the sample when fused must be in the form of an oxidized, inorganic compound. Cement mix is usually a blend of carbonates and silicates; zirconia and alumina are oxides; and so forth. Compounds without oxygen, such as sulfides, carbides, phosphides, chlorides, etc., must be oxidized before being fused; organic compounds must be ashed. Reduced metals must be oxidized as well. Platinum-group metals simply cannot be fused with borates because even their compounds reduce during fusion, and the metals will not only remain insoluble in the flux but can also alloy with platinum/5% gold crucibles.
Most fluxer users are in one of the following industries: cement, glass and ceramics, or mining and minerals. Samples analyzed include not only raw materials like dolomite, sand, basalt, iron ore, and rare-earth-element-bearing pegmatites, but also their industrial products and by-products: cement, concrete, and building materials; mining concentrates; TV tubes, cookware, and the high-tech ceramics used for rocket nozzles and engine blocks; industrial pigments such as TiO2; slags from smelters, blast furnaces, refineries, and glass plants; and many more. Most of these are naturally oxygen-rich and do not require chemical transformation prior to borate fusion. However, hybrid oxidation/fusion techniques have been developed for reliable borate fusions of sulfides, carbides, ferro-alloys, and other materials formerly considered out-of-bounds for the technique.
Borate fusion has become increasingly popular as a preparation technique for XRF sample discs. Because the bulk of X-ray data from a disc is emitted at or near its surface, the homogeneity (at the atomic level) of a fused glass disc gives it an inherent advantage over discs
pressed from pulverized samples. Even though the pressed powder method can be highly accurate when carefully done, multi-phase samples such as cements, rock, sand and ore are subject not only to segregation during grinding and pressing, but also to matrix effects (e.g., particle size and mineralogical effects) and other variables which are avoided in fused glass discs. Furthermore, borate glass discs are long-term stable if carefully stored in a desiccator, and if they become hydrated they can be polished or re-cast with little or no loss of accuracy. Synthetic standards are also easy to make from pure oxides, as glass discs don’t have the matrix-matching problems of pressed powder discs.
In preparing samples for AA, ICP, and other liquid-analyzing techniques, the major advantage of borate fusion is that it is often simpler and quicker then even dissolution with acid in a microwave pressure vessel. A complete fusion/solution procedure, from ignition of the burners to decanting of a clear solution, usually takes fifteen minutes or less in an automated fluxer. While borate fusions do require some caution in evacuating heat and fumes, and the use of dilute HCl or HNO3 to dissolve the melt, hazardous reagents such as HF and other concentrated mineral acids are not necessary.
Borate fusion has its limits, like any technique. Fusion dilutes a sample, often considerably, a consideration in trace-element analysis. It also destroys the original form of a sample, so structural/molecular information is lost. The high temperatures of borate fusion (1000º to 1100º C) drive off compounds of volatile metals such as Hg, Sn, and Sb, while other compounds formed during fusion (notoriously copper halides) are extremely volatile. And the extra steps necessary to prepare organic materials and reduced inorganics for fusion can greatly extend
turnaround time. Obviously, pulverizing a sample and pressing it into a disc is a preferable alternative in many cases. However, for many samples borate fusion is the simplest, quickest, most consistent analytical approach.
Both fusion and pulverizing/pressing are important, widely used sample preparation methods, each with its own advantages. SPEX SamplePrep, LLC has a full range of equipment for either approach. Please consult our applications specialists to help determine which method is more suitable for your laboratory. |
