Fusions are accomplished in several steps. First the sample is mixed with a flux in an appropriate ratio (usually between 1:2 and 1:20), perhaps with the addition of a non-wetting agent to prevent
the flux from sticking to the crucible and mold. Typical amounts of flux/sample mixture are 6 to 7 grams for a 31 mm glass disc, and 1 to 2 grams for a solution. Then the sample is heated past point of the flux in an inert, heat-resistant crucible. Most borate fusions are performed in crucibles made of 95% platinum and 5% gold, a standard non-wetting alloy; a minority of borate fusions is done in graphite crucibles. However, for pyrosulfate and other less common fusions, crucibles are made of zirconium and other materials.
During the fusion, of whatever sort, heating is maintained, and the crucible regularly agitated, until the sample has completely dissolved in the molten flux. At this point the melt is either poured into a mold and annealed to form a glass disc for XRF, or poured into dilute mineral
acid (e.g. 10% HNO3) which is stirred until the glass has dissolved. In some cases (notably pyrosulfate fusions) the melt is left to harden in the crucible, and the crucible and glass together are placed in an acid solution to dissolve the glass.
It is possible to fuse samples manually with simple equipment, or to achieve great sample throughput with an automated, programmable, multi-burner fluxer. The K2 Automatic Electric Fluxer is ideal for performing this technique, producing up to 25
fused glass discs in less then ten minutes, using 0.6g cement mixed with 6 g flux and 100 mg potassium iodide (as a non-wetting agent). The fusion temperature is between 1000° and 1100°C; the actual fusion takes about five minutes, and the forced cooling of the discs another four minutes. The same procedure can be carried out manually in a muffle furnace with SPEX SamplePrep graphite crucibles; the larger flat-bottomed SPEX SamplePrep crucibles can be used to cast the glass discs as well as perform the fusions, while the crucibles are handled and agitated with SPEX SamplePrep tongs. The time per sample with muffle furnaces is usually much longer than with an automated fluxer.
Each approach has its own advantages; again, please contact our applications specialists to determine the optimum equipment for your requirements. |
