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Platinum Metals Rev., 1996, 40, (2), 87

Iridium Apparatus for XRF Fusions

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The properties required of containment materials used for X-ray fluorescence analysis (XRF) applications include good chemical resistance to the borate fluxes commonly used in the preparation of samples, sufficient hot strength to give an acceptable service life, and good wetting resistance. It must also be possible to work the material into the required form. Therefore, platinum, gold-platinum and gold-rhodium-platinum were widely used for this purpose, and the good surface finish on sample beads produced in ZGS 5 per cent gold-platinum casting moulds resulted in improved analytical accuracy (Platinum Metals Rev., 1982, 26, (3), 98).

Even these materials can be damaged, however, by the fusion of certain matrices, including some sulphides and unoxidised base metals. Now a paper from the Geoservices Centre of the Ministry of Northern Development and Mines, Canada, describes preliminary tests made to find if iridium could be used for the fusion of specific samples for XRF analysis (A. Martin, in “Summary of Field Work and Other Activities”, 1995, Ontario Geological Survey, Miscellaneous Paper, 164, pp. 233–236).

The samples tested contained unoxidised base metals, sulphides, carbides and arsenides. Although the iridium crucibles contained manufacturing flaws, they did survive the fusion processes. Experiments were also performed to determine if iridium suffered from the “memory effect”, where elements present in one melt would enter into the containment material and be released into subsequent samples. While iridium possesses wetting properties, it showed little or no loss of surface integrity during these metal permeability tests.

Thus, analytical sample beads have been produced in iridium crucibles from materials that would have significantly damaged and contaminated platinum apparatus. Further tests are to be carried out on new flaw-free iridium crucibles and the results are awaited with interest.

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