| Summary: | Includes bibliographical references. === The fire assay technique may be used to extract noble metals from platinum bearing ores. It allows for the use of relatively large quantities of ore sample from which trace quantities of noble metals are concentrated. The fire assay with lead as the collector is one such procedure, where preconcentration of the noble metals allows for direct analysis of the noble metals in lead The samples may be prepared by pyrochemically treating the ore sample with a litharge-based flux. These lead buttons require an homogenising remelt and rapid cast before direct determination of the noble metals can be made using a time resolved form of spectroscopy called SAFf, or Spark Analysis for Traces. The unique characteristics of this technique is that inunediately after the electrical spark discharge has taken place radiation from atomic species continues and is termed after-glow. Special electronic techniques are required to observe this phenomenon as it takes place in microseconds. By studying the emission characteristics of various spectral wavelengths it is possible to determine the time at which the photomultiplier measures the atomic radiation. and not the background radiation. The atomic emission line to background ratio is thus improved and hence precision and limits of detection. Initial investigations into the SAFf analysis of platinum. palladium and rhodium using synthetic standards prepared from pure noble metals and lead revealed an analytical method of great promise. However the method failed when applied to real ore samples for several reasons. The SAFf analysis is particularly sensitive to the effects due to changes of the matrix of the lead depending on the type of sample used to prepare the lead buttons. Corrections for these effects could not be applied as preparation of matrix matched standards also failedAnother serious problem experienced with the SAFf technique was the inherent insensitivity of the selected platinum wavelengths together with high background emissions. The platinum atomic emission to background ratio was thus poor, and hence scientifically inappropriate practice to measure platinum in lead under these circumstances. The most sensitive platinum emission wavelength could not be used because corrections for a spectral interference from nickel could not be applied. A certain portion of base metals are collected in the lead buttons along with the noble metals during the fusion process, and since nickel is present in all ore samples associated with platinum group metals the presence of nickel and platinum together in lead after collection is inevitable. Because of the nature of the ore samples used in the investigations, the concentration range of platinum, palladium and rhodium in lead even after preconcentration using fire assay, was very limited, and attempts to create calibrations using only these samples proved unsuccessful. Additional standards prepared from different ore samples were used to extend the concentration ranges of the calibrations, however this also proved to be of no use as the different matrices of these samples detracted from any possibility of defining an accurate calibration curve.
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