Relatively low levels of metallic and non-metallic impurities can cause severe degradation of platinum components used at high temperatures, for example in equipment for manufacturing specialised glass. Thus strict control of the impurity contents is required in order to ensure that the properties of the platinum are not impaired at these high temperatures.

To help to provide this control the Johnson Matthey Technology Centre has prepared a platinum standard containing fifteen impurity elements at concentrations of 100 ppm. The preparative route selected consisted of adding the elements as solutions to a high purity platinum sponge which, after drying, milling, blending and subsequent reduction under a hydrogen atmosphere, was spin-riffle sampled into 0.5 gram portions for analysis. Part of the sponge was compacted and forged down into wire, for use in spectrographic analysis of fabricated material.

Initially the doped sponge was analysed in two Johnson Matthey laboratories using Inductively Coupled Plasma Emission Spectrometry (ICPES) and also at the Institute of Geological Sciences, London, using Inductively Coupled Plasma/Mass Spectrometry (ICPMS) to establish that the material had been homogenised correctly and to confirm that no major, unexpected losses had occurred.

As the contents of the impurities in the platinum standard had been established by means other than analysis, it was considered that a unique opportunity existed to involve other analysts outside the Johnson Matthey Group in a co-operative exercise designed to evaluate current analytical procedures. The authors are very grateful to the eight organisations that took part.

Results were received from sixteen laboratories. Six different techniques were employed, although the majority of laboratories used only one or two. Where sufficient results were available from the use of a particular technique, the means and standard deviations for that technique could be calculated separately. Four groups of results could be treated in this way. These were Atomic Absorption spectrophotometry (AA), Direct Reading Emission Spectrometry (DRES), Photographic Emission Spectrometry (PHES), and Plasma Emission Spectrometry which included results from both ICPES and Direct Current Plasma Emission Spectrometry (DCPES). Statistical analysis of the combined results was carried out using robust methods able to take account both of outliers and of the fact that some of the groups of results were non-normally distributed.

The mean values and standard deviations for each technique are presented in Table I, together with the overall mean values and 95 per cent confidence limits. The full results for two typical impurity elements, one a platinum group metal and the other a base metal, are shown in Figures 1(a) and 1(b), respectively. The data form typical “S” curves, from which it can be seen that the majority of the results cluster around the true content, although some outliers are obvious.

The elements which were deliberately added to the platinum standard were: antimony, arsenic, cobalt, copper, gold, iridium, iron, lead, magnesium, nickel, palladium, rhodium, ruthenium, silver, and zinc. Aluminium and calcium, which were also reported, may be expected as minor impurities in the starting sponge. The single outlier result of 138 ppm for calcium obtained by AA strongly influenced the overall mean for that element. Without it the overall mean was 3 ppm which is much more reasonable. As expected, silicon was picked up from the trays used for the sponge reduction stage.

The high palladium result is unlikely to be due to palladium in the starting sponge. A possible explanation may be that an error occurred during the preparation, resulting in a true content of 125 ppm. For most of the remaining elements the results are in close agreement with the true content of 100 ppm. The exceptions are antimony and arsenic, where some volatilisation during the reduction stage was expected even though the temperature was kept as low as practicable.