Determination of platinum in soil samples by inductively-coupled plasma mass spectrometry

The Pt content in soil samples taken near a highway was determined using the ICP-MS method. After sample decomposition with aqua regia, the platinum concentration was quantified using either the external calibration method or isotope dilution method (((194)pt + Pt-195)/Pt-196) and Pt-195/Pt-198 ratios were measured). Spectral interferences caused by hafnium and tungsten oxide particles were eliminated by off-line mathematical correction. The choice of proper internal standard for external calibration is discussed; iridium seems to be the best one. The isotope dilution method utilizing measurement of the ((194)pt + Pt-195)/Pt-196 ratio exhibited the smallest value of combined uncertainty (9 ng g(-1) on the 600 ng g(-1) level). The uncertainties of other methods were approximately twice higher. Major part of uncertainty of the external calibration method was caused by repeatability, a minor part consisted in calibration and spectral and nonspectral interferences. Student's t test proved the conformity of results for all methods under investigation. The off-line mathematical correction of spectral interferences is applicable only to samples with low hafnium levels causing false positive signal of tens of percent of the original Pt content. Evaluation of geological samples with elevated hafnium contents giving false positive signal of hundreds of percent of the original Pt content is questionable.