Analysis of a rhenium-osmium solid-solution spike by inductively coupled plasma mass spectrometry

The rhenium-osmium decay scheme (187Re →187Os) offers a unique opportunity to investigate the genesis of, and directly date, ultramafic rock, sulphide and platinum ore deposits. Inductively coupled plasma mass spectrometry (ICP-MS) is a viable method for Os isotopic analysis as it provides the high temperatures necessary to ionise Os. The sample can be introduced into the ICP mass spectrometer either by conventional nebulisation or by distillation using an Os “generator.” Generator-mode analysis is superior to nebulisation because (i) the total number of counts is two orders of magnitude higher for a given sample size and (ii) Os oxidises readily to volatile OsO4, which has a boiling-point lower than most Re compounds, so that Os can be efficiently separated from 187Re. The importance of Os loss during sample preparation was minimised by combining the sample powder with a powdered Re-Os isotopic spike prior to any chemical treatment. The spike is in a nickel sulphide matrix and was made by a thioacetamide co-precipitation of Os, Re and Ni followed by a fire assay. The 190Os/192Os ratio in this preparation is 51.5 ± 0.9, the 187Os/188Os ratio is 0.063 ± 0.006 and the 185Re/187Re ratio is 17.04 ± 0.41. These values are within the error limits quoted by the Oak Ridge National Laboratory and Techsnabexport, the suppliers of the metallic spikes. Parameters that significantly influence the Os analyses include the oxidising agent and the run temperature. The oxidising agent H5IO6 is preferred to H2O2 or HNO3 as it has the highest electrode potential, provides a prolonged, consistent reaction and is more stable when stored. Chilling the sample and the H5IO6, initially retards uncontrolled OsO4 emission. Heat applied later in the run releases OsO4 and helps maintain a high count rate. Thus, OsO4 can be generated in a steady, controllable and reproducible manner.