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

Abstract

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.