Sparging Os into plasma source mass spectrometers : preliminary isotopic ratio of chromite

Results of preliminary tests for rapid determination of Os concentrations and Os isotopic ratios using isotope dilution ICP-MS and HR-ICP-MS combined with HPA (High Pressure Asher) digestion are presented. With this method, volatile OsO4 is directly transferred to the ICP-torch via a flux of Ar gas bubbling into the solution (sparging). Three kinds of tubing to introduce Ar into the Os solution were tested: 1) A single 3.2 mm PFA tubes, 2) Four 0.8 mm PFA tubes and 3) A single 3.2 mm PFA tubing with holes and enclosed at the extremity in the sample solution. Sparging Os does not require any chemical separation prior to analysis which is a noticeable advantage. Sparging Os also reduces the washout time to c.a. 15 minutes between samples. Osmium concentrations determined by sparging of two reference materials (ChrPt+: a chromite powder, and WPR-1: a peridotite powder) are comparable with their published values. For the determination of 187Os/188Os isotopic ratios, preliminary results are promising. However, some adjustments must be done to improve the precision and accuracy of the analyses which are affected by the signal stability during analysis, such as a digestion of chromite powder for 15h by HPA and the use of freshly digested sample solutions. The sparging method is expected to be a powerful and efficient tool for determining osmium isotopic ratios of chromite concentrate powders from layered intrusions