Precise and accurate 186Os/188Os and 187Os/188Os measurements by multi-collector plasma ionisation mass spectrometry (MC-ICP-MS) part I:: Solution analyses

We present new high precision Os isotope data obtained by solution-mode MC-ICP-MS for 4 different Os isotope reference materials and compare the data to that obtained by the N-TIMS method. Mass fractionation effects for MC-ICP-MS are evaluated and we demonstrate excellent adherence to the exponential law. An improved and robust method for the derivation of interfering element isotope ratios is presented and evaluated via analysis of solutions with widely varying interfering element/analyte ratios. We show experiments that illustrate the extent of memory with a conventional solution introduction system and establish a protocol that reduces washout time and memory effects to the same level as those common for other elements typically analysed at high precision by MC-ICP-MS. Data for 4 Os Reference material solutions show excellent agreement between MC-ICP-MS and N-TIMS for all Os isotope ratios except Os-186/Os-188 and Os-184/Os-188 which are consistently lower by similar to 100 ppm and 3-5%. respectively for MC-ICP-MS. These differences are highly unlikely to arise from problems relating to Faraday cup or amplifier efficiency variations, residual mass fractionation effects or inaccuracies in the applied W interference corrections on Os-186 and Os-184 for MC-ICP-MS analyses. These issues require further investigation if the Pt-Os system is to find routine application as a geochemical tracer. (C) 2007 Elsevier B.V. All rights reserved.