A new method for analysis of osmium isotopes and concentrations in surface and subsurface water samples

Abstract

We describe a new method for the determination of osmium (Os) content and 187Os/ 188Os isotopic ratio in water samples, particularly adapted to analysis of reducing waters such as groundwaters. The critical feature of the method is the improved oxidation step. We use a high pressure asher (HPA-S) to achieve oxidation of all Os species at high pressure (~ 130 bars) and high temperature (250 °C). A series of tests was performed on two groundwater samples in order to study the general behavior of Os during oxidation and to optimize the various parameters of the method. At high temperature (≥ 250 °C), using hydrogen peroxide as an oxidant, complete spike-sample equilibration and oxidation of Os species to the highest oxidation state (OsO 4) are obtained in less than 10 h. A strong contrast in oxidation behavior was observed between tests performed at high (250 °C) and low temperature (100 °C). The results for both samples indicate a difference in the oxidation kinetics between spike and sample. They also demonstrate that a non-negligible proportion of each sample is easily oxidized at low temperature whereas high temperatures are required to oxidize the rest of the sample. This last observation could provide evidence for the existence of several species of osmium in groundwater samples. Our results underscore the need for high temperatures to assure spike-sample equilibration and complete oxidation of all Os species potentially present in water. The method presented here, which is adapted to various matrices, opens the possibility of analyzing Os concentrations and isotopic compositions in groundwater and thus to better constrain the potential sources of osmium delivered to the ocean.