Osmium isotope analysis by inductively coupled plasma mass spectrometry

The rhenium-osmium decay scheme offers unique opportunities to study the age and origin of base and noble metal ore deposits. This requires the precise determination of Os isotope ratios, for which inductively coupled plasma mass spectrometry (ICP-MS) is ideally suited. An apparatus was constructed for direct distillation of OsO4 into the plasma, and was tested using a pure Os solution and a natural sulphide ore standard. Individual runs gave isotope ratio results with errors as low as 0.3%(1σ), while the means of all runs gave precisions as good as 0.3%(2σ) and accuracies of 0.1–0.2% against secondary ion mass spectrometry (SIMS) data. This demonstrates that Os isotope ratios can be measured to precisions and accuracies comparable to the ultimate capability of ICP-MS, despite concentrations in the low part per billion range. The technique is now poised for geological exploitation.