A gravimetric K₂OsCl₆ standard: Application to precise andaccurate Os spike calibration

The Re-Os isotopic system is currently limited as a chronometer because of the lack of accurate gravimetric standards for Os spike calibration and uncertainty of the ¹⁸⁷Re half-life, which is also dependent on the accuracy of Os spike calibration. We demonstrate that the concentration of an Os spike can be calibrated accurately to ±0.2%. The accuracy of this calibration was achieved by using a high-purity, stoichiometric, and anhydrous K₂OsCl₆ standard. We show that (NH₄)₂OsCl₆, the standard material used by most other laboratories, is less reliable for Os spike calibration because it is hygroscopic and decomposes during heating. Nebulization and ionization in a plasma at 6500 to 8000 K does not discriminate Os of different oxidation states and chemical forms, allowing Os isotopic ratios in spike-normal mixtures to be equilibrated "on-line" and reproduced at the ∼0.02% level. Multiple isotope dilution measurements reproduce Os concentrations at the 0.04% level. The Os isotopic compositions of the standards determined by multiple collector inductively coupled plasma mass spectrometry are in excellent agreement with that of negative thermal ionization mass spectrometry. A strong linear relationship between instrumental mass fractionation factors (β) for Os and Ir in MC-ICP-MS allows us to determine the isotopic composition of the Os spike with high precision and accuracy. Application of such an accurately calibrated spike is anticipated to reduce the uncertainty of ¹⁸⁷Re half-life significantly, thereby increasing the accuracy of the ¹⁸⁷Re-¹⁸⁷Os chronometer.