Quantitative Separation of Molybdenum and Rhenium from Geological Materials for Isotopic Determination by MC-ICP-MS

We have developed a new chemical procedure for the quantitative separation of molybdenum (Mo) and rhenium (Re) from a wide variety of geological samples. A single pass anion exchange separation provided complete recovery of pure Mo and Re in a form that was ideal for subsequent isotope and abundance determination by multi-collector inductively coupled plasma-mass spectroscopy (MC-ICP-MS). An enriched Mo-100-Mo-97 solution, mixed with the sample before digestion, enabled natural mass-dependant isotopic fractionation of Mo to be determined with an external reproducibility of < 0.12 parts per thousand (delta Mo-98/Mo-95, 2s). Determination of the concentration of Mo and Re in the same sample was achieved by isotope dilution, with instrumental mass-fractionation of Re being corrected by the simultaneous measurement of the Ir-191/Ir-193 ratio. We have applied the new procedure to a variety of samples, including seawater, basalt and organic-rich mudrock. The procedure is ideally suited to palaeoredox studies requiring the precise determination of the Mo isotope composition and the Re/Mo ratio from the same sample.