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 100 Mo‐ 97 Mo 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‰ (δ 98 Mo/ 95 Mo, 2 s ). 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 191 Ir/ 193 Ir 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.