Determination of molybdenum isotope fractionation by double‐spike multicollector inductively coupled plasma mass spectrometry

Molybdenum isotopic compositions are precisely determined by MC‐ICP‐MS measurements using a Mo double spike. The double spike is added prior to chemical purification, so that laboratory and instrumental mass fractionations are separated from natural mass‐dependent fractionation. Fractionation is determined on four Mo mass ratios, providing an internal consistency check. The external standard reproducibility is at 0.06 per mil on the 98 Mo/ 95 Mo ratio (2 standard deviation)). Using a normal microconcentric nebuliser with a cyclonic spray chamber, the minimum quantity of Mo is ∼1 μg for high‐precision results. A hydrothermal molybdenite shows fractionation of −0.3 per mil on the 98 Mo/ 95 Mo ratio relative to our standard (Johnson Matthey, 1000 μg/mL (±0.3%) ICP standard solution, lot 602332B). Fine‐grained sediments show fractionation of 0.1 and −0.3 per mil on the 98 Mo/ 95 Mo ratio. The observed Mo isotope fractionation is small but resolvable with the presented high‐resolution technique.