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

Molybdenum isotopic compositions are precisely determined by MC-ICP-MS meas urements using a Mo double spike. The double spike is added prior to chemic al purification, so that the laboratory and instrumental mass fractionation s 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 Mo-98/Mo- 95 ratio (2 standard deviation)). Using a normal microconcentric nebuliser with a cyclonic spray chamber, the minimum quantity of Mo is similar to1 mu g for high-precision results. A hydrothermal molybdenite shows fractionatio n of -0.3 per mil on the Mo-98/Mo-95 ratio relative to our standard (Johnso n Matthey, 1000 mug/mL (+/-0.3%) ICP standard solution, lot 602332B). Fine- grained sediments show fractionation of 0.1 and -0.3 per mil on the the Mo- 98/Mo-95 ratio. The observed Mo isotope fractionation is small but resolvab le with the presented high-resolution technique.