Precise Isotopic Analysis of Mo in Seawater Using Multiple Collector-Inductively Coupled Mass Spectrometry Coupled with a Chelating Resin Column Preconcentration Method

It is widely recognized that the natural isotopic variation of Mo can provide crucial information about the geochemical circulation of Mo, and the ocean is an important reservoir of Mo. To obtain precise isotopic data on Mo in seawater samples using multiple collector-inductively coupled plasma mass spectrometiy (MC-ICPMS), we have developed a preconcentration technique using 8-hydroxyquinoline bonded covalently to a vinyl polymer resin (TSK-8HQ). By optimizing the procedure, Mo in seawater could be effectively separated from matrix elements such as alkali, alkaline earth, and transition metals. With this technique, even with a 50-fold enrichment factor, the changes in the (98)Mo/(95)Mo ratio during preconcentration were smaller than twice the standard deviation (SD) in this study. Mass discrimination of Mo isotopes during the measurement was externally corrected for by normalizing (86)Sr/(88)Sr to 0.1194 using an exponential law. We evaluated delta(98)/(95)Mo to a precision of +/- 0.08 parts per thousand (+/- 2 SD); this value was found to be less than one-third of previous reported values. Moreover, we were able to determine an accurate ratio for every pair of stable Mo isotopes, which was impossible with previous methods owing to the isobaric interference from the external elements (Zr and Ru). In this study, delta(92)/(98)Mo in seawater was first determined so that it had the smallest relative error. We applied the proposed method to four kinds of seawater samples. The Mo compositions were constant among them, with average delta(98)/(95)Mo and delta(92)/(98)Mo values of 2.45 +/- 0.11 and -4.94 +/- 0.09 parts per thousand ( +/- 2 SD), respectively. Our data indicate that seawater is enriched in heavy Mo isotopes than previously reported.