An enhanced method for molybdenum separation and isotopic determination in uranium-rich materials and geological samples

Abstract

The precise and accurate determination of Mo isotope ratios in geological and U-rich samples requires a thorough separation of Mo from elements that could cause matrix effects and isobaric interference. This interference can be attributed to the presence of residual Fe, Ru, Zr and W naturally present in the samples as well as PO₄ contributed by the resin used during chemical separation. Using three-stage ion-exchange chromatography, we have obtained a high degree of purification with Mo yields ranging between 42 and 80%. Mo isotopic compositions were measured at a concentration of 30 ng ml⁻¹ using a Neptune Plus MC-ICP-MS equipped with Jet cones. The sensitivity is ∼1200–1600 V ppm⁻¹ Mo with an aspiration rate of approximately 150 μl per minute. Chemical and instrumental mass dependent fractionations were both corrected using the double-spike method. The total amount of Mo necessary for a single analysis is approximately 45 ng and the typical precision for terrestrial samples is 0.02‰ (2 SE, n = 8). This precise and accurate determination of Mo isotope ratios in U-rich samples has the potential for tracing the origin of uranium ores. Another application could be in nuclear forensics, for identifying the separation processes in the nuclear fuel cycle or the provenance of nuclear materials.