Quantification of ²³⁴Th recovery in small volume sea water samples by inductively coupled plasma-mass spectrometry

Summary

Development of a small volume (2-4 liter) technique for measuring ²³⁴ Th in sea water has been instrumental in bringing to light small-scale structures in upper-ocean particle removal processes previously missed by standard ²³⁴ Th measurement techniques. Further development of this method to evaluate removal efficiencies of ²³⁴ Th via MnO₂ precipitation quantified using ICP-MS are presented in this work. Advantages to this approach are precise knowledge of ²³⁴ Th recovery, while maintaining high sample throughput afforded by ICP-MS analyzes. The improved technique includes the acidification of 4-liter sea water samples and the addition of ²³⁰ Th as a yield monitor prior to MnO₂ precipitation. Subsequent filtration and beta-counting of the high-energy daughter, ^234mPa, was followed by a final background count after 6 half-lives (144 d) of decay. Filtered precipitates were dissolved with H₂O₂, and an internal standard of ²²⁹ Th was added. Samples were purified using anion-exchange chromatography to remove high levels of manganese, and recoveries were determined by measured ratios of ²³⁰ Th/²²⁹ Th by ICP-MS. Application of this procedure for ²³⁴ Th derived export in the recent Southern Ocean Iron Experiment showed average recoveries of 91%. Corrections for rare low recoveries (25-80%) noticeably change ²³⁴ Th profiles, thus impacting subsequent elemental flux calculations.