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

Development of a small volume (2-4 liter) technique for measuring Th-234 in sea water has been instrumental in bringing to light small-scale structures in upper-ocean particle removal processes previously missed by standard Th-234 measurement techniques. Further development of this method to evaluate removal efficiencies of Th-234 via MnO2 precipitation quantified using ICP-MS are presented in this work. Advantages to this approach are precise knowledge of Th-234 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 Th-230 as a yield monitor prior to MnO2 precipitation. Subsequent filtration and beta-counting of the high-energy daughter, Pa-234m, 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 Th-229 was added. Samples were purified using anion-exchange chromatography to remove high levels of manganese, and recoveries were determined by measured ratios of Th-230/Th-229 by ICP-MS. Application of this procedure for Th-234 derived export in the recent Southern Ocean Iron Experiment showed average recoveries of 91%. Corrections for rare low recoveries (25-80%) noticeably change Th-234 profiles, thus impacting subsequent elemental flux calculations.