HIGH-PRECISION DETERMINATION OF ~(234)U/~(238)U ACTIVITY RATIOS IN NATURAL WATERS AND CARBONATES BY ICPMS

摘要

A method has been developed for precise measurement of ~(234)U/~(238)U activity ratios in natural waters and carbonates using quadrupole inductively coupled plasma mass spectrometry. A recovery of 80-85% of seawater U is achieved by Fe(III) coprecipitation followed by extraction chromatography with a supported dipentyl pentane phosphonate material; 90-95% of U is recovered from carbonates, which are are dissolved in HNO_3 and subjected to the same extraction chromatographic preparation. Isotopic measurements are made via recirculating pneumatic nebulization of small volumes of solutions containing 0.5-5 mg/L U. ~(234)U/~(235)U is measured as a proxy for determination of ~(234)U/~(238)U; iridium is added to sample solutions and the ion ratio ~(191)Ir~(40)Ar+/~(193)Ir~(40)Ar~+ is measured for internal mass discrimination correction. ~(234)U/~(238)U activity ratios in the range 1.143-1.154 are observed for 13 seawater and contemporary corals, in agreement with the established marine ~(234)U/~(238)U activity ratio. For sample sizes of 5-25 μg U, ICPMS uncertainties of +/- 0.2-0.5 % relative, 2σ standard error, approach those obtained for < 0.1 μg U by thermal ionization mass spectrometry. Measurements of ~(234)U/~(238)U activity ratios in bottled waters, Lake Erie surface waters, mollusk fossils, and fertilizers are also demonstrated.