High-Precision Determination of 234U/238U Activity Ratios in Natural Waters and Carbonates by ICPMS

Abstract

A method has been developed for precise measurement of 234U/238U 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 HNO3 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. 234U/235U measured as a proxy for determination of 234U/238U; iridium is added to sample solutions and the ion ratio 191Ir40Ar+/193Ir40Ar+is measured for internal mass discrimination correction. 234U/238U activity ratios in the range 1.143–1.154 are observed for 13 seawater and contemporary corals, in agreement with the established marine 234U/238U 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 234U/238U activity ratios in bottled waters, Lake Erie surface waters, mollusk fossils, and fertilizers are also demonstrated.

Mass Spectrometry in Environmental Sciences

“…Although liquid scintillation is still widely used for measuring radionuclides in environmental samples, mass spectrometry techniques are also commonly used. Quadrupole-based ICP-MS, double-focusing ICP-MS, electrothermal ICP-MS, and thermal ionization-MS methods have been used to measure radionuclides in environmental samples. − Detection limits for one of these methods (ICP with quadrupole MS) was reported to be 0.012−0.11 ng/L for 226 Ra, 230 Th, 237 Np, 238 U, 239 Pu and 241 Am . Becker and Dietze reviewed the performance of double-focusing sector field ICP-MS for determining long-lived radionuclides .…”