Uranium and thorium isotopic and concentration measurements by magnetic sector inductively coupled plasma mass spectrometry

We have developed techniques by sector-field inductively coupled plasma mass spectrometry (ICP-MS) for measuring the isotopic composition and concentration of uranium and thorium, focusing on the rare isotopes, ²³⁰Th and ^234U. These isotopes have been widely used as tracers in earth sciences, e.g., chronology, paleoclimatology, archeology, hydrology, geochemistry, and oceanography. Measurements made on reference materials demonstrate that the analytical precision approximates counting statistics and that the accuracy of the measurement is within error of accepted values. Routine measurement times are 20 min for U and 10 min for Th. The sensitivities (ions counted/atoms introduced) are 2-3‰ for U and 1.5-2‰ for Th. Samples of 10-40 ng of ²³⁸U (0.5-2.0 pg of ²³⁴U) give measurement precisions of 1-2‰ (2σ) for δ²³⁴U and U concentration ([U]). Only 0.4 pg of ²³⁰Th are needed to achieve [²³⁰Th] and ²³⁰Th/²³²Th data with errors less than 5 ‰ even for cases where ²³⁰Th/²³²Th is 10^-5 or less. Our ICP-MS data, including uranium standards, thorium standards, ²³⁸U-²³⁴U-²³⁰Th-²³²Th dating of speleothems and ²³⁰Th-²³²Th in oceanic particulates, replicates measurements made by thermal ionization mass spectrometry (TIMS). Compared to TIMS, the ICP-MS method allows smaller sample size and higher sample throughput due to higher sensitivity, fewer sample preparation steps and shorter measurement times. However, mass biases, intensity biases, spectral interferences and instrumental blanks are significant and must be addressed.