URANIUM AND THORIUM ISOTOPIC AND CONCENTRATION MEASUREMENTS BY MAGNETIC SECTOR INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

Abstract

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.