High precision 230Th/232Th and 234U/238U measurements using energy-filtered ICP magnetic sector multiple collector mass spectrometry

We have developed new chemical and mass spectrometric techniques for the precise determination of U-234/U-238 and Th-230/Th-232 ratios in geological materials. The isotope ratio measurements were performed using an inductively coupled plasma double focusing magnetic sector multiple-collector mass spectrometer (MC-ICPMS) equipped with an additional energy filter. Following sample dissolution, U and Th are first separated from the rock matrix by a new and highly efficient column chromatographic procedure utilizing TRU-Spec resin. The strong affinity of U and Th for this material allows the use of extremely small ( < 0.5 mi resin) columns, even for the processing of silicate samples as large as similar to 5 g. Our new mass-spectrometric techniques permit precise corrections for mass discrimination and gain variation during analysis. As a consequence, the precision and external reproducibility of uranium and thorium isotopic analysis is improved by a factor of similar to 3-5 compared with previous results by conventional thermal ionization mass spectrometry (TIMS). A sensitivity of similar to 0.04% is routinely achieved for Th and this is comparable to the best values achieved by TIMS for large sample sizes. Recent instrumental improvements, however, have further increased our sensitivity by about a factor of five. Our Th and U isotope data for standard reference materials and other geological samples are in excellent agreement with previously published values from other laboratories, further highlighting the reliability and analytical capabilities of our new techniques. (C) 1997 Elsevier Science B.V.