Determination of uranium isotope ratios by multi-static MIC-ICP-MS: method and implementation for precise U- and Th-series isotope measurements

Abstract

In order to improve and extend the application of U- and Th-series isotope measurements a new technique for MIC-ICP-MS (multiple ion counting inductively coupled plasma mass spectrometry) was developed. This method uses either two (double MIC) or three (triple MIC) multipliers to measure in static mode in turn the U-isotopes 233U, 234U, 235U and 236U. For online internal standardization, mass bias correction and cross calibration of the multipliers a 233U/236U double spike was used. Applying this method the level of 1‰ 2σ internal precision is reached in less than 30 min, consuming less than 5 ng of total U. The multi-static MIC-ICP-MS method improves the precision by a factor of 5 for U isotope measurement compared with TIMS and by a factor of 2 compared with published ICP-MS methods. Repeated measurements of uranium CRM-145 (otherwise NBL 112A) were performed to test the reproducibility and accuracy of the method. An average 234U/238U value of (5.28584 ± 0.00087) × 10−5 or a δ234U value of −36.96 ± 0.16, respectively, was determined for CRM-145 in good agreement with reference data. Based on the precisely determined U isotope ratios we furthermore developed a combined multi-static U- and Th-measurement method. This approach extends this new MIC-ICP-MS technique to those U- and Th-series isotope systems having no constant isotope ratio for normalization (e.g., 230Th/232Th, 230Th/229Th) and is the key for new approaches to determining 226Ra/228Ra and 231Pa/233Pa. The applicability of the methods presented here is demonstrated by accurate dating of very young corals (10–350 y).