The application of abundance sensitivity filters to the precise and accurate measurement of uranium series nuclides by plasma mass spectrometry

Abstract

Application of retardation filters has revolutionized the use of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) for the measurement of U-series disequilibria. Here, we present an evaluation of the application of retardation energy filters coupled with ion counting detectors for the measurement of U-series disequilibria by MC-ICPMS. We show that: 1) The RPQ minimizes backgrounds caused by ions experiencing irregular flight paths at high masses of uranium series nuclides. This reduced background is important for low count rate measurements and has enabled us to develop an improved method for the determination of 226Ra/228Ra ratios using a dual SEM/RPQ collector block. 2) Systematic biases exist for isotope ratios of 234U/238U and 230Th/232Th without the use of RPQs; thus, the RPQ is important for measurement of both of these isotope ratios. 3) That the RPQ makes possible 210Pb measurements in geologic samples using MC-ICPMS, but that this application is limited to samples that have low common Pb, such as carbonates and silica sinter. Additionally, we present mass spectrometric and gamma counting data for U-Th-Ra disequilibria in rock standard reference materials including AGV-2, AThO, BCR-1, BCR-2, BHVO-2, BIR-1a, DTS-1, PCC-1, RGM-2, TML, and W-2a. Comparing this new data with values from published literature enables us to provide consensus or recommended values for U, Th, and 226Ra concentrations and 230Th/232Th ratios for each of these standards. We note that this is the first compilation of recommended radium concentrations for rock standard reference materials.