Precise determination of low abundance isotopes (Hf-174, W-180 and Pt-190) in terrestrial materials and meteorites using multiple collector ICP-MS equipped with 10(12) Omega Faraday amplifiers

Analytical protocols for measurements of isotope ratios involving low-abundance isotopes Hf-174 (similar to 0.16%), W-180 (similar to 0.12%), and Pt-190 (similar to 0.014%) usingmulti-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) at ca. tenfold improved precision compared to previous studies are presented. The improvements in precision result in part from the use of Faraday amplifiers equipped with 10(12) Omega resistors for collection of the isotope beams of interest, as well as for the isotope beams used to monitor isobaric interferences. For ion currents >5 x 10(-13) A (>50 mV on an amplifier with 10(11) Omega resistor), uncertainties (2 s. d.) of better than 100 ppm are typically achieved for isotope ratios involving Hf-174, W-180 and Pt-190. Terrestrial isotope abundances (atomic percent +/- 2 s.d.) of Hf-174 (0.16106 +/- 0.00006; n=15); W-180(0.11910 +/- 0.00009; n=10); and Pt-190 (0.01286 +/- 0.00005; n = 9) were determined on reference solutions throughout multiple analytical sessions over similar to 3 years. The results, however, rely on the accuracy of the reference value chosen for the isotope ratios employed for mass bias correction and, to a lesser extent, on the accuracy of the exponential mass fractionation law used for mass bias correction. For the precise determination of isotope ratios involving Hf-174, W-180 and Pt-190 in natural materials, efficient chromatographic separation schemes for Hf and W from silicate-rich matrices, and for Pt from iron meteorite matrices, are presented that provide effective separation of the element of interest from interfering elements. First results for Pt suggest that IAB, IIAB and IIIAB iron meteorites exhibit terrestrial Pt-190 abundances within uncertainty. Three chondrite samples exhibit indistinguishable Hf-174 from terrestrial Hf. The isotope abundance of W-180 in meteorites is more variable due to radiogenic W-180 that was produced by Os-184-decay. (C) 2015 Elsevier B.V. All rights reserved.