Precise determination of low abundance isotopes (174Hf, 180W and 190Pt) in terrestrial materials and meteorites using multiple collector ICP-MS equipped with 1012 Ω Faraday amplifiers

Abstract

Analytical protocols for measurements of isotope ratios involving low-abundance isotopes 174Hf (~0.16%), 180W (~0.12%), and 190Pt (~0.014%) using multi-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 1012Ω resistors for collection of the isotope beams of interest, as well as for the isotope beams used to monitor isobaric interferences. For ion currents >5×10−13 A (>50mV on an amplifier with 1011Ω resistor), uncertainties (2s.d.) of better than 100ppm are typically achieved for isotope ratios involving 174Hf, 180W and 190Pt. Terrestrial isotope abundances (atomic percent ±2s.d.) of 174Hf (0.16106±0.00006; n=15); 180W (0.11910±0.00009; n=10); and 190Pt (0.01286±0.00005; n=9) were determined on reference solutions throughout multiple analytical sessions over ~3years. 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 174Hf, 180W and 190Pt 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 190Pt abundances within uncertainty. Three chondrite samples exhibit indistinguishable 174Hf from terrestrial Hf. The isotope abundance of 180W in meteorites is more variable due to radiogenic 180W that was produced by 184Os-decay.