Platinum stable isotope ratio measurements by double-spike multiple collector ICPMS
We present a new technique for the precise determination of platinum (Pt) stable isotope ratios by multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) using two different Pt double-spikes ( 192Pt-198Pt and 196Pt-198Pt). Results are expressed relative to the IRMM-010 Pt isotope standard as the parts per million difference in 198Pt/194PtPt ratios (μ198Pt). Repeated measurements of the IRMM-010 Pt standard in two different laboratories, consuming ca. 40-85 ng of Pt, show that a long-term external reproducibility for μ192Pt of ≤40 ppm (2 sd; equivalent to ≤10 ppm u-1, where u is the unified atomic mass unit) can be obtained on Pt stable isotope ratios with either double-spike. Elemental doping tests reveal that double-spike corrected Pt stable isotope ratios are insensitive to the presence of relatively high (up to 10%) levels of matrix elements, although the 192Pt-198Pt double-spike is affected by an isobaric interference on 192Pt from 192Os. The 196Pt-198Pt double-spike does not use 192Pt in the double-spike inversion and is unaffected by Os contamination, and is our recommended double-spike for use with natural samples. As part of this study, we re-determined the natural Pt isotopic composition of IRMM-010 by MC-ICPMS using external element (Pb) doping to correct for instrumental mass bias and have identified relative Pt isotope differences of up to 10% from the reference values for this standard. The new isotopic composition of the IRMM-010 standard (190Pt = 0.01289%, 192Pt = 0.7938%, 194Pt = 32.81%, 195Pt = 33.79%, 196Pt = 25.29% and 198Pt = 7.308%) results in a redefined Pt atomic weight of 195.08395 ± 0.00068. Using our technique we have measured small, reproducible and statistically significant offsets in Pt stable isotope ratios between different Pt element standards and the IRMM-010 standard, which potentially indicates that natural Pt stable isotope fractionations exist that are larger than the reproducibility of our technique.