Platinum stable isotope ratio measurements by double-spike multiple collector ICPMS

Abstract

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.