High-precision analysis of Pb isotope ratios by multi-collector ICP-MS

Abstract

We investigated high-precision Pb isotope ratio analysis by multi-collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) using added thallium as an internal isotopic standard to correct for mass dependent isotopic fractionation. We compared MC-ICP-MS analysis of both an inter-laboratory standard, NBS 981, and geological samples to conventional thermal ionization mass spectrometry (TIMS). As expected, we found that analytical error in the latter was dominated by mass fractionation. In MC-ICP-MS, we found that fractionation appears to follow the exponential law, but that the fractionation coefficients, f, of Tl and Pb were not identical. This difference in fractionation coefficients cannot be compensated for by renormalizing to a different Tl isotopic composition as done in other studies. We found, however, that the fPb/fTl ratio was constant over the course of an analytical session, allowing fPb to be calculated from fTl. An exponential law correction was then applied to the Pb isotope measurements which effectively eliminates errors associated with mass fractionation. Precision for the MC-ICP-MS analyses ranged from a factor of 2 to a factor of 6 better than for TIMS analyses for the 206Pb/204Pb and 208Pb/206Pb ratios respectively. Residual error in the MC-ICP-MS analyses was dominated by error in the analysis of 204Pb, perhaps in part due to random errors introduced by correcting for a 204Hg isobaric interference. We also found systematic errors in the MC-ICP-MS analyses compared to TIMS determinations that may be due to uneven background and collector biases in the instrument used. We found that these systematic errors were the same for both NBS 981 and the geological samples, so accurate correction factors could be generated from the standard analyses to correct the sample analyses. MC-ICP-MS has the additional advantages of requiring less preparative chemistry, less instrument time, and considerably less labor overall.