Simultaneous determination of picogram per gram concentrations of Ba, Pb and Pb isotopes in Greenland ice by thermal ionisation mass spectrometry

Abstract

A technique has been developed to simultaneously measure picogram per gram concentrations of Ba and Pb by isotope dilution mass spectrometry, as well as Pb isotopic ratios in polar ice by thermal ionisation mass spectrometry. BaPO2(+) and Pb+ ions were employed for these determinations. A calibrated mixture of enriched 205Pb and 137Ba was added to the samples providing an accuracy of better than approximately 2% for Pb/Ba element ratio determinations. Interference by molecular ions in the Pb mass spectrum occurred only at 204Pb and 205Pb, but these contributions were negligible in terms of precisions expected on picogram-sized Pb samples. The technique is illustrated with measurements on Greenland firn, using a drill-core section that includes the Laki volcanic eruption of 1783-1784. The data show deviations from the element concentrations indicating volatile metal enrichments, but the Pb isotopic signature of the Laki lava could not be identified.