A new, simple, and practical method has been developed for the accurate and precise isotopic analysis of extremely small amounts of Pb (<∼3 ng) by thermal ionization mass spectrometry. Two different types of double spikes, one consisting of 205Pb and 204Pb and another enriched in 207Pb and 204Pb, are used to reduce "204Pb error" and to correct mass fractionation during mass spectrometry. Using this technique, replicate analyses of 1.5 ng of Pb from NBS981 were performed, and an external precision of 0.02% (2σ) was attained for the 208Pb/204Pb ratio. Compared with results for 1.5 ng of Pb by the normal double spike method using a single 207Pb- 204Pb double spike, the external precision for the isotopic ratios involving 204Pb is reduced by about 60-70%, simply by addition of the 205Pb-204Pb double spike. The two double spikes method was also applied to isotopic analyses of natural rock samples, and we obtained an external precision of 0.06% (2σ) for 208Pb/204Pb in ∼ 1.5 ng of Pb separated from fresh peridotite, in which the Pb concentration is as low as 3.2 ppb. This new technique is superior to any previous method for precise and accurate isotopic analyses of extremely small amounts of Pb, and will be a highly powerful tool in the future of earth science and environmental science.
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