摘要：

A highly enriched 202Pb +205Pb double-spike (202Pb/204Pb = 41000) has been prepared and used to make measurements of lead isotopic composition with internal correction for fractionation. The ratio of 205Pb/202Pb = 0.227388 in the double spike was calibrated against the certificate abundances of 208Pb and 206Pb in NBS standard SRM-982. The effectiveness of the double spike in improving the precision of lead isotopic analysis was tested by a series of measurements of double-spiked SRM 981. These demonstrate substantial improvements in precision with standard deviations ranging from 70 to 150 ppm. Measurements were made in static mode using a Finnigan MAT-261 mass spectrometer, recently fitted with 9 new faraday cups, which allows simultaneous monitoring of all lead peaks plus the reference masses (203 and 201) for isobaric interference from thallium and BaPO2. New mutually consistent calibrations of the lead isotopic composition of the NBS lead standards SRM 981 and SRM 982 have been calculated from the double spiked measurements. In general, the new results agree closely with the certificate values, but a small difference in 208Pb abundance in SRM 981, previously observed by others and ourselves, has been confirmed.

展开