Lead-isotope data obtained on a multicollector mass spectrometer over a four year period using a 207PP04Pb double spike to correct for the effects of mass discrimination, are reported. Considerable improvements in both precision and accuracy over conventional correction procedures were noted, without recourse to rigorous loading or run conditions. An external precision in 206Pb/204Pb, 207PbPPb and 208PbP"Pb ratios +0.003,0.003 and 0.01 (2 X standard deviation), respectively, is routinely obtainable independent of minor variations in loading and run parameters.

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