Lead isotope ratio measurements using the isomass 54e in fully automatic mode

Attempts have been made to optimize the determination of Pb isotope ratios in over 700 analyses of standards and natural samples on an Isomass 54E in fully automatic operation. Amounts of Pb analysed for the N.B.S. standards 981 (common Pb) and 983 (radiogenic Pb) were initially about 100 ng Pb, but these have been increased to about 300 ng to conform with the usual amounts for natural samples. Over a period of three years, the following results have been obtained for standards (quoted errors are given as 2 standard deviations). {A table is presented}. Pooling the results of 207 analyses of 89 samples of gossans, soils, sulfides, oxides and rocks, twice the coefficients of variation (i.e. twice the relative standard deviations) for ²⁰⁸Pb/²⁰⁶Pb, ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁶PB/²⁰⁴Pb ratios are ±0.11%, ±0.05% and ±0.12%, respectively. We consider that this represents a realistic estimate of our precision as it is based on natural samples and it is also consistent with the standard data. A statistical analysis of 195 runs of SRM 981 was carried out in an attempt to evaluate the bias in the estimated ratios caused by fractionation and to quantify the contribution of fractionation to the variability of the estimated ratios. The major source of error in the measurement arises from mass fractionation rather than random fluctuations and is contingent on sample loading variability.