Evaluating the precision of Pb isotope measurement by mass spectrometry

Abstract

Analytical precision for Pb isotope measurement by thermal and plasma source mass spectrometry has improved by an order of magnitude in the last 20 years. Much of this improvement relates to a shift away from the external method of correcting instrumental mass fractionation – where samples are assumed to fractionate to the same extent as an average value of a working measurement standard. Implementation of a variety of techniques, including thallium spiking, sample–standard bracketing and double/triple spiking has provided more robust methods of fractionation correction. Isotope laboratories use one or more of these procedures, but an assessment of the measurement precision and relative merits of each system is needed to determine which is the most appropriate for the purpose. This study reviews each of these methods and provides a comparison based on an extensive analytical record covering 18 years, and using a variety of mass spectrometers. As two or three of the methods have been applied to most measurements, direct and robust comparisons can be made between correction protocols. In particular the effects of sample purity and variable sample matrices on measurement precision and accuracy have been examined. Data acquired from the measurement of rock, soil and metal are used to provide a statistical comparison of the analytical uncertainty of each technique, guiding the choice of the most appropriate method. Isotope standard data acquired over this period is also compared with other high-precision laboratories to generate a set of concordant working ratios for the NIST SRM 981 Pb standard: ²⁰⁶Pb/²⁰⁴Pb = 16.9412; ²⁰⁷Pb/²⁰⁴Pb = 15.4988; ²⁰⁸Pb/²⁰⁴Pb = 36.7233.