Precise lead isotope measurements by the double spike technique: A reconsideration |
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| Authors: | Bruno Hamelin Gérard Manhes Francis Albarede Claude J Allègre |
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| Institution: | Laboratoire de Géochimie et Cosmochimie, Institut de Physique du Globe et Département des Sciences de la Terre,Universités Paris VI et VII, 4, Place Jussieu—75230 Paris cedex 05 France |
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| Abstract: | A reappraisal of the “Double Spike Method” for correction of the mass discrimination in lead isotope analysis is presented in view of the possible efficiency of this method, especially as related to the improved performance of the new multi-collector mass spectrometers.First, the effects of mass discrimination during lead isotope analysis, using the silica-gel phosphoric acid method, are discussed in order to verify the validity of the linear discrimination law. In this discussion, we give evidence of a slight bias of the isotopic composition of the SRM 981 NBS lead standard, commonly used for mass discrimination calibrations.A rigorous way to deal with the propagation of the within-run statistical errors on the precision of the double spike correction is then presented. This allows the optimization of the parameters of the method (spike isotopic composition and precision, spiking proportion) and the calculation of the respective error associated with each corrected isotopic ratio. The perturbation of the double spike correction due to analytical contamination, which has caused previous failure in the application of the method, is avoided by preparation of the mixture between the spike and the sample at the very end of the chemical separation procedure.Using a single collector mass spectrometer, examples of application on lead standards and on a set of samples suggest a possible improvement in the precision by a factor of 3 compared to the usual method of fractionation normalization and confirm the usefulness of the method with the new generation of mass spectrometers. |
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