Determination of Eu isotopic ratio by multi-collector inductively coupled plasma mass spectrometry using a Sm internal standard

Abstract

This contribution describes methods for measuring Eu isotopic ratios in geological materials using multi-collector inductively coupled plasma mass spectrometer. These methods require an initial external correction of mass discrimination effects (between the two Eu isotopes) using a Sm isotopic spike (150Sm/154Sm = 0.3244) added to samples. Eu was then estimated according to an exponential law. We measured Eu isotope ratios (151Eu/153Eu) for six high-purity commercial Eu reagents using the NIST 3117a standard as a reference (δ151/153Eu = 0.0‰). The Eu reagents showed minor fractionation effects ranging from 0.029 ± 0.007‰ (1σ, n = 22) to 0.061 ± 0.006‰ (1σ, n = 13). We then investigated Eu isotope fractionation during cation exchange chromatography, which efficiently separates the Eu fraction (>99.99% purity). A known amount of rare earth element (REE) standard solution was eluted from columns using 2-hydroxyisobutyric acid. Fourteen sequentially collected Eu fractions gave δ151/153Eu values ranging from −1.80‰ for early eluting fractions to 2.56‰ for late eluting fractions. A numerically integrated value for the 14 fractions resembled that measured for the total eluted solution. These results indicate that different reagents give different Eu isotopic values. Fractionation may occur during reagent production or it may be inherited from geological source materials. We also measured Eu isotopic variation among five different geochemical reference rocks (BCR2, BHVO2, BIR1a, GSP2, JG1a). The reference standards also gave slightly different 151/153Eu isotope ratios. These results suggest Eu isotopic fractionation during rock-forming processes.