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

Highlights: • Eu isotopic compositions were measured from pure commercial chemical reagents. • An experimental procedure for high purity Eu separation was proposed. • A method to measure and correct mass bias for the two Eu isotopes using MC-ICP-MS was proposed. • Evidence of Eu isotopic fractionation in natural systems was observed. -- 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 (δ^151/153Eu = 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 δ^151/153Eu 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.