Gd matrix effects on Eu isotope fractionation using MC-ICP-MS: Optimizing europium isotope ratio measurements in geological rock samples

Highlights

• Optimal conditions for measuring Eu isotope ratios in natural materials by MC-ICP-MS are described.

• Measurement and estimation of Eu isotope ratios in geological samples by MC-ICP-MS using Sm or Gd as internal standards can be biased by pseudo-fractionation due to incomplete purification of Eu fractions and interference during analysis.

Abstract

Europium (Eu) has only two isotopes (¹⁵¹Eu and ¹⁵³Eu). As sequential rare earth elements, Eu and Gd do not easily separate from each other during column chromatography, making it difficult to measure accurate and precise Eu isotope ratio during mass spectral analysis. Eu isotope ratios can be determined by MC-ICP-MS using internal Sm or Gd spikes to correct for mass discrimination. In this report, we discuss and propose optimal conditions for precise and accurate Eu isotope ratio measurements in geological materials (igneous rock samples) using MC-ICP-MS. The NIST3117a ultrapure chemical reagent showed almost no Eu isotope fractionation regardless of the kind of isotope pair used in normalization. A pure Eu fraction with almost no Gd matrix extracted from rock samples also showed nearly the same degree of Eu isotope fractionation regardless of the isotope pair used in normalization. In the case of Eu fraction extracted from highly fractionated granite and high silica volcanic rocks, when ¹⁵⁴Gd interference relative to the internal ¹⁵⁴Sm standard exceeded ca. 0.1% (due to incomplete Eu separation), this Gd matrix effect created artifactual variation in Eu isotope ratios during MC-ICP-MS analysis. In order to obtain the best estimates of Eu isotope fractionation in geological samples, especially high silica rocks, our analytical methods require complete Eu separation without Gd matrix during sample preparation.