Evaluation of Sr chemical purification technique for natural geological samples using common cation-exchange and Sr-specific extraction chromatographic resin prior to MC-ICP-MS or TIMS measurement

In this paper, sample preparation protocols based on common cation-exchange and Sr-specific extraction chromatographic resin were evaluated and investigated for natural geological samples prior to ⁸⁷Sr/ ⁸⁶Sr ratio measurement using MC-ICP-MS or TIMS. Several CRMs and real geological samples were digested using HF, HNO ₃ and HClO ₄ in closed vessels prior to sequential chemical purifications and then Sr isotopic ratios were determined by MC-ICP-MS or TIMS. Because HREEs reside in the Sr fraction when common cation-exhange resin is used, this purification technique is unsuitable prior to MC-ICP-MS analysis, as doubly loaded HREE interfere on Sr masses. We observe an obviously positive relationship between the radiogenic ⁸⁷Sr/ ⁸⁶Sr ratio and the content of HREEs in the Sr fractions, as well as a negative relationship between un-radiogenic ⁸⁴Sr/ ⁸⁶Sr or ⁸⁴Sr/ ⁸⁸Sr ratios and the content of HREEs in the Sr fractions during MC-ICP-MS Sr isotopic measurement. Such effects are insignificant for TIMS measurement, because ionization temperatures are generally lower and can be well controlled during the analyses. In contrast to the traditional Sr purification method (cation exchange resin), Sr-specific extraction chromatographic resin produces high purity Sr fractions, making it feasible for both MC-ICP-MS and TIMS.