A Calibration Strategy of ⁸⁷Sr/⁸⁶Sr Ratio for Rocks with High Rb/Sr Measured by Multiple Collector-Inductively Coupled Plasma-Mass Spectrometry

The presence of ⁸⁷Rb causes isobaric interference of ⁸⁷Sr for Multiple Collector-Inductively Coupled Plasma-Mass Spectrometric (MC-ICPMS) measurements. ⁸⁷Sr/⁸⁶Sr isotopic composition of rocks with Rb/Sr≤0.0005 after chromatography separation can be effectively corrected by using a traditional Rb-interference calibration. However, the traditional calibration method is not valid for solutions with high Rb/Sr ratio which cannot completely remove isobaric interference of ⁸⁷Rb to obtain an accurate ⁸⁷Sr/⁸⁶Sr ratio. The two curves of the ⁸⁷Sr/⁸⁶Sr isotope ratio and Rb/Sr element content ratio were obtained by two sets of experiments to calibrate Rb interference in this work. Based on theoretical analysis, a new calibration method is reported, including an isotope fractionation calibration. The method yields an accurate ⁸⁷Sr/⁸⁶Sr ratio for geology samples with Rb/Sr<0.2 after chemical separation, and reduces requirements of chemical separation for isotope measurements by MC-ICPMS. However, the measurement accuracy for geological samples with Rb/Sr>0.2 after chemical separation was significantly lower due to mass fractionation and memory effect, which can not be calibrated to obtain an accurate Sr isotopic composition by any kind of calibration method.