高Rb/Sr岩石样品中Sr同位素多接收等离子体质谱分析校正方法研究

在利用多接收电感耦合等离子体质谱(MC-ICPMS)进行Sr同位素研究中,⁸⁷Rb对于⁸⁷Sr干扰严重。岩石样品经化学分离后,若Rb/Sr≤0.0005,可以采用传统的Rb干扰扣除方法对⁸⁷Sr/⁸⁶Sr测定值进行准确校正;但如果样品经化学分离后仍含有较高的Rb/Sr比,同量异位素的干扰不能完全消除,则无法准确校正⁸⁷Sr/⁸⁶Sr测定值,直接影响测试结果的准确度。本文针对Rb含量较高的地质样品设计两组实验,确定了⁸⁷Sr/⁸⁶Sr同位素比值与Rb/Sr元素含量比值的关系曲线,并在理论分析的基础上,提出包含同位素分馏校正在内的重叠干扰校正方法。通过实际地质样品验证,该校正方法在较高含量Rb元素共存(Rb/Sr<0.2)的Sr纯化液中,能够较为准确地测量⁸⁷Sr/⁸⁶Sr同位素比值,降低了MC-ICPMS分析地质样品中Sr同位素时对化学分离步骤的要求。而对于Rb/Sr>0.2的地质样品,因仪器分馏效应和记忆效应影响,测试精确度大大降低,无论采用何种校正方法均无法得到准确的Sr同位素组成。

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.