W同位素高精度负离子热电离质谱(NTIMS)测定方法

W同位素的高精度测定对于研究地球、月球和太阳系其他行星的起源和早期演化、核幔相互作用等领域具有重要意义。本文开展了负离子热电离质谱(NTIMS)高精度W同位素测定方法研究(测定WO_3~-)。采用多接收动态跳扫方式对W同位素进行测定,实时在线测定~(18)O/~(16)O,并利用实验室实时在线氧校正NTIMS Os同位素分析时获得的~(17)O/~(16)O-~(18)O/~(16)O同位素分馏趋势线计算~(17)O/~(16)O,进行氧校正计算。对多接收动态和多接收静态数据处理方式及不同的同位素分馏校正方法进行了详细对比研究。在上述工作基础上,最终建立了以~(186)W/~(184)W=0.92767进行标准化,采用多接收动态方法进行数据处理的在线氧校正W同位素NTIMS测定方法。~(182)W/~(184)W测定结果的外部精度(2RSD)可达3×10~(-6)～6×10~(-6),基本满足地球、月球和行星早期演化等W同位素研究工作的需要。

High-precision tungsten isotopic measurement by negative thermal ionization mass spectrometry (NTIMS)

High precision W isotopic measurement is of great importance for investigation of the origin and early evolution of the Earth, the Moon and other planets in the Solar System, the core-mantle interaction and other related fields. In this study, a newly high-precision W isotopic measurement method by negative thermal ionization mass spectrometry (NTIMS) (measuring WO₃^- ion beams) is presented. The W isotopes were measured in a multi-static analytical protocol with two lines of acquisition. ¹⁸O/¹⁶O ratios were measured in-run on per-integration basis and ¹⁷O/¹⁶O ratios were calculated according to the correlation trend line of mass fractionation between ¹⁷O/¹⁶O and ¹⁸O/¹⁶O obtained from OsO₃^- measurements by NTIMS (Chu et al., 2015), to perform isobaric oxide corrections. The measurement results between multi-dynamic and multi-static data reduction methods were thoroughly compared. With the multi-dynamic data reduction mode, the external precision (2RSD) of ¹⁸²W/¹⁸⁴W normalized to ¹⁸⁶W/¹⁸⁴W=0.92767 (named as (¹⁸²W/¹⁸⁴W)_N6/4-mdyn) in a measurement period could reach 3×10^-6~6×10^-6, but the difference of the average of (¹⁸²W/¹⁸⁴W)_N6/4-mdyn between two measurement periods could be up to~10×10^-6. The external precision (2RSD) of ¹⁸²W/¹⁸⁴W normalized to ¹⁸⁶W/¹⁸³W=1.9859 with the multi-static data reduction mode (named as (¹⁸²W/¹⁸⁴W)_N6/3-mstat; the same below) could reach 5×10^-6~6×10^-6 in a measurement period, and difference of the average of (¹⁸²W/¹⁸⁴W)_N6/3-mstat between two measurement periods was about 4×10^-6. However, when normalizing to ¹⁸⁶W/¹⁸⁴W=0.92767 with the multi-static data reduction mode, an obvious residual linear correlation between fractionation-corrected ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W (i.e., (¹⁸²W/¹⁸⁴W)_N6/4-mstat and (¹⁸³W/¹⁸⁴W)_N6/4-mstat) could be observed. The variation of the ¹⁸²W/¹⁸⁴W ratios during different measurement periods and the correlation between ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W mentioned above are most likely the results of Faraday cup degradation over months-long intervals. Since the multi-dynamic data reduction method can simultaneously eliminate the Faraday cup efficiency and mass fractionation effects, and can achieve~3×10^-6 external precision (2RSD) in a measurement period, we recommend to use multi-dynamic data reduction method for W isotopic measurement.