多接收器等离子体质谱法Zn同位素比值的高精度测定

The application of MC-ICP-MS to high-precision measurement of Zn isotope ratios

详细报道了Zn同位素比值的多接收器等离子体质谱(MC-ICP-MS)高精度测定方法,包括:MC-ICP-MSZn同位素测量过程中的质量歧视校正、同质异位素干扰评估、基质效应调查和同位素测量的长期重现性检验。研究表明,在测定条件下,运用标样-样品交叉法能有效地进行仪器质量歧视校正。同质异位素干扰的评估通过3种方式进行,即:在高分辨状态下同质异位数干扰信号的直接测定,低分辨状态下Zn同位素原始数据间相关关系的检验和低分辨下浓度梯度效应研究。结果表明,在低分辨模式下,尽管66Zn、67Zn、68Zn的同质异位素干扰信号很小,但的确存在,要获得准确同位素比值,必须使标样和样品的浓度在合适的范围内匹配。在基质效应方面,主要考察Fe对Zn同位素比值测定的影响。结果表明,当溶液中Fe/Zn(质量比)不大于0.2时,Fe对Zn同位素比值测定无影响。重复性测定中,δ66ZnGSB-Romil=6.96‰±0.11‰(2sd),δ67ZnGSB-Romil=10.4‰±0.20‰(2sd),δ68ZnGSB-Romil=13.8‰±0.22‰(2sd),达到国际同类实验室先进水准。运用所建立的方法,对地质岩石成分分析国家标准物质GBW07270(闪锌矿)进行了Zn同位素平均成分测定为:δ66Zn=6.71‰±0.03‰(2σ),δ67Zn=10.08‰±0.05‰(2σ),δ68Zn=13.37‰±0.07‰(2σ)。

A method for high-precision measurement of Zn isotope composition has been established, which includes correction of instrumental mass discrimination, assessment of isobaric interferences, investigation of matrix effect, and examination of long-term repeatability. It has been shown that the instrumental mass discrimination can be corrected with the standard-sample-bracketing method. The assessment of isobaric interferences was carried out using three approaches, i. e.,① the direct measurement of the interference signals in the high-resolution mode; ② the examination of correlation between Zn isotope ratios of the raw data in a low-resolution mode; ③ the investigation of the deviation of the measured Zn isotope values relative to the ＂real value＂. The matrix effect was examined by using the mixed solution of Fe and Zn relative to pure Zn solution. It is found that the content of Fe has no detectable effect on the measured values of Zn isotope when the Fe/Zn ratio is lower than or equal to 0.2. The repeatability assessed over a period of 7 months is better than 0.06% （2sd） per amu. The Zn isotope composition of a national standard reference material of sphalerite （GBW 07270） was analyzed with the techniques established, and the results are δ^66Zn = 6.71‰±0.03‰（2σ）, δ67 Zn = 10.08‰±0.05‰（2σ）, and δ^68Zn= 13.37‰±0.07‰（2σ）.