多离子计数器动态多接收方式锆石Pb同位素比值高精度测定方法

采用装配多离子计数器系统的TRITON Plus热电离质谱仪（thermal ionization mass spectrometer，TIMS），建立了多离子计数器动态多接收锆石Pb同位素（以205Pb为稀释剂）测定方法.相对多离子计数器静态多接收方法，该方法完全消除了不同离子计数器间增益差异对锆石Pb同位素测定的影响.相对传统的单个二次电子倍增器（secondary electron multiplier，SEM）五次跳峰的锆石Pb同位素测定方法，该方法两次跳峰即可测定全部Pb同位素比值，Pb同位素离子流接收效率提高2.5倍，同时，降低了离子流稳定性对Pb同位素分析结果的影响.为验证方法的可靠性，对加入205Pb稀释剂的NIST981 Pb标准和标准锆石清湖（Qinghu）进行了测定.对5×10-11 g 205Pb-NIST981 Pb混标，207Pb/206Pb测定精度达到0.079%（2RSD，n=20）；对清湖标准锆石，获得的年龄结果为159.51±0.11 Ma（2SE，n=7；MSWD=1.1），与文献报道值在误差范围内一致. 

High-Precision Pb Isotope Ratio Determination of Zircon By Multi-Ion Counter TIMS with Multi-Dynamic Collection Method

A high precision zircon Pb isotope ratio measurement method (using 205Pb as spike) was developed with a TRITON Plus thermal ionization mass spectrometer (TIMS) equipped with a multiple ion-counting (MIC) system. With the method, the Pb isotopes were measured with multi-ion counters using a multi-dynamic ion collection method. Compared to the traditional static multi-collection MIC method, the multi-dynamic MIC method can completely eliminate the influence of gain bias between different ion counters on the Pb isotope analytical results. Compared to the traditional zircon ID-TIMS Pb isotopic measurement method using a single secondary electron multiplier (SEM) with five peak jumps, the multi-dynamic MIC method can determine all the Pb isotope ratios of zircon in two peak jumps. Thus, the ion collection efficiency for the Pb isotopes were improved by 2.5 folds. In the meantime, the influences of ion beam stability on the analytical results of Pb isotopes was also reduced. To verify the reliability of the method, NIST981 Pb standards doped with 205Pb spike and Qinghu standard zircons were measured. The analytical precision of 207Pb/206Pb for 5×10-11 g of 205Pb-NIST981 Pb mixture can achieve 0.079% (2RSD, n=20). The analytical 206Pb/238U age result for Qinghu standard zircon was 159.51±0.11 Ma(2SE, n=7; MSWD=1.1), which was consistent with the previously reported values within analytical error.