Gou Long-Fei, Jin Zhangdong, Galy Albert, Sun He, Deng Li, Xu Yang
State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, 710061, China.
Centre de Recherches Pétrographiques et Géochimiques, UMR7358, CNRS, Université de Lorraine, 54500, Vandoeuvre les Nancy, France.
Rapid Commun Mass Spectrom. 2019 Feb 28;33(4):351-360. doi: 10.1002/rcm.8356.
High-precision determination of magnesium (Mg) isotopes can now be routinely achieved by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The analytical sensitivity and instrumental mass discrimination behavior of this method are, however, sensitive to the types of sample and skimmer cones used in these measurements, so it is important that these parameters should be investigated.
Using the sample-standard-bracketing method in the wet-plasma mode, four available combinations of sample and skimmer cones [Jet sample cone + H skimmer cone (Jet + H), standard sample cone + H skimmer cone (Standard + H), standard sample cone + X skimmer cone (Standard + X), and Jet sample cone + X skimmer cone (Jet + X)] were systematically investigated for peak shape, sensitivity, mass discrimination, accuracy, and precision in Mg-isotopic ratio determination using a Neptune plus MC-ICP mass spectrometer.
The results showed that different cone combinations do not affect peak shapes but would significantly change the sensitivities for Mg-isotopic determinations. Compared with using the Standard + H, the sensitivities of Mg-isotopic determinations were enhanced by approximately a factor of 1.3, 1.4, and 1.9 by using the Standard + X, the Jet + H, and the Jet + X combinations, with the most stable mass discrimination behaviors obtained by the Jet + H. The instrumental mass fractionation slope for any combination of a modified cone geometry (i.e. Standard + X, Jet + X, and Jet + H) is 0.500, while it is 0.510 for the Standard + H. In addition, the mass discrimination behavior is related to Mg concentrations once the combination is set, indicating the necessity of concentration match during Mg-isotopic determination.
The precision and accuracy of the Jet + H combination are better than those of the other combinations, and this is further supported by the validation of the Mg-isotope data for four international reference materials: Cambridge-1, NASS-6, AGV-2, and BHVO-2. As the Jet + H combination also provides a high signal, this combination gives the most robust strategy for the highly precise and accurate determination of Mg isotopes.
现在可以通过多接收电感耦合等离子体质谱法（MC-ICP-MS）常规地实现镁（Mg）同位素的高精度测定。然而，该方法的分析灵敏度和仪器质量歧视行为对这些测量中使用的样品和分离器锥的类型敏感，因此研究这些参数很重要。
在湿等离子体模式下使用样品-标准品-夹心法，使用海王星加MC-ICP质谱仪系统地研究了样品和分离器锥的四种可用组合[喷射样品锥+H分离器锥（Jet+H）、标准样品锥+H分离器锥（Standard+H）、标准样品锥+X分离器锥（Standard+X）和喷射样品锥+X分离器锥（Jet+X）]在Mg同位素比值测定中的峰形、灵敏度、质量歧视、准确性和精密度。
结果表明，不同的锥组合不会影响峰形，但会显著改变Mg同位素测定的灵敏度。与使用Standard+H相比，使用Standard+X、Jet+H和Jet+X组合时，Mg同位素测定的灵敏度分别提高了约1.3、1.4和1.9倍，其中Jet+H获得的质量歧视行为最稳定。对于任何改进的锥几何形状组合（即Standard+X、Jet+X和Jet+H），仪器质量分馏斜率为0.500，而对于Standard+H则为0.510。此外，一旦设置了组合，质量歧视行为与Mg浓度相关，这表明在Mg同位素测定过程中进行浓度匹配的必要性。
Jet+H组合的精密度和准确性优于其他组合，这得到了四种国际参考物质：剑桥-1、NASS-6、AGV-2和BHVO-2的Mg同位素数据验证的进一步支持。由于Jet+H组合也提供高信号，因此该组合为高精度和准确测定Mg同位素提供了最可靠的策略。
