热电离质谱测定钙同位素过程中双稀释剂的选择

Optimization of Calcium Double-Spike on Measuring Calcium Isotopic Compositions by Thermal Ionization Mass Spectrometer(TIMS)

本研究从理论和实践两方面详细阐述了热电离质谱法(TIMS)测定钙同位素过程中双稀释剂种类的选择,以及样品与双稀释剂最佳稀释比的确定。结果表明:在TIMS测定中,42 Ca-43 Ca双稀释剂比目前常用的42 Ca-48 Ca和43 Ca-48 Ca双稀释剂更具优势。首先,尽管使用42 Ca-43 Ca双稀释剂可能产生较大的误差,但其与样品存在较大的稀释比区间,即42 Ca-43 Ca双稀释剂的最小误差变化更为稳定,可以允许更为宽泛的样品和稀释剂的混合比例;其次,42 Ca-43 Ca双稀释剂的平均质量数与目标比值44 Ca/40 Ca的平均质量数仅相差0.5,在样品量较少或者测量过程中分馏效应较大时,使用42 Ca-43 Ca双稀释剂做仪器分馏校正产生的偏差最小;最后,42 Ca和43 Ca易被同时接收且离子光学聚焦效应较小,可降低或消除仪器本身微小改变对信号不稳定性的影响,有利于保证数据精确度。综上,42 Ca-43 Ca双稀释剂可作为TIMS测量钙同位素的首选。

Detailed investigations on how to choose the optimal double-spike and theoptimal mixture ratios of the two single spikes in the double-spike,as well as on how to determinate the optimal sample/spike ratio range during measurement of calcium isotopic compositions by thermal ionization mass spectrometer（TIMS）,were carefully carried out in both theory and practices.Compared to the 42Ca-48 Ca and 43Ca-48 Ca doublespikes,which are used widely,42Ca-43 Ca double-spike has its distinctive advantages.Firstly,although the error on the fractionation factorαis relatively bigger when using42Ca-43 Ca double-spike,but it only fluctuates a little at a much wide range of sample/spike ratio in spiked samples than using the 42Ca-48 Ca or 43Ca-48 Ca.Secondly,the average mass of the two isotopes in 42Ca-43 Ca double-spike and target 44Ca/40 Ca ratio,only differs by 0.5mass unit from each other,thus errors in the corrected ratio resulted from fractionation correction will be minimized,especially when a lower amount of sample was loaded and/or the large fractionation on the instrument occurred during mass determination.Finally,42 Ca and 43 Ca can be simultaneously collected by TIMS without significant ion optic effect,which can enhance the data quality collected.In summary,42Ca-43 Ca double-spike should be the optimal one to be used in the double-spike technique for precisely measuring calcium isotopic compositions by TIMS.