基于连续流同位素质谱的地下水无机物氯稳定同位素测试方法及影响因素研究

Determination of Chlorine Stable Isotopes in Groundwater Inorganics by Continuous Flow Isotope Mass Spectrometry Method and Analysis of Influence Factors

氯稳定同位素作为示踪剂,能指示水体演化和探索地质环境变化,在地球科学领域具有广泛的应用前景。连续流同位素质谱法(CF-IRMS)具有样品用量少和灵敏度高的优点,被广泛应用于氯等稳定同位素的测试。氯甲烷作为一种易挥发的气体,在反应过程及测试管路中容易逸出,造成样品损失。如何减少反应及测试过程中氯甲烷的流失,实现氯甲烷的分离纯化一直以来是影响CF-IRMS法测试精度的关键。本文利用稳定同位素比例质谱仪(IRMS),结合GC色谱分离技术,以氯同位素的国际标准物质(ISL-354)为基准,选择分析纯、标样、地下水样品三种类型的样品开展地下水无机物中氯稳定同位素CF-IRMS测试方法及影响因素的研究。通过采用GC手动进样和双路进参考气联合配置的进样方式,减少了反应和测试过程中氯甲烷的流失,保障了在线测试过程中氯甲烷的浓度,获得较好的测试精度。结果表明:本方法测定样品的^(37)Cl/^(35)Cl比值的标准偏差基本保持在±0.20‰范围内,不同实验室间样品测量值对比结果的误差小于0.035‰,满足地质样品再现性0.5‰的要求;在实验测试过程中,光、温度、硫酸盐的去除、持续的氦气流、分流比等是影响测试精度的关键参数,应严格控制这些参数的影响,以保障测试方法的精度。

BACKGROUND:Stable chlorine isotope,as an effective tracer,can indicate the water evolution and explore the changes of the geological environment.It has a wide application prospect in earth science.The continuous flow isotope ratio mass spectrometry(CF-IRMS)method has been widely used in the determination of stable isotopes due to the advantages of smaller sample size and higher sensitivity.As a volatile gas,methyl chloride easily escapes during the reaction and the test pipeline,causing sample loss.How to achieve the separation and purification of CH_(3)Cl and improve the conversion rate of CH_(3)Cl of the samples in the reaction and analysis is the key to successful determination of stable chlorine isotopes by CF-IRMS.OBJECTIVES:To understand the influencing factors for the determination of CH_(3)Cl by CF-IRMS.METHODS:The international standard material of chlorine isotope(ISL-354)was chosen as the standard material.The determination method of chlorine stable isotope content in groundwater was studied by using stable isotope ratio mass spectrometer(IRMS)analysis system combined with gas chromatography(GC)separation technology.Three types of samples from analytical pure,standard,and groundwater were chosen.RESULTS:By adopting the combined sampling method of GC manual injection and dual-channel reference gas injection,the loss of methyl chloride during the reaction and testing process was reduced,the concentration of methyl chloride during the online analysis was guaranteed,and better test accuracy was obtained.The results showed that the standard deviation of the ^(37)Cl/^(35)Cl ratio of the samples determined by this method was within the range of 0.20‰.The error of the results from different laboratories was less than 0.035‰,which satisfied the requirement of 0.5‰reproducibility of geological samples.Parameters such as light,temperature,sulfate removal,continuous helium flow and split ratio were the major factors affecting the test accuracy in the experiment.The influence of these parameters should be strictly controlled in the experiment to ensure the accuracy of this method.CONCLUSIONS:This method has the advantages of simple pretreatment,small sample consumption,large number of test samples and short test period,which can improve the efficiency of the analytical methods of the chlorine stable isotope of groundwater inorganics.