BACKGROUNDStable 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 CH₃Cl and improve the conversion rate of CH₃Cl of the samples in the reaction and analysis is the key to successful determination of stable chlorine isotopes by CF-IRMS.

OBJECTIVESTo understand the influencing factors for the determination of CH₃Cl by CF-IRMS.

METHODSThe 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.

RESULTSBy 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 ³⁷Cl/³⁵Cl 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.

CONCLUSIONSThis 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.