Online methodology for determining compound‐specific hydrogen stable isotope ratios of trichloroethene and 1,2‐cis‐dichloroethene by continuous‐flow isotope ratio mass spectrometry

Abstract

Carbon and chlorine compound-specific isotope analysis (CSIA) is utilized in chlorinated solvent contamination studies of soil and groundwater contaminated sites. However, in field studies, hydrogen CSIA has been used only in non-chlorinated volatile organic compound (VOC) investigations, due to low conversion yields into hydrogen gas and poor reproducibility. Therefore, it is important to develop hydrogen CSIA methodology for soil and subsurface contamination studies. A new analytical method for determining compound-specific hydrogen stable isotope ratios is presented. The isotopic ratios were measured by gas chromatography/isotope ratio mass spectrometry (GC/IRMS) coupled with a chromium reduction system. The method was used to determine the δ(2) H values of trichloroethene (TCE) and 1,2-cis-dichloroethene (cis-DCE). The accuracy of the method was verified by conducting comparison measurements of standards by the conventional offline technique and the new method. The precision of the new analytical method (better than ±7 ‰) is better than that obtained from the offline method. The quantification limits of the headspace-solid-phase microextraction (SPME) are 400 µg/L and 200 µg/L for TCE and cis-DCE, respectively. The quantification limits can be improved by adopting a more efficient pre-concentration system such as purge-and-trap or thermal adsorption. This analytical method will facilitate the use of hydrogen CSIA on chlorinated solvents, which can be beneficial in multi-isotope approaches (coupling δ(2)H values with δ(13)C and/or δ(37)Cl values) in field site investigations where source identifications and contaminant behaviours are questioned.