Online methodology for determining compound-specific hydrogen stable isotope ratios of trichloroethene and 1,2-cis-dichloroethene by continuous-flow isotope ratio mass spectrometry Orfan Shouakar-Stash 1 * and Robert J. Drimmie 2 1 Department of Earth and Environmental Sciences, University of Waterloo, 200 University Ave. West, Waterloo, Ontario, Canada, N2L 3G1 2 Isotope Tracer Technologies Inc. (IT 2 ), 695 Rupert St., Unit B, Waterloo, Ontario, Canada, N2V 1Z5 RATIONALE: 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. METHODS: 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 d 2 H values of trichloroethene (TCE) and 1,2-cis-dichloroethene (cis-DCE). RESULTS: 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 mg/L and 200 mg/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. CONCLUSIONS: This analytical method will facilitate the use of hydrogen CSIA on chlorinated solvents, which can be beneficial in multi-isotope approaches (coupling d 2 H values with d 13 C and/or d 37 Cl values) in field site investigations where source identifications and contaminant behaviours are questioned. Copyright © 2013 John Wiley & Sons, Ltd. Soil and groundwater contamination by hazardous chemicals is a widespread problem because of the high costs associated with its investigation and remediation. Compound-specific isotope analysis (CSIA) is one of the new techniques employed in investigating the fate of organic contaminants, including chlorinated volatile organic compounds (VOCs), in the subsurface. Analytical techniques to evaluate carbon and chlorine stable isotopic signatures in a variety of organic compounds are well established. [1–9] CSIA has been a powerful tool in contaminant source determination as well as in assessing the fate of contaminants in the subsurface, including natural attenuation. [10–28] Carbon has been the most commonly applied type of CSIA in both chlorinated and non-chlorinated organic contaminant field investigations. Some more recent field studies have utilized chlorine CSIA; however, hydrogen CSIA has been used only in non-chlorinated VOC investigations. The use of hydrogen CSIA in chlorinated contaminant studies has been limited due to analytical difficulties such as yield and reproducibility. Several attempts have been made to make hydrogen CSIA available to the scientific community, however with limited success. For example, in 2004, Zwank [29] used Continuous- Flow Isotope Ratio Mass Spectrometry (CF-IRMS) coupled with Gas Chromatography (GC) and a high-Temperature Conversion (TC) system to analyze trichloroethene (TCE) (C 2 HCl 3 ) and cis-dichloroethene (cis-DCE) (C 2 H 2 Cl 2 ). However, this method was associated with several challenges, including the formation of HCl as a by-product which might damage the analytical equipment and cause isotopic fractionation during the analysis. [30] In turn, this was probably the main reason for the poor precision achieved by this method (up to 91 % standard deviation for TCE). In 2007, Chartrand et al. [31] reported a hydrogen CSIA GC-TC/IRMS method for analyzing 1,2-dichloroethane (1,2-DCA), with a liquid nitrogen trap attached between the TC system and the mass spectrometer to trap any HCl by-product formed during the high-temperature conversion. The method had excellent precision (standard deviation of better than 5 %) but it was tested only on high concentration aqueous solutions. Several other methods have been introduced to determine hydrogen isotope ratios of chlorinated compounds. [30,32–34] However, they were mainly used to characterize pure phase compounds and were not capable of hydrogen CSIA of field site samples. Furthermore, these methods required several micrograms of * Correspondence to: O. Shouakar-Stash, Department of Earth and Environmental Sciences, University of Waterloo, 200 University Ave. W., Waterloo, Ontario, Canada, N2L 3G1. E-mail: orfan@uwaterloo.ca Copyright © 2013 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2013, 27, 1335–1344 Research Article Received: 29 October 2012 Revised: 20 March 2013 Accepted: 20 March 2013 Published online in Wiley Online Library Rapid Commun. Mass Spectrom. 2013, 27, 1335–1344 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6578 1335