A technique for carbon and chlorine isotope analyses of chlorinated aliphatic hydrocarbons in groundwater

Abstract

Chlorinated aliphatic hydrocarbons (CAHs), significant contaminants in groundwater, can be characterized by stable isotopic compositions of carbon and chlorine. Previously published methods were of low analytical sensitivity or not ideal for natural samples with low concentrations of CAHs. This method is reported here to carry out simultaneously carbon and chlorine isotope analyses for micromolar concentrations of dissolved CAHs. It was executed by extracting and converting CAHs to carbon dioxide and methyl chloride (CH₃Cl). Specially, a continuous-flow interface GasBench II was used to extract CH₃Cl for online chlorine isotope analysis. As a result, it greatly enhances the efficiency for isotope analysis by eliminating procedures for offline CH₃Cl preparation and separation. Sample size requirement was reduced to approximately 11 μmol chlorine. The standard deviation of δ¹³C and δ³⁷Cl for both TCE solvents and water samples was better than 0.30‰ and 0.20‰ (1σ), respectively. Carbon and chlorine isotope analyses can be used as an important tool to study the sources of organic contaminants in groundwater and their behaviors in the aquifers. The method is applicable to manufacturers’ products as well as a sample from a polluted site in principle, which will be validated in our field studies.

Keywords

chlorine isotope / carbon isotope / chlorinated aliphatic hydrocarbon (CAH) / groundwater / organic contamination

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