Calibration bias of experimentally determined chlorine isotope enrichment factors: the need for a two-point calibration in compound-specific chlorine isotope analysis.

RATIONALE: The recent development of compound-specific online chlorine isotope analysis (37 Cl-CSIA) methods has fostered dual chlorine-carbon isotope studies to gain better insights into sources and environmental transformation reactions of chlorinated ethenes. One-point and two-point calibration schemes are currently used to convert raw data to the international δ37 ClSMOC scale, but a critical evaluation of best practices to arrive at reliable δ37 ClSMOC signatures and enrichment factors was missing and is presented here. METHODS: Aqueous solutions of neat perchloroethylene and trichloroethylene (TCE) and aqueous samples from a TCE biodegradation experiment with pure cultures of Desulfitobacterium hafniense Y51 were analysed for their chlorine isotope ratios using GC/qMS and GC/IRMS. The δ37 ClSMOC values were obtained using one-point and two-point calibration schemes. Chlorine isotope enrichment factors, εCl , were calculated using both approaches and the corresponding bias of δ37 ClSMOC values introduced by the different types of calibration was determined. RESULTS: Different calibration methods resulted in significant differences (up to 30%) in both δ37 Cl signatures and εCl values. CONCLUSIONS: Our results demonstrate that a two-point calibration together with comprehensive information on reference materials is indispensable and should become standard practice for reliable 37 Cl-CSIA of organic compounds. Copyright © 2016 John Wiley & Sons, Ltd.