Calibration bias of experimentally determined chlorine isotope enrichment factors: the need for a two-point calibration in compound-specific chlorine isotope analysis.
BACKGROUND
The recent development of compound-specific online chlorine isotope analysis (37Cl-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 δ37ClSMOCscale, but a critical evaluation of best practices to arrive at reliable δ37ClSMOCsignatures 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 δ37ClSMOCvalues 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 δ37ClSMOCvalues introduced by the different types of calibration was determined.


RESULTS
Different calibration methods resulted in significant differences (up to 30%) in both δ37Cl signatures and εClvalues.


CONCLUSIONS
Our results demonstrate that a two-point calibration together with comprehensive information on reference materials is indispensable and should become standard practice for reliable37Cl-CSIA of organic compounds. Copyright © 2016 John Wiley & Sons, Ltd.
Center for Applied Geoscience, University of Tübingen, Hölderlinstr. 12, 72074, Tübingen, Germany.
Karin A
KA
Ebert
Center for Applied Geoscience, University of Tübingen, Hölderlinstr. 12, 72074, Tübingen, Germany.
Christine
C
Laskov
Institute of Groundwater Ecology, Helmholtz Zentrum München, Ingolstädter Landstr. 1, 85764, Neuherberg, Germany.
Martin
M
Elsner
Center for Applied Geoscience, University of Tübingen, Hölderlinstr. 12, 72074, Tübingen, Germany.
Stefan B
http://orcid.org/0000-0002-9309-8523
SB
Haderlein