Isotope fractionation of volatile organic compounds in porous media under variably saturated conditions

In order to investigate chlorine isotope behaviour, it was necessary to develop analytical methods and methods for data interpretation. The accuracy, precision and amount dependency of chlorine isotope ratio analyses performed with gas chromatography quadrupol mass-spectrometry (GC-qMS) were evaluated for two Agilent and Thermo instruments. The calibration of raw data to SMOC scale (Standard Mean Ocean Chloride) with external standards has been shown to deviate between instruments and over time. For that reason, at least two calibration standards are required to obtain reproducible isotopic values and true isotopic variations between samples. Precision of measurements varies from an instrument to another (1σ ≈ 0.2-0.5% for Agilent and 0.2-0.9% for Thermo GC-qMS). Additionally, different amount dependencies of δCl values were observed for each instrument, but could be eliminated by adjusting amplitudes of standards and samples. Nevertheless, δCl measurements showed good agreement between various instruments when the same external standards were used. In the second part of this chapter, a theoretical comparison between isotope and isotopologue fractionation during vaporization and diffusion was performed, as physical processes occurring in the unsaturated zone take place among isotopologues and not directly isotopes. It was demonstrated that, during vaporization, diffusion and combination of them, the isotope ratio evolves parallel to the isotopologue ratio. Partially published in Analytical Chemistry and Environmental Science and Technology: • Bernstein, A.; Shouakar-Stash, O.; Ebert, K.; Laskov, C.; Hunkeler, D.; Jeannottat, S.; SakaguchiSöder, K.; Laaks, J.; Jochmann, M. A.; Cretnik, S.; Jager, J.; Haderlein, S. B.; Schmidt, T. C.; Aravena, R.; Elsner, M. Compound-specific chlorine isotope analysis: A comparison of gas chromatography/isotope ratio mass spectrometry and gas chromatography/quadrupole mass spectrometry methods in an interlaboratory study. Anal. Chem. 2011, 83 (20), 7624-7634. • Jeannottat, S.; Hunkeler, D. Chlorine and carbon isotopes fractionation during volatilization and diffusive transport of trichloroethene in the unsaturated zone. Environ. Sci. Technol. 2012, 46 (6), 3169-3176. 2. Analytical and conceptual developments for the application of Cl isotopes in UZ studies 19 2.