Expanding the calibration range of compound-specific chlorine isotope analysis by the preparation of a 37 Cl-enriched tetrachloroethylene

Abstract

Rationale: The recent development of reliable GC/qMS methods for δ³⁷ Cl compound-specific stable isotope analysis (CSIA) paves the way for dual carbon-chlorine isotope analysis of chlorinated ethenes and thus allows deeper insights into underlying transformation processes/mechanisms. A two-point calibration is indispensable for the precise and correct conversion of raw data to the international δ³⁷ Cl_SMOC scale. The currently available calibration standards for tetrachloroethylene (PCE) span only a very narrow range from -2.52‰ (EIL2) to +0.29‰ (EIL1), which is considerably smaller than observed δ³⁷ Cl isotope enrichment in (bio-)transformation studies (up to 12‰).

Methods: We describe the preparation and evaluation of a new ³⁷ Cl-enriched PCE standard to avoid bias in δ³⁷ Cl CSIA arising from extrapolation beyond the calibration range. The preparation comprised: (i) partial PCE reduction by zero-valent zinc in a system of PCE, ethanol (initial volume ratio 3/5) and trace amounts of water followed by (ii) liquid-liquid extraction and (iii) a subsequent fractional distillation to purify the ³⁷ Cl-enriched PCE.

Results: The obtained PCE (PCE_enriched ) showed a purity of 98.8% (mole fraction) and a δ³⁷ Cl_SMOC value of +10.8 ± 0.5‰. The evaluation of an experimental dataset with and without extrapolation showed no significant variation.

Conclusions: The new PCE standard (PCE_enriched ) expands the calibration range to 13.3‰ (previously 2.8‰) and thus prevents potential bias introduced by extrapolation beyond the calibration range.