Compound-Specific Stable Carbon Isotope Analysis of Chlorofluorocarbons in Groundwater

Abstract

Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), controlled substances due to their role in stratospheric ozone loss, also occur as dissolved contaminants in groundwaters. Stable carbon isotopic signatures may provide valuable new information on the fate of these compounds as has been seen for other priority hydrocarbon contaminants, but to date no method for extraction and isotopic analysis of dissolved CFCs from groundwaters has been developed. Here we describe a cryogenic purge and trap system coupled to continuous flow compound-specific stable carbon isotope analysis mass spectrometry for concentrations as low as 35 μg/L. The method is validated by comparing isotopic signatures from water extracted CFCs against a new suite of isotopic CFC standards. Fractionation of CFCs in volatilization experiments from pure-phase CFC-11 and CFC-113 resulted in enrichment factors (ε) of +1.7 ± 0.1‰ and +1.1 ± 0.1‰, respectively, indicating that such volatile loss, if significant, would produce a more (13)C depleted signature in the remaining CFCs. Importantly, no significant fractionation was observed during volatile extraction of dissolved CFCs from aqueous solutions. δ(13)C values for groundwaters from a CFC-contaminated site were, on average, more enriched than δ(13)C values for pure compounds. Such enriched δ(13)C values have been seen in other hydrocarbon contaminants such as chlorinated ethenes and ethanes due to in situ degradation, but definitive interpretation of such enriched signatures in field samples requires additional experiments to characterize fractionation of CFCs during biodegradation. The establishment of a robust and sensitive method of extraction and analysis, as described here, provides the foundation for such future directions.

Vapor Pressure Isotope Effects in Halogenated Organic Compounds and Alcohols Dissolved in Water

“…Stable carbon isotope effects associated with nonequilibrium volatile loss of the pure phase and compounds dissolved in water were determined. Previous studies (Table ) reported inverse VPIE for pure-phase evaporation of TCE (ε = +0.24 to +0.35 ‰), , CFC-11 (ε = +1.7 ± 0.1 ‰), and CFC-113 (ε = +1.1 ± 0.1 ‰) . In the current study, pure-phase evaporation of chloroform resulted in an enrichment factor of +1.2 ± 0.1‰.…”

“…Quantification of organics dissolved in water was carried out via headspace analysis by gas chromatography (GC) with flame ionization detection (FID). Details and specifics are published elsewhere . Overall precision of this method was better than 5% (1σ).…”

“…Pure-phase volatilization experiments were carried out for methanol, ethanol, and chloroform by placing 1 g of compound in 1 mL open vials and allowing evaporation to 10% of the initial amount at 24 ± 1 °C, which was achieved after 4 to 6 h. After partial evaporation, vials were closed, and the remaining liquid was determined by weight. Stable carbon isotope analysis was carried out at each of these steps following the methods and procedures published previously for chlorofluorocarbons (CFCs) …”

“…For pure-phase evaporation, the enrichment factors shown are the result of three separate experiments (two in the case of methanol) sampled five to eight times during the course of each experiment (Table ). Enrichment factors for pure-phase evaporation of TCE , and CFCs were published previously (Table ). Error bars for δ 13 C are ±0.5 ‰ and represent total analytical uncertainty incorporating both reproducibility and accuracy …”

Volatilization, dissolution and equilibrium isotope effects (2H, 13C, 37Cl) of trichloromethane, trichloroethene and methanol dissolved in water

“…Compound specific stable carbon isotope analysis was carried out by closely following a previously published protocol. 20 Briefly, for headspace analysis, gas samples were injected directly into the GC by using a gastight syringe. After separation on a GSQ column mounted in a Varian 3400, compounds were transformed to CO2 in the connected combustion furnace and transferred to the isotope ratio mass spectrometer (Finigan MAT 252).…”