Stable Isotope (C, Cl, and H) Fractionation during Vaporization of Trichloroethylene

Stable isotope fractionation during vaporization of trichloroethylene has been measured, with possible application as a technique to investigate subsurface behavior. The equilibrium value of Δ 13 C vapor - liquid has been measured between 5 and 35 °C, and Δ 13 C vapor - liquid, Δ 37 Cl vapor - liquid, and Δ D vapor - liquid have been measured during progressive evaporation of liquid trichloroethylene at 22 ± 2 °C. Equilibrium values of Δ 13 C vapor - liquid show a total range of 0.07−0.82‰, with a trend of decreasing Δ 13 C vapor - liquid with increasing temperature, from approximately +0.7‰ at 5−15 °C to approximately +0.1‰ at 35 °C. Progressive evaporation experiments yield values of Δ 13 C vapor - liquid = +0.35‰ and +0.24‰, Δ 37 Cl vapor - liquid = −1.64‰, and Δ D vapor - liquid = +8.9‰. The positive values for carbon and hydrogen isotope fractionation, while unexpected, are consistent with available quantitative and qualititative data for trichloroethylene and other contaminant hydrocarbons, but a satisfactory explanation for these observations, particularly in combination with the negative value for chlorine, remains elusive. Vapor−liquid fractionation factors have application to the investigation of the behavior of trichloroethylene at contaminated sites, particularly sites undergoing remediation by techniques such as soil vapor extraction and soil bioventing.