δ¹³C and δ³⁷Cl on Gas-Phase TCE for Source Identification Investigation - Innovative Solvent-Based Sampling Method

The use of stable isotopes (¹³C, ²H and ³⁷Cl) has shown to be a reliable method to establish a link between the improper presence of a volatile organic compound (VOC) and the suspected emitting source, or to distinguish compound sources on the same contaminated site. While the isotopic tool is commonly applied to contaminated groundwater studies, the tool is considerably less applied for gas-phase studies such as indoor air or atmospheric investigations. The main reason is low VOC concentrations which impede collecting enough VOC mass to perform reliable isotope analysis. Accordingly, a successful isotopic investigation conducted on gas-phase VOC relies on a sampling technique that can sufficiently accumulate the compound during the sampling. Thus far, the most common sampling technique used for atmospheric or indoor air studies is the adsorption tubes. Although the latter method provides reliable isotopic measurements, the sampling method is time consuming and the results require elaborate validation for each sample. A simple and innovative solvent-based sampling method to collect VOC from the gas phase is presented. By analogy to the sorption tube method, the proposed sampling method uses an organic solvent as a sink to accumulate the VOC during the sampling process. Laboratory experiments were first carried out to evaluate the VOC dissolution efficiency in solvent during constant air flow injection, to identify suitable solvents (solvent volatility, VOC solubility) and to confirm reproducible isotopic measurements on selected VOCs (benzene and trichloroethene). After successful preliminary results, the performance of the sampling method was evaluated during an experiment carried out in a former industrial building. A TCE liquid source with known isotopic composition (δ¹³C and δ³⁷Cl) was used to create a gas-phase plume inside an isolated room. The room air was sampled by using a device developed for the solvent-based method. Summa canisters were used in parallel as the reference method. The δ¹³C value for TCE showed excellent agreement between the two sampling methods. Furthermore, the δ¹³C and δ³⁷Cl values measured for the TCE sampled with the solvent method were similar to the source values. The latter results suggested an absence of physical processes causing an isotope fractionation and consequently allowed establishing a direct link between the gas-phase TCE and the emitting source. The combination of the laboratory and field tests underlines a promising sampling method to perform CSIA measurements on gas-phase VOCs. Advantages compared to the use of sorbent tubes, method restrictions and the detection limits will also be covered.