Evaluation of the suitability of the Waterloo Membrane Sampler for sample preconcentration before compound-specific isotope analysis

Compound-specific isotope analysis (CSIA) has been used extensively for fingerprinting applications and for the evaluation of the degradation processes in organic contaminant studies in groundwater. Recently, the potential applications of CSIA in unsaturated and vapour intrusion studies have been explored. A key challenge in these studies is the development of analytical protocols for CSIA that can handle the very low concentrations of organic compounds typically found in the unsaturated zone and indoor samples. The objective of this research was to evaluate the applicability of the Waterloo Membrane Sampler (WMS) for CSIA, with intended applications in the unsaturated zone and in vapour intrusion studies. Tests were performed to evaluate isotope effects associated with sorption and desorption of the analytes under active sampling and passive sampling conditions. A standard gas mixture containing three model analytes, hexane, benzene and trichloroethene, was used in the experiments. Tests were designed to evaluate the isotope effect as a function of the time of exposure (3 to 192 hours), amount of analytes sorbed, and exposure temperature (25°C and 12°C). The results obtained in all studies showed very good reproducibility with standard deviations within the accepted analytical error of ±0.5 ‰. The data also showed that the δ13C values of the analytes collected by passive sampling were more depleted than the values obtained by active sampling. However, the degree of fractionation, ranging from 0.4 to 1.4 ‰, was practically constant and independent of the sampling time, mass adsorbed and temperature in the ranges of variables studied. The lowest concentrations that could be detected were 0.65mg/m3for hexane, 0.88mg/m3benzene and 4.38mg/m3for TCE. The method developed was applied in a field study where the results obtained for benzene and toluene collected in the unsaturated zone showed the expected values compared to carbon isotope data obtained for benzene and toluene at the water table. Results obtained in this study confirmed good data reproducibility. This indicates that CSIA coupled with WMS has the potential to become a valuable tool in unsaturated zone studies and in the environmental forensics field.