Laboratory development and validation of vapor phase PFAS methods for soil gas, sewer gas, and indoor air†

There is no standard sampling and analysis method for vapor phase per- and polyfluoroalkyl substances (PFAS) that can be routinely applied to soil gas, sewer/conduit gas, and indoor air samples. We have validated a thermal desorption GC/MS/MS method for the measurement of a set of fluorotelomer alcohols and perfluorooctanesulfonamides collected on multi-bed sorbent tubes. Applications to perfluorocarboxylic acids were also evaluated since there is debate regarding under what circumstances these compounds could be observed moving into gas phase. Perfluorooctanoic acid (PFOA) met Method TO-17 calibration requirements when calibrated using National Institute of Standards and Technology (NIST) traceable standard solutions introduced through the thermal desorption system and using multiple reaction monitoring (MRM) transitions based on precursor mass ions identified in the PFOA spectra. However, subsequent detailed studies suggested that PFOA was decomposing during the thermal desorption sample introduction step when comparing two alternative GC/MS sample introduction techniques. The primary peak resulting from the thermal desorption of PFOA standard had spectra consistent with perfluoro-1-heptene (PFHp-1), suggesting that a degradation reaction was occurring. Therefore, the identification of the PFCA compounds in this method is currently subject to a potential positive interference from the corresponding perfluoroalkene and other thermally labile PFAS. Thus, it may be beneficial to limit the application of the thermal desorption GC/MS/MS method to the fluorotelomer alcohols and perfluorooctanesulfonamides and use a parallel solvent extraction approach to quantify the PFCA-related compounds. Method validation including desorption efficiency, second source verification, storage stability and method detection limit tests were successfully completed for the fluorotelomer alcohols and perfluorooctanesulfonamides target analytes.