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

Abstract

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.

Fig. 1

Experiment 2: total ion chromatogram of PFOA vapor collected on PFAS tube analyzed using routine TD-GC/MS parameters yields multiple peaks; Y axis is area counts, X axis is acquisition time (min).

Fig. 2

Total ion chromatogram of PFOA vapor collected on tube analyzed at 200 °C and 250 °C tube and trap desorption temperatures; Y axis is area counts, X axis is acquisition time (min).

Fig. 3

Total ion chromatograms for PFOA analysis on HP-5MS column with direct injection (top panel) vs. thermal desorption (bottom panel); area counts on Y axis, and retention time in minutes on the x axis, labels on figure are X = non-perfluorinated compound; PF = perfluorinated compound, identified structures labeled.

Fig. 4

Mass spectra for primary peak measured from PFOA injection using direct injection and TD on HP-5MS column; X axis is mass to charge ratio, y axis is counts; Upper panel is direct injection method, bottom panel is thermal desorption, right inset shows chromatogram.

Fig. 5

Total ion chromatograms for PFOA analysis on DB-624 column (Y-axis scaled to match Fig. 3) with direct injection (top panel) vs. thermal desorption (bottom panel); X axis is acquisition time in minutes, Y axis is area counts, labels on figure: X = non-perfluorinated compound; PF = perfluorinated compound not otherwise identified, identified compounds labeled.

Fig. 6

Left and right mass spectral scans for peak A Measured from PFOA injection Using TD on DB-624 column. In main panel Y axis is area counts, X axis is mass to charge ratio; right inset is total ion chromatogram and selected ion chromatogram of peak of interest, with x axis retention time and y axis area counts.

Fig. 7

Total ion chromatograms for PFOA and PFHp-1 standards analyzed on HP-5MS column with thermal desorption (top and bottom panels) vs. direct injection (middle panel); X axis is acquisition time, Y axis is area counts, labels on figure: X = non-perfluorinated compound; PF = perfluorinated compound not otherwise identified, individual identified compounds labeled.

Fig. 8

Total ion chromatograms for PFOA (TD) and PFHp-1 on DB-624 column; top panel is thermal desorption analysis of PFOA, middle panel is direct injection analysis of PFHp-1, bottom panel is thermal desorption analysis of PFHp-1; in each panel X axis is acquisition time (min) and Y axis area counts. Labels on peaks X = non-perfluorinated compound; PF = perfluorinated compound; specific identified compounds labeled.