Method and validation for the analysis of perfluorinated compounds in water by pre-sampling isotope dilution-direct injection-LC/MS/MS†
A pre-sampling isotope dilution-direct injection-liquid chromatography tandem mass spectrometry (DI-LC/MS/MS) analytical method for the analysis of perfluorinated compounds (PFCs) in water is presented. This pre-sampling isotope dilution method incorporates stable isotope internal standards (ISs) and surrogate recovery standards (SRSs) added to sample bottles prior to sample collection. Pre-sampling isotope dilution corrects for PFC adsorption losses and enables a simple quantitative DI-LC/MS/MS water method with a 28 day sample holding time. Method analytes include perfluorinated carboxylic acids (PFCAs) (C4–C12), perfluorinated sulfonic acids (PFSAs) (C4, C6, and C8), perfluorooctane sulfonamide (PFOSA), and four SRSs ([2,3,4-13C3]PFBA, [1,2,3,4-13C4]PFOA, [1,2,3,4-13C4]PFOS, and [1,2-13C2]PFUnA). At 28 day sample holding times, mean recoveries of laboratory reagent water samples (Milli-Q™ water containing hardness at 165 mg equivalent CaCO3 L−1) fortified with the PFC method analytes at 0.2–40 ng mL−1 (0.1–1 ng mL−1 SRSs) are 94.9–115% with relative standard deviations of 0.97–7.9%. At 28 day sample holding times in fortified laboratory reagent water samples, single laboratory lowest concentration minimum reporting levels of 0.010–0.020 ng mL−1 (0.075 ng mL−1 PFBA) are demonstrated for method analytes and SRSs. Method applications to synthetic chlorinated drinking water samples and three environmental sample matrices are presented that demonstrate method ruggedness. Mean recoveries of synthetic chlorinated drinking water samples fortified with the PFC method analytes at 0.1–10 ng mL−1 and SRSs at 0.1–1 ng mL−1 are 97.8–113% with relative standard deviations of 2.0–18%. Mean recoveries and relative standard deviations in environmental groundwater samples, production facility non-contact cooling water effluent samples, and production facility wastewater treatment effluent samples fortified with the PFC method analytes at 0.25–2.5 ng mL−1, 0.25–10 ng mL−1, 0.25–50 ng mL−1 respectively and SRSs at 0.1 ng mL−1 are 100 ± 7.7% (RSD ± 12%, PFBA ± 23%), 100 ± 5.5% (RSD ± 11%), 100 ± 5.2% (RSD ± 11%) respectively. Excellent method correlation was obtained with USEPA Method 537 in a comparative analysis of synthetic chlorinated drinking water samples aged 7 days and fortified with C6–C12 PFCAs and C4, C6, and C8 PFSAs, and three SRSs [13C4]PFOA, [13C2]PFUnA, and [13C4]PFOS at 0.1 ng mL−1. The average absolute difference between the EPA Method 537 and DI-LC/MS/MS measurements for the method analytes and SRSs was 8%.