Implementation of a mixed-mode liquid chromatography-based analysis to assess fluorinated inorganic anions and...

Ultra-short-chain (USC) per- and polyfluoroalkyl substances (PFAS) are polar, mobile, and persistent and are increasingly focused on because of their widespread occurrence in various environmental matrices. One reason why they were previously often overlooked is their insufficient retention in reversed-phase liquid chromatography which is the standard analytical method in most laboratories for PFAS analysis. In this study, a method using mixed-mode liquid chromatography (MMLC) coupled to tandem mass spectrometry (MS/MS) was optimized for the detection and quantification of five USC PFAS (C₂ and C₃ perfluorocarboxylic acids (PFCAs) and C₁ to C₃ perfluorosulfonic acids (PFSAs)), four short-chain (SC) PFAS (C₄-C₆ PFCAs and C₄ PFSA), bis(trifluoromethane)sulfonimide (NTf2), and the three fluorinated inorganic anions (FIAs): tetrafluoroborate (BF₄^-), hexafluorophosphate (PF₆^-), and hexafluoroarsenate (AsF₆^-). To investigate the conditions for optimal retention, different eluent constitutions were tested combining varying solvents and additives. USC PFAS could be analyzed using neutral conditions in the aqueous (A) and basic conditions in the organic eluent (B) whereas FIAs were measured with acidified A and basic B. The method was validated and matrix effects were corrected with mass-labeled standards and standard addition. The method was successfully applied to three surface water samples in which eight PFAS could be quantified. TFA and PF₆^- were found with the highest concentrations at 2.70 µg/L and 0.287 µg/L, respectively, while all other target PFAS occurred in the lower ng/L range. Thus, the sum concentration of TFA and PF₆^- accounted for more than 97% of the total PFAS concentration in all samples.