Methylene blue method for analysis of perfluoroalkyl substances (PFAS): Effect of the molecular structure of PFAS compounds

This study evaluates the methylene blue method for rapid and cost-effective analysis of perfluoroalkyl substances (PFAS). By employing three perfluorosulfonic acids (PFSAs) (perfluorooctanesulfonic acid (PFOS), perfluorohexanesulfonic acid (PFHxS) and perfluorobutanesulfonic acid (PFBS)), three perfluorocarboxylic acids (PFCAs) (perfluorooctanoic acid (PFOA), perfluorohexanoic acid (PFHxA) and perfluorobutanoic acid (PFBA)) and two non-fluorinated compounds (sodium 1-octanesulfonate (SOS) and octanoic acid (OA)), the impact of PFAS structure, specifically polar group type, carbon chain length and fluorine substitution, on the efficacy of the method was assessed. The results showed that PFSAs produced reliable calibration curves (R 2 > 0.99) across all pH levels due to strong dissociation of the -SO 3 H group. In contrast, PFCAs exhibited pH-sensitive performance in that PFOA (C8) showed good linearity (R² > 0.99) at pH ≥ 4, while shorter-chain PFHxA (C6) and PFBA (C4) had poor linearity for all of the pHs tested (1.6–10.0, R² < 0.95). Hydrophobicity-driven partitioning of methylene blue active substance (MBAS) into chloroform was critical, with long-chain PFAS (e.g., PFOS, PFOA) showing stable absorbance despite that the volume of washing solution increased up to 20 mL. The method was ineffective for SOS and OA, which are non-fluorinated analogs of PFOS and PFOA. The modified MB method developed in this study provides high-quality calibration curves for PFSAs and long-chain PFCAs, with a quantification limit as low as 0.03 mg/L, offering a rapid and cost-effective alternative to HPLC-MS for analysis of some PFAS compounds.