Investigation of Ultra-short-chain PFAS and Other Per- and Polyfluoroalkyl Substances (PFAS) in the Namhangang River Using Online SPE LC-MS/MS

This study improved the analytical performance of 28 PFAS, including ultra-short-chain (USC) PFAS, and assessed their distribution in the Namhangang River. Optimization of the online SPE column and mobile-phase conditions markedly enhanced the peak intensities of USC PFAS—TFMS, PFEtS, PFPrA, and PFPrS increased by 4.9-, 13.9-, 13.0-, and 23.4-fold, respectively—allowing for reliable quantification. Using the optimized method, five monitoring surveys conducted in 2025 showed the highest PFAS concentrations at the S1 site (mean: 2,803 ng/L), where semiconductor effluent is directly discharged. PFAS concentrations consistently decreased downstream, indicating dilution as the dominant controlling factor. USC (58.9%) and short-chain PFAS (40.6%) accounted for most of the PFAS composition at the S1 site, with TFMS (47.8%) and PFPrA (51.6%) comprising the majority of the USC fraction. A very strong positive correlation was observed between the short-chain PFAS pairs PFBA–PFMPA and PFPeA–PFMBA (Spearman’s rank correlation coefficient, ρ = 1.0; p-value < 0.05), suggesting shared industrial origins and similar environmental behavior. Overall, this study strengthens the analytical reliability of USC PFAS and provides a quantitative characterization of PFAS contamination in the Namhangang River, offering essential baseline information for future PFAS monitoring and water-quality management.