Beyond Known PFAS: Emerging Analytical Strategies for Resolving the Hidden Fluorinated Chemical Space

Per- and polyfluoroalkyl substances (PFAS) occur as complex mixtures in natural and engineered systems, yet routine monitoring captures only a small fraction of the broader PFAS chemical space. Across waters, solids, and biota, measurements of total and extractable organofluorine frequently exceed the fluorine accounted for by targeted PFAS, revealing substantial unidentified organofluorine fractions that may contain unidentified PFAS, precursors, transformation products, and other fluorinated compounds. This review critically evaluates the occurrence, analytical characterization, transformation, and detection of unknown PFAS across environmental and engineered systems. We critically synthesize current knowledge on (i) the occurrence, sources, and major environmental reservoirs of unknown PFAS; (ii) complementary analytical strategies for constraining and identifying the unknown fraction, including high-resolution mass spectrometry (HRMS)-based suspect and non-target screening, ion mobility HRMS, organofluorine mass-balance approaches (EOF/AOF/TF), total oxidizable precursor (TOP) assays, Bayesian-assisted precursor inference, and fluorine-specific techniques; and (iii) current analytical uncertainties. Finally, we identify research priorities including standardized fluorine mass-balance protocols, expanded open HRMS and collision cross-section libraries, automated annotation workflows, improved quantification strategies for compounds lacking authentic standards, and hazard-informed approaches that better align monitoring and regulation with the chemical complexity and unidentified fraction of PFAS contamination.