Improved Measurement of Extractable Organic Fluorine in Biosolids and Compost and Its Implications for Fluorine Speciation

Extractable organic fluorine (EOF) analysis is increasingly used to quantify organic fluorine (OF) not captured by targeted liquid chromatography–tandem mass spectrometry (LC–MS/MS), yet method standardization and quality assurance remain unresolved for solids, where isolating low OF from much larger inorganic fluorine (IF) is analytically challenging. Here, an enhanced EOF method was developed and validated for biosolids and compost to selectively isolate anionic OF while minimizing IF carryover. Methanolic extraction alone coextracted substantial IF (22–35%), whereas incorporating a 0.1% aqueous ammonium hydroxide wash into a modified weak anion-exchange solid-phase extraction protocol reduced IF to below the limit of quantification (<0.056 μg g–1) while maintaining good OF recoveries (79–101%). Application of the validated method showed that EOF represented 0.24–0.69% of total fluorine, indicating that most fluorine was inorganic and/or nonextractable. Fluorine from targeted per- and polyfluoroalkyl substances (PFAS) accounted for 4.3–46% of EOF, suggesting that targeted PFAS explained only a subset of EOF. The improved quality-assured workflow provides more reliable EOF measurements for screening-level fluorine speciation and mass-balance evaluation, while emphasizing that EOF remains a method-defined metric and should not be interpreted as a stand-alone measure of PFAS burden.