A Novel Fluorine Mass Balance Method for Improved Characterization and Quantification of Extractable (Organo)fluorine in Drinking Water

Per- and polyfluoroalkyl substances (PFAS) are common contaminants of drinking water globally. Due to their large number and diversity, extractable organofluorine (EOF) has been employed as a sum parameter measurement to capture known and unknown PFAS in environmental samples. However, current methods for determining drinking water EOF perform poorly for trifluoroacetic acid (TFA) and provide limited insights into the nature of unidentified fluorine occurring in samples. To address this, we developed and validated a solid-phase extraction procedure for EOF determination in drinking water with improved TFA recovery, which removes and/or accounts for different species of inorganic fluorine. The method produces two fractions: one containing mostly polar fluorinated substances (e.g., TFA, tetrafluoroborate, and trifluoromethanesulfonate) and another containing longer-chain PFAS. Hexafluorophosphate was distributed across both fractions. Application of the method to Stockholm drinking water revealed a closed fluorine mass balance in fraction I, predominantly (93%) consisting of TFA. In fraction II, however, 67% of the fluorine was unidentified, pointing to unknown fluorinated substance(s) with similar physical–chemical properties to PFAS in this fraction (e.g., perfluorooctanesulfonate). In addition to providing clues for identifying EOF, the method improves estimation of “PFAS Total” for comparison to limits under the European Drinking Water Directive.