Abstract: In this study, a novel analytical method based on combustion ion chromatography (CIC) was established for the simultaneous determination of the total fluorine (TF) and the extractable organic fluorine (EOF) in water samples. By integrating the targeted quantitative analysis results of 37 per- and polyfluoroalkyl substances (PFAS), the established method can effectively quantify the proportion of unidentified organic fluorine in water samples. This approach expands the assessment scope from target PFAS congeners to a more comprehensive evaluation of the total fluorine, including both inorganic and organic fluorine. In this work, the pretreatment conditions for the determination of EOF were systematically optimized firstly. The experimental results showed that the optimal enrichment efficiency of WAX solid-phase extraction (SPE) cartridges was achieved adjusting water samples to a weakly acidic pH of 5-6. Under this pH condition, recoveries of target PFAS remained stable between 70% and 130%. After the completion of sample loading, rinsing the SPE cartridge with 20 mL of 0.01% ammonia solution can remove more than 99.0% of inorganic fluoride ions existing in the water samples, effectively eliminating the potential interference of inorganic fluorine, thereby ensuring the accurate quantification of EOF. To evaluate the performance of the developed method, a total of six actual influent and effluent samples were collected from the wastewater treatment plants of textile printing and dyeing enterprises. All samples were pretreated with the optimized procedure to obtain the measured EOF values. Meanwhile, two special groups were designed: the EOF control group and EOF correction group. The control group was used to evaluate the interference effect of residual inorganic fluorine in samples on EOF determination. In this group, the SPE cartridges were not rinsed with ammonia solution during pretreatment, so as to assess the overestimation extent of EOF content caused by residual inorganic fluorine. For the EOF correction group, the measured EOF values were calibrated by integrating the spiked recoveries from the EOF pretreatment with targeted analytical data of ultra-short chain PFAS. This approach enabled an accurate determination of the true EOF concentration in wastewater and allowed the proportional contribution of ultra-short chain PFAS to the total EOF to be quantified. The results indicated that the measured, control, and corrected EOF values in wastewater were in the range of 19.8-88.2 μg/L, 108-428 μg/L and 26.3-100 μg/L, respectively. There was a significant difference between control and measured EOF values, which suggested that the presence of inorganic fluorine would lead to obvious overestimation of EOF content in water samples. In contrast, the relatively small difference between the corrected and measured EOF values demonstrated that the loss of ultra-short chain PFAS caused by ammonia solution (used for removal of inorganic fluoride ions) had a negligible effect on EOF quantification. This finding further corroborated the necessity and appropriateness of the above-described optimized pretreatment steps. Regarding targeted PFAS analysis, a total of 22 PFAS congeners were detected in the wastewater samples, the concentration ranges of TF and ΣPFAS were 357-3590 μg/L and 5.45-24.8 μg/L, respectively. The contribution of EOF to TF was calculated to be 1.67% to 11.9%, while the proportion of quantifiable targeted PFAS in EOF was 5.23%-25.6%. These results revealed that unidentified organic fluorine accounted for a relatively high proportion in the water samples. Moreover, the conventional targeted PFAS analysis method is difficult to fully reveal the pollution characteristics of organic fluorine compounds in complex water matrices, highlighting the necessity for complementary TF and EOF measurements as established in this work.

Key words: per- and polyfluoroalkyl substances (PFAS), ultrashort-chain per- and polyfluoroalkyl substances (US-PFAS), combustion ion chromatography (CIC), total fluorine (TF), extractable organic fluorine (EOF)