Progress on analytical methods and environmental behavior of emerging per- and polyfluoroalkyl substances

Per- and polyfluoroalkyl substances (PFASs) are a group of anthropogenic chemicals widely used in manufacture and daily necessities, which are contaminants ubiquitous existed in various environmental matrices and biota. Due to the persistent, bioaccumulative, long-range transport and potential toxic behaviors, production of perfluorooctane sulfonic acid was voluntarily phased out. Perfluorooctanoic acid, its salt and related compounds were also proposed to be included the Stockholm Convention. Restrictions on production and usage of PFAS chemicals have resulted in emergence of novel PFAS compounds through direct manufacturing emissions and indirect transformation pathways, and increasing attention has been focused on these alternatives. In this review, we summarized recent investigations on major groups of emerging PFASs with various molecular structures, and current analytical strategies on the identification of unknown organic fluoride components were overviewed. Four classes of emerging PFASs are covered, including short-chain perfluoroalkyl substances, cyclic perfluoroalkyl acids, perfluoropolyethers and chlorine or hydrogen-substituted polyfluoroalkyl substances. Current knowledge on molecular structures, production and application, environmental behaviors and potential biological effects are summarized, if available. Known PFASs were noticed as a small part of organic fluorinated compounds in the environment. Consequently, new analytical strategies, such as the mass balance analysis and the oxidative conversion methods, were developed for the analysis of unknown fluorinated components. Mass balance analysis of extractable organic fluorine were used to predict the content of unknown organic fluorine in various environmental matrices including sea water, soil, and human blood. Extractable organic fluorine could be exactly quantified after conversion into inorganic fluoride by high-temperature combustion, and known fluorinated components could also be measured by liquid chromatography tandem mass spectrometry. The difference in contents between extractable organic fluorine and known fluorinated components was thus considered as components of unknown organic fluorine. The oxidative conversion method was especially superior for the analysis of fluorinated precursors such as fluorotelomer sulfonates, perfluoroalkane sulfonamides and fluorotelomer phosphate diesters, in which these poly- fluoroalkyl substances could be transformed into known perfluoroalkyl carboxylates (PFCAs) by reaction with hydroxyl radicals at basic conditions. The content of polyfluoroalkyl precursors could be determined by comparing the change of PFCAs contents before and after the oxidation assay. Compared with the mass balance analysis method, this oxidative conversion method require quantification of more PFAS terminal products, and it was not applicable for stable perfluorinated compounds. Meanwhile, occurrence of a variety of novel PFAS analogues have brought challenges on PFAS analysis. For instance, varied molecular structures of emerging PFASs result in distinct physical-chemical properties, which further complicate the analytical process including sample pretreatment and chromatographic isolation. Development of additional methods, such as hydrophilic interaction chromatography (HILIC) for the analysis of short chain PFASs and orthogonal liquid chromatography for the analysis of zwitterionic, cationic, and anionic fluorinated chemicals, are urgently needed. Statistical tools including mass spectrum deconvolution, peak picking, alignment and feature filtering would be promising for confirmation of novel PFAS molecular structures. Also, environmental transformation of PFAS precursors are still ambiguous. Application of strategies in metabolomics analysis might facilitate studies on degradation mechanism of PFAS chemicals.