PFAS: a new threat to human health Multi-analyte method development and background levels monitoring

“PFAS are defined as fluorinated substances that contain at least one fully fluorinated methyl or methylene carbon atom (without any H/Cl/Br/I atom attached to it), i.e., with a few noted exceptions, any chemical with at least a perfluorinated methyl group (−CF3) or a perfluorinated methylene group (−CF2−) is a PFAS”. The “noted exceptions” refer to a carbon atom with a H/Cl/Br/I atom attached to it (Organisation for Economic Co-operation and Development (OECD), 2021; Wang et al., 2021). The family of compounds denoted by the acronym PFAS encompass: • perfluoroalkyl substances, which are defined as aliphatic substances for which all of the H atoms attached to C atoms in the non-fluorinated substance from which they are notionally derived have been replaced by F atoms, except those H atoms whose substitution would modify the nature of any functional groups present; • polyfluoroalkyl substances, defined here as aliphatic substances for which all H atoms attached to at least one (but not all) C atoms have been replaced by F atoms, in such a manner that they contain the perfluoroalkyl moiety CnF2n+1− (Buck et al., 2011). The PFAS molecular structure (R−X) consists of a hydrophobic alkyl chain, R, of varying length (typically C4−C16) and a hydrophilic end group, X (Figure 4) (EFSA, 2020). The hydrophobic part may be fully [R = F(CF2)n−] or partially fluorinated. The hydrophilic end group can be neutral, positively, or negatively charged. The resulting substances are non-ionic, cationic, or anionic surface-active agents due to their amphiphilic character (Table 1) (EFSA, 2020).The properties of PFAS depend on the characteristics of the fluorine atom and the nature of the carbon-fluorine bond. Fluorine belongs to the group of halogens which possess seven valence electrons. This means they only need one electron to complete the octet. They have high electron affinity, ionization energy, and electronegativity. Fluorine is the element of the periodic table with the highest electronegativity which gives it a great ability to attract bonding electrons. For this reason, the bond with carbon has a decidedly more polar character than that of the carbon-hydrogen bond and has an electric dipole inversion. Fluorine forms the strongest single bond with carbon due to the optimal overlap of their orbitals during bond formation. This results from the similarity in energy levels between fluorine and carbon orbitals, especially since they belong to the same period in the periodic table. Each carbon-fluorine bond increases the strength of other C−F bonds related to the same carbon atom. Furthermore, being very small, fluorine has an occupational space like that of hydrogen and guarantees a small steric encumbrance. It has low polarizability, and this is reflected in the fact that fluorocarbons have weak intermolecular attractive forces, providing these compounds with a low surface energy, lower than that of the respective hydrocarbons. PFAS are characterized by high thermal stability and low reactivity, they can be both hydrophobic and lipophobic and their chemical- physical properties vary with the length of the carbon chain and the functional groups. These characteristics make them of great interest for industrial applications (Buck et al., 2011; Wang et al., 2017). In 2018, the "Global PFC Group" coordinated by the Organisation for Economic Co-operation and Development (OECD) and the United Nations Environment Programme (UNEP), released a comprehensive list of more than 4700 PFAS compounds. These compounds were identified as containing either a −CnF2n− (n ≥ 3) or −CnF2nOCmF2m− (n and m ≥ 1) moiety and were confirmed or presumed to have been present in the global market (Figure 5) (Organisation for Economic Co-operation and Development (OECD), 2018, 2021).