Abstract :
[en] Per- and polyfluoroalkyl substances (PFASs) are emerging pollutants of great concern, with over 5,000 compounds currently reported. Hyphenated techniques such as ion mobility spectrometry (IMS) coupled to liquid chromatography (LC) and high-resolution mass spectrometry (HRMS) hold promise for non-targeted screening of these substances. Among the many advantages of IMS, a descriptor related to molecular shape is obtained through the collision cross-section (CCS), which can be calculated from the measured ion mobility and provides an additional identification point. However, when trapped IMS (TIMS) is coupled to LC-MS to analyze legacy perfluoroalkyl carboxylic acids (PFCAs), multiple mobility peaks are observed at the mass-to-charge ratio (m/z) of a single deprotonated ion ([M-H]-), preventing the determination of an unambiguous CCS as an identifier. We determined that one of the unexpected peaks was due to a homodimeric PFCA ion ([2M-H]-) that existed prior to ion mobility separation and could dissociated after mobility separation into the corresponding deprotonated ion ([M-H]-). As CCS-m/z trendlines could be obtained for the multiple monomeric PFCA homologues and their corresponding homodimeric ions, a plausible structural conformation was hypothesized. All the PFCA monomers detected shared the same linear relationship between CCS and m/z, suggesting that the addition of CF2 units induces a growth of the ion in a cylindric shape with a constant diameter (Haler et al., JASMS 2022 33(2):273-283). For the homodimeric ions, the CCS-m/z trendline deviated from linearity and was best fitted with a power regression model. This suggests that the proton-bound PFCA homodimer ([2M-H]-) more likely adopts a V-shape with the proton bridging the carboxylate extremities, rather than a cylindric shape. To support this hypothesis, we performed IMS-MS measurements of asymmetric, but isobaric (i.e., sharing the same m/z ratio and number of CF2 units) proton-bound PFCA dimers. If PFCA dimers do indeed adopt a V-shape, the CCS values of these asymmetric dimers can be expected to differ. To this end, several mixtures of two PFCA homologues capable of forming isobaric dimers, e.g., C4+C14 and C8+C10, were analyzed in direct injection (i.e., without prior LC separation), to promote the formation of the corresponding heterodimers (e.g., C4-C14). The results show that the more asymmetric the dimer is, the higher is its CCS value. Consequently, the CCS value is influenced by the longer fluorinated chain of the two coordinated monomers, suggesting that the two chains have not folded over each other, which tends to confirm the V-shape hypothesis. To further support this, theoretical calculations will be performed to determine whether it is possible to predict the observed CCS trend. Finally, the influence of monocharged cations other than the proton (H+) on the CCS trendlines of the PFCA heterodimers was investigated to gain insight into their influence on the overall shape of these dimers. For this purpose, four alkali metal cations (Li+, Na+, K+, and Cs+) were added to the injected PFCA solutions to promote the formation of cation-bound dimers and the resulting solutions were analyzed by IMS.
