Isomer-Specific Analysis of PFOS in Wastewater and Avian Eggs Enabled by Cyclic Ion Mobility Spectrometry

Perfluorooctanesulfonic acid (PFOS) exists in the environment as a mixture of linear (L-PFOS) and branched (Br-PFOS) isomers. Although it is one of the most frequently detected per- and polyfluoroalkyl substances (PFAS), it is often analyzed and reported as total PFOS with limited attention to the distribution of individual isomers. Here, we used cyclic ion mobility spectrometry (cIMS) with a tunable drift length as an added dimension of separation combined with ultra-high performance liquid chromatography-quadrupole time-of-flight-mass spectrometry (UHPLC-cIMS-qToF-MS) to separate six PFOS isomers. By applying six passes in cIMS, the differences in drift times (DT) and collision cross sections (CCS) allowed us to distinguish L-PFOS from five Br-PFOS isomers. The disubstituted PFOS isomers were not separated from each other because they had the same DTs. Calibration curves prepared with individual isomers revealed that all Br-PFOS had higher ionization efficiencies (more than 2 to 5 times higher) in electrospray MS than L-PFOS, emphasizing the need for isomer-specific analysis for accurate PFOS quantification. The optimized UHPLC-cIMS-qToF-MS method was then used to determine PFOS isomer patterns in influent and effluent wastewater samples, as well as in avian egg yolk samples. The results showed that Br-PFOS isomers dominate in wastewater (more than 50% of total PFOS), while L-PFOS is significantly enriched in egg yolk samples (over 88%). This study highlights the effectiveness of UHPLC-cIMS-qToF-MS for separating and quantifying PFOS isomers in complex matrices and underscores the importance of evaluating the isomer distribution of other PFAS compounds in environmental and biological samples.