Illustrating the Use of Cyclic Ion Mobility to Enhance Specificity for branched-PFAS Isomer Analysis

Importance of the Topic

Per- and polyfluoroalkyl substances (PFAS) are widespread environmental contaminants with significant health and regulatory concerns. Branched PFAS isomers are notoriously difficult to distinguish from their linear counterparts using traditional chromatographic and mass spectrometric approaches. The addition of an ion mobility separation dimension based on collision cross section (CCS) enhances analytical specificity, enabling more accurate identification and quantification of both known and emerging PFAS isomers in environmental and exposomic studies.

Objectives and Study Overview

This work demonstrates the application of a SELECT SERIES™ Cyclic™ Ion Mobility Mass Spectrometer coupled to liquid chromatography (LC-cIM-MS) for the analysis of branched perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) isomers. Key objectives include:

• Measurement of travelling wave ion mobility (TWIM) CCS values for branched and linear PFAS isomers and comparison with drift tube and trapped ion mobility literature values.
• Resolution of chromatographically coeluting PFOS isomers using multipass cyclic IMS to improve peak-to-peak resolution.
• Acquisition of single-component fragmentation spectra for confident structural elucidation of unknown branched PFOS byproducts.
• Evaluation of CCS as a transferable identification descriptor for PFAS screening.

Methodology

Samples of PFOA, PFOS, and PFHxS isomer mixtures were separated by ultra-high-performance liquid chromatography (UHPLC) and introduced to the cyclic ion mobility device operating in one-pass (R~65) and multipass modes (up to R~145). CCS values were obtained from TWIM drift times. High-definition data-independent acquisition (HDMSE) and mobility-resolved MS/MS (HDMS/MS) experiments provided fragmentation spectra. Peak capacity (Pc) and peak-to-peak resolution (Rsp-p) metrics quantified separation performance.

Instrumentation Used

• ACQUITY™ Premier UPLC System with PFAS Analysis Kit and Atlantis™ Premier BEH C18 AX Isolator Column
• ACQUITY UPLC BEH C18 Column (100 x 2.1 mm, 1.8 µm)
• SELECT SERIES™ Cyclic™ IMS Mass Spectrometer
• Electrospray Ionization Source (Negative Mode)
• MassLynx™ 4.2 and waters_connect™ 3.1 Software

Main Results and Discussion

• TWIM CCS values for linear and branched PFOA and PFOS isomers agreed within 0.7% of published drift tube and trapped ion values, demonstrating CCS transferability.
• Multipass cyclic IMS improved Rsp-p from ~0.1 (one pass) to ~0.8 (multiple passes), enabling baseline separation of coeluting PFOS isomers.
• Two unknown PFOS byproducts (perfluoro-5-methyl heptane sulfonic acid and perfluoro-6-methyl heptane sulfonic acid) were resolved and identified based on distinct CCS fingerprints and mobility-resolved fragmentation patterns.
• Distinct CID fragmentation profiles for linear versus branched isomers facilitated confident structural assignments.
• CCS fingerprinting proved valuable for low-abundance analytes lacking sufficient product ion signals.

Benefits and Practical Applications

• Enhanced specificity for isomeric PFAS analysis through CCS measurements.
• Increased peak capacity by combining chromatographic, m/z, and ion mobility separations.
• Reliable non-targeted screening of known, unknown, and emerging PFAS in environmental monitoring and exposomics.
• Transferable CCS data support cross-platform comparability and long-term PFAS research.

Future Trends and Applications

Continued improvements in ion mobility resolution and data-processing algorithms will further enhance non-targeted PFAS screening in complex matrices. Integrating high-resolution MS, advanced informatics, and CCS libraries will support large-scale environmental surveys and source attribution. Future research may link isomer-specific CCS profiles with toxicity and environmental fate, advancing risk assessment and regulatory frameworks.

Conclusion

LC-cIM-MS combining cyclic ion mobility and UHPLC offers a powerful platform for resolving and identifying branched PFAS isomers with high specificity. Multipass IMS enables separation of coeluting species and acquisition of pure fragmentation spectra, while CCS values provide robust identification markers even at low concentrations. This methodology enhances non-targeted environmental PFAS screening and paves the way for exposomic and regulatory applications.

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