Structural Characterization of PFAS via Oxygen Attachment Dissociation: A Complementary Approach to CID

Importance of the topic

Per- and polyfluoroalkyl substances (PFAS) are highly persistent environmental contaminants with diverse structures and significant health concerns. Accurate structural characterization of PFAS is critical for environmental monitoring, regulatory compliance, and risk assessment. Traditional collision-induced dissociation (CID) MS/MS often fails to differentiate isomeric PFAS, especially linear versus branched forms. Oxygen Attachment Dissociation (OAD) offers a complementary radical-driven fragmentation approach capable of revealing unique diagnostic ions and cleavage pathways.

Objectives and overview of the study

This study evaluates OAD-MS/MS for detailed structural analysis of PFAS, including linear and branched perfluorooctanoic acid (PFOA) as well as an unsaturated analog (FDUEA). By comparing OAD with conventional CID, the research aims to identify distinct fragmentation patterns, elucidate cleavage mechanisms, and demonstrate OAD’s ability to resolve PFAS isomers and branching sites.

Methodology and instrumentation

Analyses were conducted using a Shimadzu LCMS-9050 quadrupole-time-of-flight mass spectrometer equipped with an OAD cell, coupled to a Shimadzu UPLC system and Shim-pack GIST-HP C18-AQ column. Negative electrospray ionization (ESI) was employed. For CID, argon was used as the collision gas at energies between 15–75 V. OAD utilized water vapor to generate neutral O• and H• radicals at 10 V. Event times were set to 200 ms. Both unlabeled and site-specific ^13C-labeled PFOA standards were used to track terminal cleavage behavior.

Main results and discussion

• CID spectra of PFOA predominantly exhibit symmetric neutral losses (–C3F6, –CO2), producing limited diagnostic ions.
• OAD induces radical-driven pathways, yielding fragment ions such as C5F9–, C6F11–, and C7F13– that are absent in CID.
• Linear PFOA under OAD shows a characteristic Δ3 Da peak pair (C7F13O–/C7F14–), while branched PFOA generates shifted fragments (C4F7O–), reflecting the influence of branching on cleavage.
• ^13C labeling confirms that OAD promotes unidirectional fragmentation from the non–carboxyl terminus, in contrast to the bidirectional cleavage seen with CID.
• OAD of the unsaturated PFAS FDUEA produces no oxygen adducts at C=C positions, indicating selective radical attack on C–F bonds and yielding unique fragments unavailable via CID.
• Density functional theory supports a mechanism involving H• attack on C–F bonds, HF loss, decarboxylation, radical recombination with OH•, and subsequent HF elimination.

Benefits and practical applications of the method

• Improved discrimination of PFAS isomers vital for environmental monitoring and industrial quality control.
• Diagnostic radical-driven fragments enable precise identification of branching sites and unsaturation.
• Direct compatibility with negative-ion MS workflows commonly used for PFAS analysis.

Future trends and potential applications

OAD-MS/MS is expected to extend to a broader range of fluorinated pollutants and integrate with untargeted high-resolution screening methods. The development of standardized OAD protocols and specialized data-analysis tools will support routine adoption in environmental and food safety laboratories. Combining OAD with complementary radical-based techniques could further enhance structural elucidation of complex organic contaminants.

Conclusion

OAD provides a powerful complement to CID by generating radical-driven, unidirectional fragmentation pathways and unique diagnostic ions. This approach significantly enhances the structural characterization of linear, branched, and unsaturated PFAS and is readily implemented on standard negative-ion MS platforms, offering broad utility in environmental and industrial analytical chemistry.

References

1. Takahashi H Toyama A Anal Chem 2018 90(12):7230–7238
2. Li et al Rapid Commun Mass Spectrom 2025 39(3):e9953