Real-Time Identification and Quantification of Per- and Polyfluoroalkyl Substances Using High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometry with Positive Reagent Ions

Per- and polyfluoroalkyl substances (PFAS) are emerging pollutants of concern, primarily due to their terminal degradation products, which exhibit environmental persistence and mobility. Several groups of PFAS, including hydrofluoroolefins (HFOs), perfluoro olefins (PFOs), perfluoro vinyl ethers (PVEs), and hydrofluoroalkanes (HF-alkanes), are volatile and reside predominantly in the gas phase. PFAS such as HFOs, PFOs, and PVEs are considered reactive and may generate short-chain degradation products that persist in the environment. Despite the importance of these gaseous PFAS, there is a lack of analytical techniques capable of providing high-resolution temporal measurements of potential precursors to terminal degradation products. This study presents the first real-time method for detecting and quantifying atmospheric HFOs, PFOs, PVEs, and HF-alkanes using a high-resolution chemical ionization mass spectrometer (HR-CIMS). Using NO⁺ mixed with O₂⁺ (NO⁺/O₂⁺), and O₂⁺ as reagent ions, the CIMS was able to identify and quantify PFAS via fluoride abstraction (M - F)⁺, hydride abstraction (M - H)⁺, or charge transfer (M⁺) mechanisms. The method achieves 10-s limits of detection (LOD) ranging from 2 to 40 ppt, enabling online monitoring in ambient air, especially near emission sources or in indoor environments. The use of NO⁺/O₂⁺ and O₂⁺ reagent ions with HR-CIMS provides a novel and sensitive approach for real-time detection of PFAS via positive reagent ion modes, especially for emerging gas-phase PFAS that currently lack suitable online measurement techniques to better constrain their atmospheric emissions and concentrations.