Optimized spin-trapping method for radicals detection induced by atmospheric fine particulate matter: Critical roles of temporal evolution and cavity quality factor

Spin-trapping coupled with electron paramagnetic resonance (EPR) is a powerful technique for characterizing short-lived free radicals generated by atmospheric particulate matter (PM) in the liquid phase. However, quantitative accuracy is frequently compromised by the poorly elucidated generation kinetics of spin adducts, solvent effects, and inconsistencies in sample pretreatment, leading to substantial uncertainty and severely limiting the comparability of existing research. To address these challenges, this study systematically investigated the impact of sample pretreatment procedures and critical EPR parameters on quantification results, as well as the kinetic evolution of spin adducts, ultimately establishing an optimized spin-trapping methodology. Results indicate that the individual spin adducts exhibit distinct formation and decay kinetics, making the selection of an optimal time window for extraction and detection is critical for accurate quantification. Furthermore, the cavity quality factor (Q-factor) was identified as a critical source of measurement bias. Correcting for Q-factor variations significantly minimizes quantification errors and enhances reproducibility. The optimized spin-trapping methodology proposed herein improves the accuracy and reproducibility of EPR-based quantification of radicals generated during PM liquid-phase reactions. This methodological improvement facilitates a more reliable characterization of the radical-generating capacity of PM, providing mechanistic insights into the atmospheric chemical behavior of PM and its associated health effects.