Unveiling Radical-Based Reactions in Photochemical Vapor Generation via Online and In Situ Electron Paramagnetic Resonance Monitoring

Photochemical vapor generation (PCVG) is an effective alternative to traditional chemical vapor generation (CVG), offering superior sample introduction and matrix separation. The mechanism of PCVG involves radical reactions initiated by photolysis of the medium, which play a crucial role in the reduction and vaporization of analytes in solution. While electron paramagnetic resonance (EPR) is widely used for free radical analysis, traditional EPR methods lack in situ and real-time monitoring and may introduce instability. In this work, a comprehensive online/in situ EPR detection strategy was developed to monitor the dynamic change of free radicals during PCVG in a formic acid system. Both methods monitored the concentrations of ^•CO₂^-, ^•OH, ^•H, and ^•CHO radicals under light exposure. The online EPR method allows for quantification of free radicals in formic acid systems and those with common transition-metal ions. Additionally, in situ EPR tracking revealed the generation and transformation of free radicals. Decay analysis of radical adducts confirmed that the observed EPR signals result from competing processes of radical formation, interconversion, and DMPO-trap addition, indicating a dynamic equilibrium among these pathways. For the first time, the interconversion of ^•OH and ^•H radicals into ^•CO₂^- during the PCVG process was observed, shedding light on the reduction mechanism of analytes. Additionally, this finding also suggests that the reductive role of a substantial amount of ^•CHO in the formic-acid-based PCVG system may have been previously underestimated. Overall, this work introduces an innovative method for real-time monitoring and could provide new insight for mechanistic studies of PCVG systems.