Identification of Organic Radicals in Aqueous Environments by DIPPMPO Spin Trapping Coupled with High-Resolution Mass Spectrometry

Organic radicals are key reactive intermediates that govern pollutant transformation and biogeochemical cycling in aquatic environments, yet their direct identification remains challenging because of structural diversity, low concentrations, and transient lifetimes. This study introduces a robust analytical workflow that integrates the spin-trapping agent 5-(diisopropoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DIPPMPO) with ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) for the systematic detection and identification of organic radicals in complex aqueous matrices. Systematic validation confirmed the high stability and linear concentration response of DIPPMPO-radical adducts, while characteristic neutral-loss patterns (42.0470 and 166.0759 Da) provided structural confirmation across multiple organic radical classes, including phenoxyl, semiquinone, acyloxyl, peroxyl, and alkyl radicals. Applied to both engineered and natural aqueous systems, the workflow successfully identified organic radicals generated during UV-irradiated bisphenol A degradation and in sunlight-exposed dissolved organic matter (DOM). The results illustrate its ability to trace pollutant-derived organic radicals and map diverse DOM-generated organic radicals, offering new insights into transformation pathways. By delivering enhanced specificity, structural insight, and semiquantitative capability, this approach advances the mechanistic understanding of radical-mediated processes and provides a versatile platform for studying transient reactive intermediates in water treatment and environmental redox systems.