New insights into the singlet oxygen-independent formation of TEMPO signals in electron paramagnetic resonance analysis

• TEMPO was formed in TEMP/PMS system over a broad pH range (3.0–11.0) • Direct electron transfer was the mechanism for TEMPO formation and PMS decomposition. • TEMPO intensity at different [TEMP]/[PMS] ratios followed a reverse parabolic curve. • Three distinct patterns were observed in the kinetics of TEMPO formation. • A strategy to mitigate the interference of direct electron transfer was proposed. Electron paramagnetic resonance (EPR) is currently the most commonly used technique for measurement of singlet oxygen ( 1 O 2 ) in advanced oxidation processes. However, the characteristic EPR signal associated with 1 O 2 (2,2,6,6-tetramethylpiperidine-N-oxide radical, TEMPO) can be generated via alternative pathways not involving 1 O 2 , leading to misinterpreted results. In this study, in-situ EPR analysis was used to re-examine the interaction between peroxymonosulfate (PMS), a common oxidant, and 2,2,6,6-tetramethyl-4-piperidinol (TEMP), the spin-trapping agent of 1 O 2 . It was found that TEMPO could be generated in TEMP/PMS system over a broad pH range (3.0–11.0). The pathway for TEMPO formation was the direct oxidation of TEMP by PMS, and 1 O 2 was not involved. Furthermore, the intensity of TEMPO ( I TEMPO ) followed a reverse parabolic pattern as the [TEMP]/[PMS] ratios changed across all pH values. Kinetic analysis unveiled three distinct patterns (continuous linear increase; linear increase followed by a lower rate of increase; increase followed by reaching a plateau) in I TEMPO . Finally, an electron transfer mechanism was proposed for the conversion of TEMP to TEMPO by PMS. The results from this study are expected to advance the understanding of 1 O 2 -independent formation of TEMPO in TEMP/PMS and to mitigate the interference during the detection of 1 O 2 by EPR.