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

Abstract

Electron paramagnetic resonance (EPR) is currently the most commonly used technique for measurement of singlet oxygen (1O2) in advanced oxidation processes. However, the characteristic EPR signal associated with 1O2 (2,2,6,6-tetramethylpiperidine-N-oxide radical, TEMPO) can be generated via alternative pathways not involving 1O2, 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 1O2. 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 1O2 was not involved. Furthermore, the intensity of TEMPO (ITEMPO) 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 ITEMPO. 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 1O2-independent formation of TEMPO in TEMP/PMS and to mitigate the interference during the detection of 1O2 by EPR.