New insights into the singlet oxygen-independent formation of TEMPO signals in electron paramagnetic resonance analysis
Yanye Tian, Yu Li, Y. J. Li, Zhiwei Zhao, Guang‐Guo Ying, Kaimin Shih, Yong Feng
Abstract
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.