Evaluation of analytical techniques for organic peroxide measurements in polar and non-polar systems.

Peroxides and hydroperoxides are generated during the oxidation of volatile organic compounds (VOCs) in both gaseous and aqueous phases. The ozonolysis of alkenes in the gas phase is a significant source of peroxy radicals (RO₂) in both nocturnal and diurnal chemistry. In the absence of NO, the recombination reactions of RO₂ radicals lead to the formation of H₂O₂ and/or organic hydroperoxides (Atkinson, 2000). Hydroperoxides are highly reactive and play a crucial role in atmospheric chemistry, such as the oxidation of SO₂ to H₂SO₄ in atmospheric aqueous droplets and heterogeneous SO₂ uptake (Wang, 2021). Moreover, peroxides are suggested to constitute a significant fraction of laboratory-generated secondary organic aerosol (SOA) (Docherty 2005) and can serve as reservoirs of HOx and ROx radicals (Li, 2016).

Various spectrophotometric methods have been employed to quantify SOA-bound peroxides offline. The Iodometry method quantifies peroxides present in any form (H₂O₂, ROOH, and ROOR) except for tertiary dialkyl peroxides. The principle of the method is A faster and more sensitive alternative to this method was developed by our group, which accelerates the reaction by microwave heating (Alba-Elena, 2023). These assays typically use water as a solvent. Here, for the first time, an alternative procedure using 1-propanol as an organic solvent is proposed and tested for H₂O₂ and various less reactive peroxides.

Other spectrophotometric methods include: the 4-nitrophenyl boronic acid (NPBA) assay, which is based on the reaction between the hydroperoxide and NPBA to form the colored nitrophenol, the absorbance of which is measured (Jiang, 2017), and the Fenton reaction-assisted ferrous-xylenol (FOX2) assay, which is based on the oxidation of Fe²⁺ by the organic hydroperoxide to form Fe³⁺, which forms a complex with Xylenol orange that can be detected spectrometrically (Morrison, 2023).

All these assays, as well as the 2,7-dichlorofluorescein-based horseradish peroxidase (DCF-HRP) assay, a widely used fluorescence method that is more sensitive to H₂O₂ compared to other organic peroxides and hydroperoxides (Badali, 2015), have been tested using water and polar organic solvents such as 1-propanol and non-polar organic solvents such as chloroform. Their response to different types of peroxide: H₂O₂, tertbutyl hydroperoxide, ditertbutyl peroxide, methyl ethyl ketone peroxide, and dibenzoyl peroxide, has been evaluated. A comparative study of the results, their different characteristics, and their possible interferences has been conducted.

 

 

ACKNOWLEDGEMENT

This research was supported by the Spanish Ministry of Science, Innovation and Universities (MICINN, https://www.ciencia.gob.es/) with grants PID2019-106468RB-I00 and PID2022-139724OB-I00, funded by MCIN/AEI/10.13039/501100011033 and co-funded by the European Union; the UCLM groups research grant No. 2022-GRIN-34199, funded by the own research plan of the UCLM for applied research projects, co-financed by the European Fund for Regional Development (FEDER). D. Alba thanks MCIN/AEI for his “Contrato Predoctoral” Ref BES-2020-094874 associated to the project indicated.