Analytical methods for investigating the presence, photoisomerisation-, and degradation kinetics of the UV-A filter avobenzone under aqueous conditions to ensure a more realistic environmental measurement

Avobenzone (AV) is one of the most widely used UV-A filter compound in sunscreen products despite showing photoinstability under UV-A radiation. Although a high number of studies have focused on the photoinstability of AV in different organic solvents, much less data are available on its ecotoxicological aspect. Despite the determination of its exact environmental concentration being essential for both environmental risk assessment and realistic ecotoxicological experiments, the methods of most of the studies aimed at this were found to be controversial or unsuitable. In the present study, we first investigated the presence of both tautomeric forms of AV with LC-MS, identified previously undetermined characteristic parent ions, and found that its exact concentration in (environmental) samples is the summed concentration of the two forms. Photostability experiments with UV-Vis spectrophotometry revealed that, in aqueous solutions, mainly photodegradation or mainly photoisomerisation occurs at neutral pH, while both processes in a similar extent might be present under acidic conditions (pH=3). Temperature (10 vs. 25°C) had no significant effect on the photostability at pH=3, but the higher temperature contributed to a faster decrease of AV concentration at pH=7. Freeze-thaw stability experiments in water with UV-Vis spectrophotometry showed an extensive degradation of AV. Our findings call attention to important methodical parameters and considerations (e.g., seasonality, sampling time, sample preparation) and contribute to a more realistic determination of AV in environmental matrices. Future studies should focus on investigating the presence of the known photoproducts in aqueous solutions to determine the exact processes.