Tracking the UV-induced photo-oxidation of polyolefins at the particle level using FT-IR imaging of immobilized microplastics

Abstract Detection of microplastics typically relies on Fourier-transform infrared (FT-IR) Imaging combined with spectral matching to reference polymer databases. However, these databases often lack truly representative spectra of environmentally aged particles, particularly those affected by UV-induced photo-oxidation. Furthermore, degradation mechanisms remain insufficiently understood. In this study, we investigate the UV-induced photo-oxidation of immobilized polyolefin microplastic particles using FT-IR Imaging to track spectral evolution over time. Unlike previous studies based on bulk polymer films, our method fixes microplastic particles on a potassium bromide (KBr) substrate, enabling controlled UV exposure and precise long-term tracking of individual particles throughout the entire process. We apply synchronous and asynchronous two-dimensional correlation spectroscopy (2D-COS), band deconvolution, and power spectrum analysis, to elucidate the evolution of key functional groups. On the example of the common polyolefins polypropylene and polyethylene, our findings show the progressive formation of carbonyl and hydroxyl functional groups, with the carbonyl stretching region undergoing complex spectral changes. Deconvolution analysis demonstrates that the spectral shift within the carbonyl band is predominantly due to the formation of peresters, γ-lactones, and γ-perlactones, rather than previously assumed ketone and ester dominance. Finally, we examine the impact of photo-oxidation on microplastic identification via spectral matching. The degradation process can significantly lower the hit quality index (HQI) when comparing degraded microplastics to standard reference spectra, raising concerns about the reliability of polymer identification in environmental samples. These results highlight the need to incorporate photo-oxidation effects into spectral reference databases to improve microplastic monitoring accuracy.