Photo-weathering effects on pyrolysis-GC/MS quantification of microplastics: Surface oxidation correlations and...

Microplastics (MPs) in the environment are typically weathered rather than pristine, and their altered physicochemical properties challenge accurate quantification. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) is widely used for MP analysis, but its reliability depends on the stability of specific pyrolyzates, which may be affected by photo-weathering. This study examined the effects of photo-weathering on Py-GC/MS signals of fragment-type MP from polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET). MPs were irradiated for 600 h using Xenon and UVC lamps. Surface oxidation was assessed by Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy, while wettability and particle morphology were evaluated via water contact angle (WCA) and particle size distribution analyses. Two diagnostic pyrolyzates per polymer were selected and their peak intensities were quantitatively evaluated against surface oxidation indices. Both irradiation sources significantly influenced Py-GC/MS quantification for all polymers. For PE under Xenon irradiation and PP under UVC irradiation, 1-decene and 2,4-dimethyl-1-heptene exhibited consistent linear declines (r² = 0.71-0.82) with exposure time that correlated with oxidation metrics. WCA proved to be the most reliable predictor for PE, while O/C ratio best captured PP degradation. Conversely, long-chain pyrolyzates (1,20-heneicosadiene, 2,4,6,8-tetramethyl-1-undecene) responded irregularly under oxidative stress, limiting their use as indicators. For PET, benzophenone depletion under UVC indicated oxidation sensitivity, but secondary conversion from benzoic acid in the presence of CaCO₃ caused signal variability. Despite suppressed signals, predictable declines in selected pyrolyzates can serve as correction indicators when integrated with polymer-specific oxidation metrics. This framework provides the first robust guideline for improving Py-GC/MS quantification of weathered MPs.