Formula-Level Turnover and Homologous-Series Patterns in Dissolved Organic Matter Associated With Sunlight-Exposed...

Sunlight exposure can alter plastic-associated material and generate a filterable dissolved organic matter (DOM) pool containing polymer fragments, oxygenated products, and additives. This study compared the molecular composition and time-dependent spectral differences of DOM associated with four widely used plastics.

Particles of poly(lactic acid) (PLA), poly(ethylene terephthalate) (PET), polyethylene (PE), and poly(butylene adipate-co-terephthalate) (PBAT) were dispersed in water and exposed to natural sunlight for 6 or 48 h. After particle removal, DOM was enriched by PPL solid-phase extraction and analyzed by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Assigned formulas were compared using elemental classes, intensity-weighted descriptors, van Krevelen distributions, Jaccard similarity, Bray-Curtis dissimilarity, and homologous-series screening.

Between 2247 and 4661 molecular formulas were assigned per sample, with maximum absolute mass errors below 0.60 ppm. Relative to corresponding 6-h spectra, formula numbers were lower at 48 h for PLA (-37.5%) and PET (-39.3%) but higher for PE (+41.8%) and PBAT (+14.0%). Formula-set similarity was lowest for PET (Jaccard 0.329; Bray-Curtis 0.787) and highest for PBAT (0.712; 0.260). In PLA, a C₃H₄O₂-spaced oxygen-rich series increased from 13.4% to 20.9% of the assigned intensity. A C₁₀H₈O₄-spaced PET-related series contributed 8.70% at 6 h and 0.17% at 48 h. PET-48 h was dominated by CHSO formulas, whereas PE showed greater CHNO abundance and higher relative contribution of lower mass oxygenated CH₂ series. PBAT retained the greatest formula-set overlap between spectra.

The 6- and 48-h spectra showed distinct polymer-dependent compositional differences. Homologous-series analysis provided information beyond van Krevelen statistics, whereas nonexclusive N- and S-containing signals highlighted the need for process blanks, dark controls, and tandem mass spectrometry before source-specific assignments. Because replicate and matched control spectra were not available in the analyzed dataset, these differences are interpreted descriptively and do not demonstrate chemical formation, disappearance, or sunlight-specific transformation.