Carbonyl Index Variability and Trends in Marine Meso-Microplastics: Implications of Instrumental Effects and Spectral Preprocessing.

This study addresses the critical need to understand variability in the carbonyl index (CI) of marine plastics to effectively assess their environmental fate. Standardization is essential to ensure data interoperability and comparability across different research laboratories. We evaluated the effect of spectral preprocessing on the comparability of four carbonyl indices derived from analyses of marine meso- and microplastics across three ATR-FTIR instruments. The fillpeaks baseline spectral preprocessing correction yielded the highest consistency and interlaboratory correlation, thus supporting its adoption as a standard for environmental CI determination. Moreover, when used with the CI-SAUB index, defined as the ratio of the carbonyl peak area to the methylene peak area, instrumental differences are effectively smoothed out. We also demonstrated that index-specific CI values derived from spectra processed differently are not directly comparable, further underscoring the need for harmonized protocols. Our analysis supports the use of the CI as a reliable proxy for surface oxidation resulting from weathering mechanisms. To enhance reproducibility in future research, we provide a cross-validated, open-access spectral library of weathered plastics, along with a reference database for estimating the CI of weathered particles. Our results emphasize the importance of standardized approaches for improving the reliability and reproducibility of plastic pollution research and monitoring.