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Challenges on microplastic identification using conventional Raman spectroscopy library
Summary
Researchers identified key challenges in using conventional Raman spectroscopy library matching for microplastic identification in complex environmental samples such as sewage sludge, demonstrating how surface degradation, contamination, and spectral interference reduce identification accuracy even after organic matter removal.
Raman microscopy offers a powerful tool for microplastics (MPs) analysis. However, MPs identification in complex environmental samples, such as sewage sludge, can be very challenging even with the organic matter reduction. That is because any change on the surface of the MPs, such as degradation and contamination, can potentially interfere on the spectrum. In Raman microscopy, the accuracy of MPs identification depends on the quality and comprehensiveness of the spectral library used for comparison. Incomplete or outdated spectral libraries may lead to misidentification or inadequate characterization of MPs. In this study, it was investigated how different processes that are involved in the analysis of MPs in sewage sludge may affect the Raman spectrum of polymers. For that, 1 mm sheets of PE, PP, PET and PA, the MPs with high occurrence in sewage sludge, were used. The polymeric samples were exposed to 6 situations: hydrogen peroxide, organic matter reduction of sewage sludge using hydrogen peroxide, simulated environmental weathering, simulated environmental weathering followed by organic matter reduction of sewage sludge using hydrogen peroxide, mesophilic and thermophilic anaerobic digestion. Then, with a 532 nm laser, the Raman spectra of the samples after being exposed to these conditions were obtained in triplicate and compared with the reference Raman spectrum of PE, PP, PET and PA. Moreover, they were compared with the spectrum library available in the Renishaw inVia Qontor Raman microscope. The comparison was based on the occurrence of the characteristic peaks of the polymers as well as the percentage quality of the spectrum match offered by the database search. It was observed that the conditions MPs in the sewage sludge can be exposed before and during the MPs study make their identification solely based on the spectral libraries available on Raman microscope inaccurate. For example, PA was extensively affected by this issue. Also see: https://micro2024.sciencesconf.org/558435/document