Interlaboratory comparison of IR spectroscopic techniques for PET microplastic characterization: defining metrological thresholds across multiple size classes

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Despite EU-driven standardization efforts, microplastic (MP, 1–1000 µm) heterogeneity and diverse analytical approaches still limit data comparability. This study evaluates infrared (IR) spectroscopic techniques—µ-FTIR, FPA-FTIR, and QCL-LDIR—using a PET representative test material in the 20–500 µm range, identifying a critical metrological threshold at 50 µm. Using µ-Raman spectroscopy as an orthogonal benchmark, independent analyses across multiple laboratories assessed methodological performance and inter-laboratory comparability.Results from a high-load PET test material demonstrate clear size-dependent performance. Above 50 µm, IR-based methods show closer agreement with Raman values and reduced dispersion compared to smaller fractions, with recoveries ranging from 28–54% for QCL-LDIR, 60–73% for FPA-FTIR, and 83–108% for µ-FTIR. Below 50 µm, a marked divergence emerges: µ-FTIR and FPA-FTIR show substantial underestimation (41–44% and 36–54%, respectively), whereas QCL-LDIR achieves 59–95%. These results indicate that instrumental configuration and software-assisted thresholds become critical at smaller scales.The interlaboratory comparison demonstrates that variability is driven not only by the analytical technique, but also by laboratory-specific factors, including sample preparation, software settings, and background control. Supplementary experiments with low-load PET test material show that reducing particle density improves agreement with the Raman benchmark, highlighting the role of particle crowding and detection limits.Overall, this study demonstrates that high internal precision may coexist with systematic bias, emphasizing that reproducibility alone is insufficient to ensure metrological reliability. These findings highlight the need for size-resolved performance evaluation and harmonized analytical workflows to achieve robust and comparable microplastic measurements.

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