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

Despite EU-driven standardization efforts, the lack of harmonized analytical workflows still limits the comparability of microplastic (MP) measurements across laboratories. This study evaluates three infrared (IR)-based techniques (& micro;-FTIR, FPA-FTIR, and QCL-LDIR) using a metrologically characterized polyethylene terephthalate (PET) representative test material (RTM) containing 1318 +/- 73 particles per tablet (20-500 & micro;m). Independent laboratories applied their routine analytical workflows, while & micro;-Raman spectroscopy served as the orthogonal reference method. The results identify 50 & micro;m as a critical metrological threshold influencing IR analytical performance. Above 50 & micro;m, IR-based methods showed 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 & micro;-FTIR. Below 50 & micro;m, recoveries decreased (& micro;-FTIR: 41-44%; FPA-FTIR: 36-54%; QCL-LDIR: 59-95%), although the broad interlaboratory range observed for QCL-LDIR indicates that optimized analytical workflows can substantially improve performance. Z-score and statistical analyses further showed that analytical performance was strongly size-dependent and influenced by both instrumental characteristics and laboratory-specific workflows. Supplementary experiments using a low-load PET RTM (96 +/- 14 particles per tablet, 20-500 & micro;m) improved agreement with the Raman reference, confirming that particle crowding may contribute to analytical bias. Overall, the study shows that high internal precision may coexist with systematic bias, emphasizing that reproducibility alone is insufficient to ensure metrological reliability and interlaboratory comparability. These findings support size-resolved performance evaluation and the harmonization of analytical workflows to achieve robust and comparable microplastic measurements.