Harmonizing infrared spectroscopic techniques for microplastic identification: a comparative evaluation of ATR and µFTIR transmission and reflection modes

This study systematically evaluated the performance of Attenuated Total Reflectance (ATR) and micro-Fourier Transform Infrared Spectroscopy (µFTIR) in transmission and reflection modes for the identification of microplastics (MPs) from real-world plastic products. Twelve common polymers were analyzed in two particle size ranges (500 µm and 100 µm). Identification quality was assessed using the Hit Quality Index (HQI), with a validation threshold of ≥ 70%. ATR analysis of 500 µm particles yielded HQI values > 80% for all polymers, with PET (97.2%), ABS (93.2%), and EVA (92.3%) achieving > 90%, demonstrating high spectral fidelity and reproducibility. µFTIR exhibited significant size-dependent variation: for 100 µm particles, reflection mode (R100) achieved HQI values > 85% for most polymers, including 94.5% for PS and 93.7% for epoxy resin. Conversely, µFTIR performance declined for 500 µm particles, with HQI values < 70% for PET, PS, epoxy resin, and PP. ANOVA (p < 0.0001) and Tukey’s post hoc test confirmed significant differences across techniques and particle sizes, with R100 performing comparably to ATR. These results highlight the influence of particle morphology and acquisition mode on spectral identification and emphasize the need for harmonized analytical protocols. These findings contribute empirical support for the refinement of current standards (e.g., ASTM D8333-20, ISO 24187:2023) recommending HQI ≥ 80% as a reliable threshold for polymer identification via µFTIR.