Instrument-dependent variability affects both microplastic polymer identification and quantification: an inter-laboratory comparison using polymer standards

Microplastic analysis is a critical tool for environmental risk assessments and policy development, aligning with several UN Sustainability Goals, however the extent to which analytical outcomes depend on instrument type and spectral reference libraries remains poorly quantified. Here, we present an inter-laboratory comparison study across seven countries evaluating both infrared polymer identification and fluorescent microscopy quantification, using pristine individual polymer standards, mixed polymer standards, and controlled recovery experiments. Participants used standardised protocols and reference materials, differing only in instrument manufacturer, type, reference library, and observer. Our results demonstrate substantial inter-laboratory variability in spectra match quality, polymer identification accuracy, polymer composition in mixed samples, and recovery rates from standardised matrices. Our findings highlight strong instrument- and library-dependent effects on data output despite the implementation of standardised protocols. Collectively, our findings demonstrate that protocol standardisation alone is insufficient to ensure inter-laboratory comparability and that substantial uncertainty persists in reported microplastic composition and abundance. We therefore propose a collaborative approach between researchers and instrument manufacturers encompassing library harmonisation, instrument calibration, and transparent reporting. Without such measures, reported microplastic composition and abundance may continue to reflect laboratory-specific artefacts rather than true environmental variability, ultimately limiting the confidence and policy impact of the reported results.