Assessing the global capacity for monitoring microplastics (0.3-5 mm) on sandy beaches: Insights from the first proficiency test.

Monitoring microplastics (MPs) has become a global priority requiring equitable analytical capacity, particularly in the Global South. Through the NUTEC Plastics initiative, the IAEA has addressed this need by developing a harmonized protocol and providing training and equipment to its Member States. The initial phase targeted large MPs (0.3-5 mm) in beach sand. To assess data comparability and analytical performance, a proficiency test was organized in which 49 laboratories from 41 countries analyzed a blind beach-sand sample spiked with a defined number and composition of MPs, reporting counts and polymer identity by size fraction. Recovery was strongly size-dependent. Performance was reliable for the large fraction (1-5 mm), where 22 laboratories (~45%) recovered the exact spiked count, in contrast to one laboratory (~2%) for the small fraction (0.3-1 mm). Of the 41 laboratories identifying polymers, 18 (~44%) failed to detect any transparent polystyrene particles. Generalized linear modelling identified the agitation method during density separation as the only significant predictor of polystyrene recovery, with mechanical shaking yielding ~1.77-fold higher recovery than magnetic stirring. Approximately one in four reported particles could not be attributed to spiked polymers. These findings identify targets and define priorities for the next capacity-building phase: improving recovery of small and transparent particles, and further standardizing agitation, contamination control, and data reporting. Altogether, this study provides a framework for method refinement and a step toward standardized global MP monitoring.