Sub-sampling strategies for analysis of small (<20 µm) microplastics in water

Quantification of microplastics (MPs) in drinking water is typically achieved using spectroscopic techniques. However, due to the time-consuming nature of these analyses researchers typically apply sub-sampling strategies whereby particles in small areas of a filter are quantified and subsequently extrapolated to the entire area. This widely applied strategy has not been evaluated in terms of potential extrapolation error despite a wide range of sub-sampling methods having been reported. The current study examined the relationship between sub-sampling and extrapolation accuracy when considering 2-100 µm low-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS) fragments, with a specific focus on particles <20 µm in size as they are the most abundant and have the potential to exert adverse health impacts. A grid-based random sub-sampling method was developed to serve as a baseline such that extrapolation accuracy could be compared to several previously published methods. Results show that as sub-sampling area increases, error decreases following a power law trend. A minimum sub-sampling threshold was identified (approximately 6-8 % of total area) corresponding to an extrapolation error ranging from 8 to 17 %. Use of a log-normal model to describe particle size distributions was evaluated and found to be applicable to particles >2-5 µm. Findings arising from this study provide insight regarding optimal sub-sampling strategies for the analysis of MP in drinking water.