Determining the appropriate number of particles on a filter to allow small microplastics to be analyzed by microscopy

The concentrations of small microplastics (diameter < 350 µm) on filters are usually determined by microscopy or micro Fourier-transform infrared spectroscopy. The presence of too many particles on a filter will cause the measured number of particles and particle sizes to be incorrect because of overlapping particles. In this study, the appropriate quantity of particles on a filter to allow effective particle analysis to be performed was determined by performing numerical experiments. The larger the number of particles and the larger the particles, the smaller the proportion of isolated particles. An isolation ratio of 99% requires a particle density on the filter of 1.28 particles mm^-2 if the particle size is 50 µm or of 0.351 particles mm^-2 if the particle size is 100 µm.•Appropriate number of particles for small plastic particle analysis was determined.•Numerical experiments to determine particle distributions on a filter were performed.•Particle number for a 99% isolation ratio was determined.