Other extraction methods: one technique that has not been used for a long time is the extraction of rapeseed oil due to the lipophilic nature of the polymers. In principle, the residual canola oil can be removed with alcohol [179]. Organic solvent extraction was also used to extract the microplastic from the sample. Liquid extraction of the sample under pressure with methanol, hexane or dichloromethane takes place at 180–190 °C under high pressure, and after extraction the solvent residue can be removed under nitrogen gas. The disadvantages of the method are the high cost, the polluting effect of the organic solvents and the fact that it requires special equipment. After evaporation of the solvent, all types of polymers of all sizes remain in a homogeneous mixture, the subsequent determination of which is difficult [191].

The separation and analytical methods used in the study of microplastics are not suitable in the nano size range, partly because of the size-related limitations and partly because of the different physical and chemical properties of nanoplastics. Ultrafiltration (UF) may be suitable for the separation of nanoplastics [178]. The ultrafilter removes all colloidal particles (0.01–1.0 µm suspended solids, viruses, proteins, etc.) from water. The pore size of the UF membrane can be selected depending on the size of the nanoplastics to be removed. The pore size of the membrane is usually between 0.005 and 0.1 µm. UF membranes are classified according to the nominal molecular weight limit of the molecules they retain. UF is ideal for removing nanoplastic particles from water that are larger than the pore size of the membrane. The density separation method, which is often used in the separation of microplastics from the matrix, cannot be used for nanoparticles because in their case the buoyancy is not large enough for separation [191], which is also hindered by the diffusion of the particles.

Few data are available on the occurrence of the smallest plastic particles in complex matrices (e.g. wastewater, soils) due to practical and, in some cases, fundamental problems in isolation and analysis. Nguyen and his colleagues address the analysis and separation of the smallest fractions of plastic particles in the most difficult matrices [200]. To separate plastics from complex matrices, their properties must be different from those of the surrounding environment, i.e., compared to aqueous matrices, plastics are generally less dense and more hydrophobic. When aqueous matrices have low solids content, plastics can be separated by size-based filtration by sequentially using filters of different sizes (e.g. 20-25 µm, 2.5 µm, 0.45 µm, and 0.1 µm). Sequential filtration only confirms the presence of nanoplastics, but not their total amount, as a significant portion of nanoparticles are lost by sticking to the filters.

Chromatographic techniques, active (e.g. flow field fractionation – FFF) and passive (e.g. hydrodynamic chromatography - HDC) separation are typically used for the separation of plastic particles smaller than 1 μm [201]. Asymmetric flow field fractionation is suitable for size fractionation of nanoparticles passed through a porous chamber using a flow field perpendicular to the flow. In a fluid flowing laminarly through a thin gap, the varying sizes and densities of the particles determine the extent to which crossflow and their diffusion slow their progress. Since there is no stationary phase, problems due to interactions with it (fragmentation, degradation) can be ruled out, but the particles may interact with the membrane of the channel. A method can be connected to an on-line detector (MS, UV, fluorescence, MALS). The efficiency of separation can be increased by applying a transverse electric field if the charge state of the particles (usually negatively charged in environmental samples) also affects their motion. HDC is a liquid chromatographic separation technique that combines the advantages of liquid dynamics and size exclusion chromatography to determine particle size in the 10–1000 nm range. After the sample enters the column filled with solid beads, a velocity profile is generated in the channels between adjacent beads. Due to the viscous force, the velocity of the liquid is highest in the center and decreases toward the wall. Larger particles remain in the center of the channel and move rapidly along the streamline, while smaller particles migrate toward the channel wall and move more slowly due to hydrodynamic effects and van der Waals attraction. In chromatography, the retention of the sample depends on its interaction with the stationary phase. This is particularly evident with porous stationary phases. Polymers exposed to environmental conditions often have a rough surface and are fragmented, which can increase the interaction with the stationary phase. Reversed-phase HPLC and size-exclusion chromatography (SEC) may be suitable for a narrower size range, HPLC 1-40 nm and SEC 1-100 nm. HDC can be used to investigate a wider size range because the stationary phase here is not porous and there are fewer interactions with the sample. In electrophoresis (capillary electrophoresis, CE), the migration of charged particles in an electric field leads to spatial separation. Stabilization of the surface charge of the particles is important, and the addition of surfactants may be necessary to stabilize the suspension [192].