## 6 Methodology for MP Analysis
### Sampling and Sample Handling  
The essential rule when sampling all types of samples for MP analysis is to use non-plastic equipment and devices to avoid possible contamination. If plastic materials are used in any part of the sampling procedure, it is necessary to include a blank sample in the analysis to prevent bias in the results [98]. Sampling for MP analysis must be planned and adjusted depending on the purpose of the analysis and research objectives. When sampling in the environment, it is necessary to consider the spatial arrangement of locations, the area of the sampled territory, the depth of sampling, the appropriate amount of sample, etc., so that the collected samples representatively reflect the pollution of the area being examined. In addition, local environmental conditions must be taken into account, which determine the distribution of MPs in space, i.e., their movement and deposition with regard to the shape, density and size of the particles [99]. When collecting biota samples, the focus is on the key species, their developmental stage, feeding habits, spatial distribution, etc. [PERSON] et al. (2019) [65] propose the blue mussel (_Mytilus edulis_) as a suitable bioindicator organism for monitoring MP contamination in the sea, considering its wide geographical distribution, sedentary lifestyle and high-water-filtering capacity. The sampling of MPs from the aquatic environment can be performed in a dynamic (with towing equipment from a vessel) or stationary manner (from a floating station or shore-fixed equipment) [100]. It is carried out using different types of nets intended for particular sampling depths, e.g., a hand net, manta trawl, plankton net, bongo net or Neuston [53,99] (Figure 7). A flow meter can be mounted on the opening of the net so that the volume of sampled water is quantified [53]. Regardless of the net type, the pore size must correspond to the research needs (typically 50-3000 \(\upmu\)m) since a larger mesh allows for smaller particles to pass through, and a smaller mesh is easily clogged. The simultaneous use of several nets of different pore sizes is recommended [98]. For MP particles of size 5-50 \(\upmu\)m, continuous flow centrifuges can be used or cascade filtering of a water sample under pressure through a series of filters or different mesh size sieves [99]. For direct water sampling at particular depth, different pumps or deep samplers can be used, e.g., Niskin, van Dorn and Kemmerer [53,99] (Figure 7). Such samples are small, spatially limited and often not representative, so a larger number of repeated samples is recommended to analyze the entire area [53,98]. In general, the load of MPs in an aqueous medium is lower compared to sediment due to water movement and dilution in the water column, so it is often necessary to analyze a larger sample volume to obtain measurable results [98]. Sediment is a static environment that accepts and accumulates settled non-floating particles [98]. For sediment sampling in the coastal area, a bentnos net, trawl net, fish net, shovel, hand rake, hand drill or bucket can be used depending on the size of the sediment particles in the sublittoral zone deep rakes (e.g., Ekman or Van Veen), box corers or deep drills [98,99]. (Figure 8). It is necessary to take at least 300-500 g of the sediment sample, which is then placed in a container made of stainless steel and transported and stored at 0-4 \({}^{\circ}\)C [101]. In the laboratory, samples are dried at 40 \({}^{\circ}\)C for 72 h [53] and thoroughly homogenized before further processing [98]. Figure 8: Sampling equipment for microplastics in aquatic environments (adapted from [98,99]). Figure 7: Sampling equipment for microplastics in aquatic environments (adapted from [53,99]). The usual equipment for soil sampling are metal (e.g., stainless steel) spatulas and aluminium sample containers [36]. The samples are dried and sieved before further processing in the laboratory [53]. A representative sample of the biota consists of at least 50 individuals.