Since the studied samples cannot be considered as homogeneous and permanent, the recording of time is important due to the daily and seasonal changes in microplastic content, the effects of natural phenomena such as rainy season and wind. Regarding daily variations, the influence of tides is important in determining the microplastic content in sediment. It has been observed that the measured microplastic concentration is higher in rainy season or windy weather than other times [176], [177], [178]. Moreover, recording the duration of sampling is important for comparing studies that use a similar sampling method. The recommended duration of sampling with a net is at least 30 minutes. The longer the sampling time, the more microparticles can be detected, but the greater the risk of clogging the net [179], [180].

Because of their ubiquitous occurrence, microplastics can be detected in oceans, seas, freshwater lakes, rivers, bottled and tap water, polar ice, soil, air, living organisms, and food [181], [182], [183], [184]. However, different methods are used for sampling from different matrices. When taking samples from natural waters, the sampling depth is crucial because microplastics behave differently in waters depending on their physical and chemical properties. Their size, density, shape, and the biofilm that forms on their surface play a major role in their sedimentation, which is why their distribution can vary in different water depths and water sections. Settled microplastic particles may behave similarly to other sediments depending on water turbulence or interaction with the riverbed [18], [179]. Nanoplastics differ from larger particles in their physical and chemical properties (e.g. surface charge). These properties are strongly influenced by the pH and salinity of the environment. Changes in the environment lead to aggregation/agglomeration and changes in hydrophobic properties, all of which affect their distribution in water [111]. The distribution of microplastics is determined not only by natural phenomena but also, of course, by human activities. The microplastic content of natural waters is usually higher in industrial, more urbanized areas near the coast. For this reason, not only the depth of withdrawal but also the measured distance from the shore is important information for the measurement data [178], [185]. When sampling tap water, if the goal is to determine general concentration, the tap should be drained for about 1 minute before sampling; however, if we are interested in the microparticle content of the first dose, which is mostly consumed when drinking tap water, it is not necessary to drain the tap. The flow rate and the origin of the tap water (from groundwater or surface water) must be recorded. For bottled water, the source, production number, and type (natural or carbonated) must be recorded. For wastewater, the water treatment processes must also be specified, as they can also influence the quantity and physicochemical behavior of microplastics [18], [186].

Regarding the amount of sample, three main methods are generally distinguished. In selective sampling, microplastics are collected directly from the environmental sample, usually sediment, using tweezers or other tools. This method is possible when the particles are large enough to be detected by the naked eye and distinguished from other materials present in the matrix. Its main disadvantage is the size limitation: only the larger particles can be detected with the naked eye and are easily confused with other non-plastic particles [170]. The other method is to take a sample with a certain volume. It is usually used for sediment and sometimes water samples from the surface or deeper layers. The sample is collected in bottles and containers by hand or with the help of pumps. This method can collect all the microplastics in the sample, regardless of size. However, the disadvantage is that the amount that can be collected is limited by the size of the container, which makes it difficult to collect a representative amount of samples [18], [170]. In reduced-volume sampling, the large volume of samples is filtered through a filter or sieve on site, and only the remaining particles are submitted for further analysis. Surface and ground waters are usually collected with nets. By measuring the volumetric flow rate, the total volume of water passing through can be determined. A relatively large, representative volume of samples can be collected using this method. However, the disadvantage is that particles smaller than the size of the filter used are lost, so the amount of microplastics determined in the quantitative determination may be much lower than the actual amount [49], [170]. An important aspect of sampling is that the volume collected is large enough to contain a sufficient number of detectable particles to obtain a representative result. For these water samples, at least 500 L is recommended as a collection volume for surface water, at least 1000 L for tap water, at least 10 L for bottled water, and at least 1 L for wastewater inflows, since a higher number of particles can be expected there than at the outflow, and at least 500 L at the outlet [18].

Sampling tools: water samples are most commonly collected with nets whose hole size is usually 300–333 µm, but recently 150– and 80-µm nets have also been used to retain the smaller fractions. The smaller hole size results in an order of magnitude higher number of microparticles, but the nets also clog much faster because the number of organic contaminants retained is also much higher. To avoid this, simultaneous sampling of multiple samples with different meshes is recommended, and the other solution is sequential sampling with meshes of decreasing pore size to obtain different fractions [180]. Trawls (Neuston and Manta nets) are used to collect surface water, Bongo nets are used for mid-depth sampling, and bottom nets are used for seafloor collection. With their help, a large volume of water can be sampled in a relatively short time [185]. To collect a certain volume of water from the surface or from mid-depth, containers that are not made of plastic – usually metal or glass bottles – are used [187]. One option for collecting smaller fractions is rotary drum sampling. A glass cylinder can be used to collect particles of 50-60 µm from the surface, taking advantage of surface tension. To collect particles smaller than 5 µm, continuous centrifugation may be suitable, during which water is pumped into a rapidly rotating cylinder, and then the particles denser than water are retained. The disadvantage of this method is that it is quite time consuming. The continuous plankton recorder used for plankton research is also suitable for collecting microplastics in the ocean because of its similarity in size [179]. When collecting with a net, it is recommended to wash the net with filtered water after each sampling. In addition to the hole size of the net and the duration of sampling, the length and diameter of the net, the speed of the vessel, the immersion depth of the net, and the distance of the net from the vessel (the role of the windward side) are also important in describing the sampling conditions [188]. For flowing water, it is necessary to record the amount of filtered water with a flow meter.

Sampling of surface water sediments is important both in the riverbed and on the coast. Sampling procedures for freshwater sediments are rarely described in detail in the literature. Coastal sediments (sand) are typically sampled manually, using squares to mark sampling areas, defining an area (e.g. a square with an edge length of 20-30 cm), and sampling to a specified depth (e.g. 5-32 cm). A metal spoon or spatula is used to remove the sediment; plastic tools are also avoided in this case. Instead of manual sampling, it is simpler and more accurate to use sediment sampling devices for the given amount of material (e.g. boxes with known area/diameter that penetrate to a given depth or cylindrical tubes, Van Veen grabs) [173]. Only the latter devices are suitable for sediment sampling from soil.