Following the preparation of target tissues, the quantity and types of microplastics should be ascertained. Of the methods currently employed, visual identification is most widely utilized, often in combination with one or more follow-up analytical techniques. Researchers can use characteristics, including morphology and density, to identify the presence of microplastics. Visual identification is based on the morphological and physical characteristics of particles whereas chemical characteristics are determined by more advanced analytical techniques.
Early reports quantifying environmental plastics primarily relied upon visual identification; this method remains an essential step in classifying microplastics and is perfectly acceptable when supported by subsequent polymer analysis of subsamples. Visual identification can be conducted using light, polarized or electron microscopy. Semi-automated methods, including ZooScan , flow cytometry , cell sorters and coulter counters  allow for a large number of samples to be analyzed rapidly; however, these require technical expertise and specialized equipment, and time must still be given to sample preparation and data analysis. Scanning electron microscopes (SEM) produce high-resolution images and have been implemented in several studies either to image recovered plastics [23,41] or as a way of identifying microbial colonization .
Visual identification is rapid, relatively cheap and can be conducted without the need for additional technical staff and consumables; however, accurately differentiating microplastics, particularly in the smaller size ranges, requires training and experience. Criteria for visually identifying microplastics include the absence of cellular or organic structures, a homogenous thickness across the particles and homogenous colors and gloss [77,123]. Manually sorting plastics under a microscope is most effective for particles >500 pm; the effort and accuracy required for sorting increases with decreasing particle size. Owing to the difficulties in handling and differentiating microplastics from organic and inorganic matter , error rates could be as high as 70%, increasing with decreasing particle size , with incorrect identification most prevalent with microfibers [123,147]. To gauge the accuracy of visual discrimination, subsamples of potential plastics should be chemically analyzed [77,123,147-150]. It has been observed that training and experience can significantly lower the error rates and misidentification stemming from visual identification .
Plastics are largely classified by their morphological characteristics: size, shape, and color. Size is typically based on the longest dimension of a particle; size categories can be used where appropriate. When reporting microplastic shape, researchers tend to use five main categories, although the nomenclature used varies between research groups (Table 8.2). Finally, colors are often reported across a wide spectrum; color differentiation is subjective and visual identification of microplastics cannot be based on color alone. Caution should be given to categorizing microplastics suffering embrittlement, fragmentation or bleaching, or encrusted with biota, as this may skew results.
TABLE 8.2 Categories Used when Classifying Microplastic by Shape
Other Terms Used
Irregular shaped particles, crystals, fluff powder, granules, shavings Filaments, microfibers, strands, threads Grains, spherical microbeads, microspheres Polystyrene, EPS
Resin pellets, nurdles, pre-production pellets, nibs