Page - 58 - in Freshwater Microplastics - Emerging Environmental Contaminants?

is scannedby an electronbeam.The contact of the electronbeamwith the sample surface results in the emission of secondary electrons and element-specificX-ray radiation. Thus, an image of the particle can be created and the elemental compo- sition canbe identifiedbyusingSEM-EDS. It is, therefore, possible to distinguish betweenmicroplastics andparticles that are composedof inorganic elements, such as aluminumsilicates [32]. Alternatively,hardness testsarereportedas inspectionof theseparatedparticles. Pressure is applied to the particles by needles or tweezers. This precludes mis- identificationsofmicroplasticswith fragilecarbonorcarbonateparticles thatbreak during the test and are not removed or formed during the sample treatment [33].However, these tests are very time-consuming, donot provide exact polymer identification, and are less accurate as other instrumentalmethods. More specialized but promisingapproaches for thedetectionofmicroplastics are described by Sgier et al. and Jungnickel et al. [34, 35]. The latter describe the measurement and identification of microplastics by time-of-flight secondary ion mass spectrometry. An imaging technique allows the visualization of the particles, and the ionization of the polymermolecules is carried out by a primary ion source, generating secondary ions of polymer fragments. As with Ramanmicroscopy, this techniqueenablestheidentificationofparticlessmallerthan10μm.Sgieretal.detected microplasticsusingflowcytometrycombinedwithvisual stochasticnetworkembed- ding(viSNE).viSNEisatoolforthevisualizationofhigh-dimensionalcytometrydata bynonlineardimensionreductionontotwodimensions[36].Thismethodwascapable of detecting microplastic particles directly in environmental samples by using nonbiologicalreferencedatasetsfor theinterpretationoftheviSNEanalysis,although the reliabilityof themicroplastic identificationneeds tobeproven in futurestudies. 2 Occurrence in theAquaticEnvironment Microplastic particles are present in surfacewater, sediments, and oceans all over theworld, for example, at the Italian, Singapore, andPortuguese coast, at beaches of Hawaii, and islands of the equatorialWestern Atlantic as well as at shores of GermanandGreek islands [3, 11, 17, 37–40].First reports of smaller plastic items were primarily focused on plastic pellets that are used in the production of bulk plastic items. Plastic pellets havebeenquantifiedonnumerous beaches and coast- lines, for instance, inNewZealand,Lebanon, andSpain [41–43].However, indus- trial plastic pellets only compose a small fraction of the numerous microscopic plastic fragments present in the ocean and other aquatic systems [15].Monitoring studies often subdividemicroplastics into categories of spheres, fibers, foams, and fragments and report range in concentrations by up to four orders ofmagnitude, spanning 1.3 particles kg 1 (German island) over 13.5 particles kg 1 (equatorial WesternAtlantic) to2175particles kg 1 (Italy).All these studieswere carriedout in the marine environment, and freshwater systems have attracted less attention until 2010. 58 S.Klein et al.