Seite - 56 - in Freshwater Microplastics - Emerging Environmental Contaminants?

pressure to increase the boiling point of the extraction solvent,which increases the extractionspeed.Theprocessusuallyusesmetalcellsofsmallvolumethatcanresist the pressure. This method bypasses the need for further sample purification and benefits of a high degree of automatization and allows for a quantitative extraction ofsmallplasticparticles.However,theidentificationofextractsconsistingofmultiple polymertypesiscomplicated,andthesizeoftheextractedsedimentsampleislimited due to thesmall sizeof theextractioncellof the instrument [20]. 1.2.2 Removal ofNaturalDebris The identificationofmicroplasticparticles isoftenpreventedbynatural debris that is present in the sample andaccompanies themicroplastics during the samplingof water samples or the density separation. Thus, the destruction of natural debris or biologicalmaterial is unavoidable tominimize the possibility ofmisidentification orunderestimationofsmallplasticparticles.Thedestructionofnaturalmaterialcan becarriedoutbychemicalorenzymaticallycatalyzedreactions.Chemicaldestruc- tionofnaturaldebris isachievedthroughthetreatmentof thesamplewithhydrogen peroxide,mixtures of hydrogen peroxide and sulfuric acid, and Fenton-like reac- tionsprior or after thedensity separation [8, 18, 21].Theseharshconditionsmight result in losses of plastics that are labile to oxidation or unstable in strong acidic solutions, such as poly(methylmethacrylate) or polycarbonates. To avoid the loss of synthetic polymers, which are not resistant against acidic treatments, usage of sodiumhydroxidewas proposed.However, Cole et al. report that the alkaline treatment with sodium hydroxide could damage some of the synthetic polymers as well [22]. Dehaut et al. showed that the application of potassium hydroxide is preferable for the destruction of organic material, as it seems toattack thesyntheticpolymers less than theabovementionedmethods [23]. Enzymatic treatmentsweredevelopedforbiota-richmarinesurfacewatersamples, whichallowthedetectionofpH-sensitivepolymers [22].Single-enzymeapproaches using proteinase K or mixtures of technical enzymes (lipase amylase, proteinase, chitinase,cellulase)wereusedfortheremovalofbiologicalmaterial,astheenzymatic digestion canbecarriedout undermoderate experimental conditions in termsofpH and temperature.Unfortunately, theuse of enzymes involves several disadvantages. Enzymatictreatmentsare,comparedtochemicaltreatments,expensiveandverytime- consumingandmightnot result inacomplete removalof thenaturaldebris. 1.3 IdentificationofMicroplastics Inmost studies,microplastics arefirst identifiedvisually,beforeanidentificationof thepolymer typeisundertaken.Largerparticlescanbeidentifiedwiththenakedeye, whereas smallmicroplastics are identifiedusing binocularmicroscopes or scanning electron microscopy (SEM) [6, 24, 25]. Early studies determined microplastic 56 S.Klein et al.