Page - 57 - in Freshwater Microplastics - Emerging Environmental Contaminants?

concentrationsaftervisualinspectionofthesampleonly.Dependingontheefficiency of the sample treatment and particle size, the visual identification is considered not stateoftheartandofteninsufficientresultinginfalse-positiveresults.Forthisreason, furtherspectroscopicorspectrometricmethodsareneededtoensuretheunambiguous identificationofparticlesmadefromsyntheticpolymers. Spectroscopic identificationmethods include Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. These methods are based on the energy absorption by characteristic functional groups of the polymer particles. For larger particles (approximately>500 μm), FTIR can be carried out using an attenuated transversereflection(ATR)unitastheparticlesneedtobetransferredonthecrystalof theATRunitmanually[9,26].CouplingofFTIRinstruments tomicroscopessuchas reflectanceor transmissionmicro-FTIRallows thedetectionofsmallermicroplastics [27].TheuseofFTIRmicroscopyintransmissionmodeisonlyapplicableforsmaller particlesorthinfilmsthatdonotfullyabsorbtheIRbeam.Moreover,specialfiltersare requiredinthesampletreatmentthataretranslucenttoIRradiation,suchasaluminum oxidemembranes. Both FTIR-based and Raman-basedmethods are limited in the minimumparticle size thatcanbedeterminedbythephysicaldiffractionof the light. FTIRmeasurements in transmittancemodeare limited for particles between10and 20μm,whileRaman instruments canmeasureparticlewith sizes that areone to two ordersofmagnitude smaller, due to the smallerwavelengths that are applied for the excitation. Identification of the polymers by FTIR and Raman is susceptible to environmentally driven changes of the polymer surface or the additive application during polymer processing. Thus, microbial fouling, soiling, adsorption of humic acids, andcoloredplasticscan interferewith theabsorbance, reflection,orexcitation of the polymer molecules and might lead to misidentification or totally prevent identificationof theparticles [28] (foran in-depthdiscussiononmicroplastic associ- atedbiofilms,see [29]).Besides the identificationof thepolymer type,visual images ofparticlesenable thedeterminationofparticle shape. Theapplicationofpyrolysis-gaschromatography/massspectrometry(Pyr-GC/MS) allows the simultaneousdeterminationof thepolymer type andpolymer additives by combustionof thesampleandthedetectionof the thermaldegradationproductsof the polymers[16,30].Theidentificationofthermaldegradationproductsservesasamarker that is specific for eachpolymer.Thedegradationproducts are separatedbyGCprior thedetectionoftheirspecificmasstochargeratiosinthemassspectrometer.Incontrast to the spectroscopic techniques, Pyr-GC/MS is a destructivemethod, preventing any further analysisof theplasticparticles.Results obtained throughPyr-GC/MSanalysis areusuallyprovidedas themass fractionormassconcentrationofplastics.Therefore, thedeterminationofparticlecountsisnotpossibleduetothecombustionofthesample. Thermal desorption GC/MS (TDS-GC/MS) in combination with thermogravimetric analysis(TGA)coupledwithasolid-phaseadsorberenableshigher initialsamplesizes compared to Pyr-GC/MS [31]. For this reason,more representative resultsmight be obtainedfor inhomogeneoussampleswithcomplexmatrices. SEM can be coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), which produces high-resolution images of the particles and provides an elemental analysisof themeasuredobjects. ForSEM-EDS, theparticle surfaceof the sample Analysis,Occurrence, andDegradation ofMicroplastics in theAqueous. . . 57