Multi-analytical approaches for the identification of microplastics in various environmental matrices

Microplastics (MP), synthetic plastic particles less than 5 mm in size, are a growing concern due to the potential negative impact they may have on the environment. Chemical characterization of MP is important for potential source tracking as well as understanding fate and transport. Spectroscopic techniques, such as Fourier-transform infrared (FTIR) and Raman spectroscopy, have been widely used for this purpose. However, the presence of additives or pigments, as well as oxidation or degradation that occurs either through natural weathering processes or as a result of sample preparation methods used to extract MP from environmental matrices, can hinder straightforward identification of MP when matching the sample spectrum to a reference database. The purpose of this work was to combine multiple analytical techniques for the characterization of MP in various environmental samples. Surface water and zooplankton samples collected at various sites in the Hudson-Raritan Estuary were analyzed using both FTIR and Raman microscopy. Use of these complementary techniques allowed for more comprehensive identification of complex MP and the analysis of particles with a wide range of morphologies and physical properties that may be limited by the use of a single technique. MP were found in surface water samples from each site and primarily were polyethylene and polypropylene, many of which exhibited surface oxidation or the presence of pigments/dyes that complicated spectral analysis. MP were also detected in all zooplankton samples, including three different species, through the combined use of Raman microscopy and FTIR imaging that allowed for sample analysis without introduction of visual bias. FTIR and Raman were also combined with thermoanalytical techniques, differential scanning calorimetry (DSC) and thermogravimetric analysis coupled with IR spectroscopy for evolved gas analysis (TGA-IR) in order to identify mixtures of known MP, rubbers and MP in river bed sediment samples. In order to aid data analysis, 40 plastic samples, consisting of 23 different polymer types, were oxidized via an established nitric acid digestion protocol and analyzed to investigate resulting changes to MP spectra and thermal properties. All spectral and thermal property changes were documented and spectra were compiled as a reference library for the benefit of future analyses.