Microplastic Occurrence in biota-rich seawater samples of the Southern North Sea - Analysis by µ-FTIR

Microplastics (MPs) are synthetic, organic polymers with a size smaller than 5 mm. It has been suggested that there is a particular danger from MPs, thus research interest in the occurrence and effects of MPs have increased tremendously in recent decades. In order to better understand effects of MPs on the environment, it is necessary to know MP occurrences in the marine environment. So far, there are various approaches which, on the one hand, investigate different size classes, and on the other hand, use strong chemical purification methods and thereby tolerate a loss of individual plastic types. Sample purification is necessary to remove the sample matrix. To facilitate a subsequent spectroscopic analysis of the extracted plastic particles, a sample of high purity is essential. A hitherto successful and plastic-friendly purification method is the enzymatic digestion of marine environmental samples, however, a complete purification of particularly biota-rich seawater samples could not be achieved so far. The aim of this work is the improvement of an existing enzymatic digestion to study biota-rich surface water samples for MPs. Surface water samples from the Southern North Sea, which had already been treated according to an existing enzymatic digestion protocol, were subjected to a Proteinase-K treatment and an extended density separation. The Proteinase-K treatment proved to be inefficient, but the extended density separation allowed spectroscopic analysis using Focal Plane Array (FPA) -based µ-Fourier Transform infra-red spectroscopy (µ-FTIR) spectroscopy. A new automated evaluation method yielded concentrations between 0 – 76 particles per cubic metres in the size range 11 – 500 µm for sixteen samples of the Southern North Sea. With regard to the size distribution of the particles, a clear trend towards small particle sizes was observed (92.3 % were < 100 µm). A total of 17 different synthetic polymers were detected, the most being rubber (43.3 %), varnish (16.1 %) and polypropylene (13.9 %). The results of this study provide an important contribution to the assessment of MP pollution in the North Sea, but the comparison between studies is difficult due to methological differences. Thus, there is a strong need for Standard operating procedures (SOP)”, which should imply a defined standard protocol for sampling, sample preparation and MP analysis. The SOPs should also take into account that sample matrices may vary strongly in their composition, containing an adapted, application-specific purification protocol.