Matrix Matters: novel insights for the extraction, preparation, and quantitation of microplastics in a freshwater mesocosm study

Abstract

The extraction and characterization of secondary microplastics, those formed through subjection to the environment, must continuously improve in accuracy and applicability in order to generate robust microplastics exposure and risk assessments. Currently, there is a dearth of reliable extraction and quantitation methods for solid microplastic particles in sediment with chemical specificity. Herein we present advances in methodologies to extract and quantify microplastic particles from sediment, utilizing an outdoor mesocosm to model a freshwater coastal environment. Model secondary microplastics of crosslinked polyurethane (PU) were studied in comparison to model secondary polypropylene (PP) microplastics. Techniques to characterize particles in sediment included pyrolysis gas chromatography mass spectrometry (py GC/MS), stereoscope microscopy, and scanning electron microscopy. To complement particle analysis, plastic-associated leachable molecules were extracted from sediment and analyzed semi-quantitatively by high performance liquid chromatography with high-resolution mass spectrometry (HPLC/HR-MS). After developing and optimizing extraction and analytical methods we quantitated PU microparticles by count and weight and discovered that model PU particles fell from the water column into the sediment over the course of a year, while no PP particles were located in sediment samples. In addition, target small molecules associated with the crosslinked PU were identified in sediment by HPLC/HR-MS but leachable molecules associated with polypropylene could not be identified in sediment samples. We share the new py GC/MS method to quantify highly crosslinked PUs in complex environmental matrices containing both inorganic and organic components. In the process of generating robust extraction methods for microplastics in sediment, we discovered important considerations for the quantitation of microplastics by py GC/MS and the impacts of sample matrix on the quantitation of PU and PP specifically. We provide guidance for the preparation of microplastics from complex environmental matrices (e.g., sediment and soil) for analysis by py GC/MS. Graphical Abstract