Matrix matters

Abstract

The extraction and characterization of secondary microplastics, those formed through subjection to the environ‑
ment, 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 specifcity. 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 chroma‑
tography mass spectrometry (py GC/MS), stereoscope microscopy, and scanning electron microscopy. To comple‑
ment particle analysis, plastic-associated leachable molecules were extracted from sediment and analyzed semiquantitatively 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 identifed in sediment by HPLC/HR-MS but leachable molecules associated with polypropylene
could not be identifed 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 quantita‑
tion of microplastics by py GC/MS and the impacts of sample matrix on the quantitation of PU and PP specifcally. We
provide guidance for the preparation of microplastics from complex environmental matrices (e.g., sediment and soil)
for analysis by py GC/MS.