Characterization of microplastics in sediment using stereomicroscopy and laser direct infrared (LDIR) spectroscopy

• Microplastics sized 20–3368 μm in lake sediment were counted and characterized. • Counts of microplastic particles increased 3.3 folds when the diameter was halved. • The mean recovery of analytical surrogates (polyethylene spheres) was 71% (N = 9). • About 14% and 46% of the total particles in two sediment samples were non-plastic. • Laser direct infrared imaging was highly automated at 7.2 sec per particle. The analysis of microplastics (MP) becomes more difficult for smaller sizes, especially in complex matrices such as sediment of natural waters. In this work, we analyzed MPs in sediment using laser direct infrared (LDIR) imaging, a relatively new technique in environmental MP studies. Sediment samples were spiked with analytical surrogates (polyethylene spheres), and subjected to density separation, wet peroxide oxidation, calcite removal, and filtration. The extracted particles were coated on to a low-emissivity glass slide which was first examined on a stereomicroscope. Six slides (two sediment samples and four different blanks) were further analyzed using an Agilent LDIR system. Approximately 520 and 430 MP/g were found in the two samples, with diameter ranging 20–3384 μm and 84% being smaller than 100 μm. The increase in particle count with decreasing particle sizes followed a power law curve, suggesting that a large portion of the smaller MPs was generated by the breakdown of larger plastic pieces. Major polymers found in this work included polyamide, polyester, polytetrafluoroethylene, and polyacetal. Most MPs were fragments, while beads and fiber were also found. In the two sediment samples, 14% and 46% of the total particles, respectively, were composed of non-plastic micro-sized particles, including natural polyamide, cellulose, chitin, rubber, and “unknown”. Challenges, potential biases, and uncertainties are discussed. This work is the first application of LDIR imaging on MPs in natural sediment.