An approach for extraction, characterization and quantitation of microplastic in natural marine snow using Raman microscopy

Marine snow is a predominant form of sinking particulate carbon in the marine water column and represents a mechanism for transporting microplastics to the sea floor. We present a new dual density separation method employing sodium iodide extraction followed by methanol precipitation, specifically designed for microplastic isolation and identification in natural marine snow samples. A total of 59 microscopic particles from eight marine snow samples collected at Avery Point, CT were confirmed as plastics and/or substances containing typical plastic manufacturing additives. Extraction efficiency of this method was determined using polyethylene microspheres of varying sizes (63-75 μm, 212-250 μm and 500-600 μm) yielding 90%, 93% and 98% recoveries, respectively. Residual organic matter which can cause interference in downstream Raman spectroscopic analyses was eliminated by employing a 15% hydrogen peroxide (H₂O₂) digestion step, which caused negligible chemical modifications to the polymer samples. Extensive precautions such as combusted glassware, a microfiltration air hood, and incorporation of process blank samples ensured that airborne microplastic contamination was avoided. A phase contrast microscope equipped with a Raman spectrophotometer system using a 785 nm laser excitation source efficiently identified anthropogenic polymer materials. Unexpectedly, plastic additives such as pigments complicated the identification of polymers but their spectra were successfully interpreted through spectral subtraction and comparison to a database and authentic standards. The protocol described can be applied to detect microplastic in marine snow samples and improve our understanding of the fate of microplastic in the ocean.