Novel method for the extraction and identification of microplastics in ocean trawl and fish gut matrices

Abstract

This work presents alternative extraction and analysis techniques to identify microplastics in the environment. This study aims to address previously noted issues with methods that use aggressive extraction treatments or optical microscopy identification techniques alone. Pulsed ultrasonic extraction with ultrapure water was used to remove microplastics from fish stomachs without dissolving the stomach tissues or microplastics. The technique is relatively simple and minimizes issues with hazardous disposal and laboratory safety. Microplastics were characterized using optical microscopy, scanning electron microscopy plus energy-dispersive X-ray spectroscopy (SEM/EDS), Fourier transform infrared (FTIR) micro-spectroscopy, and Raman micro-spectroscopy (RMS). These methods were demonstrated successfully on laboratory fish exposed to reference microplastics and on ocean surface trawl and fish samples taken from subtropical gyres. Polyethylene (PE), polypropylene (PP), and blended PE + PP microplastics were detected in the stomachs of ocean-caught lanternfish, with the majority consisting of PE. One nearly empty lanternfish stomach contained a long PE fiber that appeared to block the digestive tract. Minor amounts of fat, proteins, and carbohydrates were detected by FTIR on many microplastic surfaces. The Pacific Ocean trawl samples yielded similar plastic compositions as the fish stomachs, plus one polystyrene particle. Of the 115 ocean particles analyzed by FTIR (15 μm to 5 mm), 25 particles were microplastics (600 μm to 5 mm). The microplastic PE + PP copolymer blends were the most visibly degraded of the four observed types. FTIR and SEM/EDS identified micro-shell pieces in the ocean fish stomachs that resembled microplastics by optical microscopy alone.