Reliable methodologies to determine microplastics in mussels: Enhanced digestion protocols, transference to gold-coated filters and determination via laser-based transflectance infrared spectrometry

While several analytical strategies have been developed in the last years to determine MPs in marine ecosystems, reliable and fast analytical methods are still needed for monitoring filter-feeder seafood, such as mussels, intended for commercial use. A comprehensive review of state-of-the-art methodologies to evaluate MPs in mussels is presented. Then, an effective method to extract and characterize them in Mytilus Galloprovincialis is proposed. Six digestion procedures were studied: two 1-step alkaline-oxidative methods, two 2-steps alkaline-oxidative methods, a surfactant-oxidative method, and an enzymatic-oxidative one. The latter demonstrated the highest efficacy (recoveries >90 % for the tested polymers), though a modified 2-steps alkaline oxidation using a surfactant offers a cost-effective and slightly faster alternative (on average, 12-13 days, vs 16-17 days of the enzymatic one). The methods proposed here include a transfer protocol from stainless-steel to gold-coated filters (ca. 30 min per filter), which are then analyzed using a quantum cascade laser-based infrared imaging system, QCL-LDIR. The method was validated and its performance compared to previous works. Finally, the methodology was employed to measure MPs in both commercial (2.6 MPs/g, wet weight) and wild mussels (5.2 MPs/g, wet weight), demonstrating its applicability.