Matrix preparation and workflow for microplastics analysis in soil

One of the main difficulties in microplastic (MP) research is the lack of standardized, real-world methods such as matrix blank and simultaneous tracking of polymer particles for enumeration. Building on a previous study, a matrix preparation and experimental workflow for soil matrices is presented that addresses the challenges of purification to allow subsequent analysis using Nile Red-stained MPs as a surrogate. Key steps include peroxide digestion and density separation (NaI) followed by centrifugation for low density polyethylene (LDPE) and polyvinyl chloride (PVC) surrogates to assess recoveries in terms of number and size, based on fluorescence microscopy and Raman spectroscopy. The results yielded false positive recoveries greater than 100% for stained MPs and overall mean recovery around 80% for virgin MPs. Staining reflected the effect of pretreatment on the morphological and fluorescence characteristics of PE and PVC particles. An instrumental approach for fast Raman measurements is also presented, which facilitates counting up to 83%. Although particles down to 21 μm have been tested, this approach appears promising down to single microns due to its traceable and reliable nature for MP particles <300 μm in soil or terrestrial environments. In conclusion, the MP research community should strive to address small polymeric particles that pose an obstacle by agglomerating and interfering with particle-based quantification by spectroscopic techniques.