Microplastics are overestimated due to poor quality control of reagents.

Microplastics are widely distributed in the environment and pose potential ecological risks, increasing to be one of the most important environmental pollutants. However, when assessing the characteristics of microplastic contamination in environmental samples, inadequate quality control measures for the working solutions may introduce additional microplastic contamination and lead to an overestimation of microplastic abundance in the samples. In this study, we evaluated the microplastic contamination characteristics in commonly used flotation and digestion reagents to assess errors caused by microplastics in the reagents. The results showed that the abundance of microplastics in the reagents ranged from 0.8 to 43.4 items/g, with the abundance of microplastics in flotation reagents being lower than that in digestion reagents. The shapes of the detected microplastics included particles, fibers, and fragments, and their size and outline were generally small, with most being below 100 µm. The most common types of polymers detected were polyethylene and polypropylene. In order to improve the universality and readability of the results, the detected microplastic abundances were converted into the actual application concentration of the working fluid. It was found that the potential contamination of microplastics in untreated flotation solutions ranged from 1.5 to 30.8 items/mL, while in digestion solutions ranged from 0.1 to 2.3 items/mL. Our study emphasizes the need for quality control measures, such as suction filtration, when evaluating microplastics in environmental samples or conducting chemical and biological tests related to microplastics.