Quantification of microplastics in complex environmental matrices using a tiered approach with modulated differential scanning calorimetry (MDSC).

The widespread presence of microplastics (MPs) in biosolids raises significant concerns, primarily because biosolids are commonly used as fertilizers in soil, where MPs can accumulate, disrupt soil health and microbial activity, and potentially enter the food chain. Accurate quantification of MPs in biosolids and soil remains challenging due to their low concentrations, aging-induced property variations, and complex biosolid matrices. To address these challenges, modulated differential scanning calorimetry (MDSC), a high-sensitivity, low-detection limit, and cost-effective thermal analysis approach, was employed to quantify MPs in complex biosolid matrices. Using micron-sized polyethylene (PE), polypropylene (PP), polyamide 6 (PA6), and polyethylene terephthalate (PET) spiked into biosolid matrices, MPs were quantified based on the enthalpies generated from the melting peaks. MDSC exhibited 1.4-2.5 times higher sensitivity than conventional DSC, with a theoretical limit of quantification (LOQ) as low as 7 μg/g. An averaged recovery of 93 ± 20% for four micron-sized plastics from three different sources using MDSC demonstrated good accuracy, confirming its reliability. To highlight its applicability to real-world samples, a tiered workflow incorporating MDSC, Raman spectroscopy, and thermogravimetric analysis (TGA) was employed to identify and quantify MPs in biosolids. These findings indicate that MDSC, especially when combined with complementary techniques, is a sensitive and accurate method for identifying and quantifying MPs in complex matrices.