Analytical Challenges and Strategies for Particle-Based Analysis of Airborne Micro(nano)plastics in Size-Fractionated Samples Using Microscopy, SEM/EDX, and Raman Spectroscopy

Inhalable micro(nano)plastics (MNPs) have emerged as a significant global concern due to their abundance and persistence in the atmosphere. Despite a growing body of literature addressing the analytical requirements of airborne MNPs, the issue of inhalable fractions and analysis of slotted substrates remains unclear. Therefore, the objective of this study is to perform a systematic particle-based analysis and characterization of inhalable microplastics (MPs) collected by a high-volume sampler equipped with a five-stage cascade impactor with a size range of 10 μm to <0.49 μm. The efficacy of collection substrates (Teflon and aluminum) was evaluated, as was the impact of particle transfer from the slotted filters on the analysis area and pretreatment methods including chemical digestion for further analysis. The distribution of MNP particles across different slots of a Teflon filter was investigated using Raman microspectrometry to select an appropriate subsample. The results showed the suitability of Teflon filters without any pretreatment for particles down to a single micrometer. As observed by the SEM/EDX analysis, the airborne particles collected in a filter with a submicrometer range (<0.95 μm) showed a decrease in carbon-rich components compared to those stages with higher cutoff sizes. A minimum of 20 particles were analyzed per 1 cm2 of the slotted filter using Raman spectrometry, which revealed a homogeneous distribution of MPs across different slots and yielded a concentration of 452 ± 134 MP/m3 in the first stage of the cascade sampler. The detected MPs were morphologically classified into two main groups: fragments with a size range of 2.8–24.8 μm and fibers with a size range of 28.6–212 μm. Subsequently, the particles were chemically identified as carbon black (tires) and polypropylene. In conclusion, particle-based analysis of size-segregated airborne MNPs presents certain challenges when attempting to analyze particles as small as a single micrometer due to the fact that the aerodynamic diameter of the particles in question and the corresponding analytical limitations that result from this become particularly problematic, especially for cutoffs smaller than 3 μm.