Raman Spectral Imaging for the Detection of Inhalable Microplastics in Ambient Particulate Matter Samples

Microplastics are ubiquitous contaminants, with preliminary evidence indicating they are a novel component of air pollution. This presents a plausible inhalation exposure pathway, should microplastics occur in the inhalable size range; however, this remains an analytical challenge. Here, we develop a filter-based sampling method compatible with both air quality monitoring and Raman spectral imaging (RSI) for the detection of inhalable-sized microplastics. Clean and particulate matter (PM) contaminated filters of a range of compositions were screened. RSI was validated using a plastic microbead suspension (poly(methyl methacrylate) (5-27 μm), polyethylene (10-27 μm), and polystyrene (4 and 10 μm)). Filters were loaded with the suspension before being analyzed. RSI analysis was conducted using a univariate analysis, fitting unique plastic bands to the spectral data sets, where high spatial intensity indicated the presence of microplastics. Inhalable microplastics were not visibly detectable against quartz or spectroscopically detectable against polytetrafluoroethylene (PTFE)- and alumina-based filters. While microplastics were detectable against cellulose, the PM-contaminated filters (4 and 24 h) burned during analysis. The greatest intensities for microplastics were observed against the silver membrane filter, and inhalable microplastics were still detectable in a 24 h PM sample. These findings will facilitate the acquisition of inhalable microplastic concentrations, which are necessary for understanding microplastic exposure and, ultimately, what their potential role in PM-associated health effects might be.