EXPRESS: Time-Gated Raman Spectroscopic Characterization of Fluorescent Polyethylene Microplastics

Microplastics (MPs) have become increasingly common in nearly every natural environment. As their pervasiveness grows, so does the urgency to understand their abundance and origins. Raman spectroscopy has been used to identify MPs in natural environments. However, fluorescence from additives within MPs, other environmental matter, measurement substrates, or other factors can prevent MP identification. Polyethylene (PE) subtypes such as low-density, high-density, and ultra-high molecular weight polyethylene (LDPE, HDPE, and UHMWPE) have widely varying uses and origins (e.g. residential, industrial). In this work, we demonstrate a technique using time-correlated single photon counting (TCSPC) Raman spectroscopy to isolate the Raman signal of microplastics from competing fluorescence. Fluorescence suppression reduces spectral noise and improves quantitative spectral analysis. Decomposing Raman spectra using Voigt profiles enables quantification of PE crystallinity and estimation of its density. In artificially fluorescent and environmental samples, TCSPC Raman spectroscopy improved visualization of weak spectral features and calculation of PE crystallinity compared with continuous-wave (CW) Raman spectroscopy. For an environmental PE MP sample, CW Raman-derived crystallinity estimates often exceeded 100%. These non-physical values were attributed to a significant fluorescence background and low signal-to-noise ratio. In contrast, TCSPC measurements produced crystallinity estimates of 55-70% and 65-80% at two sampled locations on the MP. These results show the potential to quantitatively characterize MPs with TCSPC Raman spectroscopy. The ability to identify the subtype of PE MPs could assist in identifying the sources of microplastic pollution to protect natural environments.