Time-Gated Raman Spectroscopy of Environmental Microplastics*

Raman spectroscopy is a highly versatile method for identifying environmental plastics quickly and effectively. However, fluorescence of the plastics themselves as well as the substrate beneath them often overwhelms the relatively weak Raman scattering. Given the numerous additives and dyes present in environmental microplastics as well as their small size, the issue of fluorescence can easily grow to the point where spectral identification is difficult or nearly impossible. Here, we introduce the method of time-gated Raman spectroscopy and apply it to environmental microplastics. A time-gated system takes advantage of the difference in timescales between Raman scattering and fluorescence by setting a cutoff relative to the laser pulse, excluding late-arriving fluorescence photons from the final spectrum. Our initial trials on microplastics recovered from snow and caves on the Wasatch Front demonstrate the capability of time-gated Raman spectroscopy to return a clear spectrum where conventional Raman spectroscopy fails, showcasing the potential of this method to greatly improve plastic identification in the field.