Two-dimensional Identification of Microplastics: Comparison of Optical Photothermal Infrared Spectroscopy with Fourier-Transform Infrared and Raman Spectroscopy

Abstract

In recent years, vibrational spectroscopic techniques based on Fourier transform infrared (FTIR) or Raman microspectroscopy have been suggested to fulfill the unmet need for microplastic particle detection and identification. Inter-system comparison of data from reference polymers enables assessing the reproducibility between instruments and advantages of emerging quantum cascade laser infrared (IR) techniques such as optical photothermal infrared (O-PTIR) spectroscopy. In our work, IR and Raman spectra of nine plastics, namely polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polycarbonate (PC), polystyrene (PS), silicone, polylactide acid (PLA) and polymethylmethacrylate (PMMA) were simultaneously acquired using an O-PTIR based IR and Raman instrument in non-contact, reflection mode. Comprehensive band assignments were presented. We determined the agreement of O-PTIR with standalone attenuated total reflection FTIR and Raman spectrometers based on the hit quality index (HQI) and introduced the two-dimensional identification (2D-HQI) approach using both Raman- and IR-HQIs. Finally, microplastic particles were prepared as test samples from known material by wet grinding and subjected to the 2D-HQI identification approach. We concluded that this framework offers improved identification of microplastic particles in environmental, nutritious and biological materials.