Photoinduced weathering of plastics and implications for spectral inclusion into matching database for laser direct infrared spectroscopy identification.

Microplastics are a ubiquitous environmental contaminant that can be found virtually every habitat and water source around the world. Therefore, there is great interest in developing efficient and reliable methodologies for the identification and enumeration of microplastics from environmental media. Laser direct infrared spectroscopy (LDIR) utilizes the fingerprint region of infrared (IR) spectra to make identification of plastics and other materials during analysis. However, only pristine spectra of plastics are currently used in the identification process. It has been well documented that the IR spectra of plastics changes considerably upon photochemical weathering, bringing into question whether databases may lead to misidentification of certain polymer types. To test if weathering influences identification of microplastics by LDIR, nine different plastics were irradiated to simulate weathering from 3 to 13 years in the sun. The physical and chemical changes of the plastic were measured by differential scanning calorimetry (DSC) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) respectively. The weathered polymers were analyzed by LDIR-transflectance spectroscopy and resulting spectra were loaded into the LDIR matching database. This updated database was used to analyze samples from a local wastewater treatment plant with and without the inclusion of weathered spectra. This manuscript shows the inclusion of weathered transflectance spectra into the LDIR matching database can increase polymer identifications by 9.6 - 10.2% in wastewater samples on average. It also reveals that not all polymers are increased in identification uniformly, as PC and PA show the most sizeable increased spectral matches. However, most polymers tested were able to show increased medium and high-quality matches of weathered polymers using this approach.