Polymer characterization of submerged plastic litter from Lake Tahoe, United States

The polymer of each subsample (n=516) was determined through Attenuated Total Reflectance-Fourier Transform InfraRed (ATR-FTIR) spectroscopy using a Thermo Nicolet 6700 FTIR (ThermoFisher, United States) at the University of Nevada, Reno. Spectra were collected using 32 scans with a spectral resolution of 4 cm-1 within the range of 3950 and 650 cm-1. The Norton-Beer strong apodization function was applied by the instrument software (OMNIC software, ThermoFisher, United States) and the background was subtracted from each spectrum automatically by the software. The subtracted background was measured every six spectra (approximately every 10 minutes). The diamond ATR crystal was cleaned with isopropanol/methanol before every background collection. Each sample spectrum was compared to known spectra in the Hummel Polymer Library Database and a database created from the spectra of plastics in the Center for Marine Debris Research Polymer Kit 1.0 (Hawaii Pacific University, United States) collected using the same settings as described above. In the OMNIC software, we used the correlation approach for comparing the spectra to the libraries. The match value ranges from 0 to 100, with a match of 100 indicating a perfect match. The library match best fitted to the unknown sample spectrum was considered to be the identity of that sample. The polymer match was categorized as “unknown polymer” when the best match was less than a 70% similarity. All matches were additionally verified by the instrument user. For each subsample analyzed, we noted the polymer type and match.