# Microplastic Pollution: Chemical Characterization and Impact on Wildlife
## 2 The Chemical and Physical Characterization of Microplastics
### Raman Microscopy
The shift in wavelength is the variable which describes the molecular vibrational energy levels. Fortunately, various MP materials exhibit differences in their Raman spectra which can be used to identify polymeric materials. Figure 7 below depicts Stokes-shifted (lower energy) Raman spectra for a variety of materials germane to MP analysis. Clear differences appear in the fingerprint region between 400-1500 cm\({}^{-1}\) which can be exploited for polymer identification. Given the rich chemical information provided and excellent spatial resolution, the Raman approach is a popular method for finding and analyzing microplastics in various environments. For instance, in the work of [PERSON] et al., [36] the authors isolated suspected microplastic particles from various lake waters collected in China. In all, 585 particles were isolated after chemical treatment and detection by counting under an optical microscope. However, only 201 were identified as microplastic using a Raman microscope, accounting for 34% of the total suspect particles. The Raman spectra in the 300-3200 cm\({}^{-1}\) region were recorded to determine the nature of the compounds. To confirm the polymers present, the Raman spectra of the samples were compared with the library of Raman spectra and the spectra of standard polymers. The Raman spectroscopy identified six types of common microplastic polymers: polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), polyamide (PA), and polyvinyl chloride (PVC). The authors determined that 87-750 MP items per m\({}^{3}\) of water were present in the natural waters tested. The study of [PERSON] [69] considered the abundance of microplastics within marine bivalves typically consumed as food by humans. The authors found 0.3-0.5 particles per gram of tissue for oysters and mussels sampled in Europe. Herein, a micro-Raman spectrometer was used to test a subset of particles recovered to determine the type of plastics present to cover the full diversity of microparticles found. Three spectral windows covering the wavelength range of 80 to 2660 cm\({}^{-1}\) were used to record high-resolution Raman spectra using a spectrometer operating at a diode laser  
Figure 7: Raman spectra of a variety of synthetic polymers common to microplastics. Figure reproduced from Ref. [68] with permission of publisher. wavelength of 785 nm. Unfortunately, the authors report difficulty in determining polymer material because of the presence of dyes used which interfered with their signal. Since FTIR and Raman spectroscopy are complementary vibrational spectroscopic methods, their data can be directly compared. Figure 8a,b depict Raman images and FTIR images with false coloring denoting the spectral intensity in the 2780-2980 cm\({}^{-1}\) range. Figure 8b depicts the corresponding Raman spectrum (left) and IR transmission spectrum (right) of a selected particle in comparison with a reference of polypropylene. Clear similarities exist between the FTIR and Raman spectra, with polymers producing peaks at similar wavenumbers as expected. We also notice the higher spatial resolution afforded by the Raman technique, as the nuances of the shapes of particles imaged are better defined within the Raman image. Figure 8c also depicts the higher spatial resolution available for Raman spectroscopy as compared to FTIR as unknown particles of varying size were extracted from sediment samples and scanned via both FTIR and Raman microspectroscopy. The image mosaic of the FTIR instrument was acquired with an aperture size of 100 \(\upmu\)m \(\times\) 100 \(\upmu\)m and a step size of 100 \(\upmu\)m \(\times\) 100 \(\upmu\)m, while the image montage of the Raman instrument was collected under 10\(\times\) microscope objective. As shown in Figure 8c, the FTIR image mosaic presents a lower resolution compared to the Raman montage.