# Microplastic Pollution: Chemical Characterization and Impact on Wildlife
## 2 The Chemical and Physical Characterization of Microplastics
### FTIR Microscopy  
Combining optical microscopy and/or SEM with vibrational spectroscopy can reveal the molecular composition of plastic particles. The signal strength obtained, however, depends on the size of the particles being examined, and typically it is crucial to separate the suspect microplastic particle from sample matrix to avoid interfering peaks from appearing in the spectrum ([PERSON] et al., 2018). An effective and widely used method for identifying microplastics is FTIR microscopy. The signal is dependent on a change in molecular dipole moment occurring during a molecular vibration. By absorbing IR light, molecules are raised to a higher vibrational state which correlate specifically to types of bonds present in molecules under study. In contrast to SEM-EDX, IR spectroscopy is a very potent technique that offers fingerprint information about the sample's molecular makeup and specific bonds present ([PERSON] et al., 2018). Optimally, an FTIR microscope will be used for rapid spectra acquisition of microplastic particles. However, more conventional equipment can be used as well if a sufficient sample exists and can be prepared for analysis. A schematic of an FTIR microscope and photograph of a commercially available instrument is presented in Figure 4. The spatial resolution of FTIR microscopes is only roughly 10-20 um, but this is wavelength-specific and constrained by the well-known diffraction limit. Ideally, the sample must be deposited onto an IR-transparent substrate and have a minimum thickness of about 150 nm for FTIR to work well. Due to these drawbacks, FTIR works best for individual particles larger than 20 um. Agglomerates or films of smaller particles can still be examined ([PERSON] et al., 2018). Since it is more effective than other techniques at detecting microplastic particles as small as 20 um, micro-FTIR spectroscopy (micro-FTIR) is an excellent technology for identifying airborne microplastics ([PERSON] et al., 2018). Between the visible light and microwave parts of the electromagnetic spectrum is the zone known as infrared radiation (IR). While the IR region extends from roughly 700 nm-1 mm in wavelength, FTIR spectroscopy for molecular fingerprinting is generally applied in a limited range of frequencies between 4000-400 cm\({}^{-1}\) (wavenumbers) because it is this 'fingerprint' region that has significant practical value for identifying bonds within molecules. Different materials' infrared spectra differ depending on their chemical makeup, so it is possible to see the presence of various bonds or groups of atoms (functional groups) and, consequently,determine composition. To demonstrate, consider Figure 5 below which illustrates optical images and FTIR spectra of a variety of microplastics particles of different compositions. Carefully consider the wavenumbers at which dips in the transmission spectra occur. Polyethylene exhibits reasonably sharp spectral absorbance features near 3000 cm\({}^{-1}\); however, in contrast, cotton's spectrum in this region is very broad. An infrared spectrum can be used to differentiate between these two materials and/or confirm identity of a MP. The three FTIR spectral bands most used for microplastics analysis are typically 4000-2750 cm\({}^{-1}\), 2750-1850 cm\({}^{-1}\), and 1850-700 cm\({}^{-1}\). Within the fingerprint region (1850-700 cm\({}^{-1}\)), there are differences in strength and specificity (diversity of signal) which can be exploited to differentiate between various plastics. One work which demonstrates this is that of [PERSON] and [PERSON] [42] who used FTIR to detect the presence of microplastics. After comparing the FTIR spectrum results with an electronic library of spectra, the presence of PVC (polyvinyl chloride), PP (polypropylene), PET (polyethylene terephthalate), PS (polystyrene), HDPE (high-density polyethylene) and LDPE (low-density polyethylene) were found present in the sample. Figure 4: **Left–Schematic of an FTIR microscope.