# Ecotoxicological Assessment of Microplastics in Freshwater Sources--A Review
## 4 Characterization Methods for Microplastics in the Environment
### Spectroscopy-Based Methods
ATR-FTIR is appropriate for the analysis of somewhat larger particles (above 500 \(\upmu\)m) [15], although various ATR accessories provide magnification, enabling the viewing down to 70 \(\upmu\)m [87]. ATR-FTIR was successfully applied for the identification of MPs in surface waters and sediments [76]. This method is generally cheaper; however, it is unable to identify small samples and convex particles [9]. Microscopy coupled FTIR (\(\upmu\)-FTIR) has improved spatial resolution [96] and is commonly applied for MPs analysis of \(<\)100 \(\upmu\)m [87]. Its applicability for the identification of MPs presents directly on membrane filters is especially interesting [96]. [PERSON] et al. [94] applied \(\upmu\)-ATR-FTIR and Pyrolysis-Gas Chromatography (Pyr-GC-MS) in combination with thermochemolysis to characterize 27 single MPs particles and fibers of unknown material isolated from river sediments. The study showed that both methods are well suited to characterize the chemical nature of environmental MPs and can complement each other. Focal plane array-based \(\upmu\)-FTIR (FPA-\(\upmu\)-FTIR) utilizes focal plane array detectors and, due to simultaneous acquiring of a higher number of spatially resolved spectra, provides much faster identification. It can be applied for smaller MPs and at a much larger surface area [96]. Raman spectroscopy uses a characteristic Raman spectrum for the identification of MPs; the polymer is irradiated with monochromatic rays (from 500-800 nm). Once the ray comes to the target, it interacts with polymer absorbing a certain amount of the ray's energy. Consequently, Raman shift occurs, i.e., the frequency of backscattered light becomes different than input, providing a characteristic Raman spectrum of the polymer [85]. The obtained spectrum must be compared with a reference spectrum to perform identification [35]. Raman provides a complementary image of molecular vibrations to FTIR, while FTIR is sensitive to vibrations that modulate molecular dipole moment, Raman is sensitive to those that modulate molecular polarizability [95]. Raman spectroscopy offers a wide spectral range, great resolution (due to reduced linewidth between bands that are close to each other in the spectrum), and low water interference [97]. Accordingly, Raman is a powerful technique for MPs analysis not only in water samples; it is suitable for sediment, organisms, food, and cosmetics as well [86]. Furthermore, it requires a small amount of sample, providing a possibility for high throughput screening. It can resolve very small particles (\(<\)1 \(\upmu\)m). Unfortunately, Raman is highly sensitive to sample fluorescence, which can spoil the useful Raman signal. In addition, laser-induced degradation may occur when fluorescence is caused by impurities present in the sample. This may negatively affect the sample, speaking in favor of the necessity of the sample pretreatment prior to analysis [97]. However, it remains the most preferred technique for MPs analysis for particles \(<\) 10 \(\upmu\)m [85]. Accordingly, Raman was successfully used for MPs identification in sediment [98], seawater [99], freshwater [100], wastewater [101], and aquatic organisms [102]. \begin{table}
\begin{tabular}{c c c} \hline
**Polymer** & **Functional Group** & **Wavenumber (cm\({}^{-1}\))** \\ \hline \multirow{6}{*}{PS} & CH\({}_{2}\) asymmetric stretch & 2924 \\  & CH\({}_{2}\) symmetric stretch & 2850 \\  & CH\({}_{2}\) bend & 1451 \\  & aromatic CH stretch & 3024 \\  & aromatic ring stretch & 1604; 1492 \\  & aromatic CH bend & 1027 \\ \hline \end{tabular}
\end{table} 
Table 2: \(Cont.\)