# Methods of Analyzing Microsized Plastics in the Environment
## 3 Spectroscopy Analyses  
### Fourier Transform Infrared Spectroscopy  
Fourier transform infrared (FTIR) spectroscopy provides information about the specific chemical bonding of particles. Carbon-based polymers can be easily identified using this method. The chemical structure of plastics with different bonding configurations creates a unique spectrum that distinguishes them from other organic and inorganic particles [3,49]. Furthermore, plastics can be identified through well-established polymer spectral libraries [50,51]. Using infrared (IR) spectra to organize information for each plastic prevents the false-positive quantification of non-plastic particles and reduces the chance of overlooking plastic particles without a specific color or texture. Additionally, identification using FTIR reveals the polymer composition, which can provide clues to the origin and mechanism of occurrence of the sample [52]. The composition of oxygen bonds (e.g., carbonyl groups) in the IR spectrum can also provide information on the oxidation state of microplastics [26,53].  
Microplastics are easier to identify using micro-FTIR, which has both features, which together allow the microscopic observation of microscopically sized plastic-like particles prior to spectroscopic confirmation on a single platform [24]. Moreover, transmittance, reflectance, and attenuated total reflectance (ATR) modes can be used in the FTIR analysis of microplastics [42]. The reflectance and ATR modes do not require sample preparation steps for thick and opaque microplastics, unlike the transmission mode. In addition, the ATR mode can generate stable spectra on irregular microplastic surfaces. Theoretically, it is possible to detect microplastics as small as the diameter of the ATR probe's IR beam aperture (e.g., 10 \(\upmu\)m). Currently, micro-ATR-FTIR is useful for identifying microplastics in environmental samples, complementing the two functions of microscopic identification of plastic-like particles and spectroscopy of subsequent chemical identification [24]. However, it is generally difficult to obtain a clear spectrum that can be accurately identified for microplastics of size less than 50 \(\upmu\)m. Additionally, the pressure generated by the ATR probe can damage fragile microplastics because ATR-FTIR measurement is a form of surface contact analysis [37]. Small plastic particles can be difficult to detect due to adhesion to or electrostatic interactions with the probe tip. Moreover, the ATR mode takes a long time to identify each particle separately. Micro-FTIR instruments are expensive, and using ATR probes to identify individual plastic-like particle is time consuming. Microplastics in environmental samples are usually weathered, often have complex chemical structures, and sometimes exist as composite materials. Therefore, skilled operators must obtain a clear spectrum and interpret it accurately. Moreover, when using a library, it is recommended to register the spectrum of surfactants, such as sodium stearate and sodium dodecyl sulfate (SDS), which can be confused with plastic, in the FTIR library together to reduce the probability of false positives [33]. Pretreating the sample to purify it may be required to use FTIR, because there are many reports indicating that heavy metals and microorganisms are adsorbed on the surface of plastics [51; 54; 55].  
It is possible to simultaneously acquire FTIR spectra for multiple microplastics in multiple pixel arrays besides general FTIR when using a focal plane array (FPA) detector [56; 57]. Independent IR spectra can be obtained for each pixel, which can be used to distinguish small microplastic fibers and fragments [55; 58].  
The FPA detector takes a long time and utilizes the library, but errors may occur because there are not enough library data [59]. Additionally, further research is needed because PRCs, such as plasticizers, antioxidants, ink components, and adhesive from labels, can affect the FTIR results [60].