Spectroscopic methods are used to determine the chemical structure of polymers by comparing their absorption or emission spectra with a reference spectrum. They have the advantage of being non-destructive, requiring only a small amount of sample, and being less harmful to the environment. In addition to the chemical structure, they can also be used to study the size distribution of the particles [172], [178]. Among the spectroscopic techniques, only the Raman Tweezer is suitable for the study of particles in the nanometer range (> 50 nm), so this technique can be considered for the study of nanoplastics. The Raman Tweezer combines optical tweezers with Raman spectroscopy, using optical traps and chemical identification [191].

While IR absorption depends on the change of the permanent dipole moment of the chemical bond, Raman spectroscopy is based on the change of the polarizability of the chemical bond. Since the vibrational energy levels are unique for each molecule, the given molecule can be accurately determined from the IR and Raman spectra using the reference spectrum [54], [211]. In the case of Raman, the spectra are less distorted by photooxidation than in FTIR, since the resulting -C=O and -OH groups are less Raman but strongly IR-active. Raman is therefore recommended over IR when studying polymer degradation over [204].

Fourier transform infrared spectroscopy (FTIR): FTIR is currently the most used technique for the analytical study of microplastics [212]. In FTIR investigation, the sample is illuminated with infrared light, which excites molecular vibrations within it. The detected absorption spectrum contains the so-called fingerprint region, which allows the identification of the pattern. In polymers, FTIR can also be used to observe oxidation processes through the presence of characteristic carbonyl groups. Combined with a microscope, it is suitable for identifying particles up to 10-20 µm in size. The disadvantage is that the dark particles are difficult to examine, and if they are not removed beforehand, the biofilm can interfere, so it is essential to clean the sample before measurement. Water is also very IR-active, so the sample must be dried before testing. The background resulting from humidity and the presence of CO2 changes over time in the room (especially when several people are present in a small room during the measurement), so it is not sufficient to record the background once: it is recommended for each sample [190]. When operating in transmission mode, an IR-transparent filter is required, and particles thinner than 100 µm can be examined; thick plastic particles cannot be measured this way because the IR beam may be completely absorbed. However, for particles thinner than 5 µm, the detector is no longer sensitive enough in transmission mode. In reflection mode, particles with smooth surfaces can be studied well; irregularly shaped particles may have a refractive error, and ATR-FTIR (attenuated total reflection Fourier transform infrared spectroscopy) is recommended for identifying them. When operated in ATR mode, it provides information about the near surface area, it is typically used for particles 300 µm or larger, and it measures with high precision in a relatively short time. µATR-FTIR is suitable for the study of particles smaller than this. Measuring particles smaller than the ATR crystal can be problematic because the particle under investigation must cover the crystal [172]. It is usually associated with a square array MCT (mercury cadmium telluride) FPA (focal plane array) detector. These have the disadvantage that they must be cooled with liquid nitrogen. The major advantage of the FPA detector is that it can be used to examine a larger area during one measurement, allowing thousands of spectra to be recorded. This way, not only a specific part of the filter, but the entire filter can be examined in a reasonable amount of time. The correct choice of filter material is also important to reduce interference. Polycarbonate, alumina, and a novel silicone filter were found to be the most suitable for microplastic investigation [212]. Thanks to the recent developments, µFTIR can be automated. With the help of algorithms developed for this purpose, it is possible to perform automatic spectra processing faster than manual evaluation and much more accurately than before. However, automatic spectrum processing must take into account that the spectrum of partially degraded polymer particles may differ from the spectrum of the original polymer [212]. To ensure the reproducibility of the measurements, a detailed description of the system settings is required when a publication is made. In addition to the instrument, measurement technique (ATR, reflectance, transmittance), and spectral resolution, it is also advisable to disclose the background and method of spectrum processing [213].

Raman spectroscopy is based on the inelastic scattering of laser light; it is more sensitive to nonpolar and aromatic functional groups than IR, so it can be a complementary to it, since those molecular vibrations that are not IR-active can be Raman-active and vice versa. It operates at a shorter wavelength than IR and offers higher spatial resolution, thus, in conjunction with a microscope even smaller particles – up to 1–20 µm in size – can be examined. It has less water interference, and the CO2 content of the air does not interfere as much either. The thickness and shape of the sample do not affect the measurement, and even dark, opaque particles can be examined with it. The entire wavelength range can be used for identification [190]. The disadvantage is fluorescence interference, the reduction of which requires careful sample preparation, cleaning, and selection of the optimal wavelength of the laser. Removal of organic materials and dyes is essential. An automated algorithm to remove fluorescence background noise is also recommended. The material of the microplastic-bearing surface and filter is also important. An aluminum-coated polycarbonate filter is recommended because it has low fluorescence. The higher signal-to-noise ratio and long measurement time are also disadvantages. It may happen that the laser used as the light source can heat up the sample, which can cause background noise and polymer degradation. Due to the low signal-to-noise ratio, a longer measurement time is required, but the longer a sample is examined, the greater the risk of heating [204]. Nonlinear Raman techniques (coherent anti-Stokes Raman scattering [CARS], stimulated Raman scattering [SRS]) are not yet widely used in the study of microplastics due to the specialized equipment involved but they seem promising in routine studies, as their major advantage is that they are fast methods [204], [211].

X-ray photoelectron spectroscopy (XPS): when the sample is irradiated with X-rays, the photoelectrons excited by the radiation are detected and separated according to their energy, which provides information about the characteristic atom-specific binding energies. Due to the specialized equipment and higher cost, this method was also not widely used [192].

The main advantage of thermoanalytical methods is that they can be used to identify heavy metals, organic pollutants, and additives bound to microplastics in addition to the type of polymer. Biological residues are less likely to interfere with these methods, so the sample preparation required for their use is also less. They are faster than spectroscopic methods, and polymer degradation does not interfere with identification. Their disadvantage is that they provide only the weight of the individual polymer types in the sample, the number and shape of the particles cannot be determined with them, and they are destructive methods, so the given sample is destroyed during the measurement [189], [190], [199].

Pyrolysis gas chromatography-mass spectrometry (Pyr-GC-MS): Identification is based on the pyrolysis of polymers in an inert medium, which leads to the breaking of chemical bonds and produces volatile fragments of low molecular weight. These products formed during thermal decomposition can be separated by gas chromatography and identified by their mass spectrum using a spectral library. All polymers have characteristic degradation products that can be used to identify the chemical structure of the polymer. It should be noted that the individual pyrolysis products are not completely specific in all cases (benzene and styrene in the case of PVC and PS), while the polymer-specific products (chlorobenzene, styrene dimer) often do not provide a sufficiently strong signal. The strength of the received signal is strongly influenced by the matrix, so sample preparation can be important for this method. Monomers, additives, and impurities can be identified simultaneously with the polymer, but polymer subtypes (LDPE, HDPE, or PS and crosslinked PS) are difficult to distinguish [185], [193]. Because the sample must be manually inserted into the tube with tweezers, the size of the particles to be analyzed is limited; particles that are too small are difficult to insert into the tube [49].

Thermal extraction-desorption gas chromatography-mass spectrometry (TED-GC-MS): The technique is based on heating the samples in an inert environment, the volatile decomposition products generated due to heat are absorbed, so they are preconcentrating and then transferred to the GC-MS by desorption. A much larger amount can be analyzed than the former method, which is important because of the representative sample. The method also has the advantage that complex samples can be examined with less sample preparation than with the previous method because identification is not disturbed by biological impurities [178], [189], [203].