# Microplastics in Water Resources: Threats and Challenges
## 5 Methods for Determining Microl plastics
### Ldir  
Since knowledge of not only the type of polymer but also its characteristics (size, shape, surface, etc.) is very important and useful information that can affect the proper interpretation of research results, the positive aspect of growing interest in the issue of MPs is the simultaneous development of new techniques and methods for identifying MPs. Currently, in contrast to commonly used classical methods for microscopic identification of MPs, modern and advanced techniques that allow for both quantitative and qualitative descriptions of polymer particles are extremely valuable and desirable. The latest technique for identifying MPs (enabling both qualitative and quantitative determination) is laser direct infrared imaging (LDIR). The use of this technique combined with appropriate software significantly expands the current possibilities for analyzing and identifying MPs [150]. The LDIR method has recently been introduced as a new technique for analyzing polymers and MPs in environmental samples. Compared to methods such as FTIR, it has the advantage of scanning the sample for the presence of microplastics before actual imaging and ultimately only analyzing areas with detected particles [151]. This shortens the analysis time because spectra are obtained when particles are detected during the initial scan. Therefore, samples with small amounts of MPs are measured much faster. The only drawback is that if two particles are adjacent to each other, they may be recognized by the device as a single particle because only one spectrum is recorded per particle [152]. However, automation of measurement and particle recognition reduces the risk of error, which is very likely when identifying MPs using optical microscopy methods. Direct determination of samples taken from the environment using the LDIR method is mostly impossible to achieve. [PERSON] et al. [153] write about the challenge of proper sample preparation and its quantitative transfer to a reflective microscope slide, a requirement for the LDIR method. They applied this method for the first time to identify MPs in fish tissues. The researchers noted difficulties in preparing sufficiently clean samples. In another study, the same research group focused on developing a rapid and effective method for characterizing airborne microplastics [154]. They used an optimized LDIR method, achieving over 90% efficiency in differentiating fibers from particles. Quantitative recovery of particles was 82-90%, and for fibers, 62-73%. They evaluated the quantity and type of MPs deposited in northwestern Spain. The most common were polyethylene, polypropylene, and polyethylene terephthalate. The deposition rate was 98-1220 MPs/m\({}^{2}\)/day, constituting about 1.7% of the collected particles. The sizes of more than half of the depositedmicoplastic particles ranged from 20 to 50 \(\upmu\)m, and the fibers ranged from 50 to 500 \(\upmu\)m. [PERSON] et al. [155] used the LDIR method to determine microplastics collected in the northeastern Atlantic region. The analysis was performed on microplastics less than 300 \(\upmu\)m. The authors noted the relatively short analysis time and the automatic mode of operation. The device identified up to 20 different types of polymers. Thus, the ability to quickly recognize the type of plastic is an advantage of the LDIR method compared to other methods, such as the often-used Nile Red method [156]. In studies conducted in the Rzeszow reservoir, the usefulness of the LDIR system for determining microplastics extracted from bottom sediments was demonstrated. The precision of the method allowed for the identification of particles ranging in size from 30 to 1818 \(\upmu\)m. It should be noted that the high-quality optical magnification allowed for the determination of particle morphology, which is often impossible with other methods, especially for the smallest fragments. The results of the presented studies were preceded by a detailed description of the chemical sample preparation process before analysis using LDIR [12].