## 4 Discussion
The reason for this could be related to the marine litter items being covered by organic matter, and especially for smaller particles, this results in distorted NIR spectra, which is critical for polymer identification. Although no further investigations in this study were done, procedures for organic matter removal and decomposition before measurements with MicroNIR are available [11]. One drawback of the use of MicroNIR on site is that it was not possible to identify thin fibrous materials. However, if fibers are long enough, the material can be concentrated by rolling the fiber into a ball and placing it in the middle of the MicroNIR camera window. This will allow the sampler to acquire spectra with enough quality to correctly identify the polymer composition. In this work, a total of 186 items of marine plastic litter, with different colors and sizes, were studied. The MicroNIR along with the created databases and methods were able to correctly identify 96% of the larger marine macro- and mesoplastic litter (42 of 44 fragments) with exception of two black items. Seventy-three percent (104 of 142 items) of the smaller microplastic litter items were correctly identified with the MicroNIR method. Undefined items were mostly black (8%), fibers (10.5%), or fragments, which were either <1 mm or very dirty particles (8.5%). The results, however, indicate that MicroNIR can be excellently used to identify marine plastic litter with size >1 mm even on site or on board of research vessels. In addition, the use of the MicroNIR is easy and efficient when the right methods and databases are established and could be an excellent tool to be used in Citizen Science projects, adding extra value to marine registration protocols by filling the knowledge gap on the origin and sources of meso- and larger microplastics. The focus of the study was to create a method of identification of the nine most used polymer types--PET, PE, PP, PVC, PS, PA, PC, PUR, and Silicone. Combining results from five different randomly performed studies revealed that PE fragments amounted to 51% of all found plastic items: PP--24%, PVC--9%, PET--5%, PS, PA, PUR--3--4% each, and PC--1%, showing the potential of using MicroNIR methods for marine litter identification. To improve results of copolymer identification, more polymer types could be added to the library, for example, Acryl, Teflon, and ABS. Several other options to achieve even better correlation coefficient for larger datasets containing more polymers or copolymers including the use of more advanced statistics (including principal component analysis) have only been partly examined. Iterative development of this instrument allows the stored spectra of the samples to be re-analyzed with new methods or larger databases. Comparison of the described application of MicroNIR with another common technique such as FTIR spectroscopy [11; 18] shows both advantages and disadvantages of the MicroNIR method. The use of MicroNIR in the field--at beaches and on-board vessels--showed good results for marine plastic litter identification, correctly identifying 96% of macro- and mesoplastics and 73% of microplastics. However, MicroNIR is not able to identify totally black items and items smaller than 1 mm. Conventional IR spectroscopy cannot capture spectra of completely black objects that absorb the signal [17]. For black items and items smaller than 1 mm, using more advanced methods, i.e., FTIR, is needed. FTIR is also needed for identification of copolymer composition where high resolution allows a detection of differences between spectra. On the other hand, fast and inexpensive plastic identification methods are needed to support the growing number of marine litter studies. The inclusion of plastic type identification within these studies is greatly needed to support knowledge and direct mitigation strategies towards combating identified materials [11; 21].