## 4 Discussion  
Using the handheld MicroNIR spectrometer, two libraries of polymer material spectra and two methods of polymer type identification were created. One spectral library based on industrial reference materials and the other on household plastic with known polymer composition according to RIC codes. Correlation coefficients between studied polymer types are very similar for both libraries. The spectral database and method created from household plastic samples (with known polymer composition based on RIC) worked well as the method created from virgin reference polymer materials and pre-production pellets for polymer type identification of plastic litter. Color and other additives did not affect the efficiency of the polymer identification, the method also found that most dark colored plastics could be correctly identified using MicroNIR with the exception of completely black \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No** & **Color** & **Size** & **MicroNIR** & **ATR-FTIR** \\ \hline
1 & white & macro & PS & 0.99 & PS & 0.99 \\
2 & transparent & macro & PA & 0.98 & PA & 0.98 \\
3 & transparent & macro & PA & 0.96 & PA & 0.96 \\
4 & transparent & macro & PA & 0.94 & PA & 0.97 \\
5 & white & macro & PE & 0.99 & PE & 0.99 \\
6 & dark blue & meso & PE & 0.90 & PE & 0.98 \\
7 & transparent & macro & PP & 0.97 & PP & 0.97 \\
8 & transparent & macro & PP & 0.88 & PP & 0.95 \\
9 & blue & macro & PP & 0.93 & PP & 0.99 \\
10 & dark blue & meso & PP & 0.80 & PP & 0.98 \\
11 & transparent & macro & PET & 0.85 & PET & 0.98 \\
12 & transparent & meso & PC & 0.94 & PC & 0.99 \\
13 & white & macro & PVC & 0.87 & PVC & 0.76 \\
14 & yellow & macro & PVC & 0.86 & PVC & 0.73 \\
15 & black & macro & undefined & 0.11 & PVC & 0.77 \\
16 & gray & macro & PVC & 0.48 & PVC & 0.66 \\
17 & dark gray & macro & PVC & 0.95 & PVC & 0.87 \\
18 & white & macro & PVC & 0.76 & PVC & 0.87 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Results of plastic fragments identification found on the beach and in coastal seawater on Crete. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Polymer** & \multicolumn{2}{c}{**Number of Defined Items**} & \multicolumn{1}{c}{**Morphology**} \\ \cline{2-4}  & **MicroNIR on Board** & **FTIR** & \\ \hline PE & 13 & 15 & all fragments \\ PP & 7 & 8 & 7 fragments, 1 fiber \\ PUR & 3 & 7 & 4 fragments, 3 fibers \\ PVC & 2 & 6 & all fragments \\ PET & 1 & 6 & 1 fragment, 5 fibers \\ PA & 0 & 3 & all fragments \\ PS & 2 & 2 & all fragments \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Results of microplastic identification found in the surface waters of Eurasian Arctic. materials. Furthermore, the identification of samples with a copolymer composition resulted in correct identification of the main component of the copolymer although with weaker correlation coefficients with the spectra of the polymer present in the spectral library. In this study, we have defined results of polymer identification with correlation coefficient > 0.7 as true and results with correlation coefficient < 0.4 as a lower limit, indicating that there is no match with the items and polymers included in the library. Results with coefficients between 0.4-0.7 may indicate that the sample might be a copolymer with no easily distinguishable primary polymer type (if these single polymers composition is less than 50%). For example, the evaluation of a sample from Crete that looked like a kitchen oilcloth and was made of two different layers (an unknown fabric and PVC) resulted in the low correlation coefficient, 0.48 due to a \"mixed\" spectra of the combination of materials. This work shows that MicroNIR can be excellently used on site, down to the microplastic range. The lower particle size that can be detected was found to be 0.45 mm for reference materials. For real environmental samples the lower size limit was somewhat higher, and it was only possible to identify fragments larger than 1 mm.