## 3 Results
### Libraries and Methods of Identification  
#### 3.1.1 Reference Materials Library  
The initial library consisted of six main polymer types (RIC 1-6, method RM_1). Application of SMV to raw spectra did not obtain adequate polymers separation, as some spectra were very similar in this wavelength range (Figure 5). For example, the threshold coefficient for PE and PP needed to be set to 0.95. Processing the spectra first by SNV followed by applying a second derivative function, largely improved separation capacity.  
It was also shown that HDPE and LDPE could only be differentiated using a very high threshold coefficient (>0.995, Table 4). Therefore, we decided to join HDPE and LDPE in one type, PE. The highest correlation coefficient between other polymers was found for PP and HDPE, LDPE--0.53 and 0.60, respectively.  
Figure 4: Raw spectra (**a**) and processed spectra, standard normal variate (SNV) (**b**) of 7 polymer types reference material (RM) with different particles size, distance to the instrument’s lamps and detector, taken by three different MicroNIR instruments and laptops merged in one database.  
Figure 3: Examples of marine plastic litter collected at: (**a**) beach and coastal seawater on Crete; (**b**) marine sediment in the Eurasian Arctic Ocean; (**c**) Oslofjord beaches; (**d**) sea surface water in the Eurasian Arctic Ocean.  
As the second step, the other four polymers (from category RIC 7) but representing the second most produced and most frequently found in marine environments were added to the library. These were PUR, PA, PC, and Silicone, resulting in a nine-polymer library, method RM_2. For this method, the highest correlation was found between PET and PC (0.61) and for PE and PP (0.56) (Table 5). The method applied resulted in complete classification of all nine polymer types included in the library. Correlation coefficients between polymer types with RIC 1-6 were not changed significantly after the addition of the other four polymers to the library (Tables 4 and 5).  
Spectra of wood and cotton were also added to the library as they are often found and are sometimes difficult to distinguish from plastic items. Spectra of these materials differ significantly from spectra of studied polymers (Figure 5); the addition of items in the library did not affect identification of other polymers. The presence of wood and cotton in the database is useful in case of studies of plastic in seawater, since they present very often in samples, and this will reduce any uncertainty in identification.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline  & **PET** & **HDPE** & **PVC** & **LDPE** & **PP** & **PS** \\ \hline
**PET** & 1.00 & & & & & & \\
**HDPE** & \(-\)0.06 & 1.00 & & & & & \\
**PVC** & \(-\)0.01 & 0.06 & 1.00 & & & & \\
**LDPE** & \(-\)0.08 & 0.995 & 0.09 & 1.00 & & & \\
**PP** & \(-\)0.19 & 0.53 & 0.21 & 0.60 & 1.00 & & \\
**PS** & \(-\)0.37 & 0.17 & \(-\)0.12 & 0.19 & 0.23 & 1.00 \\ \hline \end{tabular}
\end{table}
Table 4: Cross-correlation table for method RM_1, reference materials library, RIC 1–6.  
Figure 5: Raw spectra of polymers with RIC 1–6 (**a**) and polymers with RIC 7 together with wood and cotton spectra (**b**).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & **PET** & **PE** & **PVC** & **PP** & **PS** & **PUR** & **PA** & **PC** & **Silicone** \\ \hline
**PET** & 1.00 & & & & & & & \\
**PE** & \(-\)0.07 & 1.00 & & & & & & \\
**PVC** & \(-\)0.01 & 0.07 & 1.00 & & & & & \\
**PP** & \(-\)0.37 & 0.56 & 0.21 & 1.00 & & & & \\
**PS** & 0.07 & 0.18 & \(-\)0.12 & 0.22 & 1.00 & & & \\
**PUR** & 0.04 & 0.04 & 0.03 & \(-\)0.12 & 0.25 & 1.00 & & \\
**PA** & \(-\)0.12 & 0.35 & 0.47 & 0.03 & 0.13 & 0.01 & 1.00 & \\
**PC** & 0.61 & \(-\)0.24 & \(-\)0.10 & 0.39 & 0.14 & 0.43 & \(-\)0.11 & 1.00 & \\
**Silicone** & 0.04 & \(-\)0.21 & \(-\)0.15 & 0.33 & \(-\)0.04 & 0.45 & \(-\)0.07 & 0.45 & 1.00 \\ \hline \end{tabular}
\end{table}
Table 5: Cross-correlation table for method RM_2, reference materials library, RIC 1–7.