## 3 Results
Figure 3: The 2 nd derivative NIR spectra of aqueous L-fucose (**a**) and 2 nd derivative spectra as a close-up from 880 to 889 nm (**b**). Each spectrum of each concentration shows the average of 3 independent solutions. The NIR spectrum of pure water is shown in comparison. The wavelength areas presenting a systematic order with the solutions of different concentrations and water (F1 \(\rightarrow\) F2 \(\rightarrow\) F3 \(\rightarrow\) F4 \(\rightarrow\) water) are marked with a black rectangle, whereas solutions appearing in the order F1 \(\rightarrow\) F2 \(\rightarrow\) F3 to F4 or F1 \(\rightarrow\) F2 \(\rightarrow\) F3 to water were marked with a light green rectangle. As observed, water overbears aqueous L-fulcose and L-proline making it hard to distinguish solutions of different concentrations from each other. However, some systematic differences between water and aqueous solutions were detected. The peaks of L-fulcose solutions seem to have shifted away from the peak of the water between 1469-1662 nm, which is within the region where pure L-fulcose differs from pure water. NIR absorption bands in this wavelength area belong to the 1 st overtone vibrational OH stretching and water (Table 3) [62; 63; 64]. Although, differences between the aqueous solutions and water were observed (Figures 3 and 4) visually, a fine k-nearest neighbors (kNN) were used to investigate spectral differences between the L-fulcose and L-proline solutions of different concentrations, as well as between the solutions and water. Optimal cross-validation accuracy (75.97%) was achieved with SNV normalized 1 st derivative spectra (Savitzky-Golay filter: degree = 3, window = 99 for 319-1100 nm, and degree = 3, window = 17 for 943-2500 nm). A confusion matrix of a fine kNN classification (Figure 6) shows the number of observations in each cell's true and predicted classes. The F1-score for each class of a fine kNN classification is presented on the right side of the confusion matrix (Figure 6). The fine kNN classification shows that the NIRS measurements in this study are not as sensitive for the low concentration solutions (i.e., F3/P3 and F4/P4) as it is for higher concentrations. Figure 4: The 2 nd derivative NIR spectra of aqueous L-proline. Each spectrum of each concentration shows the average of 3 independent solutions. The NIR spectrum of pure water is shown in comparison. The wavelength areas presenting a systematic order with the solutions of different concentrations and water (P1 \(\rightarrow\) P2 \(\rightarrow\) P3 \(\rightarrow\) P4 \(\rightarrow\) water) are marked with a black rectangle, whereas solutions appearing in the order P1 \(\rightarrow\) P2 \(\rightarrow\) P3 to P4 or P1 \(\rightarrow\) P2 \(\rightarrow\) P3 to water were marked with a light green rectangle. Both saliva samples and water have peaks between 940-1068 nm, 1400-1553 nm, and 1840-1950 nm (Figure 5). These peaks were assigned to 2 nd overtone OH stretching and water, 1 st overtone OH stretching and water, and to combination OH stretching and water, respectively. Both L-proline and saliva sample 2 have two peaks between 730-1068 nm (Figures 1 and 4), whereas water has only one strong peak between 909-1068 nm (Figures 2 and 5). In this NIR spectral region appear 3 rd overtone NH and CH stretching bands, water has an absorbance band and OH has 2 nd overtone stretching band (Tables 3 and 4). The peak transition of saliva sample 1 was observed between 1530-1650 nm (Figure 5). That region has 1 st overtone NH and OH stretching. Two peaks between 1476-1778 nm, that have higher scaled absorbance than water or saliva sample 2 were observed with saliva sample 1. In this region are 1 st overtone OH stretching, water, and 1 st overtone CH stretching bands [62].