## 4 Discussion  
This work aimed to study the ability of NIRS to measure and distinguish known oral cancer biomarkers from aqueous solutions. One hypothesis of the study was that solutions with different concentrations would be distinguishable from each other and water. The results indicate that NIRS can distinguish between the different concentrations of aqueous solutions using even a probe that is not specifically designed for measuring liquid samples. The spectra of pure solid L-tucose and L-proline (Figure 2) are noticeable different from that of water, but their diluted solutions and saliva (Figures 3-5) have more similar spectra with water. However, there are noticeable differences between solutions and water (Figures 3 and 4, and Tables 3 and 4) as noted in the present results. The differences between the spectra of L-tucose and L-proline solutions and water were also studied with a fine kNN for wavelengths between 680-1400 nm and 1550-1850 nm (Figure 6). The fine kNN, with validation accuracy of 75.97%, and the F1-scores (Figure 6) show that the sensitivity of the NIRS measurement is relatively poorer for solutions with lower concentrations. This might be due to the small number of measured spectra. The second hypothesis of this study was whether L-tucose and L-proline would be distinguishable on NIR absorbance spectra of two saliva samples of patients with diagnosed OSCC. This would indicate that the NIRS can recognize OSCC patients having higher concentrations of L-fucose and lower concentrations of L-proline from patients that do not suffer from OSCC as was the case in the NMR study by [PERSON] et al. (2015). Visual observation of the spectra shows that saliva sample 1 has two ([PERSON] et al., 2015) absorbance peaks in spectral region 1476-1778 nm, and the peaks are stronger than those of water and saliva sample 2 (Figures 2 and 5). This finding is logical as sample 1 has an order of magnitude higher L-fucose concentration  
Figure 6: Confusion matrix of a cross-validated fine kNN classifier. F1-score of each class of a fine kNN classification are presented on the right side of the figure. (694 \(\upmu\)M) than saliva sample 2 (67 \(\upmu\)M). Within the same wavelength area, saliva sample 1 has a peak transition between 1530-1650 nm that is shifted away from the nearby peak of water (Figure 5). This might be due to the high L-fucose concentration of sample 1. Saliva sample 1 has a higher L-proline concentration (799 \(\upmu\)M) than saliva sample 2 (157 \(\upmu\)M) but interestingly, the saliva sample 2 has a stronger peak between 730-915 nm (Figure 5) and sample 1 has a stronger peak between 915-1068 nm in the spectral region of 730-1068 nm (Figure 5). To confirm these findings, the measurements should be repeated with a higher number of saliva samples collected from both OSCC patients and healthy controls. [PERSON] et al. [53] developed a tube capillary method to study small volume liquid samples. In another study by [PERSON] et al. [38], authors utilized the method to measure FT-NIR spectra from saliva and distinguished oral cancer patients from healthy controls. In our study, spectral region 300-2500 nm was measured while the analysis was focused on the NIR region between 680-1950 nm. [PERSON] et al. [38] measured NIR spectra between 8000-5500 cm\({}^{-1}\) (1250-1818 nm). In their study, [PERSON] et al. [38], used only a single 2.5 \(\upmu\)L drop of saliva, which is eight times less than in our study. Similar to our study, they found the water to dominate the spectra. In the study by [PERSON] et al. [38], broadband around 6900 cm\({}^{-1}\) (1449 nm) and peaks around 7072, 6839, and 5613 cm\({}^{-1}\) (1414, 1462, 1781 nm, respectively) were assigned to water. The first overtones of CH and NH stretching modes were observed within 6500-5500 cm\({}^{-1}\) (1538-1818 nm). Compared to [PERSON] et al. [38], our study requires less preparation as the sample can be pipetted straight on top of the reflective surface. Furthermore, our method is not dependent on circulating water as [PERSON] et al. [38] were.