## 3 Results and Discussion
### Acquisition of 12 Characteristic Wavelengths  
PCA was first used to find useful information about the WAMACS. The first principal component contributes about 63.24% of the total variance. PC1 shows that as the time goes by, the score decreases which shows no difference with the ROS concentration changes. And then loading for PC1 was introduced for WAMACS identification. Figure 6A shows the wavelengths selected by the PCA method. Each wavelength corresponds to the absorption of a specific water molecular species: 1361 nm and 1366 nm--OH-(H\({}_{2}\)O)\({}_{1,2,4}\); 1378 nm--\(\upsilon_{1}+\upsilon_{3}\), 1390 nm--2\(\upsilon_{1}\), OH-(H\({}_{2}\)O)\({}_{1,4}\) and O\({}_{2}\)-(H\({}_{2}\)O)\({}_{4}\); 1395 nm and 1398 nm--water trapped in the local field of ions; 1400 nm and 1403 nm--free water molecules (S\({}_{0}\)) [27]; 1833 nm, 1844 nm, 1848 nm and 1851 nm--OH bond stretching and bending vibrations [29].  
The PLS technique was further introduced for WAMACS analysis. The spectral data obtained from the 8 h measurements were analyzed using the PLS method with storage time as the y-variable. Principal components were determined using the cross-validation method and four components were used in this study as the RMSECV is low. Then, re  
Figure 6: Load plots corresponding to PC1 in PCA analysis and regression coefficient curves in PLS analysis. **(A)**. Load plots obtained by plotting the loads derived from PC1 against wavelengths and labelled with characteristic wavelengths selected in the range 1300–1860 nm. **(B)**. Regression coefficient curves obtained by plotting the values of the regression coefficients against wavelengths and labelled with characteristic wavelengths selected in the range 1300–1860 nm.  
gression coefficient curves of the spectral were calculated and the greater the value of the regression coefficient, the more robust the correlation between the absorption generated at that wavelength. A set of characteristic wavelengths was selected based on the values of the regression coefficients, as shown in Figure 6B, where each wavelength corresponds to the absorption of a specific water molecular species: 1366 nm--OH-(H\({}_{2}\)O)\({}_{1,2,4}\); 1394 nm--water trapped in the local field of ions; 1402 nm--free water molecules (S\({}_{0}\)) [27]; 1437 nm and 1441 nm--water molecules with 1 hydrogen bond(S\({}_{0}\)); 1450 nm--OH-(H\({}_{2}\)O)\({}_{4,5}\); 1833 nm, 1844 nm and 1852 nm--OH bond stretching and bending vibrations [29]. Of these wavelengths selected by PCA and PLS, most occur in the broad absorption peak at 1300-1600 nm, which contains valuable insights into the structure of water and provides useful information. The remaining few wavelengths are in the 1800-1860 nm range and correspond to the stretching and bending vibrations of the OH bond, providing less information but are necessary to account for.  
Finally, compared with the WAMACS discovered using different methods, the WAMACS were constructed using 12 wavelengths. As described in the following section, the absorption of these wavelengths in the NIR band corresponds to the structural changes of water molecules. It is important to note that only wavelengths with positive regression coefficients were selected to guarantee the clarity and simplicity of the aquagram.