# Raman Spectroscopy and 2 DCOS Analysis of Unsaturated Fatty Acid in Edible Vegetable Oils
## 3 Results and Discussion
These are in good agreement with the results in Figure 3, further supporting that the relative intensity ratios of 1265 cm\({}^{-1}\) to 1300 cm\({}^{-1}\) and 1655 cm\({}^{-1}\) to 1440 cm\({}^{-1}\) could be applied for differentiation of MUFA (or PUFA) from the total UFA in common edible vegetable oils. Figure 4a also shows a relatively strong autopeak at 1525 cm\({}^{-1}\) and two weak autopeaks at 1325 cm\({}^{-1}\) and 1750 cm\({}^{-1}\). This suggests that the intensity of these peaks would vary when the MUFA content (or PUFA) changes. The band at 1525 cm\({}^{-1}\) is a characteristic vibration of olive oil [16], but is not observed in other types of vegetable oils, and the two bands at 1325 cm\({}^{-1}\) and 1750 cm\({}^{-1}\) are quite weak to investigate. Therefore, detailed analyses of these peaks are not included in this paper. Figure 4b shows that the cross peak at (1665 cm\({}^{-1}\), 1265 cm\({}^{-1}\)) is positive, indicating that the intensity change of 1265 cm\({}^{-1}\) occurs after that of 1665 cm\({}^{-1}\) when the MUFA content (or PUFA) changes. This suggests further systematic analyses of intensity changes between 1665 cm\({}^{-1}\) and 1265 cm\({}^{-1}\) could be helpful to differentiate between MUFA and PUFA. Figure 4: Synchronous (**a**) and asynchronous (**b**) 2D correlation spectra obtained from the series of 1D Raman spectra. Red and blue colors represent positive and negative peaks, respectively. The auto correlation spectrum extracted along the diagonal line in synchronous 2D correlation spectrum is shown at the top of Figure 4a. The above Raman spectroscopy and 2 DCOS analyses of vegetable oils indicated that the relative intensity ratio of 1655 cm\({}^{-1}\) to 1440 cm\({}^{-1}\) would be helpful for differentiating common edible vegetable oils. To further understand the differences of the relative intensity ratio of 1655 cm\({}^{-1}\) to 1440 cm\({}^{-1}\) in various vegetable oils, we investigated Raman scattering of three different types of pure UFA, one MUFA (OA) and two PUFAs (LA and ALA). The result is presented in Figure 5. OA, LA and ALA have the molecular formula of C\({}_{18}\)H\({}_{34}\)O\({}_{2}\), C\({}_{18}\)H\({}_{32}\)O\({}_{2}\) and C\({}_{18}\)H\({}_{30}\)O\({}_{2}\), respectively. The difference of these three acids is the number of double bonds in their acid chains; OA, LA and ALA have one, two, and three double bonds, respectively. Interestingly, the relative intensity ratio of 1660 cm\({}^{-1}\) to 1440 cm\({}^{-1}\) in OA, LA and ALA have the values of 1.1, 1.7, and 3.1, respectively. This not only confirms the results in Figures 3 and 4, but also indicates that the relative intensity ratio of 1655 cm\({}^{-1}\) to 1440 cm\({}^{-1}\) could be further applied for differentiating different types of PUFAs. A systematic density functional theory (DFT) calculation of the Raman spectra of UFAs with up to six carbon-carbon double bonds is currently underway. The DFT calculation indicates that the relative intensity ratio of 1655 cm\({}^{-1}\) to 1440 cm\({}^{-1}\) of UFAs with one, two, and three double bonds agrees well with the Raman scattering study of OA, LA, and ALA. However, for UFAs with four, five, and six double bonds, the relative intensity ratio of 1660 cm\({}^{-1}\) to 1440 cm\({}^{-1}\) does not increase systematically with the number of double bonds. This suggests that further systematic studies on different types of UFAs are needed to fully understand the relative intensity ratios in UFAs for future practical applications of quality evaluation of vegetable oils by Raman spectroscopy.