# Raman Spectroscopy Analysis of Free Fatty Acid in Olive Oil
## 3 Results and Discussion
The extra virgin olive oil samples were purchased from standard supermarkets in China and Korea; the samples and their mixtures were investigated without other treatment. Through relative intensity analysis, we first confirmed that the intensities of characteristic vibrational modes at 1155 cm\({}^{-1}\) and 1525 cm\({}^{-1}\) of extra virgin olive oil decrease systematically with increasing FFA content; then showed that the intensity ratio of I\({}_{1525}\)/I\({}_{1655}\) decreases linearly with FFA content. This linear correlation presents a good and rapid analysis of FFA content in extra virgin olive oil. In recent years, portable and handheld Raman systems have been developed. One advantage of our method is that it is not depending on experimental conditions. Thus, when our method is combined with portable and handheld Raman systems, it would be powerful for quick on-site quality evaluation of extra virgin olive oil in supermarkets. The main purpose of this paper is to propose that relative Raman intensity analysis is very useful for quick quality evaluation of extra virgin olive oils. For practical application of our method, a standard correlation between FFA content and relative intensity ratio I\({}_{1525}\)/I\({}_{1655}\) should be established, which would require accurate measurements of FFA% and I\({}_{1525}\)/I\({}_{1655}\). With 532 nm laser excitation, the intensity of 1525 cm\({}^{-1}\) mode is quite weak; a significant enhancement of this mode would be very helpful to improve the accuracy of I\({}_{1525}\)/I\({}_{1655}\). Thus, a resonance investigation would be necessary, as discussed below. To achieve significant enhancement of 1525 cm\({}^{-1}\) mode, the wavelength of excitation laser should be close to the absorption edge of carotenoids. The absorption study showed that the absorption edge of carotenoids is about 510 nm [32]. Therefore, the intensity of the 1525 cm\({}^{-1}\) mode would be strongly enhanced with excitation about 510 nm, and decrease with higher excitation wavelength. This is consistent with a reported study with 514.5 nm laser [6]; and our study with 532 nm, 633 nm, and 785 nm lasers, which is presented in Figure 4. It was shown that with 514.5 nm laser excitation, 1525 cm\({}^{-1}\) mode have very strong intensity, which is even higher than that of 1655 cm\({}^{-1}\) mode [6]. Figure 3: Relative intensity ratios of I\({}_{1525}\)/I\({}_{1655}\) (bullet), I\({}_{1155}\)/I\({}_{1655}\) (triangle), and I\({}_{1440}\)/I\({}_{1655}\) (square) as a function of free fatty acid (FFA) content in extra virgin olive oil. The straight line is a linear fitting of I\({}_{1525}\)/I\({}_{1655}\). While 1525 cm\({}^{-1}\) mode is much weaker than 1655 cm\({}^{-1}\) mode with 532 nm laser excitation, as can be seen in Figure 4, 1525 cm\({}^{-1}\) mode is too weak to be observed with 785 nm laser excitation, and with 633 nm laser excitation, a strong luminescence peak at about 670 nm is observed, and no vibrational modes can be observed due to the strong luminescence background. Therefore, a green laser of 514.5 nm would be ideal for practical application of relative intensity analysis of FFA in olive oils. Our above results indicate that the study of relative intensity ratio of the two characteristic vibrational modes at 1525 cm\({}^{-1}\) and 1655 cm\({}^{-1}\) with 514.5 nm green laser excitation would be of great interest for quick on-site quality evaluation of olive oils. This simple relative intensity analysis of 1525 cm\({}^{-1}\) and 1655 cm\({}^{-1}\) modes would be powerful only for quick evaluation of similar types of high-quality olive oils, such as extra virgin olive oils. For quick on-site detection of olive oil adulteration, further systematic studies would be needed. For instance, if olive oils are adulterated with other types of vegetables oils, then the mode at 1655 cm\({}^{-1}\) would not be a good reference, thus the intensity ratio of I\({}_{1525}\)/I\({}_{1655}\) cannot be simply applied for detecting olive oil adulteration. In a recent study, [PERSON] et al.