# Raman Spectroscopy Analysis of Free Fatty Acid in Olive Oil
## 3 Results and Discussion  
Figure 1 presents the Raman scattering spectra of the six olive oil samples of A, B, C, D, E, and F with 532 nm green laser excitation. All these samples show characteristic vibrational modes at 1080 cm\({}^{-1}\), 1265 cm\({}^{-1}\), 1300 cm\({}^{-1}\), 1440 cm\({}^{-1}\), 1655 cm\({}^{-1}\), and 1750 cm\({}^{-1}\). The mode at 1080 cm\({}^{-1}\) can be assigned to (C-C) stretching of the (CH\({}_{2}\))\({}_{n}\) group; the mode at 1265 cm\({}^{-1}\) can be assigned to (=C-H) deformation of cis (R-HC=CH-R); the mode at 1300 cm\({}^{-1}\) can be assigned to (C-H) bending  
twist of the CH\({}_{2}\) group; the mode at 1440 cm\({}^{-1}\) can be assigned to (C-H) scissoring of CH\({}_{2}\); the mode at 1655 cm\({}^{-1}\) can be assigned to (C=C) of cis (RHC=CHR); the mode at 1750 cm\({}^{-1}\) can be assigned to (C=O) stretching of RC=OOR [24; 25; 26; 27]. These are the common characteristic vibrational modes of almost all edible vegetable oils. The major difference between low-quality pomace olive oil and extra virgin olive oil samples is the appearance of the two vibrational modes at 1155 cm\({}^{-1}\) and 1525 cm\({}^{-1}\). In the extra virgin olive oil samples, these two modes can be observed, while in the low-quality pomace olive oil sample, these two modes are not detectable. Also, the intensity of these two modes varies with the FFA content in extra virgin olive oil samples. The mode at 1155 cm\({}^{-1}\) can be assigned to C-C stretching of carotenoids, and the mode at 1525 cm\({}^{-1}\) can be assigned to C=C stretching of carotenoids [26; 28; 29]. The carotenoids play an important role as natural antioxidants in olive oils. The observation of these two modes would be very helpful for the analyses of different quality olive oils. As can be seen in Figure 1, the intensity of the two vibrational modes at 1155 cm\({}^{-1}\) and 1525 cm\({}^{-1}\) decreases systematically with increasing FFA content in olive oil, which is consistent with reported results [6]. This intensity decrease indicates that FFA would degrade carotenoids in olive oil, thus higher FFA content is associated with lower intensity of carotenoids vibrational modes. Due to the FFA degradation of carotenoids, when an extra virgin olive oil is mixed with a lower-quality pomace olive oil, the intensity decrease of carotenoids Raman modes due to volume increase would be more significant than that of other modes in extra virgin olive oil. Below, we present a simple Raman method to confirm the FFA degradation of carotenoids by analyzing the relative intensity of characteristic vibrational modes and introducing an intensity decrease factor. This method would be helpful also to study possible degradation process in other materials. Figure 2 presents the Raman spectra of the mixtures of sample C and F with volume ratios of 2:1, 3:2, 1:1, 2:3, and 1:2. For comparison, the Raman spectra of sample C and F are also included in the figure, and the intensities of all the spectra are normalized using the intensity of the vibrational mode at 1655 cm\({}^{-1}\) as a reference. In Figure 2, the spectra were normalized only for better observation of the difference of relative intensity ratios. The mode at 1655 cm\({}^{-1}\) mode has relatively good line-shape  
Figure 1: Raman spectra of olive oil samples A, B, C, D, E, and F of different origin. Each spectrum is y-shifted for clarity. and is not much affected by the neighboring modes, thus was chosen for normalization. As can be seen in Figure 2, the intensity decrease of the two vibrational modes at 1155 cm\({}^{-1}\) and 1525 cm\({}^{-1}\) is much faster than other modes of extra virgin olive oil. Also, the intensity decrease of the two modes is faster than the volume increase when sample F is added to sample C. Furthermore, the intensity of these two modes has more significant intensity decrease when the volume ratio of sample F increases in the mixture.