# Raman Spectroscopy and 2 DCOS Analysis of Unsaturated Fatty Acid in Edible Vegetable Oils
## 3 Results and Discussion
Figure 3 shows that the intensity ratios of 1655 cm\({}^{-1}\) to 1440 cm\({}^{-1}\) and 1265 cm\({}^{-1}\) to 1300 cm\({}^{-1}\) would have a correlation with the content of PUFA or MUFA, but not the total UFA. These intensity ratios increase with increasing PUFA content (and decrease with increasing MUFA content). Table 1 showed that the variance of total UFA content is much smaller than that of PUFA or MUFA content for the studied samples. This indicates that in PUFA, the intensity ratios of 1655 cm\({}^{-1}\) to 1440 cm\({}^{-1}\) and 1265 cm\({}^{-1}\) to 1300 cm\({}^{-1}\) would be significantly larger than those in MUFA. This is consistent with the Raman analyses of oleic acid and linoleic acid, which showed that the intensity ratios of 1655 cm\({}^{-1}\) to 1440 cm\({}^{-1}\) and 1265 cm\({}^{-1}\) to 1300 cm\({}^{-1}\) are significantly larger in linoleic acid than those in oleic acid [14]. If the PUFA content (or MUFA content) is different  
Figure 2: Relative intensity ratios of I\({}_{1655}\)/I\({}_{1440}\) (square), I\({}_{1265}\)/I\({}_{1300}\) (dot), and I\({}_{1440}\)/I\({}_{1300}\) (triangle) as a function of the total unsaturated fatty acid (UFA) content. The error bars of the relative intensity ratios were estimated to be about 10%, mainly caused by baseline corrections. for vegetable oils with the same UFA content, then the intensity ratios of 1655 cm\({}^{-1}\) to 1440 cm\({}^{-1}\) and 1265 cm\({}^{-1}\) to 1300 cm\({}^{-1}\) would be very different in those samples. Therefore, the intensity ratio of a characteristic vibrational band correlated with double bond to a vibrational band not correlated with double bond could be applied for preliminary analysis of the PUFA content (or MUFA), but could not be used to analyze the total UFA content. Two-dimensional correlation spectroscopy (2 DCOS) analyses were applied to the content-dependent 1D Raman spectra in Figure 1 in order to quantify the correlation of the intensities  
Figure 3: Relative intensity ratios of I\({}_{1655}\)/I\({}_{1440}\) (square), I\({}_{1265}\)/I\({}_{1300}\) (dot), and I\({}_{1440}\)/I\({}_{1300}\) (triangle) as a function of the content of PUFA (**a**) and MUFA (**b**). The curves are not fitted, they simply show the trend how the relative intensity ratios change. The error bars of the relative intensity ratios were estimated to be about 10%, mainly caused by baseline corrections. of various vibrational bands with MUFA content (or PUFA). The raw Raman spectra were baseline corrected and the intensities were normalized using the intensity of the vibrational band at 1440 cm\({}^{-1}\) as a reference. In our 2 DCOS analyses, the average spectrum was selected as the reference spectrum, same as described in Reference [28]. Figure 4 shows the synchronous and asynchronous 2D Raman correlation spectra. The power spectrum extracted along the diagonal line in the synchronous 2D spectrum is also shown at the top of Figure 4a. The synchronous spectrum shows that there are two strong autopeaks at 1265 cm\({}^{-1}\) and 1655 cm\({}^{-1}\), but no autopeaks at 1300 cm\({}^{-1}\) and 1440 cm\({}^{-1}\). Since the intensity was normalized using the vibrational band at 1440 cm\({}^{-1}\) as a reference, it is natural that the vibrational band at 1440 cm\({}^{-1}\) shows negligible intensity change when the MUFA content (or PUFA) changes. However, it is worthwhile to note that the vibrational band at 1300 cm\({}^{-1}\) does not change at all, while the vibrational bands at 1265 cm\({}^{-1}\) and 1660 cm\({}^{-1}\) show significant intensity changes. In addition, Figure 4a shows that the vibrational band at 1665 cm\({}^{-1}\) has positive cross peak with the band at 1265 cm\({}^{-1}\). This indicates that the intensity of these two bands changes with same trend when the MUFA content (or PUFA) changes.