# Raman Spectroscopy and 2 DCOS Analysis of Unsaturated Fatty Acid in Edible Vegetable Oils
## 1 Introduction  
The main components of edible vegetable oils are triglycerides, which can be characterized in terms of fatty acids making part of their structure [1]. Fatty acids can be divided into saturated fatty acids and unsaturated fatty acids. An unsaturated fatty acid (UFA) has at least one carbon-carbon double bond within the acid chain; it is monounsaturated if it contains only one double bond, and polyunsaturated if it contains more than one double bond. Both monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) are very important for health [2; 3; 4]. Studies have shown that MUFA are helpful to improve insulin sensitivity, and can lead to reduced anger and irritability; additionally in children, consumption of MUFA is associated with healthier serum lipid profiles [5; 6; 7]. PUFA may be helpful to reduce the risk of cardiovascular disease and breast cancer [8; 9]. PUFA protects against cardiovascular disease by providing more membrane fluidity than MUFA, but it are more vulnerable to lipid peroxidation. In various common edible vegetable oils, the total content of UFA could be very similar, while the contents of PUFA or MUFA are very different. Therefore, it would be interesting to differentiate PUFA or MUFA from the total UFA for quality evaluation of edible vegetable oils.  
Raman spectroscopy and infrared spectroscopy have been extensively applied to investigate various types of oils [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. In spectroscopy analyses, studying the relative intensity ratios of characteristic bands is more powerful than investigating the intensity of a particular band, as it can provide more reliable and more detailed intrinsic information of the investigated samples [23; 24; 25; 26]. In this paper, we investigate the relative intensity ratios of characteristic vibrational bands of vegetable oils as a function of the content of totally unsaturated fatty acids, polyunsaturated fatty acids, andmonoussaturated fatty acids through Raman spectroscopy. The results suggest that characteristic relative intensity ratios could be applied for preliminary differentiation of PUFA or MUFA from the total UFA in common edible vegetable oils.  
Generalized two-dimensional correlation spectroscopy (2 DCOS) has become one of the most sensitive analysis techniques that interprets spectral data sets obtained under external perturbation such as temperature, time, pressure, potential, etc. [27; 28; 29; 30; 31; 32]. 2 DCOS has been applied to study the fine spectral features for liquid oleic acid (the most common MUFA) and linoleic acid (a common PUFA with two carbon-carbon double bonds) based on the time dependent attenuated total reflectance infrared spectra, which suggested that the carbon-carbon double bond in the alkyl chain plays an important role in the self-assembling of the carboxyl acids [33]. In this paper, 2 DCOS has been performed on the content dependent Raman spectra of common edible vegetable oils, which indicated that the analyses of vibrational bands correlated with the carbon-carbon double bond is helpful for quality evaluation of edible vegetable oils. The 2 DCOS result is consistent with that by Raman analyses of relative intensity ratios.