## 3 Results and Discussion  
Raman spectra (\(n=214\)) were acquired from 71 olive oil mixtures in the region 800-1800 cm\({}^{-1}\) under 785 nm excitation. Thirty-three Raman spectra from 11 binary blends of EVOO and rapeseed oil, 48 Raman spectra from 16 ternary blends of EVOO, rapeseed oil and corn oil, and 133 spectra from 44 quaternary mixtures of EVOO, rapeseed oil, corn oil and soybean oil were recorded. From each sample, 3 spectra were taken at different position of the droplet. Figure 2 shows the typical Raman spectra measured from pure EVOO, rapeseed oil, corn oil and soybean oil. From Figure 2, it is very obvious that the characteristic Raman peak positions of different oils are similar due to the fact that they are essentially composed of similar components such as triglycerides, saturated and unsaturated fatty acids with straight aliphatic chains and predominantly with 16 or 18 number of carbon atoms in the chains [29]. The primary Raman peak positions and the respective vibrational modes of EVOO, rapeseed, corn and soybean oil are as follows (Figure 2): 869 cm\({}^{-1}\) (C-C) stretching from the molecule -(CH\({}_{2}\))n-, 972 cm\({}^{-1}\) (C=C) bending from Trans RHC=CHR, 1079 cm\({}^{-1}\) (C-C) stretching from -(CH\({}_{2}\))n-, 1263 cm\({}^{-1}\) in-plane (=C-H) deformation from unconjugated cis (RHC=CHR), 1303 cm\({}^{-1}\) (C-H) bending (twisting) from methylene(CH\({}_{2}\)), 1441 cm\({}^{-1}\) (C-H) bending (scissoring) from methylene(CH\({}_{2}\)), 1527 cm\({}^{-1}\) (C=C) stretching attributed to carotenoids, 1657 cm\({}^{-1}\) (C=C) stretching from unsaturated banding(cis RHC=CHR), 1746 cm\({}^{-1}\) (C=O) stretching from ester (RC=OOR) [6, 8, 17, 19]. Figure 1: Fiber optic Raman spectroscopy set up. The general chemical structure of extra virgin olive oil (EVOO), where R\({}^{1}\), R\({}^{2}\) and R\({}^{3}\) are alkyl groups (approx. 20%) or alkenyl groups (approx. 80%). (DAQ-DataAcquisition). Despite the similar Raman peak positions observed for various oils, the differences can be noticed in their relative Raman peak intensities because each oil is characterized by its own specific fatty acids ratio content [29]. For instance, the fatty acids composition of the triglycerides in EVOO comprise 11% saturated fatty acids, around 80% monounsaturated fatty acids (twice the amount of oleic acid in comparison to other vegetable oils), and 9% polyunsaturated fatty acids (considerably less linoleic and linoleic acid in comparison to other vegetable oils) [7,13]. Hence, the Raman peak intensities at 972, 1263 and 1657 cm\({}^{-1}\) corresponding to the degree of unsaturation (the number of C=C bonds) are weaker and the peaks (1303 and 1441 cm\({}^{-1}\)) related to saturated fatty acids (CH\({}_{2}\)) are higher in EVOO compared to cheaper oils [13,17,18,19]. EVOO and rapeseed oil has a feature at 1118 cm\({}^{-1}\) (in-phase aliphatic all trans C-C stretch) [30], however, it has not been seen in corn and soybean oil. The distinct peak at 1527 cm\({}^{-1}\) corresponds to carotenoids, has been noticed only in EVOO [6,31]. Carotenoid is a pigment in olive oil, which is responsible for the color of olive oil and an important feature for the quality of EVOO [32]. The C=O ester carbonyl band at 1746 cm\({}^{-1}\) does not exhibit any changes. Soybean oil is the most different one from EVOO, showing the highest peak around 972, 1263, and 1657 cm\({}^{-1}\). The rapeseed oil is closer to EVOO compared to corn and soybean oil. Figure 3 shows the Raman spectra acquired from different volume percentage of EVOO blended with rapeseed oil, soybean oil and corn oil. Although there are multiple adulterants, the characteristic peaks at 972, 1263, and 1657 cm\({}^{-1}\) decrease with EVOO concentrations and the peaks at 1079, 1118, 1303, and 1527 cm\({}^{-1}\) increase with the concentrations of EVOO (Figure 3). As we know the nature of the adulterants, simple non-negative least squares fitting can be used to estimate the concentrations of EVOO and other adulterants in the oil mixtures.