Inspection of Adulteration of Walnut Oil by Laser Raman Spectrometry

Laser Raman spectrometry was applied to the inspection of walnut oil blended with other vegetable oil (i.e., soybean oil, peanut oil, corn oil, palm oil or cotton seed oil). Values of relative scattering intensity of six major peaks of Raman displacement (Δν) were taken as variables from the Raman spectra of the 6 pure vegetable oil mentioned above and the method of principal componental analysis was applied. As shown in the 2D scatter diagrams, experimental points for the 6 oil were located in different areas in each of the diagrams, and for samples of walnut oil blended with different amounts (in w%) of any of the other 5 vegetable oil, experimental points for each of the 2 oil were found in different areas in the diagram. Based on these facts, qualitative discrimination was attained. Quantification of the adulterant oil was performed on the base of linear relationship between values of decrease of relative Raman scattering intensity at wave number of 1 256 cm-1 and the contents (in w%) of the adulterant oil and the linear regression equations derived. The quantitative method was certified by analyzing 10 samples of walnut oil adulterated with plam oil with mass fractions from 8% to 95%, giving relative errors between the known values and the predicted values in the range of 1.01%-2.35%.