| According to the infrared characteristic absorption peak at about 3009 cm~-1 of=C-H bond of unsaturated fatty acid,the Fourier transform infrared spectrometer was used to analyze C=C bond quantitatively.Then a Fourier infrared spectroscopy(FTIR)method using quartz Petri dish as sample containers for determination of the iodine value(IV)of oil was proposed.Define AJc as the ratio of the absorption peak height(A)of =C-H bond of oil in carbon tetrachloride solution and the concentration(c)of samples,and there is a good linear relationship between A/c and IV of oil.Standard samples of different IV were prepared with flaxseed oil and coconut oil in a certain proportion,and the standard curve of IV determination was established.The correlation coefficient is 0.9995.The results of the oil IV of 36 samples(IVIR)determined by this method were consistent with the national standard method(IVGB),and the correlation coefficient is 0.9978,so this method can be used for rapid determination of the IV of cooking oil.The characteristic absorption positions of =C-H in oleic acid,linoleic acid and linolenic acid located at 3006,3010 and 3010 cm~-1,respectively.According to the different characteristic absorption positions of these three kinds of unsaturated fatty acids,using stretching vibration absorption band near 3009 cm~-1 of cis =C-H predicted the content of unsaturated fatty acid.According to the principle of the additivity of the absorbances,multiple linear fitting technology was employed to determinate the content of unsaturated fatty acid by FTIR spectra data and gas chromatography(GC),then the absorption curves of cis =C-H band of acyl group of oleic acid,linoleic acid and linolenic acid can be obtained.The content of oleic acid,linoleic acid and linolenic acid in 44 kinds of oils and fats predicted by FTIR were consistent with the results determined by GC,demonstrating that FTIR can be used for rapid determination of the composition of main unsaturated fatty acids with that all the olefin bonds are cis configuration in edible oil.For olive oil,soybean oil and flax seed oil with different content of trans fatty acid,Fourier infrared spectrum in 4000?400 cm~-1 was collected adopting hexane with nitrobenzene solution as internal standard,and the negative two order derivative at 966 cm~-1 was calculated.Trans fatty acids content(WGC)in the sample and the characteristic absorption(A)at 966 cm~-1 of isolated trans double bond in trans fatty acids conformed to the lambert beer's law.The quantitative relationship between the intensity of infrared absorption peak and the content of double bond in trans fatty acids determined by GC was established.This standard curve was used to determine the content of oil trans fatty acid,and the results showed that FTIR method was significantly different from GC method.The oil,soybean oil and flaxseed oil with high trans-type double bond were prepared by catalytic conversion of C=C double bond.The FTIR data of conversion of these oils showed that absorption coefficient at 966 cm~-1 of the trans oleic acid,linoleic acid and linolenic acid was different,leading to a deviation from the result.Therefore,FTIR cannot be used to measure the content of trans fatty acids accurately,but it can be used as a rapid screening for the range of fatty acid. |
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