DETERMINATION OF THE FATTY ACID PROFILE BY H-NMR SPECTROSCOPY

Authors: Knothe, Gerhard; Kenar, James - Jim

Submitted to: European Journal of Lipid Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/27/2003
Publication Date: 2/1/2004
Citation: Knothe, G.H., Kenar, J.A. 2004. Determination of the fatty acid profile by h-nmr spectroscopy. European Journal Lipid Science Technology. 106(2):88-96.

Interpretive Summary: Vegetable oils, such as soybean oil, and industrial products produced from them, such as biodiesel or lubricants, are mixtures of various components. The chief components are, in more technical terms, esters of fatty acids. Different esters of fatty acids have different properties. Therefore, the overall properties of the vegetable oil or derived product depend on the amounts of the individual esters of fatty acids present. The present work discusses an analytical method which allows us to rapidly determine the amounts of esters of fatty acids in a vegetable oil or related product. The method has the advantages that it does not consume the sample and that the sample does not need any special treatment in preparation for analysis. The results from the present method agree well with results from other analytical procedures.

Technical Abstract: The common unsaturated fatty acids present in many vegetable oils (oleic, linoleic and linolenic acids) can be quantitated by 1H nuclear magnetic resonance spectroscopy. The integration values of the signals of the allylic protons, bis-allylic protons and the terminal methyl group are employed in the present method. The integration values of the olefinic protons do not need to be used and their use actually reduces analytical accuracy. A key feature is that the signals of the terminal methyl group of linolenic acid are shifted downfield from the corresponding signals in the other fatty acids, permitting their separate integration and subsequent straightforward quantitation of linolenic acid. Then, using the integration values of the signals of the allylic and bis-allylic protons, oleic and linoleic acids can be easily quantified. The procedure was verified for mixtures of triacylglycerols (vegetable oils) and methyl esters of oleic, linoleic and linolenic acids as well as palmitic and stearic acids. The fatty acid composition determined by this method was compared to corresponding results from gas chromatography. Generally, the NMR results were in good agreement with gas chromatographic analyses. As the present 1H NMR-based procedure can be applied to neat vegetable oils, the preparation of derivatives for gas chromatography would be unnecessary. This 1H-NMR analysis is extended to quantitating the saturated fatty acids C16:0 and C18:0, although in this case 1H-NMR-based determination deviates more strongly from actual values and gas chromatographic analyses. Alternatives to the iodine value (allylic position equivalents and bis-allylic position equivalents) can be derived directly from the integration values of the allylic and bis-allylic protons.