Determination of the fatty acid profile by ¹H‐NMR spectroscopy

TLDR

Unsaturated fatty acids in vegetable oils, such as oleic, linoleic, and linolenic acids, can be quantified by 1H‑NMR spectroscopy. The method exploits the downfield shift of linolenic acid’s terminal methyl signal and the integration of allylic and bis‑allylic protons to separately quantify linolenic, oleic, and linoleic acids, derive iodine‑value equivalents, and was validated on triacylglycerol mixtures and methyl esters. 1H‑NMR results at 400 MHz closely matched gas‑chromatography data for unsaturated acids, enabling analysis of neat vegetable oils without derivatization, though saturated acid quantitation showed larger deviations.

Abstract

Abstract The common unsaturated fatty acids present in many vegetable oils (oleic, linoleic and linolenic acids) can be quantitated by 1 H‐nuclear magnetic resonance spectroscopy ( 1 H‐NMR). 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 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 quantitated. 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. Generally, the NMR (400 MHz) results were in good agreement with gas chromatographic (GC) analyses. As the present 1 H‐NMR‐based procedure can be applied to neat vegetable oils, the preparation of derivatives for GC would be unnecessary. The present method is extended to quantitating saturated (palmitic and stearic) acids, although in this case the results deviate more strongly from actual values and GC 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.