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

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 bisallylic position equivalents) can be derived directly from the integration values of the allylic and bis-allylic protons.

Analysis of oxidized biodiesel by 1 H‐NMR and effect of contact area with air

“…The data in Tab. 1 show slight deviations of the fatty acid composition when using 1 H-NMR, which was also discussed previously [26]. Therefore, a corresponding deviation of the results obtained here from those that would be theoretically obtainable by chromatography-based methods must be assumed.…”

“…The amounts of diunsaturated fatty acid moieties are slightly underestimated by this method [26,32]. …”

“…Such species have been analyzed in heated biodiesel [14]. However, 1 H-NMR can be used for fatty acid profile determination [26] and therefore it was of interest to apply this method to oxidized biodiesel and to compare these data to other easily conducted methods for assessing fuel quality, such as kinematic viscosity and acid value. Besides these analytical aspects, the influence of varying the surface area towards air of a constant volume of biodiesel was assessed.…”

“…Accordingly, the contact area between the biodiesel was varied by changing the size of the open beaker in which it was heated, but not the amount of the fuel. In order to analyze the biodiesel oxidized in this fashion, the resulting material was analyzed by proton nuclear magnetic resonance ( 1 H-NMR), which is of specific interest because the fatty acid profile of biodiesel (or a vegetable oil of fat) can be determined easily by this method [26] and changes to the peaks generated by the presence of the double bonds are affected by oxidation. Thus, the kinds of fatty compounds in oxidized biodiesel should be quantifiable by NMR, especially in light of the fact that few details are known about the fatty acid composition of oxidized biodiesel.…”

GC/MS Analysis of Fatty Acids in Italian Dry Fermented Sausages

“…All these signals were unambiguously assigned to the corresponding spin systems of the different types of protons belonging to the various acyl chains, according to the literature values [36,37]. No other signals were visible in the recorded spectrum, at least upon normally scaled graphic conditions.…”