, Volume 62, Issue 10, pp 1501-1507

Analysis of fatty acid methyl esters with high accuracy and reliability. IV. Fats with fatty acids containing four or more carbon atoms

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Those aspects of the quantitative methanolysis of fats with fatty acids containing four or more carbon atoms and the gas liquid chromatographic analysis of the methyl esters which have not been adequately dealt with to date were investigated. Accurate analysis of the esters requires the optimization of the total chromatographic system using a carefully prepared methyl ester primary standard and applying, as the only correction factors, the theoretical flame ionization detector response factors of Ackman and Sipos. These factors now have also been shown to be highly accurate for short chain saturated fatty acid methyl esters, in particular methyl butyrate and methyl caproate. Under the conditions specified by the international methods for the methylation (methanolic KOH) of fats with fatty acids containing four or more carbon atoms, saponification of the esters, once formed, proceeds much more rapidly for short chain than for long chain esters. This problem is easily overcome by neutralizing the catalyst, which then leads to very stable solutions and contributes to column life. This step should be mandatory in the international procedures. Tristearin methylated more slowly than other triacylglycerols under the standard conditions, but there was no difficulty in the case of unsaturated fatty acids of this chain length. Satisfactory results could not be obtained when sodium methoxide was the catalyst. The optimum procedure rendered unnecessary the use of methyl pentanoate as an internal standard for the accurate quantitation of methyl butyrate.