Determination of Geometrical Isomers of 9,12‐Octadecadienoic Acid Present in Food Fats

Abstract

Abstract The present work presents a method to be used in the determination of different geometrical isomers of 9,12‐octadecadienoic acid present in food fats. The method is based on isotope dilution, the radioactive isomers needed being prepared by isomerization of cis,cis‐9,12‐octadecadienoic acid. Radioactivity is acquired through esterification of the isomers by 14 C‐diazomethane. For the identification of the isomers infra‐red spectrophotometry, alkali isomerization combined with ultraviolet spectrophotometry, gas chromatography, and ozonolysis are used. The isomer to be determined is separated from other fatty acids by argentation thin layer chromatography. The standard deviation of the method was in 15 parallel analyses 0.04%. The sample of food fat to be examined was milk fat, the cis,cis‐9,12‐octadecadienoic acid content obtained being 0.83%, which corresponded to about one half of octadecadienoic acids of the sample. When a fatty acid mixture with a known composition was analyzed, the cis,cis‐9,12‐octadecadienoic acid yield was 99.7% on the average, the standard deviation being 0.04%.