Separation and Identification of Fatty Acids

Part 16. Paper Chromatography of Unsaturated Higher Fatty Acids

Abstract

To provide a simple method for the separation and identification of the component fatty acids of fats, the study of the paper chromatography of unsaturated higher fatty acids was made by using the reversed-phase technique which was developed in the preceding paper [Y. INOUYE and M. NODA: This Journal, 26, 634 (1952).]. The R_F values for fourteen unsaturated acids given with the suitable solvent systems, methanol-petroleum hydrocarbon or methanol-acetone-petroleum hydrocarbon, were measured. For detection of fatty acids on the chromatograms, a solution of 0.2% bromocresol green in alcohol was generally used, while either 2% iodine in alcohol or WIJS' iodine monochloride solution was useful to detect only unsaturated fatty acids. Therefore by pairing these spray reagents, it becomes easier to distinguish the spots of unsaturated from that of saturated acids on the paper chromatograms.
On the effects of structure of the fatty acids upon the R_F values, the following generali-zations can be made in the descending chromatography with methanol-petroleum hydroca-rbon system. Generally, the longer the aliphatic chain of a fatty acid, the lower the R_F value. With acids of equal chain length, increasing the number of double or triple bonds and hydroxyl groups also increases R_F value: saturated<monoethenoid<diethenoid<triethenoid (conj.)<monoethinoid<triethenoid <hydroxymonoethenoid acids. Differences in R_F values between cis- and trans-isomers are slight in general cases.
The method is suitable for the analysis of the component fatty acids of natural fats.