Methods for Evaluating Vitamin A Isomers

Abstract

Abstract Precise estimation of the biological activity of the vitamin A in various products is complicated if cis isomers are present because they have lower biopotencies than alltrans isomer. Two general types of vitamin A sources contain cis isomers; the trans-neo type contains all-trans and 13-cis isomers, and the trans-cis type (including fish liver oils) contains, in addition, the 9-cis and 9,13-di-cis isomers of very low biopotency. The trans-neo type cannot be satisfactorily assayed by the SbCl3 blue color procedure, but the USP assay gives a satisfactory estimate of biopotency. No colorimetric or spectrophotometric procedure alone can be used to estimate the biopotency of the transcis type. Two empirical equations involving biopotency, maleic value, and either blue color or USP analyses satisfactorily estimate the biopotency of the trans-cis type of vitamin A product.