SEPARATION AND RELATIVE DISTRIBUTION OF ALL-TRANS-BETA-CAROTENE AND ITS CIS ISOMERS IN BETA-CAROTENE PREPARATIONS

The purity and relative distribution of beta-carotene isomers in several commercially available preparations of beta-carotene was evaluated using spectrophotometric methods and reversed-phase liquid chromatography (LC). The purity of five lots of beta-carotene evaluated by comparison of spectrophotometric to gravimetric determinations was found to range from 7.1 to 82.9%. Reversed-phase LC was then used to measure the impurities due to other carotenoids and geometric isomers of beta-carotene which also absorb in the 450 nm region. Five different reversed-phase C-18 columns were examined for their ability to resolve the geometric isomers of beta-carotene which contribute to the apparent concentration of trans-beta-carotene standard solutions. The columns packed with wide-pore, C-18 polymerically-modified silica exhibited adequate selectivity to separate the geometric isomers from all-trans-beta-carotene. Impurities separated by LC accounted for 16-75% of the absorbance of commercial beta-carotene preparations at 450 nm. Based on these observations, all-trans-beta-carotene measurements could potentially be only 1/50 of reported values.