Comparison of the separation of cis/trans isomers of tretinoin with different stationary phases by liquid chromatography nuclear magnetic resonance coupling

The comparison of two different types of RP stationary phases in their ability to separate cis/trans isomers of retinoic acid (tretinoin) has been investigated by LC-NMR coupling. Only by recording of H-1 NMR spectra the structural identification of the separated compounds was possible, since their absorption coefficients are very similar and their mass is identical, and therefore identification by UV-Vis is not unambiguous and identification with LC-MS fails due to identical fragmentation patterns. A commonly used C-18 phase and a recently developed C-30 phase have been used for the separation of a mixture of thermal isomerized retinoic acids. Three isomers could be separated and identified with the separation on a C-18 column, whereas five cis/trans isomers could be identified by the use of a C-30 column. It could be shown that even under optimized chromatographic conditions the separation efficiency of the two phases varies and that these differences are probably due to differences in the alkyl chain organization of the stationary phases.