Anomalous behaviour of lycopene by HPLC on cyano-amino polar phase

By reversed phase HPLC the highly pure lycopene produces only one peak, that corresponds to the all-trans polyunsaturated system. Commercial standards, however, show generally two peaks that correspond to the molecule in all-trans configuration (about 90%) and probably to its 13-cis geometrical isomer (about 10%), respectively, independent of the solvent used to inject the sample. An anomalous HPLC behaviour was observed by analysing a commercial sample of lycopene by normal-phase HPLC. In this case the chromatogram showed different profiles depending on the solvent used to prepare the solution to be injected. A relationship between such unusual behaviour and the polarity of the solvent used to dissolve the sample was tentatively found. It is worth emphasizing that in the same HPLC conditions the behaviour of beta-carotene does not show the above described phenomenon.