Chiral ion-pair chromatography on porous graphitized carbon using N-blocked dipeptides as counter ions

Two newly synthesized chiral di-anionic counter ions were tested for enantiomeric resolution of a set of amino alcohols on porous graphitized carbon, Hypercarb. Z-l-Aspartyl–l-proline dissolved in methanol baseline resolved nine of 12 tested racemates. One of its diastereoisomers, Z-l-aspartyl–d-proline was also tested but resulted in low separation factors, <1.1. Sodium hydroxide was added to the mobile phase in order to titrate the counter ion to its mono- or di-anionic form. Results show that the di-anionic form was found to be superior to the mono-anionic form regarding enantioselectivity. Increased content of the counter ion in the mobile phase, with constant ratio between counter ion and sodium hydroxide concentration, decreased retention but only slightly affected enantioselectivity. Increased retention and enantioselectivity were observed with decreased column temperature. Resolution factors >3 were obtained between the enantiomers in atenolol and metoprolol with a total retention time of less than 15 min. Further, all four stereoisomers of an analogue to metoprolol were separated using Hypercarb and a mobile phase of 5 mM Z-l-aspartyl–l-proline and 9 mM sodium hydroxide in methanol.