RETENTION MECHANISMS OF POLYCYCLIC AROMATIC NITROGEN HETEROCYCLICS ON BONDED AMINO PHASES IN NORMAL-PHASE LIQUID-CHROMATOGRAPHY

Two aminoalkyl bonded phases, aminopropylsilica and dimenthylaminopropylsilica, operated in the normal-phase mode were investigated regarding the retention mechanisms of polycyclic aromatic nitrogen heterocyclics (PANHs). By blocking active interaction sites with methyl substituents, the retention mechanisms could be ascertained. The bonded functional groups of the aminoalkyl stationary phases were found to be the primary adsorption sites. Hydrogen bonding was found to be the dominant retention mechanism for carbazole-type PANHs on both stationary phases and for acridine-type PANHs on the aminopropyl phase. For acridine-type PANHs on dimethylaminopropylsilica, retention was found to be mainly due to electron donor-acceptor interaction. Residual silanols on the stationary phase support material on these non-end-capped bonded phases were found to contribute only slightly to the retention of both acridine- and carbazole-type PANHs. A strong dependence of retention on steric hindrance of the n-electrons on the nitrogen of acridine-type solutes was shown. Further, the necessity to consider not only the polarity and solvent strength but also the selectivity of the mobile phase is demonstrated.