Classification of special octadecyl-bonded phases by the carotenoid test

Amongst the numerous base-deactivated ODS phases obtained by increasing the bonding density or/and by efficient endcapping treatments, some particular stationary phases have been developed, to limit the additional interactions of basic compounds with residual silanols, to work at extreme pH or with rich water mobile phases. Horizontal polymeric phases, sterically protected ones, hybrid silicas, propylene bridge, are particularly used for this purpose. Octadecyl chains with embedded polar groups and hydrophilic endcapping are also used in this goal. The properties of these phases were studied with a simple test consisting in the injection of carotenoid pigments in Subcritical Fluid Chromatography. The molecules used and the nature of the mobile phase allow the determination of hydrophobicity, polar site accessibility and type or/and bonding density of the stationary phases. Whatever the type of the phases, the particular stationary phases do not show any remarkable property, in comparison to other base-deactivated C18-bonded phases. On the other hand, embedded and polar-endcapped phases display a specific behaviour in regard of hydrophilic interactions, which are highlighted by the absence of water in the subcritical fluid. Additional properties of these phases are described, such as steric recognition and retention performances. As expected, polar-embedded phases are less retentive than classical ODS ones, but are sometimes able to provide greater steric recognition. On the other hand, the polar-endcapped phases display greater hydrophobicity than polar-embedded ones. From a simple classification diagram based on chromatographic properties, differences can be noticed between the polar-embedded groups (amide, carbamate, ether, sulfonamide) and between embedded and endcapped phases. Surprising behaviours are also noticed for some on the tested phases.