Comparison of the performance of a few packing materials designed to minimize the thermodynamic band tailing of basic compounds in reversed-phase liquid chromatography.

The adsorption isotherms of phenol, caffeine, propranolol chloride, and amitriptyline chloride were measured on three new brands of C(18)-bonded silica that have been designed to be more resistant than conventional C18-bonded silica at high pHs (>8). These columns were the 4 microm Bidendate Cogent-C18 (Microsolv Technology Corporation, Long Branch, NJ, USA), the 3.5 microm Zorbax Extend-C18 (Agilent Technologies, Palo Alto, CA, USA), and the 5 microm XTerra-C18 (Waters, Milford, MA, USA). The originality of these adsorbents is due to their surface chemistry, which protects them from rapid hydrolysis or dissolution at extreme pH conditions. Their adsorption properties were compared to those of the 3 microm Luna-C18 (Phenomenex, Torrance, CA), which is a more conventional monofunctional material. The adsorption data were acquired by frontal analysis (FA) and the adsorption energy distributions (AEDs) of all systems studied were calculated by the expectation-maximization (EM) method. The experimental results show that neither a simple surface protection (Extend-C18) nor the elimination of most of the silanol groups (Cogent-C18) is sufficient to avoid a peak tailing of the basic compounds at pH 8 that is of thermodynamic origin. The incorporation of organic moieties in the silica matrix, which was achieved in XTerra-C18, the first generation of hybrid methyl/silica material, reduces the silanols activity and is more successful in reducing this peak tailing.