Adsorption isotherms on high-performance liquid chromatographic sorbents. I. Peptides and nucleic acid constituents on octadecyl-silica.

A miniaturized chromatographic system with short, narrow-bone columns was used for the measurement of adsorption isotherms of peptides and of nucleic acid constituents on octadecyl-silica by frontal chromatography. The range of mobile phase composition was similar to that used in elution chromatography of the substances investigated. Adsorption isotherms were measured under different experimental conditions as far as the mobile phase pH and composition as well as the column temperature are concerned. Most isotherms were concave down, and the data were fitted to both the Langmuir and Jovanović models, the former yielding a somewhat better fit. Under certain conditions, sigmoidal isotherms were also observed and this behavior was attributed to solvent-mediated conformational changes and molecular stacking. The results shed light on the adsorption behavior of the substances under investigation on octadecyl-silica, the most commonly employed sorbent in reversed-phase chromatography, over a wide range of concentrations. This is of interest in preparative and process-scale chromatography.