Simple relationship concerning mobile and stationary phases in normal- and reversed-phase chromatography

Abstract

For normal-phase chromatography on aminoproply-bonded silicas, the adsorption of the polar modifier of various mixtures of hexane with n-alkanols, benzyl alcohol, acetone, dimethylformamide, nitromethane, chloroform and diisopropyl ether were measured. One to three molecules of the polar solvent per bonded aminopropyl chain can be fixed. Simple calculations allow the determination of the relative proportions of amino groups free or bonded with one, two or three modifier molecules for each mobile phase composition. The amount of the polar modifier in the stationary phase increases with its ability to form hydrogen bonds with the amino groups for a given composition of the mobile phase. The amount is larger with any alcohol than with nitromethane, acetone or dimethylformamide. The capacity factor ( k′) of various polar solutes (phenols, amines and pyridines) and non-polar solutes (polyaromatic hydrocarbons) decreases when the concentration of polar solvent in the mobile phase increases. When the average number of molecules of the polar modifier in the stationary phase remains less than one per amino group, a linear relationship exists between 1/ k′ and the volume fraction of the polar solvent in the mobile phase. When more than one molecule per amino chain is fixed, the variations of 1/ k′ are not easily linked to the eluent composition. For reversed-phase chromatography on alkyl-bonded silicas, the ln k′ and the selectivity for non-polar and polar solutes vary linearly with NS ( N being the total number of chains bonded on the silica surface and S the hydrocarbonaceous surface area of a chain). For instance, the retention time of a solute will be the same using C 8, C 12 or C 22 bonded phases with the same value of NS and consequently the same carbon content. Retention of polar and non-polar solutes were measured with five different water-miscible organic modifiers (methanol, ethanol, 1-propanol, acetonitrile and tetrahydrofuran). The variation of k′ can be related to the volume fraction ( x) of water by ln k′ = ax n + b (O < x < 0.85), n depending on the organic solvent nature and a on the solute properties. For each organic modifier, a linear relationship exists between a and the molar volume of the solute. The solubility of solutes was measured in the same mixtures used as mobile phases. The solubility of a given solute in any water-modifier mixture is related to x by ln s = − ax n + b′, a and n having the same values as those obtained for the retention variation. This equation is verified for x values corresponding to 1 < k′ < 200. Hence a very simple relationship between k′ and s ( k′ s = C) is valid for all mobile phase compositions.