Solute Retention in Column Liquid Chromatography. XI. Exposition of the Minor-Disturbance Method of Measurement of the Void Volume

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Abstract
The retention volumes corresponding to the disturbance-peaks produced by injection of the components of aqueous blends of organic liquid carriers (reverse-phase stationary packings) have been claimed by some groups to agree to within an experimental error of ca. 1% while, in other instances, discrepancies of slightly greater than this figure have been reported. These results are accounted for in terms of the concentration-based solute partition coefficients KR. Thus, taking water-methanol mobile phases as an example, the injection of a small amount of either compound is said to alter its partition coefficient due, if nothing else, to the resultant change of the composition of the bulk mobile phase. Further, the respective partition coefficients of the mobile-phase components (hence their retentions) will in all likelihood not be identical. Moreover, for liquid-liquid and reverse-phase LC systems at least, while the magnitude of the change of each of the component KR is difficult to forecast, the direction is said to be predicated on the finite-concentration activity coefficients of the solutes in the blended solvent. For example, because water gives substantially larger negative deviations from Raoult's law with methanol-rich solvents than does methanol, the former is expected to be retained relative to the latter when such blends are employed as LC mobile phases; the converse should then obtain for water-rich carriers. Both situations have been verified experimentally.