Influence of organic modifiers on the rentention behaviour in reversed-phase liquid chromatography and its consequences for gradient elution

Abstract

In a previous paper it was shown that the key to the description and understanding of retention behaviour under gradient conditions in reversed-phase liquid chromatography lies in the relationship between the isocratic capacity factor ( K) and the volume fraciton of organic modifier (ϕ). Hence, the presentation of extensive data on this relationship in different organic modifier systems offers possibilities for evaluating and refining existing gradient elution models, such as the one of Snyder and co-workers. An extensive study of 32 aromatic compounds in aqueous system containing 10–100% of methanol, acetonitrile and tetrahydrofuran as organic modifiers under isocratic conditions is described. Two major conclusions are drawn that have consequences for gradient elution. Firstly, the relationship between in k and (ϕ) is generally non-linear, in accordance with previous statements. Secondly, the rate of change of in k and (ϕ) can be solute dependent. Systematic changes in the slope of the in k versus (ϕ) curves with absolute retention are observed for methanol and tetrahydro- furan as organic modifiers. It is shown that this conclusion leads to optimal gradients, which are convex rather than linear.