Retention in reversed-phase liquid chromatography as a function of mobile-phase composition

The dependence of solute retention (k′) on mobile phase composition (%B for binary-solvent mixtures A–B) is reviewed and compared with various empirical and theoretical equations that have been proposed for this relationship. Because the functional dependence of k′ on organic modifier composition varies from one system to another, it is not possible from these data to draw any overall conclusions as to the nature of the retention process in reversed-phase chromatography. Likewise, there is probably no one best equation for extrapolating all retention data to 0 %B for purposes of predicting log Po/w values from chromatographic data. The relative change in k′ with change in %B can be described in terms of the parameter S = −d(log k′)/dϕ (ϕ = 0.01 %B). Values of S as a function of solute structure, mobile-phase composition, column type and experimental conditions are of interest for several reasons: insight into the retention process or “mechanism”, mobile phase optimization, etc. Previous work relating to this question is reviewed here and some conclusions are presented.