Two- and three-parameter equations for representation of retention data in reversed-phase liquid chromatography

Two-parameter equations that describe the dependence of ln kappa upon psi, where kappa is the retention factor and psi the volume fraction of the organic modifier in the mobile phase, are examined in what concerns the underlying approximations and their performance to fit experimental data obtained from reversed-phase liquid chromatography. Using 293 experimental systems, it was found that the performance of these equations to describe ln kappa versus psi data is rather low, since the percentage of the systems that can be described satisfactorily ranges from 40 to 60% depending on the fitting equation. This percentage may be raised to 75%, if the discreteness effect is properly taken into account. A further improvement to 90% of the systems studied can be achieved only by the use of three-parameter equations, which may arise by refinements of the rough approximations of the two-parameter equations. Although the refinements do not lead always to better equations, we developed a new three-parameter expression of In kappa that works more satisfactorily, since it combines simplicity, linearity of its adjustable parameters and the highest applicability.