Effects of peak compression in gradient elution of liquid chromatography

Peak compression is a unique feature of gradient elution and is non-existent in isocratic elution. Since the classical plate height equation, which is also called as van Deemter equation, is derived by assuming isocratic elution, it cannot be used to account for the effects of peak compression. As opposed to the case of isocratic elution, the retention factor (k) varies with the mobile phase composition (φ) under gradient elution, thereby complicating mathematical analysis. Herein, the research progress on peak compression in the past decade, especially the effect of the nonlinear feature of solvent strength models (i. e., expressions for ln k vs. φ), is reviewed. A general expression for the peak compression factor (G) is introduced, for which the variation in plate height (H) with φ is ignored. Based on this equation, the classical equation for G, which was first proposed by Poppe and assumes the linear solvent strength model (LSSM) and linear gradient elution, can be derived. The effects of pre-elution of the solute in the initial mobile phase on G, which are attributed to the dwelling time of the system, are included in the Poppe equation. When the solvent strength model is nonlinear, e. g., the quadratic solvent strength model (QSSM), the analytical expressions for G can also be obtained from the general expression. Under ideal chromatographic conditions, where H=0 and the adsorption isotherm is linear, the peak compression is determined by the ratio of the retention factor of the solute in the initial mobile phase to that at the eluted mobile phase composition.