Journal of Chromatographic Science, Vol. 41, July 2003

Influence of Mobile Phase Composition on Retention
Factors in Different HPLC Systems with Chemically
Bonded Stationary Phases
Wojciech Zapala*
Department of Chemical Engineering and Process Control, Chemical Faculty, Rzeszów University of Technology, Al. Powstan´ców
Warszawy 6, 35-959 Rzeszów, Poland

Abstract
The influence of mobile phase composition on the retention of selected test analytes in different normal- and reversed-phase chromatographic systems is studied. A novel adsorption model for an accurate prediction of the analyte retention in the column chromatography with binary mobile phase is proposed.
Performance of the model is compared with the retention model reported in the literature. Both models are verified for different
HPLC systems by use of three criteria: (a) the sum of squared differences between the experimental and theoretical data, (b) approximation of the standard deviation, and (c) the Fisher test.

Introduction
The study of the influence of a mobile phase modifier concentration on the retention in high-performance liquid chromatography (HPLC) is important for understanding the regularities of the retention and mechanisms of substances separation in a chromatographic process. Binary eluents consisting of a weak and a strong solvent are widely used in HPLC.
In the reversed-phase (RP) liquid chromatography (LC) mode, for example, the major constituent of the mixed eluent is a highly polar solvent (e.g., water), whereas a less polar solvent
(e.g., methanol, acetonitrile, etc.) is an organic modifier added to control the process of elution. The composition of the mobile phase determines the retention volume and time of solutes, both in the normal-phase (NP) and RP chromatographic modes. Composition changes and the nature of mobile phases enable tuning of the separated analytes’ retention in a wide range of the retention parameters and optimization of the chromatographic processes, as well. One factor, which undoubtedly contributes to the quality of separation, is selectivity. Among the factors that strongly affect the selectivity of separation in LC is the composition of the mobile phase. Optimization of separation selectivity can be achieved by several different methods, one of them is the so-called interpretative
* Email: ichwz@prz.rzeszow.pl.

strategy (1,2). The key role in this strategy is the implementation of adequate models of retention that couple the retention of a solute with the composition of a mixed eluent (3,5,6,9,13).
The comparison of literature-known retention models for different thin-layer chromatography (TLC) and HPLC systems was presented in detail in a previous work (6). The results of the investigations showed that for a strong parabolic k = f(ϕ) dependence (for crown compounds), a majority of retention models [except models proposed by Schoenmakers (1) and
Kaczmarski et al. (2,5)] gave significant errors of computation.
At the same time, the models correctly describing strong nonlinear k = f(ϕ) dependences gave meaningful errors for low concentration of organic modifier. Therefore, the search for the most universal and accurate retention models for describing both linear and strong parabolic k = f(ϕ) dependences is reasonable, from the practical and theoretical point of view.
In this paper, a novel adsorption model for an accurate prediction of the analyte retention in the column chromatography with binary mobile phase is proposed. This model has been formulated theoretically, according to methodology reported in literature (5). The presented model was experimentally examined by the author. In addition, the retention data taken from literature [and contained in papers by Kaczmarski et al. (5), Kahie et al. (7), Lanin et al. (8), and Nikitas et al. (9)] have been also used for the examination.

Theory
In the case of chemically bonded stationary phases, some of the surface silanols remain nonbonded. For such stationary
phases,