Sucrose effect on reversed-phase liquid chromatography solute retention

The reversed-phase liquid chromatography retention of a series of benzoate ester molecules was investigated over a wide range of sucrose concentrations (c) 0.01-0.8 M using a C18 as the stationary phase;md a methanol-water mixture with high organic fraction (60/40, v/v) as the mobile phase. A theoretical treatment was developed to investigate the effect of sucrose molecules on the equilibrium between the solutes and the C18 phase and the metlhanol-water medium, respectively. This was found to be adequate to accurately describe the benzoate ester retention behavior when c varied. It was expected that the addition of sucrose was responsible for two main opposite contributions to solute retention, (i) a net interaction between the solute and the modifier in the mobile phase determining a decrease in k' values with c up to 0.2 M; and (ii) an increase in the solute affinity for the C18 stationary phase due to the salting-out effect governing a retention increase above 0.2 M. Thermodynamic parameter variations were calculated using van't Hoff plots and discussed in relation to this retention model to confirm the respective effects of the modifier on the solute affinity towards the two phases. (C) 2001 Elsevier Science B.V. All rights reserved.