Analytica Chimica Acta 537 (2005) 53–61 Modelling retention in liquid chromatography of polyphenolic acids Prediction of solvent composition and pH of the mobile phase N. Sanli a , G. Fonrodona b , J. Barbosa b , G.A. ¨ Ozkan a , J.L. Beltran b, ∗ a Department of Chemistry, S¨ uleyman Demirel University, 32010 Isparta, Turkey b Department of Analytical Chemistry, Barcelona University, Diagonal 647, 08028 Barcelona, Spain Received 30 October 2004; received in revised form 7 January 2005; accepted 7 January 2005 Available online 4 February 2005 Abstract The retention behavior of a series of polyphenolic acids has been modeled in water–acetonitrile mobile phases, in order to optimize the chromatographic resolution between these compounds. The retention model is based on the relationships between retention factors of solutes with the pH and solvatochromic parameter of the mobile phase (E N T ), and between the dissociation constants of the compounds and the mole fraction of organic modifier. The retention of these ionizable compounds has been studied at three levels of acetonitrile percentage (10, 20 and 30%), and at six different pH values ranging between 3.5 and 7. Three different fitting procedures have been applied: model I consists of a nonlinear regression to all data points, whereas models II and III use only four and two data points, respectively, taken from the extreme pH/E N T values. The results obtained from these models indicate a good agreement between real and predicted retention factors for each compound, with errors about 5% for model I and about 10% for models II and III. The results of model III are of particular interest, because of only two experiments for each compound are needed to describe the retention behavior in a relatively large pH and percentage of acetonitrile intervals; this is especially valuable in the optimization of the mobile phase for chromatographic separation of several compounds. © 2005 Elsevier B.V. All rights reserved. Keywords: Retention modelling; Mobile phase composition; Polyphenolic acids; Benzoic acids; Cinnamic acids 1. Introduction Polyphenolic acids are a complex group of substances which are present in both plants and fruits and in their derived products. These substances have gained enormous attention in the last years, because of their biological properties. They have been demonstrated to act as antioxidants and are as- sumed to contribute to the beneficial health effects such as anti-inflammatory, anti-histaminic and anti-tumor activities, and as free radical scavengers and protection against car- diovascular diseases [1–3]. The determination of polyphe- nolic acids is carried out mainly by liquid chromatography [4–7], in most cases coupled to diode array detectors (DAD). Although liquid chromatography (LC) is a routine method ∗ Corresponding author. Tel.: +34 93 403 9120; fax: +34 93 402 1233. E-mail addresses: joseluis@apolo.qui.ub.es, jlbeltran@ub.edu (J.L. Beltran). for solving many practical analytical problems, separations are still being developed in a non-systematic manner, often by trial-and-error, which involves several disadvantages. The most evident disadvantage is the long development time that is required to select experimental conditions that are not nec- essarily the optimum ones, and often they are not as good as might be expected from this powerful technique. The most important aspect of method development in LC is the achievement of sufficient selectivity, relative retention. Usually, the methods are focused on optimizing the concen- tration of organic modifier in the mobile phase. However, the pH of the mobile phase is also an important parameter for op- timizing selectivity when separating ionizable compounds. Variations in the pH of mobile phase may easily lead to impor- tant variations in the selectivity because the degree of ioniza- tion of solute, stationary phases and mobile phase additives (e.g., ion-pairing reagents) may be affected by pH. Therefore, the most useful way to maintain a reasonable retention factor 0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.01.006