Journal of Chromatography A, 1133 (2006) 69–75 Effect of chaotropic mobile phase additives on retention behaviour of beta-blockers on various reversed-phase high-performance liquid chromatography columns Hisham Hashem, Thomas Jira ∗ Institute of Pharmacy, Pharmaceutical/Medicinal Chemistry, Ernst-Moritz-Arndt-University Greifswald, Friedrich-Ludwig-Jahn-Str. 17, D-17487 Greifswald, Germany Received 15 March 2006; received in revised form 26 July 2006; accepted 31 July 2006 Available online 22 August 2006 Abstract In this study the effect of type and concentration of chaotropic counter anion in buffered mobile phase on retention behaviour of some beta- blockers was studied. Two types of anions (perchlorate, dihydrogen phosphate) were examined under different concentrations. Further different pH-values for each anion were used to investigate the effect of pH changes on chaotropic behaviour of these anions. The role of organic modifier (methanol, acetonitrile) type was also considered. All these parameters were studied for three different RP-columns (calixarene modified silica gel, monolithic RP-column and a conventional RP-column). The results indicate that all studied factors affected the chaotropic behaviour of the tested anions. Also the type of the used stationary phase has found to play an important role in retention behaviour of beta-blockers. © 2006 Elsevier B.V. All rights reserved. Keywords: Column liquid chromatography; Beta-blocker; Chaotropic counter-anions; Calixarene-bonded phase; Monolithic columns 1. Introduction Counter anions that have been shown to increase the disor- der of water are called chaotropic counter anions [1]. Although it was reported that the increase of the ionic strength of the mobile phase leads to decrease the retention time of the basic compounds due to the competitive interaction of the counter- cation with the residual silanol [2]. Horvath et al. [3] found that increase of anion salt concentration led to increase the surface tension of the mobile phase and so increasing of the reten- tion factors. LoBrutto et al. [4] found that under increasing of counter-anion concentration the retention factor increased for protonated basic compounds due to their electrostatic interac- tion with counter-anion (perchlorate and trifluoroacetate). This in turn led to increased hydrophobicity of the ion-associated neu- tral complex. Solvation shell of the basic analyte is disrupted due to formation of the ionic complex therefore causing an increase of apparent analyte hydrophobicity. Kazakevich et al. [5] found that a multilayer-type adsorption of the organic modifier (ace- ∗ Corresponding author. Tel.: +49 3834 864850; fax: +49 3834 864843. E-mail address: jira@uni-greifswald.de (T. Jira). tonitrile) on the reversed-phase surface and its strong dispersive (or -) interactions with liophilic ions are responsible for sig- nificant retention of chaotropic counterions. Pan et al. [6] found that increase of chaotropic counter-anion concentration led to increase of symmetry of basic compounds and also increase the efficiency of the C8-bonded silica column. Dai and Carr [7] pointed out that the retention of basic compounds on conven- tional reversed phase can increase, decrease or effectively remain constant as the concentration of the additives is increased. They found that there are two opposing effects, one of them leads to elongate the retention times (ion pairing of the analytes with the chaotropic counter-anion) and the other results in shorten- ing the retention times (competition of the buffer counter-cation with the analyte for the ionized silanols). In previous studies for other authors, the effect of chaotropic mobile phase addi- tives on retention behaviour of basic drugs (beta-blockers) on conventional RP-columns was studied [4,8]. We have examined the retention behaviour of beta-blockers on monolithic column in a previous study and we have found that beside the type and concentration of the buffer counter-anion the properties of the basic analyte especially its hydrophobic- ity have an effective role determining to which extent the type and concentration of chaotropic anions will affect the retention 0021-9673/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2006.07.074