Please cite this article in press as: A. Vemi ´ c, et al., The inﬂuence of salt chaotropicity, column hydrophobicity and analytes’ molecular properties on the retention of pramipexole and its impurities, J. Chromatogr. A (2015), http://dx.doi.org/10.1016/j.chroma.2015.01.078 ARTICLE IN PRESS G Model CHROMA-356239; No. of Pages 8 Journal of Chromatography A, xxx (2015) xxx–xxx Contents lists available at ScienceDirect Journal of Chromatography A j o ur na l ho me page: www.elsevier.com/locate/chroma The inﬂuence of salt chaotropicity, column hydrophobicity and analytes’ molecular properties on the retention of pramipexole and its impurities Ana Vemi ´ c a , Marko Kalini ´ c b , Slavica Eri ´ c b , And ¯elija Malenovi ´ c a,∗ , Mirjana Medenica c a University of Belgrade–Faculty of Pharmacy, Department of Drug Analysis, Vojvode Stepe 450, Belgrade, Serbia b University of Belgrade–Faculty of Pharmacy, Department of Pharmaceutical Chemistry, Vojvode Stepe 450, Belgrade, Serbia c University of Belgrade–Faculty of Pharmacy, Department of Physical Chemistry and Instrumental Methods, Vojvode Stepe 450, Belgrade, Serbia a r t i c l e i n f o Article history: Received 11 December 2014 Received in revised form 26 January 2015 Accepted 27 January 2015 Available online xxx Keywords: Chaotropic salts Column hydrophobicity Thermodynamic approach Molecular descriptors Pramipexole a b s t r a c t The aim of this study was to examine the interaction of the chaotropic salts of different position in Hofmeister series (CF 3 COONa, NaClO 4 , NaPF 6 ) added to the mobile phase with the stationary phases of different hydrophobicity (C8 and C18 XTerra ® columns), as well as their common inﬂuence on the retention behavior of pramipexole and its structurally related impurities. The extended thermodynamic approach enabled the understanding of the underlying separation mechanism. Comparing six different column-salt systems it was observed that general system hydrophobicity presented by salt chaotropicity and column hydrophobicity favors stationary phase ion-pairing over the ion-pair formation in the eluent. Further, an attempt was made to describe the inﬂuence of analytes’ nature on their retention behavior in such chromatographic systems. An analysis is performed in order to select and elucidate the molecular descriptors (electrostatical, quantum-chemical, geometrical, topological, and constitutional) that best explain the experimental evidence and ﬁndings obtained by the thermodynamic approach. The results of this analysis suggest that analytes’ charge distribution and its complementarity to the structure of the electric double layer formed on the surface of the stationary phase upon the addition of chaotropic additives can be useful for understanding the differences in retention of structurally related analytes. These ﬁndings provide a novel understanding of the interactions between all the components of the chromatographic system containing chaotropic additive and a good basis for further investigations sug- gesting the development of generally applicable predictors in structure-retention relationship studies in related chromatographic systems. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Chaotropic chromatography is a widely recognized approach for the separation of positively charged or protonated analytes. Many efforts were made to elucidate its beneﬁts in separation selectivity and efﬁciency of small molecules [1,2], -blockers [3,4], differ- ent amines [5,6], antibiotics [7], alkaloids [8,9], phenothiazine and thioxanthene derivatives [10], and antiparkinsonian drugs [11–13]. Different models were put forward with the aim to rationalize analytes’ retention behavior in such systems [1,14–17]. The inﬂu- ence of chaotropic salts on the retention of neutral, zwitterionic and negatively charged analytes was also investigated [15,18]. Besides, chaotropic effect was used for the chiral recognition of ∗ Corresponding author. Tel.: +381 11 3951 333. E-mail address: andja@pharmacy.bg.ac.rs (A. Malenovi ´ c). enantiomers [19,20] and found its place in sample preparation for chromatographic analysis [21]. However, few attempts were made to describe the inﬂuence of analytes’ nature on their retention in the presence of chaotropic salts [4,10,22]. This study aimed to investigate the interaction of the mobile phase chaotropic additives of different rank in Hofmeister series with the stationary phases of different hydrophobicity, as well as their joint impact on the retention behavior of pramipexole and its structurally related impurities, a model mixture that fairly represent a usual chromatographic challenge in pharmaceuti- cal analysis. The comprehensive understanding of mechanisms underlying the mixture separation was achieved through the extended thermodynamic approach suggested by Cecchi [15,16]. Finally, in order to gain insights in the structural features that inﬂuence analytes’ retention, an analysis was performed to identify and interpret molecular descriptors which can best account for the experimentally obtained retention factors. Results of this http://dx.doi.org/10.1016/j.chroma.2015.01.078 0021-9673/© 2015 Elsevier B.V. All rights reserved.