Received: 6 December 2018 Revised: 22 December 2018 Accepted: 23 December 2018 DOI: 10.1002/sscp.201800144 RESEARCH ARTICLE Variability of temperature dependences of the retention of strongly polar compounds under ZIC-HILIC liquid chromatographic mechanism Maria Tanase 1 Elena Bacalum 2 Victor David 1 1 University of Bucharest, Faculty of Chemistry, Department of Analytical Chemistry, Bucharest, Romania 2 Research Institute - ICUB, Blvd. M. Kogalniceanu, University of Bucharest, Bucharest, Romania Correspondence Dr. Victor David, University of Bucharest, Faculty of Chemistry, Department of Analytical Chemistry, Sos. Panduri, no 90, Bucharest 050663, Romania. Email: victor.david@chimie.unibuc.ro The retentions of 13 strongly polar compounds (aminocids; dipeptides, drugs, and other biologically active species) were studied for two SeQuant ZIC-HILIC (Merck) stationary phases containing a zwitterionic sulfobetaine moiety covalently attached to porous silica (3.5 and 5 μm particle size). Methanol and acetonitrile were used as organic components in the mobile phase composition, whose aqueous phase con- sisted in buffers of various controled pH values based on acetate-acetic acid. This study shows various dependences of the retention on the absolute column temper- ature, some being obeyed to van't Hoff dependence, while others not following this rule. The values of standard enthalpy for the transfer of the analyte from mobile phase to the stationary phase were calculated from linear van't Hoff plots, which were below 10 kJ/mol. From the polynomial regression applied to the experimental retention data that not obeyed to a linear fit, the value of absolute temperature was calculated that corresponds to a maximum retention for studied compounds. KEYWORDS non-linear plots, temperature influence, van't Hoff plots, zIC-HILIC mechanism, zwitterions 1 INTRODUCTION Hydrophilic interaction liquid chromatography (HILIC), which is typically based on a hydrophilic stationary phase and used for separating very polar compounds, represents an alternative HPLC to the reversed-phase LC [1,2]. Retention of polar analytes on HILIC stationary phase depends on the intesity of polar interactions between polar groups of analytes and polar moieties from stationary phase [3,4]. At their turn, these interactions depend on the composition of the mobile phase and temperature [5–9]. Zwitterionc liquid chromatography (ZIC-HILIC) is a particular HILIC mechanism in liquid chromatography that uses a stationary phase containing both anionic and cationic groups in their structure, such as embedded quaternary amine and sulfonic or carboxyl terminal groups [10,11]. A major role in this mechanism is also played by the water layer adsorbed onto the surface of the stationary phase, for which a partition model has been proposed to explain the retention of polar analytes [3]. Temperature may influence the retention of the analytes separated by HILIC and the chromatographic performances, mainly the peak shape and separation selectivity [9,12,13]. The dependence of retention on the column temperature is generally represented by the linear van't Hoff function between the natural logarithm of the retention factor (ln k) and the inverse value of absolute temperature of the studied column (1/T) [14–17]: ln  =- Δ 0  ⋅  + Δ 0  + ln φ (1) In eq. (1), ΔH 0 is the standard enthalpy for the transfer of the analyte from mobile phase to the stationary phase, ΔS 0 is standard entropy for the same partition process, R is the universal gas constant (8.314 J K -1 mol -1 ), T is the absolute column temperature (namely t + 273.15, t being column tem- perature in Celsius degree) and ϕ represents the phase volume ratio (the volume of stationary phase to that of the mobile phase). According to this equation the dependence between ln k and 1/T should be linear, but in several circumstances a 12 © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Sep Sci plus 2019;2:12–17. www.sscp-journal.com