Research Article Characterization of cyclofructan-based chiral stationary phases by linear free energy relationship Cyclofructans (CFs), a new class of chiral selectors, have been recently introduced for application in liquid chromatography and capillary electrophoresis. So far, derivatized CFs have performed interesting separation possibilities for a variety of compounds. The current work is focused on characterization of three different CF-based chiral stationary phases (CF-based CSPs), i.e. isopropyl carbamate cyclofructan 6 (IP-CF6), R-naphthylethyl carbamate cyclofructan 6 (RN-CF6) and dimethylphenyl carbamate cyclofructan 7 (DMP- CF7). The linear free energy relationship (LFER) model was used to reveal the dominant interactions participating in the complex retention mechanism. A set of 44 different test solutes, with known solvation parameters, was used to determine the regression coeffi- cients of the LFER equation under two mobile-phase compositions in normal separation mode. The LFER results showed that hydrogen bond acidity, hydrophobicity and dipo- larity/polarizibility mostly affect the retention and separation process on the CF-based columns in the studied separation systems. Keywords: Cyclofructan-based chiral stationary phases / HPLC / LFER / Normal- phase mode DOI 10.1002/jssc.201100462 1 Introduction Interest in the field of the enantiomeric separation and development of chiral separation media (chiral selectors, chiral stationary phase (CSPs)) has increased greatly in the past few decades due to the demand of pharmaceutical, agrochemical and food analysis. HPLC with CSPs has become the most powerful method for separating racemic samples at analytical and preparative scales and/or deter- mining enantiomeric purity. A variety of CSPs with complex interaction mechanisms have been reported by many research groups, which dominate the works of Davankov, Pirkle, Okamoto, Blaschke, Allenmark, Hermansson, Armstrong, Gasparrini, and Lindner [1–5]. Numerous applications have been reported that involve different chiral selectors chemically bonded to silica gel (or polymeric support) such as polysaccharides [6, 7], proteins [8], macrocyclic antibiotics [9, 10], crown ethers [11] and cyclodextrins [12]. Despite the applicability and broad selectivity of many existing CSPs the research effort for the development of new or improved chiral selectors continues. In 2009 a novel class of CSPs based on cyclofructans (CFs) was introduced by Armstrong [13]. This group of chiral selectors was shown to have potential both for HPLC [13–17] and CZE [18]. CFs are macrocyclic oligosaccharides as cyclodextrins. However, CFs are quite different in both their structure and behavior. They consist of six or more b-(2-1) linked D-fructofuranose units [19, 20]. Their abbreviations CF6, CF7, CF8, etc. indicate the number of fructofuranose units in the macro- cyclic ring. Each fructofuranose unit contains four stereo- genic centers and three hydroxyl groups, which can be utilized for derivatization. While native CFs have rather limited enantioselectivity in HPLC [13] their derivatized forms show improved and unique chiral recognition abilities for a wide range of analytes [13, 14, 16, 21]. Aliphatic or aromatic functionalization of a native chiral selector is a common strategy used to develop new CSPs and improve the enantioseparation performance. Figure 1 shows the molecular structure of cyclofructans CF6 and CF7 and the derivatization groups studied in this work. These CSPs can be operated in all common separation modes (normal, reversed-phase and polar organic) but mostly higher selectivity was obtained in the normal-phase mode. A comprehesive method for characterization and comparison of separation systems is the model of linear free energy relationship (LFER) [22–26], which can Lucie Janec ˇ kova ´ 1 Kve ˇ ta Kalı ´kova ´ 2 Jir ˇı ´ Vozka 2 Daniel W. Armstrong 3 Zuzana Bosa ´ kova ´ 1 Eva Tesar ˇova ´ 2 1 Department of Analytical Chemistry, Faculty of Science, Charles University in Prague, Prague, Czech Republic 2 Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University in Prague, Prague, Czech Republic 3 Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, TX, USA Received May 25, 2011 Revised July 11, 2011 Accepted July 11, 2011 Abbreviation: CSP, chiral stationary phase; DMP-CF7, dimethylphenyl carbamate cyclofructan 7; hex, n-hexane; IPA, propane-2-ol; IP-CF6, isopropyl carbamate cyclofructan 6; LFER, linear free energy relationship; RN-CF6, R-naphthylethyl carbamate cyclofructan 6 Correspondence: Dr. Eva Tesar ˇova ´ , Charles University in Prague, Faculty of Science, Department of Physical and Macromolecular Chemistry, Albertov 2030, 12843 Prague, Czech Republic E-mail: tesarove@natur.cuni.cz Fax: 1420-224-919-752 & 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com J. Sep. Sci. 2011, 34, 2639–2644 2639