Spanik, Oswald, Krupcik, Benicka, Sandra, Armstrong 45 Ivan Spanik a) , Peter Oswald b) , Jan Krupcik a) , Eva Benicka a) , Pat Sandra c) , Daniel W. Armstrong d) a) Slovak University of Technology, Faculty of Chemical Technology, Department of Analytical Chemistry, Radlinskeho 9, SK- 812 37 Bratislava, Slovak Republic b) Environmental Institute, Okruzna 784/42, 97241 Kos, Slovak Republic c) Department of Organic Chemistry, University of Ghent, Krijgslaan 281 S4, B-9000 Ghent, Belgium d) Department of Chemistry, Gilman Hall, Iowa State University, Ames, IA 50011 – 3111, USA Evaluation of non-polar interactions in chiral recognition by alkylated b- and c-cyclodextrin chiral stationary phases The gas chromatographic separation of enantiomers of seven N-TFA-O-alkyl amino acid derivatives was studied on four different permethyl- and 2,6-di-O-methyl-3-O- pentyl-b- and -c-CD stationary phases.It was shown that the separation of enantio- mers N-TFA-O-alkyl amino acid derivatives depends both on the length of the linear alkyl chain attached to the stereogenic carbon (R 1 ) and to the ester part of the amino acid derivative (R 2 ). The cyclodextrin cavity size also affected selectivity. The separa- tion of the amino acid derivatives decreases with increasing length of both the R 1 and R 2 alkyl chains on b-CD stationary phases, but improves on c-CD stationary phases. The separation of enantiomers of all N-TFA-O-methyl amino acid esters, is better on the larger c-cyclodextrin CSPs except for enantiomers of N-TFA-O-alkyl esters of ala- nine which are better separated on b-CD stationary phases. Key Words: Capillary GC; Enantiomer separation; TFA-O-alkyl amino acid deriva- tives; Received: August 23, 2001; revised: September 13, 2001; accepted: September 27, 2001 1 Introduction Different chiral stationary phases, based on amino acid derivatives, cyclodextrins, calixarenes, crown ethers, and saccharides [1 – 6] were introduced in the last decade for the direct gas chromatographic separation of enantio- mers. Cyclodextrin derivatives are among the most fre- quently used stationary phases for the direct GC separa- tion of enantiomers of different types of chiral compounds. Native cyclodextrins are cyclic glucose oligomers in which all glucose units have the chair conformation and the D- configuration. The glucose contains three hydroxyl groups bonded to the 2, 3, and 6 carbon atoms which can easily be substituted with similar or different groups. This allows one to tailor the cyclodextrin chiral selector for the separa- tion of enantiomers of a variety of compounds [7]. The type of cyclodextrin, the nature of its substituents, the con- centration of the cyclodextrin derivative, and the polarity of the associated achiral solvent all belong to the basic parameters which determine the enantioselective proper- ties of GC columns coated with mixed cyclodextrin sta- tionary phases [8 – 11]. Ultimately, chiral recognition is dependent on the nature of the cyclodextrin and its substituents. Thus, one must consider the cyclodextrin cavity size, given by the number of glucose units in the cyclodextrin molecule, and the type of substituents and their location in or on the cyclodextrin molecules. The sub- stituents at the 6-position of the glucopyranose units are located at the narrow end of the cyclodextrin cavity. Some workers believe that these substituents are not active in the chiral recognition process, because they do not inter- act directly with the enantiomers. Others consider that chiral recognition in GC can be a more varied process, particularly for different cyclodextrin derivatives and differ- ent analytes [12]. Also, the character of these substituents can influence the conformation of the cyclodextrin cavity [13]. The substituents at the 2-positions of the glucopyra- nose units are located at the wider mouth of the cyclodex- trin cavity. These substituents are oriented away from the cyclodextrin cavity and play an important role in chiral recognition, particularly if the enantiomeric analyte is adsorbed or interacts with the surface of the cyclodextrins. Substituents in the 3-positions of the glucopyranose units also are located at the wider mouth of the cyclodextrin cavity. These substituents are oriented more toward the cyclodextrin cavity. They may be more important if inclu- J. Sep. Sci. 2002, 25, 45–52 Correspondence: Prof. Jan Krupcik, Slovak University of Technology, Faculty of Chemical Technology, Department of Analytical Chemistry, Radlinskeho 9, SK-812 37 Bratislava, Slovak Republic. E-mail: Krupcik@cvt.stuba.sk Fax: + 421 2 52926 043 i WILEY-VCH Verlag GmbH, 69469 Weinheim 2002 1615-9306/2002/0101–0045$17.50+.50/0 Microcolumn Separations Peer-review of papers in the section “Microcolumn Separations” was supervised by Milton L. Lee and Pat Sandra. Their editorial support is gratefully acknowl- edged.