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The isocratic retention of enantiomers of chiral analytes (1-methyl-2-piperidinoethylesters of 2-, 3-, and 4-alkoxyphenylcarbamic acid, potential local anaesthetic drugs) was studied on a methylated teicoplanin CSP, at different temperatures (in the range of 0–50°C with 10°C increments) in the polar organic mode (the mobile phase composition consisted of methanol with 17.5 mmol/L acetic acid and 4.8 mmol/L diethylamine). The natural logarithms of the retention factors (ln ki) of the investigated chiral compounds depended linearly on the inverse of temperature (1/T). van't Hoff plots afforded thermodynamic parameters, such as the apparent change in enthalpy (ΔHi), the apparent change in entropy (ΔSi), and the apparent change in Gibbs free energy (ΔGi) for the transfer of analyte between the mobile phase and the stationary phase. The thermodynamic parameters were calculated in order to promote an understanding of the thermodynamic driving forces for retention in this chromatographic system. Enthalpy–entropy compensation plots showed that all of the compounds in this study separate via the same enthalpy driven chiral recognition mechanism. [ABSTRACT FROM AUTHOR]

The Whelk-O 1 chiral stationary phase (CSP) is an useful column for the high performance liquid chromatographic (HPLC) resolution of enantiomers of 2-methoxy-1-[(4-methylpiperazino)methyl]ethyl esters of N-(2-,3-, and 4-alkoxyphenyl)carbamic acid (group of the local anaesthetic drugs) in the polar organic mode and the reversed mode. According to the results of enantiomeric separations in different mobile phases [containing organic modifiers (methanol, acetonitrile, water), with different additives of acetic acid and trimethylamine], it can be postulated that the piperazino part of the phenylcarbamic acid compound is essential because the separation of piperidino, pyrrolidino, and perhydroazepino esters of alkoxy phenylcarbamic acid were not observed. Also the influence of the environment near the analyte's stereogenic is important for enantioseparation. A comparison of the enantiomeric elution order and the configuration of the chiral selector shows that R-(−) enantiomers are more retained on the (R,R) Whelk-O 1 column and S-(+) enantiomers are more retained on the (S,S) Whelk-O 1 column. [ABSTRACT FROM AUTHOR]

Abstract: The enantiomers of dialkyl 2,3-pentadienedioate undergo interconversion during gas chromatographic separation on chiral stationary phases. In this paper the on-column apparent interconversion kinetic and thermodynamic activation data were determined for dimethyl, diethyl, propylbutyl and dibutyl 2,3-pentadienedioate enantiomers by gas chromatographic separation of the racemic mixtures on a capillary column containing a polydimethylsiloxane stationary phase coupled to 2,3-di-O-methyl-6-O-tertbutyldimethylsilyl-β-cyclodextrin. A deconvolution method was used to determine the individual enantiomer peak areas and retention times that are needed to calculate the interconversion rate constants and the energy barriers. The apparent rate constants and interconversion energy barriers decrease slightly with an increase in the alkyl chain length of the dialkyl 2,3-pentadienedioate esters. The optimum conformation of the dialkyl 2,3-pentadienedioate molecules, their separation selectivity factors and apparent interconversion enthalpy and entropy data changes with the alkyl chain length. The dependence of the apparent interconversion energy barrier on temperature was used to determine the apparent activation enthalpy and apparent entropy (where a denotes the first and b second eluted enantiomer). The comparison of the activation enthalpy and entropy indicated that the interconversion of dialkyl 2,3-pentadienedioate enantiomers on the HP-5+Chiraldex B-DM column series is an entropy driven process at 160°C. Data obtained for dimethyl 2,3-pentadienedioate enantiomers on the HP-5+Chiraldex B-DM column series at 120°C corresponds (at the 95% confidence interval) with the value of ΔG # =128±1kJmol−1 found at this temperature by gas chromatography using a two-dimensional stop flow technique on an empty capillary column [V. Schurig, F. Keller, S. Reich, M. Fluck, Tetrahedron: Asymmetry 8 (1997) 3475]. [Copyright &y& Elsevier]

Abstract: This paper describes how different multivariate analysis and classification methods can be used, to characterize the gas chromatographic separation of complex hydrocarbon mixtures in three columns coupled in series. Principal component analysis (PCA), correspondence factor analysis (CFA), and hierarchical ascending classification (HAC) were used as potential tools for evaluating the experiments on single columns and on column series. It has been demonstrated that: (1) multivariate analysis with PCA and CFA offers a powerful strategy to search for the main factors influencing the separation of hydrocarbons without a priori knowledge of the key factors of the separation. (2) With CFA the contribution of retention due to vapour pressure can be minimized. The use of retention indices, which use the n-alkanes as reference compounds, also helps to decrease the dominant focus on vapour pressure in favor of the more selectivity-based interaction forces. (3) CFA helps to analyze the degree of relevance of the chosen experimental design to the most important factors, controlling chromatographic selectivity. [Copyright &y& Elsevier]

Separation methods have become versatile tools for the determination of kinetic activation parameters and energy barriers to interconversion of isomers and enantiomers in the last 20 years. New computer-aided evaluation systems allow the on-line determination of these data after separating minute amount of pure compounds or mixture of isomers or enantiomers, respectively. Both dynamic interconversion during the separation process as well as static stopped-flow techniques have been applied to determine the kinetic activation parameters and interconversion energy barriers by separation methods. The use of (1) combinations of batchwise kinetic studies with enantioselective separations, (2) a continuous flow model, (3) a comparison of real chromatograms with simulated ones, (4) stopped-flow techniques, (5) stochastic methods, (6) approximation functions and (7) deconvolution methods, for the determination of interconversion energy barriers by separation methods is summarized in detail. [Copyright &y& Elsevier]