Immobilized strychnine as a new chiral stationary phase for HPLC.

A new ion-exchanger type chiral stationary phase for high-performance liquid chromatography was prepared. The synthetic protocol is based on derivatization of silica with (3-iodopropyl)trimethoxysilane in the first step followed by immobilization of strychnine via quaternization of nitrogen atom of the alkaloid strychnine. The synthesized chiral stationary phase was chromatographically characterized. The main effort was headed towards the evaluation of the enantioselectivity of the novel sorbent. For that purpose a set of suitable chiral probes, specifically, binaphthyl derivatives, was employed. The influence of methanol content, concentration of aqueous ammonium acetate buffer, and its pH on retention factors, separation selectivity, and resolution of the atropoisomers of the mentioned chiral solutes was studied in detail. It was demonstrated that the new chiral stationary phase was capable to separate atropoisomers of four out of seven testing compounds. Despite the strong influence of the above mentioned variables on retention, their impact on selectivity and resolution was rather moderate. Concerning retention mechanism, it seems that electrostatic interaction between the positively charged quaternary nitrogen of the chiral stationary phase and anionic solute participates significantly in the retention process.

Chromatographic methods enabling the characterization of stationary phases and retention prediction in high-performance liquid chromatography and supercritical fluid chromatography.

In the scope of the present review, the current status of high-performance liquid chromatography/ultra-high performance liquid chromatography and supercritical fluid chromatography is briefly provided. These techniques and their retention mechanisms are compared. Various alternative approaches utilized for the determination and description of the retention processes in these two systems are mapped. Two frequently used concepts, linear-free energy relationships, and hydrophobic subtraction models, used for the characterization of the retention interactions, are discussed. Principles and selected applications of the both methods are also covered. Then the models applied for the prediction of retention behavior of solutes on stationary phases are outlined. The procedures utilized for the sorbent/column classification are also covered. Simple chromatographic tests frequently used for the basic characterization and mutual comparison of stationary phases are summarized and briefly commented on. The importance of a statistical evaluation of complex retention data obtained from the chromatographic measurements is outlined. Finally, computer simulations aiming at the facilitation of the quest to optimize separation conditions for a given mixture of analytes are touched upon.

Evaluation of differences between Chiralpak IA and Chiralpak AD-RH amylose-based chiral stationary phases in reversed-phase high-performance liquid chromatography.

Polysaccharide-based chiral stationary phases can be used for the enantioselective separation of a wide range of structurally different compounds. These phases are available with chiral selectors coated or immobilized on silica gel support. The means of attachment of the chiral selector to the carrier can influence the separation performance of these stationary phases. This paper deals with evaluation of differences in the separation abilities of coated Chiralpak AD-RH versus immobilized Chiralpak IA amylose-based stationary phases in the reversed-phase mode of high-performance liquid chromatography. A set of chiral analytes was separated under acidic and basic conditions. Differences were observed in the enantioseparation potential of the tested phases. The linear-free energy relationship and additional evaluation of ionic interactions were used to ascertain whether the interactions that participate in retention and enantioseparation are affected by the means of preparation of these phases. All the interactions covered by the linear-free energy relationship were significant for the studied phases and their absolute values were almost always higher for the coated phase. Ionic interactions were found to be more important on the immobilized stationary phase but did not contribute to any improvement in the enantioselective separation performance.

Enantioselective potential of chiral stationary phases based on immobilized polysaccharides in reversed phase mode.

Derivatized polysaccharide-based columns have high enantiodiscrimination potential mainly in normal phase separation mode. In this work chiral recognition ability of four immobilized polysaccharide-derived chiral stationary phases was evaluated under reversed phase conditions. A set of 30 chiral compounds, particularly drugs possessing various functional groups was used for testing. Baseline enantioseparation was achieved for 17 of them. In general, amylose-based chiral stationary phases showed higher enantioselectivity than the cellulose-based ones, mainly for acidic and bifunctional compounds. The influence of the type and pH of the aqueous mobile phase constituents as well as the role of the organic modifier on the enantioselective separation ability of the stationary phases were also investigated and compared. Complementary separations were obtained on the amylose- and cellulose-based columns. The immobilized polysaccharide-based chiral stationary phases were shown to be useful tool for the enantioseparation of a broad spectrum of chiral analytes in reversed phase separation mode.

Supercritical fluid chromatography as a tool for enantioselective separation; a review.

Supercritical fluid chromatography (SFC) has become popular in the field of enantioselective separations. Many works have been reported during the last years. This review covers the period from 2000 till August 2013. The article is divided into three main chapters. The first one comprises a basic introduction to SFC. The authors provide a brief explanation of general principles and possibilities of this method. The advantages and drawbacks are also listed. Next part deals with chiral separation systems available in SFC, namely with the commonly used chiral stationary phases. Properties and interaction possibilities of the chiral separation systems are described. Recent theoretical papers are emphasized in this chapter. The last part of the paper gives an overview of applications of enantioselective SFC in analytical chemistry, in both analytical and preparative scales. Separation systems and conditions are summed up in tables so that they provide a helpful tool for analysts who search for a particular method of analysis.

Isopropyl derivative of cyclofructan 6 as chiral selector in liquid chromatography and capillary electrophoresis.

Cyclofructans and preferentially their derivatives can serve as chiral selectors for the separation of different enantiomers/atropisomers. Moreover, the strong ionophoric nature of the 18-crown-6 ether core of cyclofructan 6 for barium cations may be exploited to enhance or modify enantioselectivity. In this work isopropyl-cyclofructan-6 was used as a chiral selector for the separation of binaphthyl atropisomers in HPLC and CE. The data from both separation systems were compared with each other. While in HPLC the chiral selector was bonded to silica gel to afford a chiral stationary phase, in capillary electrophoresis it was freely mobile in the background electrolytes (BGE). This significant difference is reflected in the separation potential of the two separation systems. All five analytes could be baseline separated in HPLC (reversed phase mode) while only one derivative was baseline resolved in CE. This result was attributed to the more rigid nature of the immobilized chiral selector. Addition of Ba(2+) to the mobile phase or BGE improved chiral separations in both systems. The results may help to elucidate the interaction mechanism in these systems with cyclofructan derivatives and to gain some general knowledge of their separation potential.

An insight into the use of dimethylphenyl carbamate cyclofructan 7 chiral stationary phase in supercritical fluid chromatography: the basic comparison with HPLC.

Cyclofructan-based chiral stationary phases were previously shown as a promising possibility for separation of chiral compounds in high performance liquid chromatography. In this work retention and enantiodiscrimination properties of the 3,5-dimethylphenyl carbamate cyclofructan 7 chiral stationary phase are described in supercritical fluid chromatography. The results obtained in both of the separation methods were compared. A set of compounds with axial or central chirality was used as analytes. The effect of mobile phase composition, that is, addition of different alcohol modifiers and/or trifluoroacetic acid to carbon dioxide, was examined in the supercritical system. Similarly, mobile phases composed of hexane modified with propan-2-ol and/or trifluoracetic acid were used in liquid chromatography. A linear free energy relationship model was utilized for characterization of interactions that are decisive for retention and separation in both techniques. Dispersion interactions showed similar negative values using both methods. The main contribution of hydrogen bond acidity was also comparable for both methods. The propensity to interact with n- and/or π-electron pairs of solutes was significant only in the supercritical system.

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 coefficients of the LFER equation under two mobile-phase compositions in normal separation mode. The LFER results showed that hydrogen bond acidity, hydrophobicity and dipolarity/polarizibility mostly affect the retention and separation process on the CF-based columns in the studied separation systems.