Enantiomer separation of amino acids as their N-alkyloxycarbonyl alkylamide derivatives by chiral phase capillary GC

The enantiomers of amino acids were first converted into N-alkyloxycarbonyl 2,2,2-trifluoroethyl esters, and then into N-alkyloxycarbonyl alkylamides by nucleophilic substitution of the ester group with amines. The first reaction proceeds instantaneously, while the second substitution occurs smoothly with n-propylamine and isobutylamine. The final derivatives were produced for separation on a capillary column coated with Chirasil-Val by GC. Pro, which is difficult to separate completely as its N-perfluoroacyl alkyl ester derivative, showed complete separation of the enantiomeric pair. All amino acids examined in this study showed an increased separation factor.     

Liquid chromatographic separation of the enantiomers of beta-amino acids on a ligand exchange chiral stationary phase

A liquid chromatographic ligand exchange chiral stationary phase (CSP) derived from (S)-leucinol was applied in the separation of the enantiomers of 12 beta-amino acids. The resolution was quite successful especially for the enantiomers of beta-amino acids containing aromatic functional group in the side chain. The chromatographic resolution behaviors were dependent on the organic modifier and Cu(II) concentration in aqueous mobile phase and the column temperature.     

Cyclodextrin dimer derivatives used as stationary phase for capillary gas chromatography

Summary Four cyclodetrin dimer derivatives were synthesized by linking two single cyclodextrin derivatives with difunctional spacer at the primary side of cyclodetrin. The separation properties of these cylodextrin dimer derivatives as CGC stationary phases were investigated and compared with those of the unbridged native cyclodextrin derivative. The results show that two recognition sites and one link spacer of these cyclodextrin dimer derivatives cooperate in separation and affect the separation of disubstituted benzene positional isomers.     

Capillary gas chromatography separation of pyrethroic acid methyl esters using four acylated cyclodextrin derivatives as chiral stationary phases

Summary Four cyclodextrin derivatives (CDs) were synthesized by substituting 3-OH of 2,6-di-O-pentyl-β-cyclodextrin with four different chain lengths of acyl groups (butyryl, valeryl, heptanoyl, octanoyl). The chromatographic properties of the four CD derivatives as stationary phases of capillary gas chromatography (CGC) were investigated. These CDs exhibit a wide range of application. Not only five pairs of enantiomers of pyrethroic acid methyl esters were separated on the four CDs, but also some other racemic compounds. Among the four CDs, 2,6-di-O-pentyl-3-O-butyryl-β-CD possesses better enantiomer separation abilities to the studied enantiomers of pyrethroic acid methyl esters than the other studied CDs. The extension of chain length of the acyl groups in 3-position of CDs cannot improve the enantiomer separation abilities of the CD derivatives.     

Synthesis and characterization of β-cyclodextrin modified hyperbranched carbosilane as stationary phase for GC

A new β-cyclodextrin modified hyperbranched carbosilane stationary phase for gas chromatography was synthesized by substituting the -OH groups ofβ-cyclodextrin with hyperbranched carbosilane and was coated on the inner wall of fused silica capillary column for gas chromatography.The chromatographic behaviors of the stationary phase were studied.The initial testing results showed mat it possessed good separation abilities for several kinds of mixtures,such as benzenes,acrylates,ketones and alkylchlorides.     

Enantiomeric separation of four methylenedioxylated amphetamines on β-cyclodextrin chiral stationary phases

Summary Native and derivatized β-cyclodextrins such as chiral stationary phases (CSP) were used for the simultaneous enantiomeric separation of four methylenedioxylated amphetamines (MDA, MDMA, MDEA and MBDB) by liquid chromatography. Fluorimetric detection was used in order to enhance sensitivity and selectivity. The mobile phase was, optimised by studying the influence of pH, triethylamine concentration, organic solvent type, column temperature and flow rate of the mobile phase. This method was validated by determining linearity, precision, accuracy, limits of detection and quantification, and was applied to the stereoselective analysis of illicit tablets (23 samples) and of human whole blood samples (spiked samples and two post-mortem cases). Whereas no significant deviation from a racemic ratio was observed in the tablets contents, the analysis of blood samples showed an enantioselective metabolism of MDMA.     

Immobilized versus coated amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phases for the enantioselective separation of cyclopropane derivatives by liquid chromatography

The solvent versatility of Chiralpak IA, a new chiral stationary phase (CSP) containing amylose tris(3,5-dimethylphenylcarabamate) immobilized onto silica gel, is investigated for the enantioselective separation of a set of cyclopropane derivatives using ethyl acetate or dichloromethane (DCM) as non-standard mobile phase eluent and diluent, respectively in high-performance liquid chromatography (HPLC). A comparison of the separation of cyclopropanes on both immobilized and coated amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phases (Chiralpak IA and Chiralpak AD, respectively) in HPLC using a mixture of n-hexane/2-propanol (90/10 and 99/1, v/v) as mobile phase with a flow rate of 0.5 ml/min and UV detection at 254 nm, is demonstrated. The optimized method of separation is used for an online HPLC monitoring for the Rh(II)-catalyzed asymmetric intermolecular cyclopropanations in dichloromethane. Direct analysis techniques without further purification, workup or removal of dichloromethane were summarized. The method provides an easy and direct determination of the enantiomeric excess of the cyclopropanes and selectivity of the catalyst used without any further work up.     

A new gas chromatographic stationary phase: Polysiloxane with β-cyclodextrin side chain for the separation of chiral and positional isomers

Summary A new stationary phase [bikis(2,6-di-O-pentyl-3-O-hex-6-enyl)-pentakis(2,6-di-O-pentyl-3-O-methyl)-β-CD-polysiloxane] was synthesized and successfully applied in GC for the separation of chiral and achiral isomers. It possesses high column efficiency and exhibits excellent separation ability for disubstituted benzenes. Some typical enantiomers and optical isomers are well separated. The separation behavior of this new phase is characterized and discussed.     

Single-walled carbon nanotubes as stationary phase in gas chromatographic separation and determination of argon, carbon dioxide and hydrogen

A chromatographic technique is introduced based on single-walled carbon nanotubes (SWCNTs) as stationary phase for separation of Ar, CO₂ and H₂ at parts per million (ppm) levels. The efficiency of SWCNTs was compared with solid materials such as molecular sieve, charcoal, multi-walled carbon nanotubes and carbon nanofibers. The morphology of SWCNTs was optimized for maximum adsorption of H₂, CO₂ and Ar and minimum adsorption of gases such as N₂, O₂, CO and H₂O vapour. To control temperature of the gas chromatography column, peltier cooler was used. Mixtures of Ar, CO₂ and H₂ were separated according to column temperature program. Relative standard deviation for nine replicate analyses of 0.2 mL H₂ containing 10 μL of each Ar or CO₂ was 2.5% for Ar, 2.8% for CO₂ and 3.6% for H₂. The interfering effects of CO, and O₂ were investigated. Working ranges were evaluated as 40-600 ppm for Ar, 30-850 ppm for CO₂ and 10-1200 ppm for H₂. Significant sensitivity, small relative standard deviation (RSD) and acceptable limit of detection (LOD) were obtained for each analyte, showing capability of SWCNTs for gas separation and determination processes. Finally, the method was used to evaluate the contents of CO₂ in air sample.     

Gas-chromatographic separation of underivatized steroids using a BPhBT liquid crystal stationary phase

A gas-chromatographic separation of underivatized steroids is described, in which the liquid crystal N,?-bis(p-phenylbenzylidene)α,α′-bi-p-toluidine (BPhBT) was used isothermally at 265°C for the stationary phase. The nematic range of this crystal is 247 to 403°C. Use of the BPhBT crystal allowed operations in a temperature range where steroids are volatile and unlikely to display broad elution peaks and long retention times. With columns precoated with 2.5 weight percent of BPhBT, steroids containing minor structural differences in the ring system were readily differentiated. For example, 5-androsten-17α-ethinyl-3β, 17β-diol was found to elute more rapidly than its 3-keto analog, ethisterone. Ethisterone was distinguished readily also from its 19-nor analog, norethisterone. Additionally, differences in side-chain functionality and resultant molecular dimensions facilitated the separation of steroids of identical ring structure, as in the comparison of cholesterol with methyl 3β-hydroxy-5-cholenate and 25-hydroxycholesterol. An enhancement of separation according to the molecular length-to-breadth ratio was observed; in general, the larger this length-to-breadth ratio, the greater the retention period on the column.     

Cyclodextrins as chiral stationary phases in capillary gas chromatography. Part VI: Octakis (2,3,6-tri-O-pentyl)-γ-cyclodextrin

Perpentylated γ-cyclodextrin is a very hydrophobic liquid which exhibits enantioselectivity toward a number of non-polar chiral substrates including the saturated hydrocarbons cis- and trans-pinane, β-chiral olefins, iron(O) tricarbonyl complexes of prochiral olefins and 3,3,8,8-tetramethyl-trans-cyclooctene, a compound with planar chirality.     

Rapid determination of ambrisentan enantiomers by enantioselective liquid chromatography using cellulose-based chiral stationary phase in reverse phase mode

A sensitive, specific, and rapid high-performance liquid chromatography (HPLC) method for the determination of ambrisentan enantiomers has been developed and validated. Six chiral columns were tested in a reversed-phase system. Excellent enantioseparation with the resolution more than 2.5 was achieved on Chiralcel OZ-3R (cellulose 3-chloro-4-methylphenylcarbamate) using mixture of 20 mM sodium formate (pH 3.0) with acetonitrile (55:45; v/v). Validation of the HPLC method including linearity, limit of detection, limit of quantification, precision, accuracy, and selectivity was performed according to the International Conference on Harmonisation (ICH) guidelines. The method has an advantage of a very quick chromatographic separation (less than 6 min) and therefore is highly suitable for routine determination of (R)-ambrisentan in enantiopure active pharmaceutical ingredient (S)-ambrisentan.     

Enantiomeric separation of racemic sulphoxides on chiral stationary phases by gas and liquid chromatography

Summary The enantiomeric resolution of seven racemic sulphoxides on chiral stationary phases has been investigated by gas and liquid chromatography. In gas chromatography the separations were performed on octakis-(2,6-di-O-pentyl-3-O-butyryl)-γ-cyclodextrin (FS Lipodex-E) and heptakis-(2,6-di-O-methyl-3-O-pentyl)-β-cyclodextrin (DMP-β-CD). Both stationary phases were suitable for separation of the enantiomers of the sulphoxides. With one exception for each series all racemetes could be resolved on both stationary phases; FS Lipodex-E was more enantioselective than DMP-β-CD, whereas the latter seemed more generally applicable. Liquid chromatographic separations with Chiralcel-OB as stationary phase were significantly improved by optimization of mobile phase composition and temperature. Resolution factors up to R_s=6 were achieved indicating that the improved separations could now be easily used for preparative purposes.     

Synthesis and characterization of benzyl β-cyclodextrins used as GC stationary phases

Summary Heptakis(2,6-di-O-benzyl)-β-CD(I), heptakis(2,6-di-O-benzyl-3-O-pentyl)-β-CD(II), heptakis(2,6-di-O-benzyl-3-O-methyl)-β-CD(III) and heptakis(2,6-di-O-benzyl-3-O-acetyl)-β-CD(IV) derivatives were synthesized and identified. Their thermal stabilities were tested using PyGC. These CD derivatives are stable up to 300°C and suitable for use as gas chromatographic stationary phases. The fused silica capillary columns coated with heptakis(2,6-di-O-benzyl-3-O-methyl)-β-CD and heptakis(2,6-di-O-benzyl-3-O-pentyl)-β-CD showed excellent chromatographic properties in separating positional isomers.