Immobilisation and evaluation of a vancomycin chiral stationary phase for capillary electrochromatography

Recent advances in chiral selectors immobilization and chiral mobile phase additives in liquid chromatographic enantio-separations: A review

For decades now, the separation of chiral enantiomers of drugs has been gaining the interest and attention of researchers. In 1991, the first guidelines for development of chiral drugs were firstly released by the US-FDA. Since then, the development in chromatographic enantioseparation tools has been fast and variable, aiming at creating a suitable environment where the physically and chemically identical enantiomers can be separated. Among those tools, the immobilization of chiral selectors (CS) on different stationary phases and the chiral mobile phase additives (CMPA) which have been progressed and studied extensively. This review article highlights the major advances in immobilization of CS together with their different recognition mechanisms as well as CMPA as a cheaper and successful alternative for chiral stationary phases. Moreover, the role of molecular modeling tool as a pre-step in the choice of CS for evaluating possible interactions with different ligands has been pointed up. Illustrations of reported methods and updates for immobilized CS and CMPA have been included.

Recognition of Cis-Trans and Chiral Proline and Its Derivatives by Ion Mobility Measurement of Their Complexes with Natamycin and Metal Ion

Discrimination of isomers is an important and valuable feature in many analytical applications, and the identification of chiral isomers and cis-trans isomers is the current research focus. In this work, a simple method for direct, simultaneous recognition of d-/l-proline (P), d-/l-/cis-/trans-4-hydroxyproline (4-HP), and d-/l-/cis-/trans-N-tert-butoxycarbony (N-Boc-4-HP) was investigated by means of trapped ion mobility spectrometry-mass spectrometry (TIMS-MS). The isomers with cis-/trans-/d-/l-configuration can be directly recognized based on their mobility upon reaction with natamycin (Nat) and metal ions through noncovalent interactions. The results indicate that the recognition of the enantiomers has certain specificity, and the structural difference of the enantiomers was increased in a complex with Nat and metal ions. Herein, d-/l-P can be recognized through the ternary complexes [P + Nat + Mg - H]⁺, [P + 2Nat + Ca - H]⁺, [P + 2Nat + Mn - H]⁺, and [P + Nat + Cu - H]⁺. Similarly, c-4-HP_L, c-4-HP_D, t-4-HP_L, and t-4-HP_D can be recognized by [4-HP + Nat + Ca - H]⁺, [4-HP + 2Nat + Ca - H]⁺, and [4-HP + Nat + Cu - H]⁺, while N-Boc-c-4-HP_L, N-Boc-c-4-HP_D, N-Boc-t-4-HP_L, and N-Boc-t-4-HP_D were recognized through the enantiomer complexes [N-Boc-4-HP + Nat + Li]⁺, [N-Boc-4-HP + Nat + 2Na - H]⁺, [N-Boc-4-HP + Nat + K]⁺, [N-Boc-4-HP + Nat + Mn - H]⁺, and [N-Boc-4-HP + Nat + Ba - H]⁺. Moreover, tandem mass spectrometry (MS/MS) results indicated that different collision energies were obtained for the same fragment ions, which implied that the enantiomer complexes that contributed to their mobility separation shared identical interaction mode but had different gas-phase rigid geometries. Furthermore, the relative quantification for the enantiomers was performed, and the results were supported by a satisfactory coefficient (R² > 0.99). The developed method can provide a promising and powerful strategy for the separation of chiral proline and its d-/l-/cis-/trans derivatives, bearing the advantages of higher speed, better accuracy, high selectivity, and no need for chemical derivatization and chromatographic separation.

A Polymer-based Monolithic Capillary Column with Polymyxin-B Chiral Selector for the Enantioselective Nano-High Performance Liquid Chromatographic Pharmaceutical Analysis

Herein, we report the first use of Polymyxin-B antibiotic as a enantio-selector in polymer monolithic capillary. The capillaries were functionalised, characterized and tested for the enantioselective nano-HPLC separation of 50 racemic pharmaceutical drugs. They have been easily prepared by immobilizing Polymyxin-B over the organic polymer for 48 h (P1) or encapsulating Polymyxin-B within the organic polymer (P2) and tested for the enantioselective resolution of racemic drugs. Acceptable resolution was achieved for 21 drugs using RP-HPLC conditions on both (P1) and (P2) capillary columns, while no separation was observed under NP-HPLC conditions. Polymyxin-B is commercially available, easily solubilized and stable in both acidic and neutral media. The developed Polymyxin-B-based polymer monolithic capillaries provide a promising expansion of platform in enantioselective HPLC separations.

Application of Experimental Design Methodologies in the Enantioseparation of Pharmaceuticals by Capillary Electrophoresis: A Review

Chirality is one of the major issues in pharmaceutical research and industry. Capillary electrophoresis (CE) is an interesting alternative to the more frequently used chromatographic techniques in the enantioseparation of pharmaceuticals, and is used for the determination of enantiomeric ratio, enantiomeric purity, and in pharmacokinetic studies. Traditionally, optimization of CE methods is performed using a univariate one factor at a time (OFAT) approach; however, this strategy does not allow for the evaluation of interactions between experimental factors, which may result in ineffective method development and optimization. In the last two decades, Design of Experiments (DoE) has been frequently employed to better understand the multidimensional effects and interactions of the input factors on the output responses of analytical CE methods. DoE can be divided into two types: screening and optimization designs. Furthermore, using Quality by Design (QbD) methodology to develop CE-based enantioselective techniques is becoming increasingly popular. The review presents the current use of DoE methodologies in CE-based enantioresolution method development and provides an overview of DoE applications in the optimization and validation of CE enantioselective procedures in the last 25 years. Moreover, a critical perspective on how different DoE strategies can aid in the optimization of enantioseparation procedures is presented.

Daptomycin: A Novel Macrocyclic Antibiotic as a Chiral Selector in an Organic Polymer Monolithic Capillary for the Enantioselective Analysis of a Set of Pharmaceuticals

Daptomycin, a macrocyclic antibiotic, is here used as a new chiral selector in preparation of chiral stationary phase (CSP) in a recently prepared polymer monolithic capillary. The latter is prepared using the copolymerization of the monomers glycidyl methacrylate (GMA) and ethylene glycol dimethacrylate (EGDMA) in the presence of daptomycin in water. Under reversed phase conditions (RP), the prepared capillaries were tested for the enantioselective nanoliquid chromatographic separation of fifty of the racemic drugs of different pharmacological groups, such as adrenergic blockers, H1-blockers, NSAIDs, antifungal drugs, and others. Baseline separation was attained for many drugs under RP-HPLC. Daptomycin expands the horizon of chiral selectors in HPLC.

Colistin Sulfate Chiral Stationary Phase for the Enantioselective Separation of Pharmaceuticals Using Organic Polymer Monolithic Capillary Chromatography

A new functionalized polymer monolithic capillary with a macrocyclic antibiotic, namely colistin sulfate, as chiral selector was prepared via the copolymerization of binary monomer mixtures consisting of glycidyl methacrylate (GMA) and ethylene glycol dimethacrylate (EGDMA) in porogenic solvents namely 1-propanol and 1,4-butanediol, in the presence of azobisiso-butyronitrile (AIBN) as initiator and colistin sulfate. The prepared capillaries were investigated for the enantioselective nano-LC separation of a group of racemic pharmaceuticals, namely, α- and β-blockers, anti-inflammatory drugs, antifungal drugs, norepinephrine-dopamine reuptake inhibitors, catecholamines, sedative hypnotics, antihistaminics, anticancer drugs, and antiarrhythmic drugs. Acceptable separation was achieved for many drugs using reversed phase chromatographic conditions with no separation achieved under normal phase conditions. Colistin sulfate appears to be useful addition to the available macrocyclic antibiotic chiral phases used in liquid chromatography.

High-Performance Liquid Chromatography Enantioseparations Using Macrocyclic Glycopeptide-Based Chiral Stationary Phases: An Overview

Since their introduction by Daniel W. Armstrong in 1994, antibiotic-based chiral stationary phases have proven their applicability for the chiral resolution of various types of racemates. The unique structure of macrocyclic glycopeptides and their large variety of interactive sites (e.g., hydrophobic pockets, hydroxy, amino and carboxyl groups, halogen atoms, aromatic moieties) are the reasons for their wide-ranging selectivity. The commercially available Chirobiotic™ phases, which display complementary characteristics, are capable of separating a broad variety of enantiomeric compounds with good efficiency, good column loadability, high reproducibility, and long-term stability. These are the major reasons for the frequent use of macrocyclic antibiotic-based stationary phases in HPLC enantioseparations.This overview chapter provides a brief summary of general aspects of antibiotic-based chiral stationary phases including their preparation and their application to direct enantioseparations of various racemates focusing on the literature published since 2004.

Single-step approach for fabrication of vancomycin-bonded silica monolith as chiral stationary phase

A vancomycin-bonded silica monolithic column for capillary electrochromatography (CEC) was prepared by a single-step in situ sol-gel approach. This sol-gel process incorporates a synthetic sol-gel precursor which contains a macrocyclic antibiotic, vancomycin, to form a porous silica network inside a fused-silica capillary. To avoid degradation of vancomycin during the column fabrication, a mild step was adopted into the sol-gel process. The performance of the vancomycin chiral stationary phase was investigated by CEC in both the reversed-phase mode and the normal-phase mode. The vancomycin chiral stationary phase was optimized with respect to vancomycin loading in the reversed-phase mode for chiral separation of thalidomide enantiomers. The best efficiency and resolution values of 94600plates/m and 5.79, respectively, were achieved. The optimized column was further applied to chiral separation of alprenolol enantiomers. A plate height of less than 7μm for the first eluted enantiomer of alprenolol was obtained in an aqueous mobile phase at a flow rate of 0.74mm/s. Using enantiomers of seven β-blockers and some other basic enantiomers as test analytes, separation efficiencies of up to 148100plates/m in the reversed-phase mode and up to 138100plates/m in the normal-phase mode were achieved.

Enantioseparations by high-performance liquid chromatography using macrocyclic glycopeptide-based chiral stationary phases: an overview

Since their introduction by Armstrong in 1994, macrocyclic antibiotic-based chiral stationary phases have proven their applicability for the chiral resolution of various types of racemates. The unique structure of macrocyclic glycopeptides and their large variety of interactive sites (e.g., hydrophobic pockets, hydroxyl, amino and carboxyl groups, halogen atoms, aromatic moieties, etc.) are the reason for their wide-ranging selectivity. The commercially available Chirobiotic™ phases, which display complementary characteristics, are capable of separating a broad variety of enantiomeric compounds with good efficiency, good column loadability, high reproducibility, and long-term stability. These are the major reasons for the use of macrocyclic antibiotic-based stationary phases in HPLC enantioseparations. This overview chapter provides a brief summary of general aspects of macrocyclic antibiotic-based chiral stationary phases including their preparation and their application to direct enantioseparations of various racemates focusing on the literature published since 2004.

Advances in the enantioseparation of β-blocker drugs by capillary electromigration techniques

β-Blocker drugs or β-adrenergic blocking agents are an important class of drugs, prescribed with great frequency. They are used for various diseases, particularly for the treatment of cardiac arrhythmias, cardioprotection after myocardial infarction (heart attack), and hypertension. Almost all β-blocker drugs possess one or more stereogenic centers; however; only some of them are administered as single enantiomers. Since both enantiomers can differ in their pharmacological and toxicological properties, enantioselective analytical methods are required not only for pharmacodynamic and pharmacokinetic studies but also for quality control of pharmaceutical preparations with the determination of enantiomeric purity. In addition to the chromatographic tools, in recent years, capillary electromigration techniques (CE, CEC, and MEKC) have been widely used for enantioselective purposes employing a variety of chiral selectors, e.g. CDs, polysaccharides, macrocyclic antibiotics, proteins, chiral ion-paring agents, etc. The high separation efficiency, rapid analysi,s and low consumption of reagents of electromigration methods make them a very attractive alternative to the conventional chromatographic methods. In this review, the development and applications of electrodriven methods for the enantioseparation of β-blocker drugs are reported. The papers concerning this topic, published from January 2000 until December 2010, are summarised here. Particular attention is given to the coupling of chiral CE and CEC methods to MS, as this detector provides high sensitivity and selectivity.

Enantioseparation of dansyl amino acids by HPLC on a monolithic column dynamically coated with a vancomycin derivative

In this work a chiral stationary phase was prepared by dynamically coating a monolithic reversed-phase HPLC column with a vancomycin-derivative as chiral selector. A hydrophobic alkyl-chain was attached to the vancomycin molecule, providing the immobilization of the chiral selector on the reversed-phase material. Dansyl amino acids were chosen as model analytes for testing the separation power of the dynamically coated phase. All investigated compounds were separated into their enantiomers. Compared with a conventionally packed vancomycin-CSP, a reversal of the enantiomer elution order was obtained.

Recent advances in analytical determination of thalidomide and its metabolites

Thalidomide, a racemate, is coming into clinical use as immuno-modulating and anti-inflammatory drug. Thalidomide was approved by the FDA in July 1998 for the treatment of erythema nodusum leprosum associated with leprosy. Recently, thalidomide is proving to be a promising drug in the treatment of a number of cancers and inflammatory diseases, such as multiple myeloma, inflammatory bowel disease (Crohn's disease), HIV and cancer associated cachexia. These effects may chiefly be exerted by S-thalidomide, but the enantiomers are inter-converted in vivo. Thalidomide is given orally, although parenteral administration would be desirable in some clinical situations. Thalidomide has been determined in formulations and, principally in biological fluids by a variety of methods such as high-performance liquid chromatography with ultraviolet detection and liquid chromatography coupled with tandem mass spectrometry. The overview includes the most relevant analytical methodologies used in its determination.

High-performance liquid chromatographic enantioseparations on capillary columns containing crosslinked polysaccharide phenylcarbamate derivatives attached to monolithic silica

Monolithic capillary columns containing native silica gel were covalently modified with 3,5-disubstituted phenylcarbamate derivatives of cellulose and amylose and applied for enantioseparations in capillary LC. The method previously used for covalent immobilization of polysaccharide phenylcarbamate derivatives onto the surface of microparticulate silica gel was successfully adapted for in situ modification of monolithic fused-silica capillary columns. The effects of the nature of polysaccharide and the substituents, as well as of multiple covalent immobilization of polysaccharide derivative on chromatographic performance of capillary columns were studied. The capillary columns obtained using this technique are stable in all solvents commonly used in LC and exhibit promising enantiomer resolving ability.

Preparation and evaluation of a vancomycin-immobilized silica monolith as chiral stationary phase for CEC

Enantiomeric separations in CEC with the macrocyclic antibiotic vancomycin immobilized silica monolith as a chiral stationary phase are presented. The monolithic silica capillary columns were prepared by a sol-gel process in fused-silica capillaries with an inner diameter of 50 mum and subsequently in situ immobilization of vancomycin as a chiral selector by reductive amination. Enantioselectivity was obtained for eight pairs of enantiomers in nonaqueous polar organic or aqueous mobile phases and most of them were baseline-separated with high column efficiencies. It was observed that the organic modifier ratio (MeOH/ACN) in the polar organic mobile phase played a significant role in controlling the resolution and efficiency of the enantiomers. In enantiomeric separation of propranolol, repeatability for column efficiency and resolution in the nonaqueous mobile phase was given in terms of RSD values at 1.1 and 2.3% (n = 5) for run-to-run injections and 7.2 and 9.6% (n = 5) for column-to-column testing while repeatability for the separation of thalidomide in the aqueous mobile phase was given in terms of RSD values at 1.5, 2.8% and 6.1, 10.5%, respectively.

Considerations on influence of charge distribution on determination of biomolecules and microorganisms and tailoring the monolithic (continuous bed) materials for bioseparations

The importance of continuous beds (monoliths) as separation materials is connected with their better chromatographic properties and easier preparation in comparison to particulate-packed columns. Moreover the tuning of porosity as well as surface chemistry can lead to obtaining of highly selective materials, especially useful in separation of biologically important compounds or even microorganisms. To obtain high selectivity for such analytes as e.g. proteins, it is often important to have a knowledge about their shape, size, charge and finally charge distribution. This article presents our considerations on the charge distribution on the monolithic stationary phase and surface of such species as proteins or microorganisms as well as its eventual influence on the separation or sample preparation processes and tuning of their selectivity.

Preparation of a monolithic capillary column with immobilized α-mannose for affinity chromatography of lectins

A simple method for the preparation of an affinity monolithic (also called continuous bed) capillary column for alpha-mannose-specific lectins is described. 2-Hydroxyethyl methacrylate in combination with (+)-N,N -diallyltartardiamide (DATD) and piperazine diacrylamide (PDA, 1,4-bisacryloyl-piperazine) as crosslinkers, were used as monomers for the monolith. After oxidation of DATD with periodate, alpha-mannose with spacer was bound to the aldehyde groups of the polymeric skeleton via reductive amination to form an affinity column for the separation, enrichment or binding studies of mannose-specific lectins. The permeability of the column was excellent. The porosity of the monolith was investigated by scanning electron microscope (SEM) and inverse size exclusion chromatography (ISEC). The affinity of the monolith was evaluated by frontal analysis (FA) and fluorescence microscopy (FM) using fluorescently labeled concanavalin (Con A). Frontal affinity chromatography showed a specific interaction of two different lectins with the alpha-mannose-modified monolith. According to FM the affinity sites were evenly distributed over the monolithic bed.

Enantioselective aptameric molecular recognition material: Design of a novel chiral stationary phase for enantioseparation of a series of chiral herbicides by capillary electrochromatography

A chiral stationary phase derived from an L-RNA aptamer is evaluated for the enantiomer separation of a series of herbicide molecules (aryloxypropionic, aryloxyphenoxypropionic, and aminopropionic acid) by CEC after binding to biotin and grafting upon streptavidin-modified porous glass beads. We demonstrated that the aptamer capillary was stable in term of efficiency and retention during a long period. The influences of the mobile phase constitution and its flow-velocity on the enantioseparation were also investigated. The results suggest that the interactions of the enantiomer during CEC are solely based on chromatographic mechanisms and that the electrophoresis plays only a minor role. The separation efficiency and peak shape could be improved by Mg2+ divalent cation that stabilized the aptamer secondary structure and thus enhanced the mass transfer kinetics during the ligand-aptamer binding process. In addition, it was demonstrated that the determination of the enantiomerization barrier of flamprop was possible using this chiral stationary phase.

Chiral separation principles in chromatographic and electromigration techniques

Almost half of the drugs in use today are chiral. It is well established that the pharmacological activity is mostly restricted to one of the enantiomers (eutomer). There can be qualitative and quantitative differences in the activity of the enantiomers. In many cases, the inactive enantiomer (distomer) shows unwanted side effects or even toxic effects. Even if the side effects are not that drastic, the distomer has to be metabolized and this represents an unnecessary burden for the organism. Therefore, the development of methods for the separation of enantiomers, both on analytical and preparative scale, has become increasingly important. Chromatographic techniques such as thin layer chromatography (TLC), gas chromatography (GC), supercritical fluid chromatography (SFC), and above all high-performance liquid chromatography (HPLC) have been used for enantiomer separation for about two decades. More recently, electromigration techniques, such as capillary electrophoresis and capillary electrochromatography, have been shown to be powerful alternatives to chromatographic methods. This review gives a short overview of different chiral separation principles and their application. Several new developments are discussed.

High-performance liquid chromatographic enantioseparations on capillary columns containing monolithic silica modified with amylose tris(3,5-dimethylphenylcarbamate)

Monolithic capillary columns containing native silica were modified by in situ coating with amylose tris(3,5-dimethylphenylcarbamate) and applied for enantioseparations in capillary liquid chromatography. Capillary columns were examined for 10 standard racemic compounds in order to compare the performance of monolithic silica columns with the common, 4.6mm I.D. high-performance liquid chromatographic columns packed with particulate silica. The effects of polysaccharide coating and of the linear velocity of the mobile phase on peak performance were studied. Enantioseparations with an analysis time below 1min were achieved for some chiral analytes.

Enantioseparation of glycyl-dipeptides by CEC using particle-loaded monoliths prepared by ring-opening metathesis polymerization (ROMP)

Novel particle-loaded monolithic capillary electrochromatography (CEC) phases for chiral separations were prepared via ring-opening metathesis polymerization (ROMP) within the confines of fused silica columns with 200 microm i.d. using norborn-2-ene (NBE), 1,4,4a,5,8,8a-hexahydro-1,4,5,8,exo,endo-dimethanonaphthalene (DMN-H6) as monomers, 2-propanol and toluene as porogens, RuCl2(PCy3)2(CHPh) as initiator and silica-based particles containing the chiral selector. By suspending silica particles bearing the chiral selector in the polymerization mixture, particle-based monoliths are easily prepared. This approach has several advantages compared to particle-based separation media: (i) the concept of particle-based monoliths is broadly applicable, as any silica-based chiral phase can be used; (ii) they are inexpensive to prepare; and (iii) the manufacturing process is very simple, no sophisticated packing procedures or the preparation of end frits are required. To show the usefulness of this concept for chiral CEC, the chiral separation performance of particle-loaded CEC monoliths bearing teicoplanin aglycone, chemically bonded to 3 microm silica gel, was investigated for a set of glycyl-dipeptides. Particle-loaded ROMP CEC monoliths showed good separation performance for glycyl-dipeptides.

Thalidomide enantiomers: Determination in biological samples by HPLC and vancomycin-CSP

Thalidomide is a racemate with potentially different pharmacokinetics and pharmacodynamics of the component (+)-(R)- and (-)-(S)-thalidomide enantiomers. As part of a project on the adjunctive effects of thalidomide and cytotoxic agents, a method for the chiral separation and quantitation of thalidomide was developed and validated. Thalidomide in relevant serum and tissue homogenate samples was stabilized by buffering with an equal volume of citrate-phosphate buffer (pH 2, 0.2M), and stored at -80 degrees C pending assay. The thalidomide enantiomers, extracted from the samples with diethyl ether, were well separated on a chiral HPLC column of vancomycin stationary phase and a mobile phase of 14% acetonitrile in 20 mM ammonium formate adjusted to pH 5.4; their concentrations were determined with phenacetin as internal standard at 220 nm detection. Over a thalidomide concentration range of 0.1-20 microg/ml, assay precision was 1-5% (CV) for both enantiomers, and calibration curves were linear with all correlation coefficients being >0.99. The estimated limit of quantification for both enantiomers was 0.05 microg/ml with 0.2-0.6 ml serum samples. Thalidomide in rat and human serum, acidified and stored as described above, was found to be chemically and chirally stable over 1 year. The method has been successfully applied to serum samples from human patients undergoing thalidomide treatment for mesothelioma, and to serum, blood and tissue samples from a laboratory rodent model using transplanted 9l gliosarcoma. Enantioselectivity in thalidomide pharmacokinetics has been found, thereby reinforcing the need for considering the relevance of chirality in thalidomide pharmacology.

Recent developments in the field of monolithic stationary phases for capillary electrochromatography

This review summarizes the contributions to the rapidly growing area of monolithic columns based on both silica and synthetic polymers for capillary electrochromatography and chip electrochromatography, with a focus on those published during the year 2004. A wide variety of both modified approaches to the "old" monoliths and new monoliths have been reported despite the very short period of time covered. This demonstrates that monolithic stationary phases have become a well-established format in the field of electrochromatography. The simplicity of their preparation as well as the good control over their porous properties and surface chemistries make the monolithic separation media an attractive alternative to capillary columns packed with particulate materials.

Recent advances in chiral separation principles in capillary electrophoresis and capillary electrochromatography

This review summarizes recent developments in chiral separation in capillary zone electrophoresis (CZE), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC) covering literature published since the year 2000. New chiral selectors and innovative approaches for CE and CEC are introduced. Recent progress in column technology for CEC is highlighted and the development of new chiral stationary phases is discussed. This review is not dedicated to list applications but will focus on new developments.

Capillary electrochromatographic chiral separations with potential for pharmaceutical analysis

The use of capillary electrochromatography as a chiral separation technique for pharmaceutical applications is reviewed. Publications of the past 10 years that provide a potential practical application in pharmaceutical analysis are considered. Method development or validation, separation strategies, and potential routine analysis by the methods/applications cited are the main subjects on which we focused our attention. The indirect chiral separation method was only used once in CEC mode. In the direct chiral separations, the use of chiral stationary phases was obviously preferred over the use of chiral mobile phases with non-chiral stationary phases. Amongst the chiral stationary phases, those based on macrocyclic antibiotics and polysaccharide selectors were the most frequently used. Monolithic stationary phases also have several applications, but not so extended as those with packed capillary electrochromatography. The considered papers not only describe the applicability of the technique for relatively large sets of chiral analytes, they also showed that various types of stationary phases can be produced in-house in a simple manner. However, to survive as a mature separation technique, considerable time and effort are still needed to solve some disadvantages currently characterizing capillary electrochromatography.

Chiral separations by capillary electromigration techniques in nonaqueous media

A review on the advantages, peculiarities, and the potential of enantioselective capillary electrochromatography (CEC) in nonaqueous media is presented. Some fundamentals on CEC with particular focus on enantioselective CEC are discussed. The strategies, concepts, preferentially utilized chiral selectors and column technologies that have been utilized to succeed in highly efficient enantiomer separations by nonaqueous CEC are described thoroughly.

Enantiomer separations on a vancomycin stationary phase and retention mechanism of pressurized capillary electrochromatography

Several chiral drugs, promethazine, carteolol, celiprolol, and albuterol, were resolved with vancomycin as the chiral stationary phase by pressurized capillary electrochromatography (pressurized CEC) and capillary HPLC. The effects of pressure and electrical field strength on efficiency, resolution, and capacity factor in pressurized CEC were investigated. A mathematical model describing the relationship of capacity factor in pressurized CEC with voltage, pressurized flow velocity, electroosmotic mobility, and electrophoretic mobility was established, which was in good agreement with the experimental data.

High-performance liquid chromatographic enantioseparations on capillary columns containing monolithic silica modified with cellulose tris(3,5-dimethylphenylcarbamate)

Monolithic capillary columns containing native silica gel were modified with cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC) and used for enantioseparations in capillary liquid chromatography. The method adopted for in situ enantioselective modification of monolithic fused silica capillary columns by coating with CDPMC appears to be fairly simple and fast. High efficiency enantioseparations of test racemic compounds and s(everal chiral drugs were achieved in a short time. It was possible to increase the amount of chiral selector present by multiple coating of monoliths with CDMPC. The baseline enantioseparation of 2,2,2-trifluoro-1-(9-anthryl)ethanol was achieved in an analysis time less than 30 s with this capillary column. In addition, reproducible enantioseparations were obtained when the chiral selector was removed from the monolithic column by flushing it with appropriate solvent and the column recoated.

Capillary electrochromatography of methylated benzo[a]pyrene isomers. I. Effect of mobile phase tuning

It is of increasing importance that chromatographic methods be developed for the separation and identification of biological and environmentally harmful compounds such as methylated polycyclic aromatic hydrocarbons (PAH). Capillary electrochromatography (CEC) is fast becoming a useful technique for analysis of PAH, as it offers both high efficiency and superior resolution. The separation of 12 methylated benzo[a]pyrene (MBAP) isomers is a challenge due to the extreme hydrophobicity and structural similarity of these compounds. In this work, we present Part I of our ongoing study, a method for the systematic mobile phase tuning for CEC separation of the 12 MBAP isomers. The CEC experiments were conducted utilizing a CEC-octadecylsilica (ODS) stationary phase and fused-silica capillary [(75 microm I.D., 363 microm O.D.) 36.5 cm total length, 25.0 cm effective length] which was slurry pressure packed in our laboratory. Several mobile phase parameters were manipulated to provide optimum separation. These included acetonitrile (ACN) concentration, tris(hydroxymethyl) aminomethane (Tris) concentration, pH, and addition of a tertiary buffer constituent such as tetrahydrofuran (THF) and isopropanol (IPA) to ACN-aqueous buffer mixtures. Optimum CEC separation conditions were achieved using 75% (v/v) ACN-25% (v/v) 12.5 mM Tris, pH 8.0, and 30 kV at 25 degrees C. These mobile phase conditions were then utilized for Part II of our study, the CEC stationary phase optimization for the separation of 12 MBAP isomers.

Brush-type chiral stationary phase for enantioseparation of acidic compounds. Optimization of chiral capillary electrochromatographic parameters

The capillary electrochromatographic separations of three acidic enantiomers (carprofen, coumachlor and warfarin) were studied on a capillary column packed with 5 microm (3R,4S)-Whelk-O 1 chiral stationary phase. The influence of several experimental parameters (mobile phase pH, type of background electrolyte, acetonitrile ratio, temperature, applied voltage and ionic strength) on electroosmotic flow velocity, retention factor, selectivity factor, efficiency, resolution and effectiveness of chiral separation was evaluated. It was notable that the optimum resolution of the acidic enantiomers was achieved at pH 3.0 phosphate buffer, suggesting that capillary electrochromatography in the ion-suppressed mode can be applied for chiral separations of a range of acidic compounds.

Enantiomeric separation of acidic compounds of pharmaceutical interest by capillary electrochromatography employing glycopeptide antibiotic stationary phases

Enantiomeric separation of some selected acidic compounds of pharmaceutical interest belonging to the group of non-steroidal anti-inflammatory drugs were separated by capillary electrochromatography employing silica based glycopeptide antibiotic stationary phases, namely vancomycin or a teicoplanin derivatives (Hepta-Tyr). The vancomycin stationary phase allowed to achieve the chiral resolution of some racemic studied compounds only using mobile phases containing ammonium formate at a relatively low pH 2.5-3.5 and acetonitrile. Employing the teicoplanin derivative stationary phase, good enantiomeric resolution was achieved eluting with mobile phases containing sodium phosphate pH 6-acetonitrile. Enantiomers were moved to the detector because a relatively high reversed electroosmotic flow (due to the positive charge of the stationary phase) and to the electrophoretic mobility of analytes.

Enantioseparation of dipeptides by capillary electrochromatography on a teicoplanin aglycone chiral stationary phase

This work deals with investigations on the enantioseparation of glycyl-dipeptides by capillary electrochromatography (CEC) on a capillary packed with teicoplanin aglycone immobilized on 3.5 microm silica gel. The results were compared to those obtained with micro-HPLC using the same chiral stationary phase. Polar organic and reversed-phase mode were checked, whereby the latter showed better results. Out of 12 glycyldipetides investigated, all compounds showed baseline separation with Rs values up to 20. Plate numbers were in the range of 10,000-300,000/m. The choice of organic modifier was found to be crucial. While methanol increased retention time, acetonitrile reduced it. A ternary mixture of ethanol-acetonitrile-aqueous triethylamine acetate solution pH 4.1 was found to be a useful compromise, providing excellent resolution with retention times less than 25 min. Efficiency and resolution were generally found to be higher in CEC than with micro-HPLC.

Use of short-end injection capillary packed with a glycopeptide antibiotic stationary phase in electrochromatography and capillary liquid chromatography for the enantiomeric separation of hydroxy acids

A new chiral stationary phase (CSP) was prepared by reacting MDL 63,246 (Hepta-Tyr), a glycopeptide antibiotic belonging to the teicoplanin family, with 5-microm diol-silica particles. The CSP mixed with 5-microm amino silica particles (3:1) was packed into 75-microm fused-silica capillaries for only 6.6 cm and used for electrochromatographic experiments analyzing several hydroxy acid enantiomers. A reversed electroosmotic flow carried both analytes and mobile phase towards the anode in a short time (1-3 min), being baseline resolved all the studied analytes. In order to achieve the fastest enantiomeric resolution of the studied hydroxy acids, the effect of several experimental parameters such as mobile phase composition (organic modifier type and concentration, pH of the buffer and ionic strength), capillary temperature and applied voltage on enantioresolution factor, retention time, enantioselectivity were evaluated. The packed capillary column allowed the separation of mandelic acid enantiomers in less than 72 s with resolution factor Rs=2.18 applying a voltage of 30 kV and eluting with a mobile phase composed by 50 mM ammonium acetate (pH 6)-water-acetonitrile (1:4:5, v/v). The CSP was also tested in the capillary liquid chromatography mode resolving all the studied enantiomers applying 12 bar pressure to the mobile phase [50 mM ammonium acetate (pH 6)-water-methanol-acetonitrile, 1:4:2:3, v/v)], however, relatively long analysis times were observed (12-20 min).

Direct chiral separation of unnatural amino acids by high-performance liquid chromatography on a ristocetin a-bonded stationary phase

Direct high-performance liquid chromatographic chiral separation of numerous underivatized unnatural amino acids on a ristocetin A-bonded chiral stationary phase used in the reversed-phase and in the polar organic chromatographic modes is reported. The effects of different parameters such as mobile phase composition, temperature, and the structure of the analytes on the selectivity in both chromatographic modes are discussed. By variation of the parameters, the separation of the stereoisomers was optimized and, as a result, baseline resolution was achieved in most cases.

Molecular interactions of glycopeptide antibiotics investigated by affinity capillary electrophoresis and bioaffinity electrospray ionization-mass spectrometry

Many analytical approaches are available to evaluate (bio)molecular interactions, all of which have their particular advantages and disadvantages. In recent years, two relatively new techniques have emerged that may be used by the bioanalytical community to evaluate such interactions, namely affinity capillary electrophoresis (ACE) and bioaffinity electrospray ionization-mass spectrometry (ESI-MS). In this paper, we describe and evaluate the use of both these techniques for the investigation of the interactions of glycopeptide antibiotics with peptides that mimic the bacterial cell wall binding site. We focus particularly on the effect of the sugar moieties attached to the antibiotic peptide backbone and on the noncovalent dimerization of these glycopeptide antibiotics.