Chiral separations by capillary electrophoresis: present state of the art

Mechanism of signal suppression by anionic surfactants in capillary electrophoresis-electrospray ionization mass spectrometry

Micellar-mediated capillary electrophoresis (CE) is used for a wide variety of applications, including the separation of pharmaceuticals, environmental contaminants, illicit drugs, DNA fragments, and many other biological samples. The electrospray ionization interface is one of the most common CE-MS interfaces. Coupling micellar-mediated CE separations with MS detection combines two very powerful, widely applicable analytical techniques. Some types of surfactants strongly interfere with electrospray ionization mass spectrometric (ESI-MS) detection of analytes, and in many cases the ESI-MS analyte signals are completely quenched. Only a few reports have appeared that describe the ESI-MS detection of analytes in the presence of surfactants; however, the exact mechanism of ionization suppression has not yet been addressed. In this work, a modified aerosol ionic redistribution (AIR) model is presented that qualitatively explains the results of previous studies, including those using "polymeric surfactants". Analyte ionization suppression by surfactants appears to be caused by Coulombic interaction between oppositely charged solute and surfactant ions in the ESI-produced offspring droplets. It appears that the ability of surfactants to quench electrospray ionization is directly related to the surface activity and the charge of the surfactant. Also, highly surface active components tend to be enriched in ESI-produced offspring droplets. Analyte ion signals can be detected under conditions that lower the surface concentration of oppositely charged surfactant ions in aerosol droplets. The mechanistic information outlined here may be used to design micellar-mediated CE separations that allow detection of analyte ions by ESI-MS.

Chiral separations by CE employing CDs

This paper summarizes the history of chiral separations done by using electromigration methods with CDs. Several enantioresolution mechanisms and a wide number of chiral selectors have been applied to the separation of optical isomers by CE. Among them inclusion-complexation with CDs or their derivatives played a very important role in CE. Since the beginning our group was involved in studying method optimization for enantiomer resolution by using these chiral selectors. One of our publications was the basis for further development in the field, at least for us. New chiral selectors, development of theory, new methodological approaches and a wide number of practical applications are the main results achieved in the last almost 25 years using CE as an enantioseparative technique.

Chiral separation of acidic drug components by open tubular electrochromatography using avidin immobilized capillaries

Avidin was immobilized covalently onto the inner surface of fused silica capillaries as a stationary phase in an open tubular capillary electrochromatography (OT-CEC) for chiral separations. The modification was attained by a combination of glutaraldehyde with both an amino-silylated fused silica surface and avidin using a Schiff base formation reaction. This method couples the advantage of high efficiency and small consumption of a chiral selector with the possibility of UV detection without limitations of protein absorption. In addition, the prepared capillary was stable for 50 days with over 100 runs. To evaluate the electrochromatographic performance of the prepared capillaries, the chiral separation of abscisic acid and arylpropionic acids were investigated. Effects of the modification condition of protein, pH of running buffer, and an organic modifier on the enantioseparation were also investigated. In addition, the avidin immobilized capillary was employed for the selective chiral analysis with an electrospray ionization mass spectrometry (ESI-MS) detection scheme.

Fundamental aspects of chiral separations by capillary electrophoresis

A review is presented that surveys the basic theory of direct separation of enantiomers by capillary electrophoretic (CE) techniques. These separations are based on the formation of diastereomeric complexes between the enantiomeric analytes and a chiral selector added to the electrolyte solution. The review covers a comprehensive treatment of the equations needed for optimization of selectivity coefficients, resolution and analysis time in the zone electrophoretic mode. In this context, it takes into account combined equilibria of complexation and protonation/deprotonation as well as complexation and paritition into micelles. On the basis of these equations, the benefits of charged selectors and the optimization potential inherent to pH tuning can be documented. In addition, the review deals with some basic aspects of chiral isoelectric focusing and briefly discusses indirect enantioseparation. In a subsequent section a survey is given on particularfeatures of the various types of chiral selectors. Finally, the recent developments in preparative enantioseparation in continuous free-flow system and by use of isoelectric membranes are discussed.

Development and evaluation of a linear regression method for the prediction of maximal chiral separation of basic drug racemates by cyclodextrin-mediated capillary zone electrophoresis

An important step in method development of chiral separations with neutral cyclodextrins (CDs) as chiral selectors is the estimation of the CD concentration that gives the highest degree of separation. From the equation [S]opt=1/(K1K2)(1/2) this optimal CD concentration can be calculated if any knowledge is available about the binding constants K1 and K2 of both enantiomer complexes. These values can be obtained by measuring the effective velocities of each enantiomer as a function of the selector concentration and fitting these profiles by non-linear least-square regression. An alternative approach has been developed which makes it possible to predict the optimal CD concentration from a few experiments performed at low CD concentrations. The model is developed using some antimycotic imidazole derivatives (econazole, miconazole and isoconazole) as test substances and hydroxypropyl-beta-CD as chiral selector. The results obtained by this method are in good agreement with those from non-linear least-square regression.

Recent progress in chiral separation principles in capillary electrophoresis

This review summarizes recent developments in the field of chiral separations by electromigration techniques including capillary zone electrophoresis (CZE), capillary gel electrophoresis (CGE), isotachophoresis (ITP), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC). This overview focuses on the development of new chiral selectors and the introduction of new techniques rather than applications of already established selectors and methods. The mechanisms of the different chiral separation principles are discussed.

Determination of enantiomeric excess by capillary electrophoresis

Capillary electrophoresis (CE) is becoming an established method for the determination of chiral trace impurities. This paper provides an overview of the state of the art of CE for such determinations. Detection limits of 0.1% impurity is widely accepted as a minimum requirement for chiral trace impurity determinations. This can be relatively easily achieved with CE. However, determination of lower concentrations requires careful optimization of the separation system. Four factors that are of particular significance for trace enantiomeric determinations: resolution, limit of detection, linear range and type of detection, are discussed. Further, the advantages and disadvantages of derivatization in this context are treated as well as the separation approach, ie., direct chiral separation or separation after the formation of diastereomers. It is concluded that the limit of impurity detection can be about 0.05% when UV detection is employed. Using laser-induced fluorescence detection, a quantitative determination at the 0.005% level is often possible.

Enantioselective determination by capillary electrophoresis with cyclodextrins as chiral selectors

This review surveys the separation of enantiomers by capillary electrophoresis using cyclodextrins as chiral selector. Cyclodextrins or their derivatives have been widely employed for the direct chiral resolution of a wide number of enantiomers, mainly of pharmaceutical interest, selected examples are reported in the tables. For method optimisation, several parameters influencing the enantioresolution, e.g., cyclodextrin type and concentration, buffer pH and composition, presence of organic solvents or complexing additives in the buffer were considered and discussed. Finally, selected applications to real samples such as pharmaceutical formulations, biological and medical samples are also discussed.

Chiral separations in capillary electrophoresis

The marked increase in the number of communications on the utilization of electrophoresis for practical chiral separations within the last three years is the most evident, and the most important fact. It reveals that the basic period of intensive research in the field is finished. The search for chiral selectors discriminating racemates in a reasonably analytical manner and the study of both the mechanism and physicochemical aspects of the chiral discrimination process were the main features of that period. Here, we review the state of the art in the field and state the references of the related literature up to the end of 1998.

Chirality and drugs used in psychiatry: nice to know or need to know?

1. Many drugs used to treat psychiatric disorders contain a chiral center or a center of unsaturation and are marketed as a mixture of the resultant enantiomers or geometric isomers, respectively. These enantiomers or geometric isomers may differ markedly with regard to their pharmacodynamic and/or pharmacokinetic properties. 2. Examples of the effects of chiral centers or geometric centers on such properties are given for drugs from the following classes: antidepressants (tricyclics, selective serotonin reuptake inhibitors, monoamine oxidase inhibitors, viloxazine, bupropion, trazodone, mianserin, venlaflaxine); benzodiazepines, zoplicone, and antipsychotics. 3. As described in this review, there are several notable examples of psychiatric drugs currently available where the individual enantiomers or geometric isomers differ considerably with regard to factors such as effects on amine transport systems, interactions with receptors and metabolizing enzymes, and clearance rates from the body. Indeed, relatively recent developments in analytical and preparative resolution of racemic and geometric drug mixtures and increased interest in developing new drugs which interact with specific targets, which have been described in detail at the molecular level, have resulted in increased emphasis on stereochemistry in drug development.

Method development strategies for the enantioseparation of drugs by capillary electrophoresis using cyclodextrins as chiral additives

General strategies for the development of capillary electrophoretic methods for the enantiomeric separation of basic, acidic or neutral drugs were developed. For all kinds of compounds, the use of a buffer made of 100 mM phosphoric acid adjusted to pH 3 with triethanolamine and containing anionic and/or uncharged cyclodextrin (CD) derivatives as chiral selectors was recommended. Two different optimization schemes depending on the acidic or basic character of the analytes, were elaborated. For most basic compounds present in cationic form at pH 3, enantiomeric separation could be achieved in the normal polarity mode. Different beta-cyclodextrin derivatives were first tested at a given concentration. Five derivatives were found to be particularly useful for enantioseparations in capillary electrophoresis (CE): the anionic carboxymethyl-beta-CD (CMCD) and sulfobutyl-beta-CD (SBCD) and the neutral dimethyl-beta-CD (DMCD), trimethyl-beta-CD (TMCD) and hydroxypropyl-beta-CD (HPCD). After selection of the most suitable CD, its concentration was optimized with respect to chiral resolution. If necessary, a further improvement in resolution could often be obtained for the enantiomers of cationic solutes by increasing the buffer pH from 3 to 5 using CMCD as chiral additive. Another possible alternative for enhancement in chiral resolution was the addition of metharlol or cyclohexanol to the buffer. For acidic drugs, essentially present in uncharged form at pH 3, and for neutral solutes, anionic CD derivatives such as SBCD or CMCD were first tested at a given concentration in the reversed polarity mode. Dual systems, based on the simultaneous addition of a charged CD (SBCD or CMCD) and a neutral CD (TMCD or DMCD), could then be investigated for resolution improvement. After optimization of the CD concentrations, the use of dual systems with CMCD at pH 5 could also be tested if necessary, especially for very weak acidic and neutral drugs. By applying these optimization strategies, 48 of the 50 drugs examined as model compounds could be fully enantioseparated by CE in short analysis times (usually less than 10 min).

Separation of chiral compounds by capillary electrophoresis

This review presents the different chiral selectors used in capillary electrophoresis (CE) for the separation of enantiomers. The use of charged cyclodextrins, crown ethers, polysaccharides, proteins, natural and synthetic micelles, macrocyclic antibiotics and ergot alkaloids is discussed in detail. Neutral native and derivatized cyclodextrins are not treated because several review articles have already been published on this topic. Recent developments like the application of two chiral selectors in the same background electrolyte are highlighted.

Advances in capillary electrophoresis

This review summarizes the advancement in operational modes and selected applications of the title technique over the past five years. Regarding operational modes particular emphasis is put upon increasing selectivity and resolution, hyphenation of capillary electrophoresis with techniques based on other than electromigration principles, the so-called chip technology and new ways of detection. In applications selected examples of chiral separation and separation of biopolymers (proteins, nucleic acids) are emphasized. It is demonstrated that capillary electrophoresis represents a complementary technique to high-performance column chromatography and in a number of cases it offers better separations than standard chromatographic procedures.

The Principle of Maximum Chiral Discrimination: Chiral Recognition in Permethyl-beta-cyclodextrin

Five guest molecules, isomenthone, pulegone, 1-fluoro-1-phenylethane, 1-phenylethanol, and 2-methylbutanoic acid, binding to permethyl-beta-cyclodextrin, a chiral host molecule, have been simulated by molecular dynamics techniques. From the simulations we find the preferred binding site to be the interior of the macrocyclic cavity. A new technique was used for locating the host's most enantiodiscriminating domain, which was also found to be inside the macrocyclic cavity. It is concluded that this particular host molecule displays its enhanced chiral discriminating capacity because of this spatial coincidence. Also evaluated in this paper are the types and magnitudes of intermolecular forces responsible for diastereomeric complexation and chiral discrimination; in both cases the short-range dispersion forces dominate. This study illustrates the "principle of maximum chiral recognition", the idea that maximum chiral recognition can be achieved by maintaining a spatial congruence between the host's domain of greatest enantiodifferentiation with the guest's preferred binding site.

Controlling enantioselectivity in chiral capillary electrophoresis with inclusion-complexation

The separation of chiral compounds is of key importance in different fields of application, e.g., pharmaceutical, industrial, forensic, biological, clinical etc. Capillary electrophoresis (CE) is a powerful analytical method applied in chiral analysis and inclusion-complexation is one of the most frequently used mechanism to improve the selectivity of the enantiomeric separation. Cyclodextrins and their derivatives or modified crown-ethers have been successfully applied in CE for the enantiomeric separation of a wide number of analytes. This review surveys the separation of enantiomers by CE when chiral selectors, forming inclusion-complexation, are used. The control of enantioselectivity can be done carefully by considering several experimental parameters such as chiral selector type and concentration, pH, ionic strength and concentration of the background electrolyte, electroosmotic flow, organic modifier etc. The review presents a list of the latest separation of enantiomers by CE where inclusion-complexation plays a key role in the stereoselective separation mechanism.

Pre-, on- and post-column derivatization in capillary electrophoresis

This survey gives a short overview of the various reagents and procedures that can be used for pre-, post- and on-column derivatization in capillary electrophoresis. First there is an introduction about capillary electrophoresis as an analytical technique; this is followed by a discussion of the pros and cons of the various modes of derivatization and a comparison with liquid chromatography. In the following paragraphs the reagents for a number of functional groups are discussed. The emphasis is on derivatization of the amino group. Most of the information on the reagents and derivatization procedures is listed in tables together with information on the detection mode, analytes, sensitivity and samples. In addition to the amino group, information is given on labeling of aldehyde, keto, carboxyl, hydroxyl and sulfhydryl groups.

Chiral separation of primary amino compounds using a non-chiral crown ether with beta-cyclodextrin by capillary electrophoresis

A non-chiral crown ether (18-crown-6) along with beta-cyclodextrin (beta-CD) was used to achieve enantioselective separations of primary amino compounds in capillary electrophoresis. In this new method, the amino group of these compounds is protonated in a low pH separation buffer and forms a selective host-guest complex with the crown ether (amino compound+18-crown-6). The hydrophobic portion of the host-guest complex is then incorporated into the cavity of the beta-cyclodextrin. The amino compound is sandwiched between the crown ether and the cyclodextrin (18-crown-6+amino compound+beta-CD) and thus determines or enhances the enantioselective recognition. It is postulated that the formation of this sandwich results in a more selective chiral interaction between the molecule and beta-cyclodextrin. The chiral recognition is dependent upon the formation of this sandwich complex. This method has been used to achieve enantioselectivity of primary amino compounds with a wide variety of substitutions.

Resolution of acylated dipeptide stereoisomers by capillary electrophoresis using sulfobutylether derivatized beta-cyclodextrin

The separation of enantiomerically and diastereomerically related stereoisomers of acylated Asp-Phe dipeptides was explored using capillary electrophoresis (CE). This series of dipeptides included the alpha-L,L parent compound and the three other potential Asp containing stereoisomers (alpha-D,D, alpha-L,D, and alpha-D,L), as well the four possible isoAsp containing stereoisomers (beta-L,L, beta-D,D, beta-L,D and beta-D,L). The separation of these substances was explored using both neutral and charged cyclodextrins as the stereoisomer selector added to the running electrolyte. The major experimental parameters investigated included pH, the cyclodextrin type, and the cyclodextrin concentration. Due to differences in the pKa values of the carboxylic acid groups, adjustment of the separation buffer to between pH 3.0 and 4.0 provided for sufficient electrophoretic mobility differences to result in excellent separations of the diastereomerically related peptides in this pH region. The resolution of the enantiomerically related peptide stereoisomers was accomplished using low concentrations (1 mM) of the anionic cyclodextrin derivative, sulfobutylether-beta-cyclodextrin (SBE-beta-CD). This negatively charged cyclodextrin was found to be superior for the resolution of the enantiomerically related peptides as compared to native beta-cyclodextrin or the neutral derivatives, dimethyl beta-cyclodextrin and hydroxypropyl beta-cyclodextrin. An alternative approach using anionic or neutral surfactants in conjunction with the SBE-beta-CDs was also explored and found to be successful but problematic.

Three approaches to enantiomer separation of beta-adrenergic antagonists by capillary electrochromatography

Three different capillary electrochromatographic methods for the enantiomer separation of beta-adrenergic antagonists (acebutolol, alprenolol, atenolol, metoprolol, pindolol, prenalterol, and propranolol) were applied using different cyclodextrins (beta-cyclodextrin, carboxymethyl-beta-cyclodextrin and dimethyl-beta-cyclodextrin) added to the electrolyte, a cross-linked protein-gel (cellobiohydrolase I) and a molecularly imprinted ((R)-enantiomer of propranolol) superporous polymer as chiral selectors. Through use of these different separation strategies, all the beta-adrenergic antagonists studied could be resolved into their enantiomers, although the three methods were carried out without extensive optimization. The protein and molecularly imprinted phases gave the highest selectivities whereas employing cyclodextrins resulted in the highest separation efficiency. Proteins and cyclodextrins are primarily natural products, albeit the cyclodextrins can be derivatized. In contrast, the molecularly imprinted chiral stationary phase can be highly customized when produced.

Capillary electrophoresis in pharmaceutical analysis

Capillary electrophoresis (CE) is a separation technique particularly suited to the analysis of pharmaceutical compounds. This review offers a detailed discussion of the four common modes of detection coupled to CE-UV absorption, fluorescence, electrochemical, and mass spectrometry-and gives examples of the use of these methods in pharmaceutical analyses. Sample preparation and pretreatment techniques used for CE separations are described, as well as methods of preconcentration including hydrophobic retention, affinity concentration, sample stacking, and isotachophoresis. The use of affinity CE, chiral CE, and capillary gel electrophoresis for analysis of pharmaceuticals is covered in detail, and recent advances in capillary electrochromatography and CE on a chip are also discussed.

Cyclodextrins and enantiomeric separations of drugs by liquid chromatography and capillary electrophoresis: basic principles and new developments

Investigation of individual drug enantiomers is required in pharmacokinetic and pharmacodynamic studies of drugs with a chiral centre. Cyclodextrins (CDs) are extensively used in high-performance liquid chromatography as stationary phases bonded to a solid support or as mobile phase additives in HPLC and capillary electrophoresis (CE) for the separation of chiral compounds. We describe here the basis for the liquid chromatographic and capillary electrophoretic resolution of drug enantiomers and the factors affecting their enantiomeric separation. This review covers the use of CDs and some of their derivatives in studies of compounds of pharmacological interest.

Stereoselective interaction of drug enantiomers with human serum transferrin in capillary zone electrophoresis

Stereoselective interaction of drugs with human serum transferrin in capillary zone electrophoresis is described. The substances passed a pseudo-stationary protein zone applied in a coated capillary and the possible chiral separation of the optical isomers was followed. Drugs with different structures were screened and the enantiomers of bupivacaine, propranolol and promethazine as well as the diastereomers of labetalol were resolved. Racemic mixtures of atenolol and pindolol enantiomers could not be resolved under these conditions.

A practical procedure for the determination of association constants of the analyte-chiral selector equilibria by capillary zone electrophoresis

A practical procedure is proposed for the determination of association constants and mobility of the associate of a solute with a chiral selector in chiral separations by capillary zone electrophoresis. The procedure is based on the measurement of the effective mobility of a solute at zero and two different nonzero concentrations of the chiral selector. Simple explicit formulas have been derived in order to calculate the required data. The essence of the procedure is that all mobility data are adjusted to the background electrolyte (BGE) without chiral selector, serving as the viscosity reference. A simple procedure is described for measuring the viscosity of the operational electrolytes directly with the commercial capillary electrophoresis instrumentation, and Walden's rule has been utilized for adjusting the experimental mobility data to constant reference viscosity. The use of the procedure is exemplified by a separation of D,L-tryptophan in BGE containing alpha-cyclodextrin as chiral selector.

Characterization of the stereoselective metabolism of methadone and its primary metabolite via cyclodextrin capillary electrophoretic determination of their urinary enantiomers

Using capillary zone electrophoresis with a phosphate buffer at pH 3, containing 4.3 mM hydroxypropyl-beta-cyclodextrin as chiral selector, the simultaneous separation of the enantiomers of methadone and its primary metabolite, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP), is reported. After solid-phase extraction and analysis of the reconstituted extracts in a 60 cm fused-silica capillary of 50 microns ID within about 25 min, the mean S/R enantiomeric ratio of methadone in the urines of eight patients undergoing methadone therapy is shown to be 0.653 (range: 0.502-0.842). The mean enantiomeric ratio for the metabolite is 0.630 (range: 0.517-0.729). These data document the stereoselectivity of the methadone metabolism. Finally, the EDDP/ methadone ratio is shown to vary between 0.357 and 2.917 with a mean value of 1.731. The capillary electrophoretic assay described is simple, inexpensive and neither requires any sample derivatization, nor large amounts of organic solvents or expensive separation columns.

Resolution improvement by use of carboxymethyl-beta-cyclodextrin as chiral additive for the enantiomeric separation of basic drugs by capillary electrophoresis

Three beta-cyclodextrin derivatives--carboxymethyl-, dimethyl- and hydroxypropyl-beta-cyclodextrin--were tested as chiral selectors for the enantioseparation of seven basic drugs in free solution capillary electrophoresis, using buffers made of 100 mM phosphoric acid adjusted to pH 3.0 with triethanolamine in fused silica capillaries thermostatted at 15 degrees C. The best results with respect to chiral resolution were obtained with carboxymethyl-beta-cyclodextrin (CMCD): the enantiomers of all compounds examined were completely resolved with this beta-cyclodextrin derivative. The influence of the CMCD concentration on the migration times, the apparent electrophoretic mobility difference and the resolution of the drug enantiomers was investigated thoroughly. Particularly impressive resolution values, up to 23.7, were obtained for several compounds in these capillary electrophoretic systems, using CMCD in the 5-15 mM concentration range.

Identification of chiral drug isomers by capillary electrophoresis

Separation of optical isomers of compounds of pharmaceutical interest by capillary electrophoretic techniques is reviewed. The direct and indirect separation method, as well as the main resolution mechanisms and the parameters influencing the stereoselectivity are discussed considering capillary zone electrophoresis, micellar electrokinetic chromatography, isotachophoresis and electrochromatography. Several chiral selectors have been successfully used in CE for chiral separation, including cyclodextrins and their derivatives, modified crown-ethers, proteins, antibiotics, linear saccharides and chiral surfactants. Only applications in the pharmaceutical field with the most important experimental conditions are summarised in the Tables reported in this paper. The chiral analyses of drugs in real samples like biological fluids or pharmaceutical formulations are also reported.

Control of enantiomer migration order in capillary electrophoresis separations using sulfobutyl ether beta-cyclodextrin

The control of enantiomer migration order in capillary electrophoresis (CE) by use of sulfobutyl ether beta-cyclodextrin (SBE-beta-CD) was investigated. At high pH, electroosmotic flow (EOF) dominated and the enantiomer most strongly associated with the anionic cyclodextrin was detected last. At low pH (and reversed polarity), EOF was minimal and SBE-beta-CD functioned as a carrier. Under such conditions, the enantiomer migration order for a neutral chiral compound was reversed. Factors involved in optimization (cyclodextrin and organic modifier concentration) were studied. The impact of migration order on quantitation of low levels (below 1%) of one enantiomer in the presence of the other was also investigated. The precision of peak area ratios (minor/major enantiomer) was evaluated for samples of both enantiomers run by each method. The migration of minor before major enantiomer yielded better quantitation precision in each case.

Dynamic control to improve the separation performance in capillary electrophoresis

Dynamic control in capillary electrophoresis (CE) is able to provide better resolution, shorter separation time, reduced band broadening and better reproducibility for the separations of organic and inorganic ions and large molecules such as proteins and DNA. This article provides an overview of dynamic control in CE by several techniques: pH, temperature, external electric field, and field amplification. These techniques take advantage of the changes in electroosmotic flow coefficient, electrophoretic mobilities of the analytes due to changes in equilibrium, viscosity of the buffer solution, and/or electric field strength. The basic theories and applications to the improvement of the separation performance for standard samples in CE by using these dynamic control techniques are presented. Finally, the advantages and shortcomings of these techniques and the future trends of dynamic control for the separations of real samples are discussed.