Selectivity in capillary electrokinetic separations

Strategies for capillary electrophoresis: Method development and validation for pharmaceutical and biological applications-Updated and completely revised edition

This review is in support of the development of selective, precise, fast, and validated capillary electrophoresis (CE) methods. It follows up a similar article from 1998, Wätzig H, Degenhardt M, Kunkel A. "Strategies for capillary electrophoresis: method development and validation for pharmaceutical and biological applications," pointing out which fundamentals are still valid and at the same time showing the enormous achievements in the last 25 years. The structures of both reviews are widely similar, in order to facilitate their simultaneous use. Focusing on pharmaceutical and biological applications, the successful use of CE is now demonstrated by more than 600 carefully selected references. Many of those are recent reviews; therefore, a significant overview about the field is provided. There are extra sections about sample pretreatment related to CE and microchip CE, and a completely revised section about method development for protein analytes and biomolecules in general. The general strategies for method development are summed up with regard to selectivity, efficiency, precision, analysis time, limit of detection, sample pretreatment requirements, and validation.

Selection of background electrolyte for CZE analysis by a chemometric approach

In the first part of this study a chemometric approach to choose a suitable background electrolyte for CZE analysis was introduced. Two hundred and twenty-two possible electrolytes were previously characterized by means of the descriptors pH, conductivity, ionic strength and relative viscosity and the approach was applied to the separation of a mixture of acidic drugs. In this second part, another application concerning the analysis of basic drugs is presented. The test mixture was made of eight beta-blocker drugs. According to the basic nature of the analytes, the original data set was reduced to a new subset of 117 objects with pH less than or equal to 7, and after computing principal components the new set of objects was represented in a two-dimensional space. Ten objects to be tested in CZE, capable of covering homogeneously the principal component space, were selected by means of Kennard-Stone algorithm. The data set was further reduced around the BGEs which gave the best results, and a new set of electrolytes to be tested was selected. Using pH 4 citrate buffer, an electropherogram with baseline resolution was obtained in 10 min. A Doehlert design was run to further reduce analysis time, and applying the optimized conditions (voltage, 23 kV; temperature, 26 degrees C) the separation was obtained in about 7 min.

Selection of background electrolyte for CZE analysis by a chemometric approach

This paper is the first part of the presentation of a chemometric approach for the rapid selection of a suitable background electrolyte (BGE) in CZE analysis of small drug molecules. The strategy is based on principal component analysis and experimental design. In this first section, the approach is applied to the analysis of a mixture of six arylpropionic anti-inflammatory drugs. Initially, 222 possible aqueous background electrolytes (objects) were characterized using as descriptors pH, conductivity, ionic strength and relative viscosity. In order to allow the dissociation of the acidic analytes, this original data set was reduced to 154 background electrolytes with pH values higher than or equal to 5. Principal component analysis made it possible to graphically represent the new set of objects, described by the four variables, in a two-dimensional space. Among these electrolytes, Kennard-Stone algorithm selected ten objects to be tested by CZE, covering homogeneously principal component space. CZE analyses were carried out with the selected electrolytes, and 0.1 M borax was identified as the most suitable one for the specified application. Finally, the characteristics of the analysis were finely tuned by means of a response surface study, which allowed the best conditions to be determined: borax concentration, 0.09 M; methanol, 6% (v/v); temperature, 24 degrees C, voltage, 20 kV. Applying these conditions, a baseline resolution among the six compounds was obtained in less than 10 min.

Molecular imprinting science and technology: a survey of the literature for the years up to and including 2003

Over 1450 references to original papers, reviews and monographs have herein been collected to document the development of molecular imprinting science and technology from the serendipitous discovery of Polyakov in 1931 to recent attempts to implement and understand the principles underlying the technique and its use in a range of application areas. In the presentation of the assembled references, a section presenting reviews and monographs covering the area is followed by papers dealing with fundamental aspects of molecular imprinting and the development of novel polymer formats. Thereafter, literature describing attempts to apply these polymeric materials to a range of application areas is presented.

Mixed-mode electrokinetic chromatography of aromatic bases with two pseudo-stationary phases and pH control

The electrokinetic chromatographic (EKC) separation of a series of aromatic bases was achieved utilising an electrolyte system comprising an anionic soluble polymer (polyvinylsulfonic acid, PVS) and a neutral beta-cyclodextrin (beta-CD) as pseudo-stationary phases. The separation mechanism was based on a combination of electrophoresis, ion-exchange interactions with PVS, and hydrophobic interactions with beta-CD. The extent of each chromatographic interaction was independently variable, allowing for control of the separation selectivity of the system. The ion-exchange and the hydrophobic interactions could be varied by changing the PVS and the beta-CD concentrations, respectively. Additionally, mobilities of the bases could be controlled by varying pH, due to their large range of pKa values. The separation system was very robust with reproducibility of migration times being <2% RSD. The two-dimensional parameter space defined by the two variables, [beta-CD] and %PVS, was modelled using a physical model derived from first principles. This model gave very good correlation between predicted and observed mobilities (r2=0.999) for the 13 aromatic bases and parameters derived from the model agreed with the expected ion-exchange and hydrophobic character of each analyte. The complexity of the mathematical model was increased to include pH and this three-dimensional system was modelled successfully using an artificial neural network (ANN). Optimisation of both the two-dimensional and three-dimensional systems was achieved using the normalised resolution product and minimum resolution criteria. An example of using the ANN to predict conditions needed to obtain a separation with a desired migration order between two of the analytes is also shown.

Spherical molecularly imprinted polymer particles: a promising tool for molecular recognition in capillary electrokinetic separations

Spherical molecularly imprinted polymer particles obtained via precipitation polymerization, were introduced as a pseudostationary phase in capillary electrophoresis (CE) to study molecular recognition. Analyses were performed via a partial filling technique using (+)-ephedrine-imprinted microspheres (100-200 nm) which were polymerized from methacrylic acid and 1,1,1-Tris(hydroxymethyl)propanetrimethacrylate using acetonitrile as the solvent. The influence of pH and the modifier content on the separation was investigated. A 0.1% w/v suspension in an aqueous 10 mM phosphate buffer (pH 2.5 with 40% acetonitrile) was hydrodynamically injected into the CE system (80% of the effective capillary length) and led to full baseline separation of racemic ephedrine within 10 min.

Electrokinetic chromatography utilizing two pseudostationary phases providing ion-exchange and hydrophobic interactions

The electrokinetic chromatographic separation of a series of inorganic and organic anions was achieved by utilizing an electrolyte system comprising a cationic soluble polymer (poly(diallydimethylammonium chloride, PDDAC) and a neutral beta-cyclodextrin (beta-CD) as pseudostationary phases. The separation mechanism was a combination of electrophoresis, ion-exchange (IE) interactions with PDDAC, and hydrophobic interactions with beta-CD. The extent of each chromatographic interaction was independently variable, allowing for control of the separation selectivity of the system. IE interactions could be varied by changing either the PDDAC concentration or the concentration of a competing ion (e.g., chloride) in the BGE, while the hydrophobic interactions could be varied by changing the concentration of beta-CD. The separation system was very robust, with the reproducibility of the migration times being <0.7% RSD. A mathematical model that predicted the mobilities of analytes under varying experimental conditions was derived and was shown to give good correlation (r2 = 0.9804) between predicted and experimental migration times. Parameters derived from the model were in good agreement with the ion-exchange and hydrophobic characteristics of the analytes. The model was also applied successfully to the optimization of conditions for the separation of a mixture of analytes or for conditions under which particular analytes migrated in a desired order. That is, the opportunity to tune the separation selectivity has been demonstrated.

Selected fundamental aspects of chiral electromigration techniques and their application to pharmaceutical and biomedical analysis

While capillary electrophoresis has been established as a major enantioseparation technique within the last decade, the potential of capillary electrochromatography is still studied extensively. This review summarizes recent applications of electromigration techniques with regard to the enantioseparation of chiral drugs. The first part discusses the general aspects of migration models and the enantiomer migration order. The application of capillary electrophoresis to chiral pharmaceutical analysis considers recent literature on: (1) chiral resolutions of non-racemic mixtures of enantiomers for the development of assays and the determination of the stereochemical purity of the drugs, (2) chiral separations of compounds in pharmaceutical formulations and products, and (3) enantioseparations of drugs in biological samples. A shorter section devoted to chiral electrochromatography discusses some fundamental aspects as well as the application to the chiral analysis of drugs including bioanalysis.

Enantioselective analysis of albendazole sulfoxide in cerebrospinal fluid by capillary electrophoresis

Albendazole (ABZ) is a benzimidazole anthelmintic drug used in the treatment of neurocysticercosis. After oral administration, ABZ is rapidly oxidized to albendazole sulfoxide (ABZSO), which has an asymmetric sulfur center, and later to albendazole sulfone (ABZSO2). ABZSO is the active metabolite responsible for the therapeutic effect of the drug. Previous studies have demonstrated pharmacokinetic differences between the two enantiomers, with the predominance of (+)-ABZSO in human biological fluids. This article describes for the first time the enantioselective analysis of ABZSO in cerebrospinal fluid (CSF) using capillary electrophoresis. The samples were prepared by liquid-liquid extraction using chloroform:isopropanol (8:2 v/v). The resolution of ABZSO enantiomers was obtained with a fused-silica capillary (60 cm x 75 microm ID) using 20 mmol/L Tris, pH 7.0, with 3.0% w/w sulfated beta-cyclodextrin as running buffer. The coefficient of variations and % relative error obtained for both within-day and between-days assays were lower than 15%. The method was linear over the concentration range of 100 to 2,500 ng/mL for each enantiomer, indicating that it is suitable for the analysis of ABZSO enantiomers in CSF from patients medicated with ABZ.

Separation and selectivity of benzophenones in micellar electrokinetic chromatography using sodium dodecyl sulfate micelles or sodium cholate modified mixed micelles

The separation and selectivity of nine benzophenones in micellar electrokinetic chromatography (MEKC) using sodium dodecyl sulfate (SDS) micelles or sodium cholate (SC) modified mixed micelles were investigated in the pH range 6.5-8.0. The results indicate that the combined effects of buffer pH and SC concentration can greatly affect the separation and selectivity of benzophenones, particularly for benzophenones possessing a hydroxyl substituent at the 4-position of the aromatic ring with respect to the carbonyl moiety when using SDS-SC mixed micelles. Better separability can be obtained with SDS-SC mixed micelles than with SDS micelles. Complete separation of nine benzophenones in MEKC can be achieved with an appropriate choice of buffer pH and the concentration of SDS micelles or SC modified mixed micelles. The dependence of the migration order of those benzophenones based on their structures and solute-micelle interactions is discussed.

Capillary-electrochromatographic separations with copolymeric reversed-stationary phase and ion-exchanger-packed columns

A macroporous, spherical, 7 microm, polystyrene-divinylbenzene (PS-DVB), reversed-phase adsorbent (PRP-1) was evaluated as a stationary phase for the capillary electrochromatographic (CEC) separation of neutral, acidic, and basic analytes of pharmaceutical interest. Electroosmotic flow (EOF) for a PRP-1 packed capillary is nearly constant over the pH 2 to 10 range and is higher than for a silica-based C18 packed capillary on the acidic side. EOF increases with an increase in buffer acetonitrile concentration or as applied potential increases. As analyte hydrophobicity increases, analyte retention and migration time increases. Increasing buffer acetonitrile concentration reduces analyte partitioning with the PS-DVB stationary phase and analyte retention and migration time decreases. When exchange sites are present on the PS-DVB copolymer, EOF (EOF is reversed for the anion-exchanger) increases as the exchange capacity increases. An increased exchange capacity also reduces partitioning of the analyte with the PS-DVB matrix and analyte retention and migration time decrease. Because of excellent stability in an acid environment, the PRP-1 packed capillary can be used in strong acid buffer solution and weak acid and base analytes depending on pKa values can be separated as neutral species and cations, respectively. CEC separations on a PRP-1 capillary of neutral steroids, weak base pharmaceuticals (separation as cations), purines and pyrimidines (as cations), fatty acids (as undissociated species), and sulfa derivatives (as cations) are described. Efficiency for the PRP-1 packed capillary for acetone or thiourea as the analyte is about 6 x 10(4) plates m(-1).

Enantioseparation of ofloxacin in urine by capillary electrokinetic chromatography using charged cyclodextrins as chiral selectors and assessment of enantioconversion

A method was developed for the enantioseparation of ofloxacin, a member of the fluoroquinolones, using an anionic cyclodextrin-derivative with or without combination with a neutral cyclodextrin-derivative, as the chiral selector (s) in an electrokinetic chromatography system. The best results were obtained with 0.35 mM sulfated beta-cyclodextrin dissolved in a 50 mM phosphate buffer, pH 2.5, and at 15 degrees C. Under these conditions, a resolution of 2 was readily achieved. Furthermore, under adequate separation conditions, studies were performed in order to assess possible in vitro and in vivo enantioconversion of levofloxacin. The current method allows detection of 2 microg R-(+)-ofloxacine/mL diluted urine without the necessity of sample cleanup.

Capillary electrochromatography with fused-silica capillaries packed with copolymeric reversed-phase adsorbent and ion exchangers

Macroporous poly(styrene-divinylbenzene) (PSDVB), PRP-1, a reversed-phase adsorbent, and PSDVB-based strong acid cation exchangers and strong base and weak base anion exchangers were evaluated as stationary phases for capillary electrochromatography (CEC). Electroosmotic flow (EOF) for adsorbent and exchanger packed fused-silica capillaries for acetone as the marker increases with increasing ion exchange capacity, buffer organic solvent concentration, and applied voltage, is nearly independent of pH, and decreases with increased buffer ionic strength. For anion exchangers, EOF is reversed. Thiourea, acetone, acrylamide, nitromethane, propanal, and acetic acid were evaluated as EOF markers and undergo weak interaction with the PSDVB-based stationary phases. EOF in a basic buffer is greater than or equal to silica-based C-18 and cation exchanger packed capillaries. For an acidic buffer, EOF for a PRP-1 capillary is almost twice the C-18 packed capillary. As analyte hydrophobicity increases, retention and migration time increases for the PSDVB-based stationary phases. As exchange capacity increases, availability of the polymeric matrix for analyte partitioning decreases, causing analyte migration time to decrease. Increasing buffer organic solvent concentration decreases analyte retention. The PSDVB-based stationary phases provide good resolving power and reproducibility and are applicable to the CEC separation of neutral, weakly acidic, and basic analytes. Efficiency, however, is less than obtained with silica-based stationary phases. Because of stability in a strong acid buffer, the CEC separation of weak acids, where dissociation is suppressed, and weak bases as cations is possible. Separations of short-chain alkyl aldehydes, methyl ketones, aromatic hydrocarbons, substituted benzene derivatives, and short-chain carboxylic acids are described.

Determination of immunoreactive gonadotropin-releasing hormone in serum and urine by on-line immunoaffinity capillary electrophoresis coupled to mass spectrometry

The need for urgent diagnoses has propelled the development of automated analyses that can be performed in a short time at reasonable cost. One such method is immunoaffinity capillary electrophoresis. This emerging hybrid technology employs two powerful techniques coupled on-line for the direct and rapid determination of analytes present in biological fluids. The first technique, immunoaffinity, is used for the selective extraction of a molecule present in a complex matrix, utilizing a microscale-format chamber affinity device. An analyte (affinity target) present in serum or urine is captured by an immobilized molecular recognition antibody molecule (affinity ligand) bound to a solid support constituent (glass beads or an appropriate porous structure) of a microchamber affinity device. The second technique, capillary electrophoresis, is used for the high-resolution analytical separation of the purified and concentrated affinity target material after elution from the microchamber affinity device. In this work, immunoaffinity capillary electrophoresis was developed for the identification and characterization of a single constituent of a complex matrix. Immunoreactive gonadotropin-releasing hormone was determined in serum and urine specimens derived from a normal individual and from a patient suffering from benign prostatic hyperplasia. Furthermore, the on-line immuno-separation system was coupled in tandem to mass spectrometry to obtain molecular mass information of the affinity isolated and CE separated neuropeptide. This hybrid immuno-analytical technology is simple, rapid, selective and sensitive. In addition, an attempt was also made to characterize other urinary constituents by CE-MS that may lead to marker activity in the urine of the diseased subject. The hyphenation of analytical techniques has proved valuable in enhancing their individual features. The future of bioanalysis using miniaturized affinity systems is discussed in this paper.

Recent innovation in capillary electrokinetic chromatography with replaceable charged pseudostationary phases or additives

Recent developments of separation of neutral analytes in capillary systems with the mobile phase driven by the electroosmotic flow (EOF) and charged additives acting as a pseudostationary phase are reviewed. As pseudostationary phases a number of additives are used. Soluble polymers, either anionic or cationic, were applied as alternatives to micelles. Monomeric charged additives are also intended to form associates with the analytes, leading to selective retention and separation in a similar way as the polymeric pseudostationary phases. Dendrimers, spherical macromolecules with highly branched chains and charged terminal groups, are successfully applied for the separation of lipophilic analytes. Polymers with covalently stabilized structures are introduced in the form of permanent micelles and are therefore insensitive to the mobile phase composition, enlarging the applicability of micellar electrokinetic capillary chromatography (MEKC).

Recent innovations in the use of charged cyclodextrins in capillary electrophoresis for chiral separations in pharmaceutical analysis

A review is presented on the use of charged cyclodextrins (CDs) as chiral selectors in capillary electrophoresis (CE) for the separation of analytes in pharmaceutical analysis. An overview is given of theoretical models that have been developed for a better prediction of the enantiomeric resolution and for a better understanding of the separation mechanism. Several types of charged CDs have been used in chiral capillary electrophoretic separation (anionic, cationic, and amphoteric CDs). Especially the anionic CDs seem to be valuable due to the fact that many pharmaceutically interesting compounds can easily be protonated (e.g., amine groups). For that reason several anionic CDs are now commercially available. Cationic and amphoteric CDs are less common in chiral analysis and only a few are commercially available. Attention is paid to the most common synthesis routes and the characterization of the CDs used in chiral capillary electrophoretic separations. The degree of substitution in the synthesized CDs may vary from one manufacturer to another or even from batch to batch, which may have a detrimental effect on the reproducibility and ruggedness of the separation system. In Sections 4, 5, and 6 the applications of anionic, cationic, and amphoteric CDs for the chiral separation in CE are described. Many interesting examples are shown and the influence of important parameters on the enantioselectivity is discussed.