Separation of enantiomers in capillary electrophoresis with contactless conductivity detection

Determination of amino acids by capillary and microchip electrophoresis with contactless conductivity detection - Theory, instrumentation and applications

This review article summarises aspects of the determination of amino acids using capillary and chip electrophoresis in combination with contactless conductivity detection from their historical beginnings to the present time. Discussion is included of the theory of conductivity detection in electromigration techniques, the design of contactless conductivity cells for detection in capillaries and on microchips, including the use of computer programs for simulation of the conductivity response and the process of the electrophoretic separation of amino acids. Emphasis is placed on optimisation of the background electrolyte composition, chiral separation, multidimensional separation, stacking techniques and the use of multidetection systems. There is also a description of clinical applications, the determination of amino acids in foodstuffs, waters, soils and composts with emphasis on modern techniques of sample treatment, such as microdialysis, liquid membrane extraction and many other techniques.

New Kind of Polymer Materials Based on Selected Complexing Star-Shaped Polyethers

In today’s analytical trends, there is an ever-increasing importance of polymeric materials for low molecular weight compounds including amines and drugs because they can act as carriers or capture amines or drugs. The use of this type of materials will allow the development of modern materials for the chromatographic column beds and the substrates of selective sensors. Moreover, these kinds of materials could be used as a drug carrier. Therefore, the aim of this study is presenting the synthesis and complexing properties of star-shaped oxiranes as a new sensor for the selective complexation of low molecular weight compounds. Propylene oxide and selected oxirane monomers with carbazolyl in the substituent were selected as the monomers in this case and tetrahydrofuran as its solvent. The obtained polymer structures were characterized using the MALDI-TOF. It was found that in the initiation step potassium hydride deprotonates the monomer molecule and takes also part in the nucleophilic substitution. The resulting polymeric material preferably cross-linked with selected di-oxiranes (1,2,7,8-diepoksyoktan in respect ratio 3:1 according to active center) was then used as a stationary phase in the column and thin layer chromatography for amine separation and identification. Sorption ability of the resulting deposits was determined using a quartz microbalance (QCMB). The study was carried out in stationary mode and flow cells to simulate actual operating phase conditions. Based on changes in electrode vibration frequency, the maximum amount of adsorbed analyte and the best conditions for its sorption were determined.

Enantiomeric separation of some common controlled stimulants by capillary electrophoresis with contactless conductivity detection

CE methods with capacitively coupled contactless conductivity detection (C(4)D) were developed for the enantiomeric separation of the following stimulants: amphetamine (AP), methamphetamine (MA), ephedrine (EP), pseudoephedrine (PE), norephedrine (NE) and norpseudoephedrine (NPE). Acetic acid (pH 2.5 and 2.8) was found to be the optimal background electrolyte for the CE-C(4)D system. The chiral selectors, carboxymethyl-β-cyclodextrin (CMBCD), heptakis(2,6-di-O-methyl)-β-cyclodextrin (DMBCD) and chiral crown ether (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18C6H(4)), were investigated for their enantioseparation properties in the BGE. The use of either a single or a combination of two chiral selectors was chosen to obtain optimal condition of enantiomeric selectivity. Enantiomeric separation of AP and MA was achieved using the single chiral selector CMBCD and (hydroxypropyl)methyl cellulose (HPMC) as the modifier. A combination of the two chiral selectors, CMBCD and DMBCD and HPMC as the modifier, was required for enantiomeric separation of EP and PE. In addition, a combination of DMBCD and 18C6H(4) was successfully applied for the enantiomeric separation of NE and NPE. The detection limits of the enantiomers were found to be in the range of 2.3-5.7 μmol/L. Good precisions of migration time and peak area were obtained. The developed CE-C(4)D method was successfully applied to urine samples of athletes for the identification of enantiomers of the detected stimulants.

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.

The role of cyclodextrins in chiral capillary electrophoresis

The members of the enantiomeric pairs frequently show rather different biological effects, so their chiral selective synthesis, pharmacological studies and analysis are necessary. CE has unique advantages in chiral analysis. The most frequently used chiral selectors are CDs in this field. This paper gives a short view on the advantages on CE in direct chiral separations, emphasizing the role of CDs. The reason for the broad selectivity spectra of CDs is discussed in detail. The physical background of chiral selective separations is briefly shown in CE. Their interaction mechanisms are shortly defined. The general trend of their use is statistically evaluated. Most frequently used CDs and CD derivatives are characterized. Advantages of ionizable CDs and single-isomer derivatives are shown. The general trend of their use is established.

Enantioseparation of dopa and related compounds by cyclodextrin-modified microemulsion electrokinetic chromatography

A chiral microemulsion electrokinetic chromatography method has been developed for the enantiomeric separation of 3,4-dihydroxyphenylalanine (dopa), its precursors phenylalanine and tyrosine, and the structurally related substance methyldopa. The separations were achieved using an oil-in-water microemulsion, which consisted of the oil-compound ethyl acetate, the surfactant sodium dodecylsulfate (SDS), the co-surfactant 1-butanol, the organic modifier propan-2-ol and 20mM phosphate buffer pH 2.5 or 2.0 as aqueous phase. For enantioseparation sulfated beta-cyclodextrin was added. The resolution of each racemate was optimized by varying the concentration of the buffer and all components of the microemulsion. Enantioseparation could be achieved for dl-dopa, dl-phenylalanine and dl-tyrosine within 13 min with a resolution of 4.3, 3.1 and 3.3, respectively, and for methyldopa in 17 min (Rs: 1.4). The established methods allowed the detection of dopa, phenylalanine, tyrosine and methyldopa with a limit at 0.5, 1.0, 0.2 and 2.0 microg/ml.

Review of aqueous chiral electrokinetic chromatography (EKC) with an emphasis on chiral microemulsion EKC

The separation of enantiomers using electrokinetic chromatography (EKC) with chiral microemulsions is comprehensively reviewed through December 1, 2006. Aqueous chiral EKC separations based on other pseudostationary phases such as micelles and vesicles or on other chiral selectors such as CDs, crown ethers, glycopeptides, ligand exchange moeities are also reviewed from both mechanistic and applications perspective for the period of January 2005 to December 1, 2006.

Capillary electrophoresis and microchip capillary electrophoresis with electrochemical and electrochemiluminescence detection

Recent advances and key strategies in capillary electrophoresis and microchip CE with electrochemical detection (ECD) and electrochemiluminescence (ECL) detection are reviewed. This article consists of four main parts: CE-ECD; microchip CE-ECD; CE-ECL; and microchip CE-ECL. It is expected that ECD and ECL will become powerful tools for CE microchip systems and will lead to the creation of truly disposable devices. The focus is on papers published in the last two years (from 2005 to 2006).

Electrokinetic partial filling technique as a powerful tool for enantiomeric separation ofDL-lactic acid by CE with contactless conductivity detection

A modified partial filling method for chiral separation of DL-lactic acid as the model chiral compound with vancomycin chloride as the chiral selector was developed by CE with contactless conductivity detection. Electrokinetic partial filling technique (EK-PFT) was used as an alternative method to the conventional hydrodynamic partial filling method. EK-PFT, in contrast to the hydrodynamic partial filling technique, allowed the removal of the chloride counterions from the chiral selector which otherwise led to poor sensitivity in conductivity detection. The baseline separation of DL-lactic acid as the model analyte was achieved in 5 min in a polyacrylamide-coated capillary. The best resolution was achieved by electrokinetic partial filling of vancomycin cations from the injection solution containing 5 mmol/L oxalate L-histidinium at pH 4.5 with 10 mmol/L vancomycin chloride. Computer simulation was used to explain the observed phenomena in the boundary between the inject vial and the capillary during the EK-PFT of vancomycin cations.

Simultaneous contactless conductivity detection and UV detection for the study of separation of tamsulosin enantiomers in discontinuous electrolyte systems by CE

This work shows the potential of using discontinuous electrolyte systems for the separation of tamsulosin enantiomers by CE. Sulfated beta-cyclodextrin was used as a chiral selector. In acidic electrolytes, sulfated beta-cyclodextrin migrates as an anion and the analyte (tamsulosin) migrates as a cation. Due to this, four experimental arrangements were proposed. These arrangements differ in composition of electrolytes in the inlet compartment, in the capillary and in the outlet compartment. The separation of tamsulosin enantiomers in acetate buffers with sodium and Tris counterions was studied. Simultaneous contactless conductivity detection and UV detection were used for the study of the separation mechanism in these systems. Mobilities of sulfated beta-cyclodextrin were used for the calculation of the time when the analyte migrates through the BGE zone with the selector. The simulation program Simul 4.0 was used for the calculations of the concentration profiles of the electrolyte components dependent on the time of the separation. The mechanism of enantioseparation in these arrangements was suggested.

Enantiomeric separation of underivatized small amines in conventional and on-chip capillary electrophoresis with contactless conductivity detection

The determination of the enantiomers of small non-UV-absorbing amines which otherwise can only be achieved with difficulty was possible by using a combination of the chiral crown ether (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18C6H4) and dimethyl-beta-CD as selectors in CE and contactless conductivity measurement for detection. Alkylamines without any other functional group, amino alcohols, species with ether or ester groups and with a cyclic moiety were investigated. The detection limits were found to be about 1.0 microM and the determination is possible up to at least 1.0 mM. The determination of enantiomeric ratios of up to 99.5:0.5 was also found feasible.

Recent applications of conductivity detection in capillary and chip electrophoresis

The review provides a comprehensive survey of the recent applications of contact and contactless conductivity detection in capillary electrophoretic and chip electrophoretic analyses of a broad scale of compounds, from low-molecular-mass highly mobile small inorganic and organic ions, via medium-molecular-mass peptides and oligo- and polynucleotides up to high-molecular-mass biopolymers, proteins and nucleic acids fragments. The review presents also the recent developments in the construction of different types of conductivity detectors (detectors with galvanic contact of the sensing electrodes with the BGE and sample components, contactless conductivity detectors with capacitively coupled tubular and semitubular electrodes and combined conductivity/optical detectors) applied in the capillary electromigration methods performed in classical fused silica, polytetrafluorethylene, and polyetheretherketone capillaries or on glass and polymethylmethacrylate microchips. In addition, the principle and theoretical bases of conductivity detection in capillary electromigration techniques, zone electrophoresis, ITP, micellar EKC, and electrochromatography are briefly described.

Rapid electrophoretic separations in short capillaries using contactless conductivity detection and a sequential injection analysis manifold for hydrodynamic sample loading

A sequential injection-capillary electrophoresis (SI-CE) system for the fast and automated quantitative analysis of anions and cations is described. Because of the low sample load in capillary electrophoresis a split injection approach had to be used to achieve reliable hydrodynamic injection. The use of a capillary of 8 cm effective length allowed for the separation of five inorganic cations within 11 s. One common electrolyte solution containing 12 mM l-histidine and 2 mM 18-crown-6 whose pH value was adjusted to 4.0 with 10% v/v acetic acid was used for anions and cations, thus the analysis of both groups of analytes could be carried out in rapid sequence simply by switching the polarity of the high voltage supply. The system also allows automated flushing of the capillary. Detection limits between about 2 and 5 micromol l(-1) could be achieved with the contactless conductivity detector employed.

Structural Scaffold of 18-Crown-6 Tetracarboxylic Acid for Optical Resolution of Chiral Amino Acid: X-Ray Crystal Analyses of Complexes of D- and L-Isomers of Serine and Glutamic Acid

(+)-18-crown-6 tetracarboxylic acid (18C6H4) has been used as a chiral selector for D/L-amino acids in HPLC, where L-isomer is usually eluted prior to D-isomer, except for the case of serine. To clarify why serine exhibits the reverse order for the elusion, the chiral interactions of D- and L-serines with (+)-18C6H4 were investigated by the X-ray single crystal analyses, together with the case of D- and L-glutamic acids, which exhibit the usual elution order in HPLC. The backbone structures (amino, Calpha-H and carboxyl groups) of these four amino acids showed the nearly same interaction with (+)-18C6H4 despite their different chirality. In contrast, the hydroxyl group of L-serine side chain formed a hydrogen bond with the carboxyl group of (+)-18C6H4, whereas such a interaction was not formed for the side chain of D-serine and D- and L-glutamic acids. Thus, it was shown that the exception of D/L-serine from the first elution rule of L-isomer in HPLC is due to the presence and absence of a hydrogen bond formation of its side chain OH group.

Evaluation of capillary ion exchange stationary phase coating distribution and stability using radial capillary column contactless conductivity detection

The use of radial (across) capillary column capacitively coupled contactless conductivity detection is demonstrated as a simple and rapid technique for visualisation of stationary phase longitudinal coverage and coating stability in capillary ion exchange chromatography.

Enantiomeric separation of 1-phenylethylamine and 1-cyclohexylethylamine in capillary electrophoresis with contactless conductivity detection

Contactless conductivity detection was employed for the detection of the enantiomers of 1-phenylethylamine and 1-cyclohexylethylamine which were separated in capillary electrophoresis with unprecedented high resolutions R(s) of 2.3 and 3.3, respectively, by using a combination of dimethyl-beta-cyclodextrin and the chiral crown ether 18C6H4 as chiral selectors in a citric acid buffer of pH 2.4. The conductivity measurement enabled the direct detection, i.e. without having to derivatize or resort to indirect methods, of all species including the non-UV-absorbing enantiomers of cyclohexylamine. Detection limits of 0.5 microM were achieved and the determination of enantiomeric ratios of up to 99:1 was found possible.