Cations In Capillary Electrophoresis Research Articles

Development of Electrophoretic Methods for Simultaneous Determination of Enantiomeric Ratio and Composition of Diastereomeric Salt Mixtures

Obtaining enantiomeric pure compounds is—among other techniques—possible in a resolvation experiment via diastereomeric salt formation, excellently exemplified by a modified Pope–Peachy method performed in supercritical carbon dioxide as solvent. The salt precipitation is followed by supercritical fluid extraction (SFE) to separate the diastereomeric salts and the unreacted enantiomers. To evaluate the extraction efficiency, conversion and enantioselectivity achieved, it is essential to determine the enantiomer excess and the residual resolving agent content in extracts and raffinates. Carefully chosen experimental parameters enable the simultaneous determination of certain anions and cations in capillary electrophoresis in a single run, which has not been reported for diastereomeric mixtures so far. In this paper, a partially validated chiral selective cyclodextrin enabled capillary electrophoresis method is presented for the characterization of cis-permethrinic acid samples resolved with (R)-1-phenylethylamine prepared by the SFE-based resolvation technique. To evaluate the efficiency of the resolvation, a cyclodextrin enabled chiral separation method was developed applying permethylated-β-cyclodextrin as chiral selector. The theoretical possibility of the widespread application of the developed method (with minor adjustments) is justified for other selectands and selectors. The developed methods can be thereby applied for the fast and reliable control of resolvation experiments.

Referenced Capacitively Coupled Conductivity Detector for Capillary Electrophoresis

AbstractA dual capacitively coupled contactless conductivity detector for capillary electrophoresis was developed. The two channels are arranged in a bridge configuration so that one of them acts as a reference whose signal is subtracted. This effectively compensates for the baseline conductivity of the separation buffer so that the electronic zero setting is not necessary. Changes in the buffer composition are automatically accounted for, as are temperature drifts. The system is demonstrated for the detection of inorganic model cations in capillary electrophoresis. Besides the use with two separate capillaries, one of which solely serves as reference, it was also found possible to use a single capillary which is looped back through the reference cell.

Utilisation of crown ethers in microemulsion electrokinetic chromatography for the separation of inorganic cations

A new method to influence the separation selectivity of inorganic cations in capillary electrophoresis is presented. This method combines the use of certain crown ethers to form complexes with a specific cation (changing its ionic radius/charge ratio and thereby its electrophoretic mobility) with partitioning of the crown ether/analyte complex between an aqueous phase and a pseudo-stationary phase, such as the oil droplet of a microemulsion. Several microemulsions, including uncharged oil droplets and oil droplets with different degrees of surface charge were tested to evaluate their ability to improve the separation of the selected analytes.

Light-emitting diode-based indirect fluorescence detection for simultaneous determination of anions and cations in capillary electrophoresis

This report presents simultaneous analysis of cations and anions by capillary electrophoresis (CE) in conjunction with indirect fluorescence detection using a blue light-emitting diode (LED), based on the displacement of fluorescein with anionic EDTA–metal complexes and anions. A new focusing system combined with a plastic lens and a 40× objective was developed and used effectively to focus the diverging beam of the LED on the capillary. The optimum compositions for simultaneous analysis of metal ions and anions are the samples prepared in 5 mM borate, pH 9.2, containing 2 mM EDTA and the background electrolytes (BGEs) consisting of 5 mM borate buffer, 5 μM fluorescein, and 1 μM NaCl at pH 9.2. Using this pre-capillary complexation method, the analysis of a sample containing five metal ions and eight anions was accomplished in 8 min, with the relative standard deviation values for the migration times less than 2.0%. The peak heights against the concentrations of the metal ions and anions are linear in 10–1000 and 50–2000 μM, with correlation coefficients better than 0.998, and 0.982, respectively. The limits of detection at a signal-to-noise ratio 3 of up to 14.6 μM for formate and as low as 3.7 μM for Ni 2+. The results of the analyses of pond water and a Chinese herbal soup present the advantages of this method, including simplicity, rapidity, reproducibility, and low costs.

Enhancement of detection sensitivity for indirect photometric detection of anions and cations in capillary electrophoresis.

This review focuses on the indirect photometric detection of anions and cations by capillary electrophoresis. Special emphasis has been placed on the sensitivity of the technique and approaches taken to enhance detection limits. Theoretical considerations and requirements have been discussed, including buffering, detection sensitivity, separation of cations, and detector linearity. A series of tables detailing highly absorbing probes and the conditions of their use for indirect photometric detection are included.

Influence of methanol as a buffer additive on the mobilities of organic cations in capillary electrophoresis.

The mobilities of a series of aromatic ammonium ions, ranging in charge from +1 to + 3, were investigated by capillary electrophoresis using buffers consisting of 0-75% v/v methanol. This is an extension of our previous studies involving anion mobility in methanol-water media [1]. Absolute mobilities were determined by extrapolation of the effective mobilities to zero ionic strength according to the Pitts' equation. For all of the buffer compositions studied, the ionic strength effect increased with increasing cation charge, and varied as a function of solvent 1/eta epsilon (1/2) as predicted by the electrophoretic term within the Pitts' equation. In the presence of methanol, the ionic strength effects became more dramatic. The absolute mobilities of the cations were altered by the addition of methanol to the electrophoretic media. For example, at 75% MeOH, a migration order reversal was observed between the + 2 and + 3 ammonium ions. These solvent-induced selectivity changes are attributed to dielectric friction. As predicted by the Hubbard-Onsager dielectric friction model, dielectric friction increased with increasing methanol content and with increasing analyte charge. Further, the changes in cation mobility correlated to the changes in solvent relaxation time (tau), epsilon and eta. Although not predicted by the Hubbard-Onsager theory, the + 3 ammonium ion experienced more dielectric friction than the - 3 sulfonate and - 3 carboxylate investigated previously [1]. This apparent failure of the Hubbard-Onsager model results from its continuum nature, whereby ion-solvent interactions are not taken into account.

Determination of nonelectroactive cations by capillary electrophoresis with amperometric detection at a graphite electrode modified with a thin film of mixed-valent ruthenium-iron cyanide

A graphite electrode modified with a thin film of mixed-valent ruthenium-iron cyanide was prepared and evaluated as an amperometric sensor for nonelectroactive cations in capillary electrophoresis. This electrode was very stable and showed a selective and sensitive response to Cs +. Based on three times signal-to-noise ratio, a detection limit of 2×10 −8 M for Cs + was obtained under the optimized conditions. The applicability of this method was demonstrated for the analysis of synthetically constituted and real samples.

Use of cyclofructan as a potential complexing agent in capillary electrophoresis

A novel selective complex-forming agent for metal cations in capillary electrophoresis (CE) was investigated. The complex former is cyclofructan (cyclic d-fructohexaose or cycloinulohexaose), obtained from enzymatic conversion of inulin using the enzyme, cycloinulo-oligosaccharide fructanotransferase (CFTase), obtained from a strain of Bacillus circulans, MCI-2554. It has a circular hexameric structure resembling that of 18-crown-6. Complex formation with a number of cations was determined using CE with indirect UV detection at 200 nm and a background electrolyte consisting of 10 m M 2-aminopyridine, buffered at pH 4.50 with acetic acid. Cyclofructan was added to the electrolyte at different concentrations and complexation constants with a number of cations were determined. Permethylated cyclofructan was compared with 18-crown-6 in a 50% aqueous methanol buffer.

Potentiometric Detection of Alkali and Alkaline Earth Metal Cations in Capillary Electrophoresis with Simplified Electrode Alignment and Enhanced Separation and Sensitivity

Coated-wire electrodes were employed as potentiometric detectors for the analysis of alkali and alkaline earth metals by capillary electrophoresis. Easy electrode alignment was achieved by a Plexiglas capillary-electrode holder, which makes it unnecessary to employ a microscope and a micromanipulator. The addition of a crown ether to the running electrolyte not only renders possible the complete separation of ammonium and potassium but also improves considerably the sensitivity of the coated-wire electrode. We studied the dependence of the migration times on the crown ether concentration as well as the change in sensitivity by means of calibration curves for potassium and calcium.

Potentiometric detection of inorganic anions and cations in capillary electrophoresis with coated-wire ion-selective electrodes

The construction of coated-wire ion-selective electrodes of small size with two different PVC-based membranes is described. The electrodes are used as potentiometric end-column detectors for monovalent anions and alkali and alkaline earth metal cations separated by capillary electrophoresis. In comparison to conventional micropipette electrodes these all solid state electrodes showed a similar performance but much higher mechanical robustness and a longer lifetime which was found to be 9 and 8 days, respectively. Calibration curves for Cl − and Ca 2+ demonstrate the wide dynamic range of these detectors and detection limits of lower than 10 −7 and 10 −8 M could be achieved using the electrostacking technique.

Sodium dodecyl sulfate-Tween 20 mixed micellar electrokinetic chromatography for separation of hydrophobic cations: application to adrenaline and its precursors.

Separation of hydrophobic cations in capillary electrophoresis under the mixed micellar system composed of SDS and Tween 20 was investigated from the viewpoints of thermodynamics and practical application. Hydrophobic cations interact strongly with the anionic SDS micelle, and this often lead to predominant dissolution of the analytes into the micellar phase, resulting in poor resolution. The ionic interaction was evaluated to be close in strength to the hydrophobic interaction between benzenes and the SDS micelle. Tween 20 as a component to the mixed micelle was found to weaken the attractive ionic interactions between the cationic solutes and SDS. In addition, the polyether chain of Tween 20 serves also as a hydrogen acceptor to cause the attractive hydrogen-bonding interactions with hydrogen-donating analytes. The two different functions of Tween 20 improve the separation of hydrophobic cations remarkably. Separation patterns are well controlled by varying the mixing ratio of the two surfactants. Adrenaline and its six precursors were successfully separated with this mixed micellar system. The improved separation efficiency was not affected by the presence of bovine serum matrix, realizing the direct MEKC analysis of serum sample without deproteination, in which proteins were comigrated with the micelle.

Prediction and optimization of the separation of metal cations by capillary electrophoresis with indirect UV detection

The migration of metal cations in capillary electrophoresis with indirect UV detection was investigated. A mathematical model was used to relate the electrophoretic mobility of a metal cation with two background electrolyte factors: the pH and the concentration of a complexing agent. Imidazole, which has an electrophoretic mobility close to those of metal cations, was used as the visualization agent, and 2-hydroxyisobutyric acid (HIBA) was used as the complexing agent. Through computer modeling, the electrophoretic mobilities of fourteen metal cations were predicted on the basis of five initial experiments within the factor space of pH and HIBA concentration. Good agreement was observed between the predicted electrophoretic mobilities and the observed values of the metal cations. Subsequently, the separation of a complex mixture of metal cations was successfully optimized. A complete separation of 14 metal cations including alkali, alkaline earth and transition metal(II) cations was achieved within 4 min. In addition, the effectiveness of other visualization agents and complexing agents for the separation of metal cations was examined.