Determination of triazine compounds in ground water samples by micellar electrokinetic capillary chromatography

In this work, a capillary electrophoresis (CE) method for the determination of a group of eleven triazine compounds by micellar electrokinetic capillary chromatography (MEKC) with diode array detection was developed. The eleven herbicides studied were: desethylatrazin-2-hydroxy (DEA), simazine, prometon, atrazine, simetryn, ametryn, propazine, prometryn, trietazine, terbutylazine, and terbutryn The separation of these compounds was optimised as a function of buffer concentration and pH, concentration of sodium dodecyl sulphate (SDS) and voltage applied. To increase the selectivity of the separation and the resolution of the solutes, different organic solvents were tested as buffer additives, obtaining the best results when 1-propanol was used. The optimised buffer (24 mM of sodium borate, 18 mM of disodium hydrogen phosphate, 25 mM of SDS, pH 9.5, and 5% of 1-propanol) provides the best separation in terms of resolution and migration time. This method allowed the determination of these compounds at concentrations of 0.05 μg l⁻¹ in ground water samples pretreated using solid-phase extraction (SPE).     

Separation of anabolic steroids by micellar electrokinetic capillary chromatography

Summary The separation of seven analogous anabolic steroids was studied by micellar electrokinetic capillary chromatography (MECC). The retention order was found to be dependent on polarity. All of these steroids were well separated by the addition of organic modifiers to the separation buffer. Of the organic modifiers tested, 1-propanol gave the best separation, better than methanol or acetonitrile.     

Systematic optimization of micellar electrokinetic chromatographic separation of flavonoids

Summary A simple and rapid systematic optimization scheme was described for the micellar electrokinetic chromatographic separation of a group of flavonoids. The scheme employed an interpretative optimization approach to predict the optimum conditions for the separation of a group of flavonoids by micellar electrokinetic chromatography. By performing a set of nine pre-planned experiments conducted over the maximum working range for the system, global optimum separation conditions could be determined. To validate the optimization procedure, additional experiments were performed using the optimum experimental conditions derived from the optimization scheme. The results showed that satisfactory separation of all the peaks could be obtained. In addition, the application of the method in micropreparative micellar electrokinetic chromatography of the flavonoids was demonstrated.     

Optimization strategies in micellar electrokinetic capillary chromatography. Optimization of the temperature of the separation capillary

Summary The influence of the temperature of the capillary on the separation of small molecules by micellar electrokinetic chromatography with buffers containing sodium dodecyl sulfate was examined. Investigated parameters were the electroosmotic velocity, the migration velocity of the micelles, the strength of the electric current, retention factors and plate numbers. Thermodynamic parameters associated with micelle solubilization were evaluated from the variation of the retention factor with temperature. An investigation was carried out as to whether the dependence of the electroosmotic velocity and the migration velocity of the micelles are mainly due to the decrease in viscosity and dielectric constant with increase in temperature. The temperature of the capillary strongly affects the plate numbers for various solutes obtained with the same chromatographic system. In order to exclude secondary effects by alteration of the instrumental band broadening and the electroosmotic velocity with temperature, a method is presented that permits the development of chromatograms in MEKC at various temperatures with constant injected plug length and constant electroosmotic velocity. The results are discussed from the point of view of minimizing the analysis time and optimizing the resolution.     

Optimization of the separation of flavonoids using solvent-modified micellar electrokinetic chromatography

In this work the separation of eighteen flavonoids was attempted using reduced-flow micellar elektrokinetic chromatography (RF-MEKC) electrolytes modified by selected solvents with differing properties: methanol (MeOH), acetonitrile (ACN) and tetrahydrofuran (THF). Structural aspects such as unsaturation of the C ring, number and position of OH groups, methylation and glycosylation as well as solvent effects and their impact on the electrophoretic behavior of flavonoids were addressed. By evaluating the electropherograms obtained from mixture-designed electrolytes and searching for changes in the critical pairs, a favorable separation condition was achieved using 20 mmol/L phosphate buffer at pH 2.5 containing 50 mmol/L sodium dodecyl sulfate (SDS), 15% ACN and 5% THF (one critical pair) in less than 12 min with 1.5% coefficient of variation (CV) for retention factor and 3% CV for peak area (n = 5). The applicability of the proposed separation condition was demonstrated by the inspection of flavonoids in herbal extracts of Neem.     

Effect of organic modifiers on the capacity factor in micellar electrokinetic chromatography

Organic modifiers were effective both to extend the migration time window and to improve the separation of very hydrophobic compounds in MEKC.An iteration method was used to determine the migration time of micelles.The quantitative relationship between the capacity factor k' and the concentration of organic modifiers was derived,which was investigated experimentally.The linear solvation energy relationships (LSER) methodology was applied to MEKC,and good linear relationships between Ink' and solvatochromic parameters of 15 solutes were obtained in the presence of organic modifier in different concentrations,which indicated a new access in MEKC to predict k' from the structural parameters of solutes.The effect of column temperature T on k' was also investigated.     

Profiling propolis flavonoids by means of micellar electrokinetic capillary chromatography, capillary gas chromatography and bactericidal action

Summary There is no proportional correlation between increasing organic solvent polarity from hexane to methanol and the extractability of propolis solids by the solvents or the bactericidal action of the propolis extracts obtained. Hexane (17.7%) and chloroform (62.1%) were the poorest and the best extractants, respectively, for propolis solids. The antibiotic activity of the extracts againstStaphylococcus strains decreased in the order hexane≥ethanol>methanol. Different capillary GC profiles were obtained for persilyl derivatives of propolis extracted with hexane and methanol and for propolis collected in different Brazilian provinces, suggesting the influence of flora variability on propolis composition. Wavelength-selected DAD detection revealed MEKC to be an innovative and sensitive method for monitoring the occurrence in propolis of the flavonoids and phenolcarboxylic acids thought to be responsible for the antimicrobial activity of propolis.     

Optimization strategies in micellar electrokinetic capillary chromatography

Summary Computer-assisted procedures for the one-parameter optimization of the surfactant concentration and the concentration of urea or D-glucose as modifiers in micellar electrokinetic capillary chromatography have been developed. These procedures permit a rapid optimization of one parameter on the basis of only two experiments. Predicted values are compared to empirically obtained optimum values. The influence of the modifier concentration on the critical micelle concentration of sodium dodecyl sulfate was experimentally determined in buffers commonly employed in micellar electrokinetic chromatography. The alteration of retention factors of solutes caused by the influence of urea addition on the critical micelle concentration of sodium dodecyl sulfate was calculated under the assumption of constant distribution coefficients and compared to experimentally obtained values. It was demonstrated that the addition of urea or of D-glucose does not alter the phase ratio substantially.     

Comparative study of capillary zone electrophoresis and micellar electrokinetic chromatography for the separation of twelve aromatic sulphonate compounds

Summary Two modes of capillary electrophoresis (CE), capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC), were investigated for the separation of 12 aromatic sulphonate compounds. In CZE, although the voltage applied, the buffer concentration and the pH were optimized for effective separation of the compounds studied, under the best conditions four of the five amino compounds coeluted, as did naphthalene-1-sulphonic acid and naphthalene-2-sulphonic acid. In MEKC, sodium dodecyl sulphate (SDS) and Brij 35 were chosen as the anionic and nonionic surfactants and the effect of the concentration of micelles was examined. The effect of adding methanol as the organic modifier was also investigated with each of these micellar systems. All the analytes, including the isomers, were completely separated by use of MEKC with Brij 35 but when SDS was used only 11 compounds were separated because two amino compounds coeluted.     

Multivariate evaluation of the separation performance in micellar electrokinetic capillary chromatography of peptides: Optimization

Summary A simultaneous optimization of resolution, efficiency and migration times of enkephalin-related peptides in micellar electrokinetic capillary chromatography (MEKC) was performed. Six experimental variables; the surfactant concentration, the percentage of organic modifier, the ionic strength of the buffer, the injected plug length, the applied temperature and the sample solution composition were studied via central composite design (CCD). Large differences in separation performance were observed at a commonly used level of organic modifier in the background electrolyte. Partial least squares regression of the responses revealed that the experimental domain was too large and complicated to be explained by the model. A new CCD model was obtained with improved prediction ability at narrower ranges of the experimental factors, especially of the organic modifier. A conflict between maximum resolution and efficiency within the shortest analysis time was observed. Therefore, constraints were set on maximal resolution and analysis time, while solving for maximum efficiency. Optimal operating conditions were found at 35 mM sodium dodecyl sulfate (SDS), 5% v/v of acetonitrile, 1.9 mm injected plug length, 35°C and sample solution with no added micelles, giving high efficiency and resolution at short analysis time. The value predicted by the model was found to agree very well to the observed values, at the optimal experimental conditions, even on a new capillary.     

Surfactant-assisted pressurized liquid extraction for determination of flavonoids from Costus speciosus by micellar electrokinetic chromatography

Micellar electrokinetic chromatography (MEKC), a mode of capillary electrophoresis (CE), is considered an efficient analytical technique allowing for the reduction of organic solvent consumption during the experimental procedure. However, during sample preparation of natural products, the usage of large amount of organic solvent is generally unavoidable. In this article, therefore, a fast, simple, efficient, highly automatic and organic solvent-free sample preparation method, namely surfactant-assisted pressurized liquid extraction (PLE), was developed for the extraction of flavonoids in Costus speciosus flowers before MEKC analysis. The various experimental parameters such as the type and concentration of surfactant, and extraction time were evaluated systematically. Under the optimized conditions, the extraction efficiencies of surfactant-assisted PLE methods were comparable with Soxhlet extraction using organic solvent. The combination of surfactant-assisted PLE and MEKC was shown to be a green, rapid and effective approach for extraction and analysis of flavonoids in C. speciosus flowers.     

Pesticide analysis by micellar electrokinetic capillary chromatography

On-capillary sample concentration using sample stacking for the improvement of detection limits for various pesticides separated by micellar electrokinetic capillary chromatography was examined. The dependence of the stacking on different parameters was investigated. An approximately 30-fold preconcentration was achieved by applying sample stacking. Employing a two-step enrichment process (solid-phase extraction combined with sample stacking), detection limits were improved and the sample volume for SPE was reduced. In addition, the total time for the analysis was considerably reduced. Detection limits were between 0.01 and 0.1 ng/ml under these enrichment conditions.     

Determination of piribedil in pharmaceutical formulations by micellar electrokinetic capillary chromatography

A fast and simple micellar electrokinetic capillary chromatographic method was developed for the analysis of piribedil in pharmaceutical formulations. The effects of buffer concentration, buffer pH, sodium dodecyl sulphate (SDS) concentration, organic modifier, applied voltage and injection time were investigated. Optimum results were obtained with a 50 mM borate buffer at pH 8.0 containing 50 mM SDS by using a fused silica capillary (50 m internal diameter, 72 cm effective length). The sample was injected hydrodynamically for 4 s at 50 mbar pressure and the applied voltage was +30 kV. The detection wavelength was set at 205 nm. Diflunisal was used as an internal standard. The analysis was performed at 25 °C and the total run time was 14 min. The method was suitably validated with respect to linearity range, limit of detection and quantification, precision, accuracy, specificity and robustness. The linear calibration range was 5–100 g mL^–1 and the limit of detection was determined as 1 g mL^–1. The method developed was successfully applied to the determination of piribedil in pharmaceutical formulations. The results were compared with a spectrophotometric method reported in the literature and no significant difference was found statistically.     

A comparison of ionic liquids to molecular organic solvents as additives for chiral separations in micellar electrokinetic chromatography

In this study, we report the effects of adding ionic liquids (ILs), as compared to adding conventional molecular organic solvents (MOSs), to aqueous buffer solutions containing molecular micelles in the separation of chiral analyte mixtures in micellar EKC (MEKC). The molecular micelle used in this study was polysodium oleyl-L-leucylvalinate (poly-L-SOLV). The ILs were 1-alkyl-3-methylimidazolium tetrafluoroborate, where the alkyl group was ethyl, butyl, hexyl, or octyl. These ILs were chosen due to their hydrophobicity, good solvating, and electrolyte properties. Thus, it was expected that these ILs would have favorable interactions with chiral analytes and not adversely affect the background current. Common CE buffers, mixed with a molecular micelle, and an IL or a MOS, were used for these chiral separations. The buffers containing an IL in the concentration range of 0.02-0.1 v/v were found to support a reasonable current when an electric field strength of 500 V/cm was applied across the capillary. However, a current break down was observed for the buffers containing more than 60% v/v MOS on application of the above-mentioned electric field. The chiral resolution and selectivity of the analytes were dependent on the concentration and type of IL or MOS used.     

The separation of herbicides by micellar electrokinetic capillary chromatography

Summary A method for the separation of a number of herbicides consisting of chlorophenoxy acids by micellar electrokinetic capillary chromatography (MECC) was developed. Sodium dodecyl sulphate (SDS), Brij 35, cetyltrimethylammonium bromide (CTAB) and methanol were introduced into the buffers to investigate their effects on the separation of the herbicides. SDS combined with Brij 35 as the micellar agent was found to provide the best overall separation of these components.     

Simultaneous separation of nitrofuran antibiotics and their metabolites by using micellar electrokinetic capillary chromatography

Mixtures comprising nitrofuran antibiotics (NFA) and nitrofuran metabolites (NFM) were resolved for the first time by using MEKC. Sodium deoxycholate (SDC) was chosen as the micelle-forming surfactant. Optimization of separation conditions was achieved by using a central composite experimental design (CCD) approach. Experimental parameters such as concentration ratio of borate to phosphate in the buffer, pH of the running electrolyte and voltage were investigated. The effect of concentration of the surfactant on resolution was significant. Under optimal conditions of 80 mM SDC, pH 9.0, (20 mM borate + 20 mM phosphate) and 16 kV, the resolution between eight consecutive peak pairs ranged from 1.9 to 11.8. Due to the absence of a UV-active chromophore in the metabolites, they were derivatized with 2-nitrobenzaldehyde (2-NBA). In order to mimic a proposed extraction procedure for the analysis of both NFA and/or derivatized NFM in a sample, aqueous samples (prederivatized with 2-NBA) were extracted by using C(18) SPE cartridges. After washing with H(2)O, the cartridges were eluted with a small portion of organic solvent with weak elution characteristics to remove excess 2-NBA (hexane was chosen). Target analytes were then recovered with ACN. Excellent reproducibility of migration time (t(mig)) was achieved for all analytes using the developed MECC approach, with absolute t(mig) <1% RSD and t(mig) ratio <0.2% RSD, and peak area ratio was 4% RSD. The LOD for each compound, calculated by extrapolating to an S/N of 3, were found to be 0.19-2.0 microg/mL.