Ching-Erh Lin

Migration behavior and separation of sulfonamides in capillary zone electrophoresis III. Citrate buffer as a background electrolyte

Abstract To separate sulfonamides using capillary zone electrophoresis, citrate buffer is superior to phosphate buffer as a background electrolyte. Separation parameters affecting the selectivity and resolution of sulfonamides were optimized. Complete separation of thirteen sulfonamides was rapidly and efficiently achieved within 2.1 min with citrate buffer at pH 6.9. The resolution of peaks between sulfamethoxypyridazine and sulfathiazole, is enhanced either on heating the capillary to 35°C or on adding methanol to the buffer electrolyte in an appropriate proportion. In combination with mobility data obtained at low pH, two p K a values of each individual sulfonamide are satisfactorily determined. Electrophoretic mobilities of sulfonamides measured at optimum pH of the buffer correlate well with those calculated from Offords equation. Thus the order of migration of these sulfonamides, reflected in the magnitudes of their electrophoretic mobilities, depends on their ratios of charge to mass, and is primarily determined by their p K a values.

Migration behavior and selectivity of sulfonamides in capillary electrophoresis

The migration behavior and selectivity of thirteen sulfonamides in capillary electrophoresis (CE), with emphasis on micellar electrokinetic chromatography (MEKC) were systematically investigated using a phosphate-borate buffer electrolyte, with sodium dodecyl sulfate (SDS) as an anionic surfactant in MEKC. The optimization strategies for the separation of sulfonamides in capillary zone electrophoresis (CZE) and in MEKC are described. The migration behavior and selectivity of sulfonamides in CZE are mainly manipulated by the pH of the buffer. The migration order of sulfonamides depends on the ratios of charge to mass (q/M2/3) and is primarily determined by their pKa values. Thus precise optimization of buffer pH is crucial to further improve the separation of some closely migrating sulfonamides. On the other hand, buffer pH and micelle concentration greatly affect the migration and selectivity of sulfonamides in MEKC. The migration order of sulfonamides is mainly determined by their pKa values and the magnitude of the binding constants of solutes-to-micelles. The influences of buffer pH and micelle concentration correlate with each other. The magnitude of the binding constants correlates with the differences between the electrophoretic mobility of sulfonamides measured at a pH below pKa-2 in CZE and that in MEKC. In this work, acid dissociation constants of these sulfonamides and binding constants of sulfonamides to SDS micelles in a phosphate-borate buffer are reported.

Migration behavior and separation of sulfonamides in capillary zone electrophoresis. I. Influence of buffer pH and electrolyte modifier.

The influence of buffer pH and electrolyte modifier on the migration behavior and separation of thirteen sulfonamides was investigated by capillary zone electrophoresis. The results indicate that precise optimization of buffer pH is crucial in improving the separation of some closely migrating sulfonamides. On the addition of either an appropriate amount of an organic modifier (methanol or acetonitrile) or a low concentration of beta-cyclodextrin (beta-CD, 0.5 mM) to a phosphate-borate buffer at pH 6.85 and an applied voltage of 20 kV, the resolution of peaks between sulfathiazole and sulfamethoxypyridarine is markedly enhanced and effective separations of thirteen sulfonamides are achieved within a relatively short time. Methanol gives better resolution than acetonitrile as an organic modifier. Weak inclusion complexation occurs between beta-CD and sulfonamides, with the exception of sulfathiazole. The formation constants of thirteen sulfonamides with beta-CD are reported.

Separation of benzenediamines, benzenediols and aminophenols in oxidative hair dyes by micellar electrokinetic chromatography using cationic surfactants

The separation of 13 dye intermediates, including benzenediamine, benzenediols and aminophenols, in oxidative hair dyes was investigated by micellar electrokinetic chromatography (MEKC) using tetradecyl- and hexadecyl-trimethylammonium bromides (TTAB and CTAB) as cationic surfactants in a phosphate buffer at acidic pH. The influences of separation parameters such as micelle concentration, buffer pH, buffer concentration and organic modifiers on the selectivity and migration order of these dye intermediates were examined. The results indicate that micelle concentration and buffer pH are the two most important factors that affect greatly the migration and the separation of these dye intermediates. The electrophoretic mobility of 6-methyl-3-aminophenol, 1,3-benzenediol and 1,2-benzenediol, owing to solute-micelle interactions, increases markedly with increasing micelle concentration, but decreases to a greater extent than that of the other analytes with increasing the proportion of organic modifiers. Thus the selectivity and the order of migration of the analytes which migrate consecutively with these three analytes may be altered on varying these separation parameters. Complete separation was achieved either with TTAB at a concentration of 18 mM or with CTAB at a concentration of 13 mM at pH 5.0. The analysis of a real sample of a commercial product of oxidative hair dyes was performed.

On-line concentration of s-triazine herbicides in micellar electrokinetic chromatography using a cationic surfactant.

On-line concentration of neutral species of s-triazine herbicides in micellar electrokinetic chromatography using tetradecylammonium bromide (TTAB) as a cationic surfactant was investigated. Factors affecting the stacking of analytes were examined. The results indicate that the stacking efficiency is markedly improved with addition of phosphate buffer in the sample matrix. It was found that, depending on the nature of the analytes, the most effective stacking of these analytes occurs when the ratio of the conductivity of buffer electrolyte to that of sample matrix is in the range 1.4-1.2, with sample matrix containing phosphate buffer. Micelle concentration in the separation buffer is also a crucial factor to enhance the stacking efficiency and detection sensitivity of analytes. Moreover, the stacking efficiency of each individual analyte depends on its binding constant to TTAB micelles. The concentration effect is primarily based on sweeping mechanism which is operated in a normal stacking mode with reversed electrode polarity in the presence of reversed electroosmotic flow. As a result of concentration enhancement, the detection limits of these herbicides can reach about 9-15 ng/ml with UV detection.

Separation and migration behavior of structurally related phenothiazines in cyclodextrin-modified capillary zone electrophoresis

The influences of buffer pH and the concentration of beta-cyclodextrins (beta-CDs) on the separation and migration behavior of 13 structurally related phenothiazines in CD-modified capillary zone electrophoresis (CD-CZE) using a phosphate background electrolyte at low pH were investigated. We focused on the separation of these phenothiazines, including the enantiomers of chiral analytes, with the use of beta-CD and hydroxypropyl-beta-CD (HP-beta-CD) as electrolyte modifiers or chiral selectors at concentrations less than 8 mM. The results indicate that the interactions of phenothiazines with beta-CDs are very strong and that effective separations of 13 analytes can be achieved with addition of 0.3 mM beta-CD or 0.5 mM HP-beta-CD in a phosphate buffer at pH 3.0. Binding constants of phenothiazines to beta-CDs were evaluated for a better understanding of the interactions of phenothiazines with beta-CDs.

Capillary zone electrophoretic separation of alkylbenzyl quaternary ammonium compounds: effect of organic modifier

Capillary zone electrophoretic separations of alkylphenyl- and alkylbenzyl ammonium compounds with the polar organic solvents acetonitrile, tetrahydrofuran, acetone, and methanol as micelle disruptors were investigated. The results indicate that addition of acetonitrile (30%, v/v), tetrahydrofuran (40%, v/v), or acetone (50%, v/v) to a phosphate buffer is necessary for effective separation of alkylbenzyldimethyl ammonium compounds in a sample (0.01 mM) dissolved in a methanolic solution (60%, v/v). The effectiveness of the organic modifier to disrupt micelles decreases in the order acetonitrile > tetrahydrofuran > acetone > methanol. With a smaller concentration of sample and a sufficient concentration of buffer, less organic modifier is required. The most effective separation was achieved within three minutes with the addition of acetonitrile (30%, v/v) in a phosphate buffer (20 mM, pH 5.0) using a fused-silica capillary at 15 kV for a sample of concentration 0.01 mM. The selectivity and resolution of quaternary ammonium compounds were slightly affected on variation of the concentration and pH of the buffer when electrophoretic conditions with concentrations of the phosphate buffer greater than 20 mM and of the sample less than 0.01 mM were employed.

A capillary electrophoresis study on the influence of β-cyclodextrin on the critical micelle concentration of sodium dodecyl sulfate

The influence of beta-cyclodextrin (beta-CD) on the critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS) was investigated by capillary electrophoresis using anionic chlorophenols as probe molecules at pH 7.0. The variations of the electrophoretic mobility of probe molecules as a function of surfactant concentration in both premicellar and micellar regions in the absence and presence of beta-CD was analyzed. The results indicate that, as a consequence of a strong inclusion complexation between beta-CD and SDS, the encapsulation of beta-CD with probe molecules is greatly diminished, or even vanished, in the presence of SDS. The complexes formed between beta-CD and SDS monomers exist predominantly in the form of a 1:1 stoichiometry, while the complexes with a 2:1 stoichiometry reported previously in the literature as a minor component may exist by less than 10%. The elevation of the CMC value of SDS depends not only on the concentration of beta-CD in the buffer electrolyte but also on methanol content in the sample solution. The binding constants of probe molecules to beta-CD, to surfactant molecules, and to the complexes formed between beta-CD and SDS are reported.

Migration behavior and separation of benzenediamines, aminophenols and benzenediols by capillary zone electrophoresis.

The migration behavior and separation of five benzendiamines, five aminophenols and three benzenediols were investigated in capillary zone electrophoresis. The results indicate that benzendiamines and aminophenols are optimally separated with a phosphate buffer at pH 5, whereas benzenediol isomers are best separated at pH about 12. The addition of surfactant monomers of tetradecyltrimethylammonium bromide to a phosphate buffer at pH 5 under the conditions of reversed electroosmotic flow is effective for separating these dye intermediates, except for the separation of 1,2-benzenediol from 1,3-benzenediol. The addition of sodium tetraborate as an electrolyte modifier is effective in the separation of 1,2-benzenediol from 1,3-benzenediol, but the latter comigrates with the 1,4-benzenediol isomer at pH 5.0. The electrophoretic mobility of ionized analytes can be described with Offords equation, and the migration order depends on their ratios of charge to mass. In addition, the pKa values of these analytes in 50 mM phosphate buffer are reported.

Enantioseparation of phenothiazines in capillary zone electrophoresis using cyclodextrins as chiral selectors

In this study, enantioseparations of five phenothiazines, including promethazine, ethopropazine, trimeprazine, methotrimeprazine, and thioridazine, in cyclodextrin (CD)-modified capillary zone electrophoresis were investigated using a phosphate buffer (40 mM) at pH 3.0. We focussed on the separation of phenothiazines with the use of CDs at low concentrations. Three different CDs, including beta-CD, hydroxypropyl-beta-CD (HP-beta-CD) and gamma-CD, were chosen as chiral selectors. The results indicate that effective enantioseparation of phenothiazines, except for methotrimeprazine, is simultaneously achievable with addition of gamma-CD at a concentration of 2.5-6.0 mM. The enantiomers of ethopropazine and trimeprazine are effectively separated with addition of HP-beta-CD at low concentrations, in the range 0.4-6.0 mM, whereas those of promethazine and trimeprazine are baseline resolved with beta-CD at much lower concentrations (0.02-3.0 mM) than with HP-beta-CD. The results also confirm that the separation window is greatly enlarged at low CD concentrations. Moreover, the drastic variations of the electrophoretic mobility of phenothiazines as a function of CD concentration reveal that phenothiazines interact very strongly with CDs in the order gamma-CD