Gyula Vigh

Fast, accurate mobility determination method for capillary electrophoresis.

A new method for accurately determining effective mobilities and electroosmotic flow rates for capillary electrophoresis is described. The proposed method can be performed using most commercial capillary electrophoresis instruments. Problems inherent to the conventional mobility determination method such as a variable electroosmotic flow during the run and migration through unthermostated regions of the capillary are eliminated with the use of the proposed method. In addition, very low effective mobilities and electroosmotic flow rates can be measured quickly and reproducibly. Also, cation mobilities and anion mobilities can be measured in a single run regardless of the magnitude or direction of the electroosmotic flow.

Capillary electrophoretic chiral separations with cyclodextrin additives: I. acids: Chiral selectivity as a function of pH and the concentration of β-cyclodextrin for fenoprofen and ibuprofen

Abstract An equilibrium model has been developed to describe the pH and cyclodextrin concentration dependence of the electrophoretic mobilities as well as the c

On the Determination of the Hold-Up Time in Reversed Phase Liquid Chromatography

Abstract The determination of the hold-up time in reversed phase liquid chromatography has been studied extensively for the mobile phase system methanol-water. Hold-up times obtained by static methods, linearization of homologous series and so-called “unretained compounds” are discussed and mutually compared. Several n-alkyldimethylsilyl bonded phases have been used for this investigation. A rough estimate of the hold-up time can be obtained by using components of the mobile phase or highly concentrated salt solutions, but only for mobile phase compositions around 60% (v/v) methanol. Hold-up times accurate to 1% can be obtained over the complete range of mobile phase compositions from the linearization of net retention times of homologous series.

A family of single-isomer chiral resolving agents for capillary electrophoresis. 2. Hepta-6-sulfato-β-cyclodextrin

A new, hydrophilic, single-isomer charged cyclodextrin, the sodium salt of hepta-6-sulfato-β-cyclodextrin has been synthesized, characterized, and used for the capillary electrophoretic separation of the enantiomers of numerous noncharged, acidic, basic, and zwitterionic analytes. Hepta-6-sulfato-β-cyclodextrin proved to be a much stronger complexing agent for all the analytes tested, in both low-pH and high-pH background electrolytes, than the previously synthesized, moderately hydrophobic heptakis(2,3-diacetyl-6-sulfato)-β-cyclodextrin. The separation selectivities of the two single-isomer, differently functionalized charged cyclodextrins often proved to be complementary. In agreement with the predictions of the charged resolving agent migration model, separation selectivity for the noncharged analytes decreased as the concentration of hepta-6-sulfato-β-cyclodextrin was increased. For acidic, basic, and zwitterionic analytes, selectivity could increase, decrease, or pass a maximum, depending on the binding strength of the enantiomers and ionic mobilities of both the complexed and noncomplexed forms of the enantiomers.

A Family of Single-Isomer Chiral Resolving Agents for Capillary Electrophoresis. 3. Heptakis(2,3-dimethyl-6-sulfato)-β-cyclodextrin

The third member of a new family of single-isomer charged cyclodextrins, the sodium salt of heptakis(2,3-dimethyl-6-sulfato)-β-cycldextrin, has been synthesized, characterized, and used for the capillary electrophoretic separation of the enantiomers of neutral, acidic, basic, and zwitterionic analytes. Though heptakis(2,3-dimethyl-6-sulfato)-β-cyclodextrin complexes much less strongly with any of the analytes tested here than the previously synthesized heptakis(2,3-diacetyl-6-sulfato)-β-cyclodextrin and heptakis-6-sulfato-β-cyclodextrin, it offers excellent enantioselectivities, complementary to those of the other two single-isomer, differently functionalized charged cyclodextrins. Confirming the predictions of the charged resolving agent migration model, heptakis(2,3-dimethyl-6-sulfato)-β-cyclodextrin allowed for the reversal of the migration order of the enantiomers of neutral analytes as the cyclodextrin concentration was increased. Just as with the previous two single-isomer charged resolving agents, separation selectivity for the acidic, basic, and zwitterionic analytes could increase, decrease, or pass a maximum as the cyclodextrin concentration was increased, depending on the respective binding strength of the enantiomers and the ionic mobilities of both the complexed and noncomplexed forms of the enantiomers.

Capillary electrophoretic chiral separations using β-cyclodextrin as resolving agent: II. Bases: Chiral selectivity as a function of pH and the concentration of β-cyclodextrin

Abstract An equilibrium model, first developed to describe the pH and cyclodextrin (CD) concentration dependence of the electrophoretic mobilities as well as the separation selectivities observed during the capillary electrophoretic (CE) separation of the enantiomers of weak acids, is extended to cover the separation of the enantiomers of weak bases as well. Model parameters are derived from three series of CE experiments: (i) one with cyclodextrin-free background electrolytes of varying pH values to obtain the ionic mobilities and base dissociation constant of the weak base enantiomers, (ii) one with constant, low pH background electrolytes with varying concentrations of β-cyclodextrin to obtain the mobilities and formation constants for the protonated base enantiomer-β-cyclodextrin complexes, and (iii) one with constant, high pH background electrolytes with varying concentrations of β-cyclodextrin to obtain the formation constants for the nonprotonated base enantiomer-β-cyclodextrin complexes. Homatropine was used as a model substance to test the validity of the model and an excellent agreement has been found between the calculated and the measured mobility and selectivity values. Baseline separation of the enantiomers of homatropine could be achieved in less than 15 min using a pH 6.25 phosphate buffer, which contained 15 mM β-cyclodextrin.

Dry look at the CHARM (charged resolving agent migration) model of enantiomer separations by capillary electrophoresis

An analysis of the capillary electrophoretic separation of enantiomers using permanently charged cyclodextrins as resolving agents is presented. The charged resolving agent migration model (CHARM model) is based on the consideration of the simultaneous protonation and 1:1-type complexation equilibria that take place in such background electrolytes. Analytical expressions are derived that allow the calculation of the effective charge, effective mobility, separation selectivity and peak resolution values for various enantiomeric analyte classes: non-electrolytes, strong electrolytes and weak electrolytes. Analysis of the resulting surfaces as a function of the pH and the charged cyclodextrin concentration of the background electrolyte indicates that, in the absence of electroosmotic flow (e.g. in neutral-coated capillaries), only two series of measurements have to be carried out to locate the best separation conditions as a function of the concentration of the charged cyclodextrin: one in low-pH background electrolytes (e.g. pH 2.2) and one in high-pH background electrolytes (e.g. pH 9.5 or above). By taking advantage of the predictions of the CHARM model, the utility of a large number of charged cyclodextrins, each offering different intermolecular (enantioselective) interactions can be evaluated in a short period of time for any chiral analyte.

Sample concentration by sample stacking and sweeping using a microemulsion and a single-isomer sulfated β-cyclodextrin as pseudostationary phases in electrokinetic chromatography

Two of the powerful on-line sample concentration techniques, sample stacking and sweeping under pH-suppressed electroosmotic flow, have been evaluated using a microemulsion and a single-isomer sulfated β-cyclodextrin derivative in electrokinetic chromatography. Several clinically relevant steroids have been separated and concentrated using a microemulsion consisting of 100 mM sodium dodecyl sulfate, 41 mM n-heptane and 700 mM 1-butanol in 50 mM phosphoric acid (pH 1.9). Three environmentally relevant phenoxy acid herbicides and their enantiomers have been separated and concentrated using a background electrolyte consisting of 20 mM hepta-6-sulfato-β-cyclodextrin in 15 mM phosphoric acid (pH 1.9). Significant detector response improvements have been achieved and utilized for analysis of a relatively clean matrix, lake water.

Chiral separations by capillary electrophoresis using cyclodextrin-containing gels

Abstract Allyl carbamoylated β-cyclodextrin derivatives were synthesized to be used as chiral resolving agents when copolymerized with acrylamide to form gels suitable for enantiomer separations in capillary electrophoresis. Both solid and liquid gels have been produced by adjusting the concentrations of acrylamide, bis-acrylamide and the allyl cyclodextrin derivative. The liquid gels were free of the problems associated with solid gels: bubble formation, short lifetime and poor reproducibility. The chiral selectivity of the liquid cyclodextrin gels in the separation of dansylated amino acid enantiomers depended on the cyclodextrin concentration of the gel. The liquid gels were successfully used to separate the enantiomers of several chiral molecules.

Capillary electrophoretic chiral separations using cyclodextrin additives. III: Peak resolution surfaces for ibuprofen and homatropine as a function of the pH and the concentration of β-cyclodextrin

Our recent extended peak resolution equation of capillary electrophoresis has been combined with the multiple equilibria-based electrophoretic mobility model of chiral separations to describe peak resolution as a function of the composition of the background electrolyte (pH and the β-cyclodextrin concentration) and a function of the operating variables (effective portion of the applied potential, dimensionless electroosmotic flow coefficient). Using the previously determined model parameters, the resolution surfaces were calculated for a Type I chiral separation (ibuprofen), and a Type III chiral separation (homatropine). In Type I separations resolution can be obtained only over a narrow pH range in the vicinity of the pKa value, and above a minimum value, the concentration of β-cyclodextrin plays a lesser role. In Type III separations, the pH- and β-cyclodextrin concentration-dependent resolution surface has two lobes, on which the migration order of the enantiomers is opposite. This can be an advantage in trace component analysis. In both Type I and Type III separations, peak resolution varies strongly with the dimensionless electroosmotic flow coefficient when its value is changed in the − 1 to 1 range. The loci of the pH-dependent and the β-cyclodextrin concentration-dependent resolution maxima do not shift significantly when the dimensionless electroosmotic flow coefficient is changed. This fact provides the analyst with an additional resolution enhancement tool that does not alter the selectivity of the separation. The utility of the model and its theoretical predictions has been demonstrated by comparing measured and calculated Rs values for ibuprofen and homatropine.