Bohuslav Gaš

Optimization of the high-frequency contactless conductivity detector for capillary electrophoresis.

Two constructions of the high‐frequency contactless conductivity detector that are fitted to the specific demands of capillary zone electrophoresis are described. The axial arrangement of the electrodes of the conductivity cell with two cylindrical electrodes placed around the outer wall of the capillary column is used. We propose an equivalent electrical model of the axial contactless conductivity cell, which explains the features of its behavior including overshooting phenomena. We give the computer numerical solution of the model enabling simulation of real experimental runs. The role of many parameters can be evaluated in this way, such as the dimension of the separation channel, dimension of the electrodes, length of the gap between electrodes, influence of the shielding, etc. The conception of model allows its use for the optimization of the construction of the conductivity cell, either in the cylindrical format or in the microchip format. The ability of the high‐frequency contactless conductivity detector is demonstrated on separation of inorganic ions.

High-frequency contactless conductivity detection in isotachophoresis

Summary A new detection system for isotachophoresis, the high-frequency contactless conductivity detector, is described. This detector has a high resolving power and gives good reproducibility.

Optimization of background electrolytes for capillary electrophoresis I. Mathematical and computational model.

A mathematical and computational model is introduced for optimization of background electrolyte systems for capillary zone electrophoresis of anions. The model takes into account mono- or di- or trivalent ions and allows also for modeling of highly acidic or alkaline electrolytes, where a presence of hydrogen and hydroxide ions is significant. At maximum, the electrolyte can contain two co-anions and two counter-cations. The mathematical relations of the model are formulated to enable an easy algorithmization and programming in a computer language. The model assesses the composition of the background electrolyte in the analyte zone, which enables prediction of the parameters of the system that are experimentally available, like the transfer ratio, which is a measure of the sensitivity in the indirect photometric detection or the molar conductivity detection response, which expresses the sensitivity of the conductivity detection. Furthermore, the model also enables the evaluation of a tendency of the analyte to undergo electromigration dispersion and allows the optimization of the composition of the background electrolyte to reach a good sensitivity of detection while still having the dispersion properties in the acceptable range. Although the model presented is aimed towards the separation of anions, it can be straightforwardly rearranged to serve for simulation of electromigration of cationic analytes. The suitability of the model is checked by inspecting the behavior of a phosphate buffer for analysis of anions. It is shown that parameters of the phosphate buffer when used at neutral and alkaline pH values possess singularities that indicate a possible occurrence of system peaks. Moreover, if the mobility of any analyte of the sample is close to the mobilities of the system peaks, the indirect detector signals following the background electrolyte properties will be heavily amplified and distorted. When a specific detector sensitive on presence of the analyte were used, the signal would be almost lost due to the excessive dispersion of the peak.

Optimization of background electrolytes for capillary electrophoresis: II. Computer simulation and comparison with experiments.

A mathematical and computational model described in the previous paper (Gaš, B., Coufal, P., Jaroš, M., Muzikář, J., Jelínek, L., J. Chromatogr. A 2001, 905, 269–279) is adapted, algorithmized, and a computer program PeakMaster having a status of freeware (http://natur.cuni.cz/∼gas) is introduced. The model enables optimization of background electrolyte (BGE) systems for capillary zone electrophoresis. The model allows putting to use uni‐ or di‐ or trivalent electrolytes and allows also for modeling highly acidic or alkaline BGEs. It takes into account the dependence of ionic mobilities and dissociation of weak electrolytes on the ionic strength. The model calculates the effective mobility of analytes and predicts parameters of the system that are experimentally available, such as the transfer ratio, which is a measure of the sensitivity in the indirect UV detection or the molar conductivity detection response, which expresses the sensitivity of the conductivity detection. Further, the model enables evaluation of a tendency of the analyte to undergo electromigration dispersion or peak broadening. The suitability of the model is verified by comparison of the predicted results with experiments, even under conditions that are far from ideal (under extreme pH and a high ionic strength).

Dispersive phenomena in electromigration separation methods

A review on dispersive effects and on peak broadening in electromigration separation methods (capillary electrophoresis and electrochromatography) is presented, mainly covering papers published between the beginning of 1997 and the beginning of 2000. Most attention is drawn to work dealing with nonlinear effects that cause anomalous electromigration dispersion in electrolyte systems with two or multiple coions. Further, topics cover the comparison of electroosmotic and pressure‐driven modes in electrochromatography, dispersive effects due to nonhomogeneous velocity fields in packed electrochromatography columns, to nonuniform electroosmotic flow, to sorption of analytes (mainly proteins) at the column wall or the stationary phase, and due to the influence of the nonideal column geometry like coiling or irregularities in shape.

Electroosmosis in capillary zone electrophoresis with non-uniform zeta potential

Abstract The influence of the longitudinally non-uniform zeta potential on processes in capillary zone electrophoresis was studied. The velocity field of the electroosmotic flow in capillary tubes is modelled by the Navier-Strokes equations. Their stationary solution represents connective transport of a solute which is taken into account in the continuity equation for the concentration distribution. All equations are studied numerically. The results represent the time evolution of initials forms of sample peaks. These are presented in graphical form for several cases of zeta potentials which are either instructive or closely related to situations encountered in practice. It is shown that plug-like flow in the capillary cannot be expected and that a non-uniform zeta potential generally leads to significant dispersion of peaks.

Extension of the application range of UV-absorbing organic solvents in capillary electrophoresis by the use of a contactless conductivity detector

A contactless conductivity detection (CCD) system is used for capillary zone electrophoresis (CZE) with non-aqueous solvents of the buffering background electrolyte, which exhibit strong UV absorbance below 230 nm. It is found that the CCD characteristics with such solvents (propylene carbonate, N,N-dimethylformamide and N.N-dimethylacetamide as examples) is the same as with aqueous solutions: the same signal and noise is obtained for a given electric conductance of the background electrolyte, independent of the kind of the solvent. Therefore CCD enables the extension of the application range to solvents with restricted use for common UV detection in CZE due to their unfavourable or even unfitting optical properties. The applicability of CCD is demonstrated by CZE of aliphatic ammonium compounds in these solvents.

Simulation of the effects of complex‐ formation equilibria in electrophoresis: I. Mathematical model

Simul 5 Complex is a one‐dimensional dynamic simulation software designed for electrophoresis, and it is based on a numerical solution of the governing equations, which include electromigration, diffusion and acid–base equilibria. A new mathematical model has been derived and implemented that extends the simulation capabilities of the program by complexation equilibria. The simulation can be set up with any number of constituents (analytes), which are complexed by one complex‐forming agent (ligand). The complexation stoichiometry is 1:1, which is typical for systems containing cyclodextrins as the ligand. Both the analytes and the ligand can have multiple dissociation states. Simul 5 Complex with the complexation mode runs under Windows and can be freely downloaded from our web page http://natur.cuni.cz/gas. The article has two separate parts. Here, the mathematical model is derived and tested by simulating the published results obtained by several methods used for the determination of complexation equilibrium constants: affinity capillary electrophoresis, vacancy affinity capillary electrophoresis, Hummel–Dreyer method, vacancy peak method, frontal analysis, and frontal analysis continuous capillary electrophoresis. In the second part of the paper, the agreement of the simulated and the experimental data is shown and discussed.

Enhanced selectivity in CZE multi-chiral selector enantioseparation systems: proposed separation mechanism.

It has been reported many times that the commercial mixtures of chiral selectors (CS), namely highly sulfated β‐CDs (HS‐β‐CDs), provide remarkable enantioselectivity in CZE when compared with single‐isomer CDs, even single‐isomer HS‐β‐CDs. This enhanced enantioselectivity of multi‐CS enantioseparative CZE is discussed in the light of multi‐CS model that we have introduced earlier. It is proposed on a theoretical basis and verified experimentally that the two enantiomers of a chiral analyte under interaction with a mixture of CSs are very likely to differ in their limit mobilities, which is opposite to single‐CS systems where the two limit mobilities are likely to be the same. Thus while the enantioseparation is usually controlled by different distribution constants between the two enantiomers and CS used in single‐CS systems, an additional, electrophoretic, enantioselective mechanism resulting from different limit mobilities may play a significant role in multi‐CS systems. This additional mechanism generally makes the multi‐CS systems more selective than the single‐CS systems. The possible inequality of limit mobilities is also significant for optimization of separation conditions using mixtures of CSs. A practical example supporting our considerations is shown on enantioseparation of lorazepam in the presence of a commercial mixture of HS‐β‐CDs and a single‐isomer HS‐β‐CD, heptakis(6‐O‐sulfo)‐β‐CD.

Computer-aided simulation of electromigration

Abstract Equations that describe the electrophoretic migration of monovalent ionic substances in solution with a significant presence of H + or OH − ions are formulated. A derivation of the Kohlrausch regulating function for these conditions is presented. The model of electromigration consists of a set of continuity equations, together with a set of algebraic equations describing the chemical equilibria involved, and is implemented on a personal computer. Simulation of some experimental phenomena in electrophoretic methods, e.g ., the sharpening effect in capillary zone electrophoresis or anomalous spikes in isotachophoretic systems, is presented.