Migration Behavior Of Analytes Research Articles

Analyte Distribution at Channel Intersections of Electro-Fluid-Dynamic Devices

Mass conservation is the guiding principle for analyte distribution at channel intersections of microfluidic devices, where analyte migration is mainly driven by an applied electric field, and in electro-fluid-dynamic (EFD) devices, where multiple fields and pressures can be applied simultaneously on the same channel network. This paper introduces another type of conservation, the conservation of effective volumetric flow rate, at channel intersections when the conductivity of the solution in the intersecting channels is maintained constant. This conservation principle provides an additional criterion needed to describe analyte migration in channels connecting to a common intersection and to predict how analyte is distributed into individual channels in the channel network of EFD devices, when multiple voltages and pressures are applied. The theoretical bases of effective volumetric flow rate balance are discussed, and the potential use of this principle in conjunction with the principle of mass conservation to predict the migration behavior of analytes is demonstrated. Junctions of different geometry in EFD devices are used to demonstrate the validity of these equations, and the measured velocities and numbers of microbeads in each channel agree with the predicted values.

Reverse of Mixing Process with a Two-Dimensional Electro-Fluid-Dynamic Device

Mixing of two solutions into one is a spontaneous process with a net increase in entropy. However, the reverse of the mixing process is usually not possible unless certain conditions are met. A continuous solution stream containing a mixture of two compounds can be separated into two channels, each containing a pure compound, thus reversing the mixing process using a two-dimensional microfluidic electro-fluid-dynamic (EFD) device. When the electric field is strategically applied in the interconnecting channels of an EFD device, the pressure required to direct an analyte into a certain channel can be calculated by using the solutions of electric field and fluid dynamics in the mass balance equation. If the pressure and electric potential at various inlets and outlets satisfy these predetermined conditions, the reverse of a mixing process is observed. Conventional microfluidic devices have been used to introduce samples from interconnecting channels or efficiently mix different solutions into a single channel. The EFD devices expand the spatial separation of analytes from one dimension to two using both the differential migration behavior of analytes and the velocity field distribution in different channel geometries. The devices designed according to these basic physicochemical principles can be used for complete processing of minute samples and to obtain pure chemical species from complex mixtures.

The preparation of background electrolytes in capillary zone electrophoresis: golden rules and pitfalls.

In this article the methodology of the design of suitable background electrolytes (BGEs) in capillary zone electrophoresis (CZE) is described. The principal aspects of the role of a BGE in CZE are discussed with respect to an appropiate migration behavior of analytes, including the transport of the electric current, the buffering of pH, the Joule heat, the electro-endosmotic flow (EOF) and the principal migration and detection modes. The impact of the composition of the BGE upon migration and detection is discussed. It is shown that the total concentration of the BGE is a principal factor and the adjustment of migrating analyte zones according to the Kohlrausch regulating function (KRF) is the principal effect in most of the sample stacking techniques. The number of co-ions and their properties are of key importance for peak shapes of the analyte peaks and for the existence of system zones. The detection of UV-transparent analytes may advanteously be done in the indirect UV mode, by using UV-absorbing co-ions, however, both peaks and dips may be expected in the UV trace in case of multiple co-ionic BGEs. Properties of BGEs can be predicted applying mathematical models and it is shown that with SystCharts, predictions can be given concerning the existence of system zones, detection modes and the peak shapes of analytes for a given BGE. Practical examples of methodological considerations are given in the design of suitable BGEs for four principal combinations of migration and detection modes. The properties of the BGEs selected are exemplified with experimental results. Golden rules are summarized for the preparation of suitable BGEs in CZE.

Titanium dioxide coated surfaces for capillary electrophoresis and capillary electrochromatography.

The potential of titanium dioxide coatings to control the electroosmotic flow and to affect the migration behavior of analytes in capillary electrophoresis and open-tubular capillary electrochromatography was evaluated. The inner wall of a fused-silica capillary was applied with a solution of a titanium peroxo complex, followed by heating at an elevated temperature. The resultant product was ascertained to be titanium dioxide in a crystalline form of anatase by the results of Fourier transform-infrared spectrometry (FT-IR), X-ray photoelectron spectroscopy, and Raman spectroscopy. The capillary thus made had anodic or cathodic electroosmotic flow, depending on the pH and composition of the background electrolyte used. The titanium dioxide surface of the capillary was readily modified by a silanizing reagent. The performance of the titanium dioxide surfaces with or without chemical modifications was examined with inorganic anions, neutral compounds and peptides.

Analytical Chemistry for Environmental and Human Health. Separation of structural isomers of chlorinated phenols by capillary electrophoresis with a volatile running solution.

The simultaneous separation of chlorinated phenols including structural isomers by capillary electrophoresis was studied.Structural isomers of mono- and dichlorophenols and phenol were selected as analytes because of their low hydrophobicity.In order to detect them using mass spectrometry in the future, three volatile running solution(ammonium acetate, ammonium hydrogencarbonate and ammonium carbonate)were investigated.The migration behavior of analytes was found to be governed by their dissosiation constants.When an ammonium hydrogencarbonate solution(pH8.1)was used, the peak shapes were the best, and all dichlorophenol isomers completely separated.The optimized concentration of the solution was 20mM.All analytes were completely separated within 8 minutes.

Predicting Peak Symmetry in Capillary Zone Electrophoresis. Background Electrolytes with Two Co-ions: Schizophrenic Zone Broadening and the Role of System Peaks

This paper investigates fundamental phenomena in the migration behavior of analytes in the very frequent case in current capillary zone electrophoresis practice of using background electrolytes with two co-ions. It is shown that in such background electrolyte systems there always exists a regular system phenomenon which brings enormous electromigration dispersion to certain analytes. The analytes which yield to this phenomenon may be predicted by calculating the so-called velocity slope S X , which is a measure of zone symmetry and broadening. On the example of a simplified model system, it is shown explicitly that these analytes coincide by their mobilities with the mobility of the system peak. The explanation for these effects is given on the basis of schizophrenic behavior where one and the other co-ions force the analyte zone to be fronting and tailing simultaneously. Detailed theoretical description of the behavior of analyte zones in binary co-ion background electrolytes is presented using peak shape diagrams that provide a comprehensive view of their electromigration dispersion characteristics. Experimental verification of the theory is given, together with a detailed discussion of the effect of the background electrolyte composition on the magnitude and extent of appearance of schizophrenic zone broadening.

Higher Order Equilibria and Their Effect on Analyte Migration Behavior in Capillary Electrophoresis

This paper presents a quantitative investigation into the effect of analyte-additive interactions on analyte migration behavior in capillary electrophoresis (CE) when both 1:1 and 1:2 stoichiometries are present. Equations based on the individual capacity factors for each interaction are derived to account for the effect of both first- and second-order equilibria. The analyte migration behavior is described using these equations with a full account of how the microscopic equilibrium constants and microscopic mobilities are combined to give the macroscopic values. The binding isotherms of interactions with both 1:1 and 1:2 stoichiometries are compared with those of a 1:1 stoichiometry. 4,4'-Biphenol and 4-phenylphenol were chosen as analytes that undergo complexation with one and two hydroxypropyl-β-cyclodextrin (HP-β-CD) molecules; phenol was used as an analyte that interacts with only one HP-β-CD molecule. The process of calculating higher order equilibrium constants and complex mobilities from the binding isotherms is demonstrated. The effects of experimental conditions, such as the additive concentration range and the number of data points, on the error in the calculated constants and the ability of the equations to accurately describe the experimental data are discussed. A comparison of the linear transformations of the binding isotherm with respect to their ability to detect higher order equilibria is made, and the advantage of using the capacity factor in CE is illustrated.

The effect of complexation additives on analyte migration behavior in capillary electrochromatography.

In capillary electrochromatography (CEC), analytes often have different mobilities in the mobile phase, and often are involved in multiple equilibria. In this paper, the migration behavior of an analyte in CEC is described by a general equation in which individual capacity factors are used to describe the tendency of the analyte to exist as the various analyte species present in a separation system, and the effects of both field and equilibrium are accounted for. The resolution of two analytes is shown to be related linearly to the ratio of their migration rates. The effect of the electroosmotic flow (EOF) in CEC is more complicated than in CE because it is experienced only by a fraction of the analyte, whereas in CE, it is experienced by all analyte species. A procedure for calculating the electrophoretic mobility of the analyte based on the fraction of the analyte in the buffer is demonstrated. The effect of the EOF on resolution is also discussed.

The effects of a mixture of charged and neutral additives on analyte migration behavior in capillary electrophoresis.

Multicomponent additives, such as derivatized cyclodextrins with various degrees of substitution, can be considered single-component additives as long as the fraction of each component remains constant. In this paper, equations are derived describing the effect of such additives on the migration behavior of analytes. These equations are used in the study of capillary electrophoresis (CE) systems with differentially charged cyclodextrins as additives. For weakly acidic analytes, the binding with highly negatively charged sulfobutyl ether beta-cyclodextrin (SBE-beta-CD) increases their negative electrophoretic mobility, while the binding with neutral hydroxypropyl-beta-cyclodextrin (HP-beta-CD) decreases their negative mobility. By obtaining the equilibrium constants and mobilities for each additive with each analyte (in this case, phenol, 2-naphthol and 1-naphthol), the migration behavior of these analytes in CE systems is quantitatively predicted at various concentrations of mixtures of the two additives. The properties of the contour lines in the binding isotherm surfaces of such CE systems are discussed.

Dynamic complexation of solutes in capillary electrophoresis.

The analyte migration behavior in any chemical separation system can be described using a single equation that unifies all areas of separation science. This equation can be used in capillary electrophoresis (CE) to design separation systems, and to study interactions between analytes and additives. By using individual capacity factors for each analyte species present in the system, and with the knowledge of the characteristics of each interaction, one can predict the analyte migration behavior in complicated CE systems, including systems with multiple 1:1 interactions and/or higher order interactions.

High-performance capillary electrophoretic analysis of synthetic food colorants

A high-performance capillary electrophoresis method with diode-array detection has been developed for analysis of synthetic food colorants. The influence of buffer composition on the separation of the food colorants was examined, as were the effects of α-, β- and γ-c-yclodextrins on analyte migration behavior. Eight food colorants were completely separated within 10 min using pH 9.5 borax—NaOH buffer containing 5 mM β-cyclodextrin. Experimental results indicate that the relative standard deviations of analyte migration times were<0.88% under the optimized separation condition. Correlation coefficients of the linear calibration plots of the analytes exceeded 0.998. The method was suitable for determination of the quantities of synthetic food colorantsi in ice cream bars and fruit soda drinks.

Prediction of the migration behavior of analytes in capillary electrophoresis based on three fundamental parameters

The prediction of analyte migration behavior in capillary electrophoresis (CE) is essential for rapid method development. The dynamic complexation model, based on 1:1 interactions, was used to accurately predict the apparent electrophoretic mobilities and the migration times of a group of deoxyribonucleotides (dNPs) at various concentrations of β-cyclodextrin (β-CD). The electrophoretic mobility of the analyte, the electrophoretic mobility of the analyte-additive complex and the equilibrium constant are the three fundamental parameters required to determine the mobility of an analyte. The apparent migration time of the analyte can be predicted once the electroosmotic mobility and relative viscosity of the solution are known. Optimum separation conditions can be determined based on these parameters. Excellent agreement between observed analyte migration behavior and predicted values was demonstrated, with relative errors being often less than 1%. The theory was tested repeatedly under various conditions in order to assess its predictive capabilities and limitations. Analysis by molecular modeling, in conjunction with calculated electrophoretic parameters and equilibrium constants, provided deeper insight into the probable mechanisms of the separation process at the molecular level.

Quantitative description of analyte migration behavior based on the dynamic complexation model in capillary electrophoresis

A theory based on dynamic complexation is used to describe analyte migration behavior in capillary electrophoresis (CE). This theory is based on a one-phase system, instead of the commonly accepted two-phase system. The migration behavior of an analyte is described by three parameters (the electrophoretic mobility of the free analyte, the electrophoretic mobility of the analyte–additive complex, and the equilibrium constant (formation constant) that determines the fractions of the free analyte and the complex at a certain additive concentration). Varying the additive concentration shifts the equilibrium and changes the viscosity of the background electrolyte. Viscosity correction is crucial in interpreting the observed migration behavior of analytes. While electroosmotic flow in a capillary often varies from one capillary to another, the viscosity of a buffer is characteristic of the buffer composition and is constant for each buffer. The electrophoretic mobility of a certain species and the equilibrium constant are intrinsic properties and are less sensitive to changes in the environment. Understanding these relationships is indispensable in CE method development and method validation. A universal resolution equation is proposed, with a separation factor that has taken both the electrophoretic mobilities and equilibria into consideration. This resolution equation gives clear guidance for the optimization of CE separations. A group of nucleosides and their phosphates are used as analytes, and β-cyclodextrin is used as the additive in the model system studied in this paper. Both the observed analyte migration behavior and the resolution of analytes agree well with this theory. Keywords: dynamic complexation capillary electrophoresis, nucleoside and nucleotide separation, capacity factor, resolution equation, viscosity correction.

Analysis of prostaglandins by micellar electrokinetic capillary chromatography

This paper presents a rapid and reliable micellar electrokinetic capillary chromatography (MEKC) method to separate major prostaglandins and thromboxane B 2. The running buffer was modified with sodium dodecyl sulfate (SDS). The effects of the SDS concentration on the migration behavior of analytes was also examined. Moreover, the influences of electrolyte concentration and capillary temperature on the separation were investigated. In optimum conditions, seven major prostaglandins and thromboxane B 2 could be separated within 8 min. The relative standard deviations of the migration times (reproducibility) of the analytes were less than 0.82%.

Quantitative description of analyte migration behavior based on dynamic complexation in capillary electrophoresis with one or more additives.

A comprehensive theory is proposed to describe the migration behavior of analytes in capillary electrophoresis (CE) when one or more additives are present in the buffer solution. This theory amalgamates and extends the previous work done by others. The capacity factor (k') in this theory is defined as the product of the equilibrium constant and the additive concentration, thus, k' changes linearly with additive concentration. The net electrophoretic mobility of an analyte is a function of k', therefore, it can be changed by varying the additive concentration. Three parameters are needed to predict the mobility of an analyte in a one-additive CE system: the mobility of the free analyte, the mobility of the complex, and the equilibrium constant for the analyte-additive interaction (which determines the fraction of the free analyte at different additive concentrations). When additives are used, the change in viscosity obscures this relationship, therefore, a viscosity correction factor is required to convert all mobilities to an ideal state where the viscosity remains constant. The migration behavior of an analyte in a solution with multiple additives can be predicted and controlled, once the equilibrium constants of the interactions between the analyte and each of the additives are obtained separately. beta-Cyclodextrin and hydroxypropyl-beta-cyclodextrin are used as additives and the migration behavior of phenol, p-nitrophenol, and benzoic acid are studied as a model system to verify this theory. When the necessary viscosity correction factor is included, the net electrophoretic mobilities of the analytes obtained from experimental results agree with the values predicted by the theory based on dynamic complexation. Although only experiments with one and two additives were carried out to verify the theory, the equations apply to situations when more than two additives are used. The relationship between the theories of electrophoresis and chromatography is clarified.

Migration behavior of dichlorophenols for replicate separations without replenishment of buffer electrolyte in micellar electrokinetic capillary chromatography

The migration behavior of dichlorophenols in replicate separations, without replenishment of buffer electrolyte between runs, in micellar electrokinetic capillary chromatography (MECC) was investigated. The results indicated that the pH of the buffer electrolyte drifts as a result of electrolysis of the buffer solution during electrophoresis and that the pH drift, being independent of the micelle concentration, is responsible for the variation in electrophoretic mobilities of analytes in MECC when the buffer is not replenished. The influences of the buffer pH and micelle concentration on the migration behavior of analytes were demonstrated to be correlated at small micelle concentrations. The variation of electrophoretic mobility of an analyte as a function of buffer pH at a given micelle concentration is quantitatively described. 2,6-Dichlorophenol is selected for illustration. The variation of electrophoretic mobility agrees satisfactorily between predicted and observed data.