The dynamics of band (peak) shape development in capillary zone electrophoresis in light of the linear theory of electromigration

The continuity equations that describe the movement of ions in liquid solutions under the influence of an external stationary electric field, as it is utilized in electrophoresis, were introduced a long time ago starting with Kohlrausch in 1897. From that time on, there have been many attempts to solve the equations and to discuss the results. In electrophoresis, special attention has always been devoted to the peak shapes obtained by the detector since the shapes have a tight connection with the phenomena taking place during electromigration and influence the efficiency and selectivity of the separation. Among these phenomena, the most important is electromigration dispersion. In this commented review paper, we compare various models of electromigration, try to find points that connect them, and discuss the range of their validity in light of the linear and nonlinear theory of electromigration.     

Peak shapes in open tubular ion-exchange capillary electrochromatography of inorganic anions

An experimental study of parameters influencing peak shapes in ion-exchange open tubular (OT) capillary electrochromatography (CEC) was conducted using adsorbed quaternary aminated latex particles as the stationary phase. The combination of separation mechanisms from both capillary electrophoresis and ion-exchange chromatography results in peak broadening in OT-CEC arising from both these techniques. The sources of peak broadening that were considered included the relative electrophoretic mobilities of the eluent co-ion and analyte, and resistance to mass transfer in both the mobile and stationary phases. The parameters investigated were the mobility of the eluent co-ion, column diameter, separation temperature and secondary interactions between the analyte and the stationary phase. The electromigration dispersion was found to influence peak shapes to a minor extent, indicating that chromatographic retention was the dominant source of dispersion. Improving the resistance to mass transfer in the mobile phase by decreasing the capillary diameter improved peak shapes, with symmetrical peaks being obtained in a 25 microm I.D. column. However, an increase in temperature from 25 degrees C to 55 degrees C failed to show any significant improvement. The addition of p-cyanophenol to the mobile phase to suppress secondary interactions with the stationary phase did not result in the expected improvement in efficiency.     

Conductivity detection in capillary zone electrophoresis: inspection by PeakMaster

A simple rule stating that the signal in conductivity detection in capillary zone electrophoresis is proportional to the difference between the analyte mobility and mobility of the background electrolyte (BGE) co-ion is valid only for systems with fully ionized electrolytes. In zone electrophoresis systems with weak electrolytes both conductivity signal and electromigration dispersion of analyte peaks depend on the conductivity and pH effects. This allows optimization of the composition of BGEs to give a good conductivity signal of analytes while still keeping electromigration dispersion near zero, regardless of the injected amount of sample. The demands to achieve minimum electromigration dispersion and high sensitivity in conductivity detection can be accomplished at the same time. PeakMaster software is used for inspection of BGEs commonly used for separation of sugars (carbohydrates, saccharides) at highly alkaline pH. It is shown that the terms direct and indirect conductivity detection are misleading and should not be used.     

Optimization of injection technique in capillary ion electrophoresis for the determination of trace level anions in environmental samples

Capillary ion electrophoresis (Waters' trade name: Capillary Ion Analysis) offers several advantages compared to ion chromatography for the analysis of ionic solutes, primarily simplicity, matrix independence, and a different separation selectivity. The use of electromigration sample introduction leads to on-capillary enrichment of ionic analytes at the sample—buffer interface, permitting the determination of low ng/ml levels of anions in environmental samples of moderate ionic strength. This injection method allows improved sensitivity compared to hydrostatic injection; and is significantly more rapid than precolumn concentration ion chromatography. The analyte enrichment rate, hence peak response, is strongly dependent upon ionic strength and appropriate measures, such as standard addition or internal standard techniques, must be used to account for differences in standard and sample conductance.     

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.     

Electromigration peak dispersion of isotachophoretic protein zones during capillary zone electrophoresis

The relationships between electromigration dispersion (EMD) and on-line isotachophoresis-capillary zone electrophoresis (ITP-CZE) are described for several basic model proteins and interleukin-6 (rhIL-6). During CZE separation of the highly concentrated analyte zones which were generated during the initial ITP step EMD evolves from intrinsic differences in conductivity between the focused ITP zones and the leading electrolyte. Nearly triangular peaks with a sharp front and diffuse rear side were observed. An electromigration dispersion factor (F_EMD) was introduced to measure peak asymmetry. EMD of individual peaks was shown to increase with the absolute amount of the respective analyte injected and with analyte mobility. Good linearity was observed when F_EMD was plotted against protein mobility (r > 0.95). The slope of the graphs describing this relationship increased with the amount of analyte injected. The influence of EMD on the separation efficiency of neighboring peaks appeared to be less pronounced than expected. Consecutive release from the ITP-stack during transition from ITP to CZE might be an explanation for this observation.     

Generalized model of the linear theory of electromigration and its application to electrokinetic chromatography: Theory and software PeakMaster 6—Next Generation

The linear theory of electromigration, including the first‐order nonlinear approximation, is generalized to systems with any equilibria fast enough to be considered instantaneous in comparison with the timescale of peak movement. For example, this theory is practically applied in the electrokinetic chromatography (EKC) mode of the CZE. The model enables the calculation of positions and shapes of analyte and system peaks without restricting the number of selectors, the complexation stoichiometry, or simultaneous acid–base equilibria. The latest version of our PeakMaster software, PeakMaster 6—Next Generation, implements the theory in a user‐friendly way. It is a free and open‐source software that performs all calculations and shows the properties of the background electrolyte and the expected electropherogram within a few seconds. In this paper, we mathematically derive the model, discuss its applicability to EKC systems, and introduce the PeakMaster 6 software.     

Selectivity in capillary electrokinetic separations

This review gives a survey of selectivity modes in capillary electrophoresis separations in pharmaceutical analysis and bioanalysis. Despite the high efficiencies of these separation techniques, good selectivity is required to allow quantitation or identification of a particular analyte. Selectivity in capillary electrophoresis is defined and described for different separation mechanisms, which are divided into two major areas: (i) capillary zone electrophoresis and (ii) electrokinetic chromatography. The first area describes aqueous (with or without organic modifiers) and nonaqueous modes. The second area discusses all capillary electrophoretic separation modes in which interaction with a (pseudo)stationary phase results in a change in migration rate of the analytes. These can be divided in micellar electrokinetic chromatography and capillary electrochromatography. The latter category can range from fully packed capillaries, via open-tubular coated capillaries to the addition of microparticles with multiple or single binding sites. Furthermore, an attempt is made to differentiate between methods in which molecular recognition plays a predominant role and methods in which the selectivity depends on overall differences in physicochemical properties between the analytes. The calculation of the resolution for the different separation modes and the requirements for qualitative and quantitative analysis are discussed. It is anticipated that selectivity tuning is easier in separation modes in which molecular recognition plays a role. However, sufficient attention needs to be paid to the efficiency of the system in that it not only affects resolution but also detectability of the analyte of interest.     

Evaluation of analyte protectants to improve gas chromatographic analysis of pesticides

A common problem in gas chromatography (GC) applications is the analyte losses and/or peak tailing due to undesired interactions with active sites in the inlet and column. Analytes that give poor peak shapes or degrade have higher detection limits, are more difficult to identify and integrate, and are more prone to interferences than stable analytes that give narrow peaks. For susceptible analytes, significant peak quality improvements are obtained when matrix components are present because they fill active sites, thus reducing analyte interactions. This phenomenon is called "matrix-induced chromatographic response enhancement." Several approaches have been proposed to minimize peak distortion phenomena and compensate for matrix-induced effects, which is especially important for accurate quantitation, but each approach has serious limitations for routine multi-pesticide analysis. In this study, we demonstrate the feasibility of using "analyte protectants" to provide a more convenient and effective solution to the problem than other approaches developed thus far. The protecting agents are added to extracts and matrix-free standards alike to provide the chromatographic enhancement effect even for the most susceptible analytes in a very dirty GC system. In this study, we evaluated 93 different compounds to find the most suitable ones for improving chromatographic quality of the signal. Because hydrogen bonding has been shown to be an important factor in analyte interactions with active sites, we mainly focused on additives with strong hydrogen bonding capabilities. Dramatic peak enhancements were achieved using compounds containing multiple hydroxy groups, such as sugars and sugar derivatives, and gulonolactone appears to be the most effective protecting agent for the most pesticides that we tested. The benefits of using analyte protectants versus alternative procedures for overcoming matrix-induced effects in quantitation include: (a) simpler procedure; (b) easier integration of peaks; (c) lower detection limits; (d) better quantitation; (e) less maintenance of the GC inlet; and (e) lower cost. However, long-term influences on the performance of the chromatographic system have yet to be established.     

Simul 5 - free dynamic simulator of electrophoresis

We introduce the mathematical model of electromigration of electrolytes in free solution together with free software Simul, version 5, designed for simulation of electrophoresis. The mathematical model is based on principles of mass conservation, acid-base equilibria, and electroneutrality. It accounts for any number of multivalent electrolytes or ampholytes and yields a complete picture about dynamics of electromigration and diffusion in the separation channel. Additionally, the model accounts for the influence of ionic strength on ionic mobilities and electrolyte activities. The typical use of Simul is: inspection of system peaks (zones), stacking and preconcentrating analytes, resonance phenomena, and optimization of separation conditions, in either CZE, ITP, or IEF.     

Indirect spectrophotometric detection of inorganic anions in ion-exchange capillary electrochromatography

The application of indirect spectrophotometric detection was investigated for a capillary electrochromatographic system in which an anion-exchange stationary phase (in the form of aminated latex particles) was coated onto the wall of a fused-silica capillary. The study has focused on the choice of the type and concentration of the absorbing coion (probe) added to the background electrolyte and the role of this species in manipulating the ion-exchange contributions to the separation with a view to controlling the selectivity of the separation. Common inorganic anions were used as analytes and nitrate, p-toluenesulfonate, nicotinate, and chromate were investigated as probes. It was found that most of these probes produced only a limited range of separation selectivities when their concentration was varied over the practically accessible range. p-Toluenesulfonate provided the greatest variation in selectivity, but peak distortion due to electromigration dispersion was evident for the faster ions. When variation of the separation selectivity - from predominantly electrophoretic in nature to predominantly ion-exchange in nature - was desired, this was best achieved by varying the type of probe rather than its concentration. For example, the nitrate probe provided predominantly electrophoretic separations with good peak shapes and high efficiencies. A comprehensive list of probes, ranked in order of ion-exchange selectivity coefficients determined by ion chromatography, was compiled and this proved to be a useful tool to assist in the selection of a probe for a desired separation selectivity. The limits of detection for the analytes and probes studied ranged from 20-55 micromol for the chromate system to 230-600 micromol for the nicotinate system, with nitrate and p-toluenesulfonate giving intermediate values.     

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.     

Micellar electrokinetic chromatography for high‐performance analytical separation

Capillary electrophoresis (CE) is a relatively new method of analytical separation having the advantages of high separation efficiency, requirement of a small sample amount, low operating cost, and fast separation time. CE is a separation method where the analyte migrates under an electric field due to a charge on the analyte. Hence, CE was unable to separate neutral analytes until the advent of micellar electrokinetic chromatography (MEKC). MEKC is performed with an addition of ionic micelles to an electrophoretic medium, where a portion of the analyte is incorporated into the micelle and has an apparent charge, which can be subject to electrophoretic separation. The migration velocity of the neutral analyte in MEKC depends on what portion of the analyte is incorporated into the micelle. Thus, the separation principle of MEKC is similar to that of chromatography, although the micelle corresponding to the stationary phase in chromatography is not stationary inside the capillary. The fundamental characteristics and theoretical treatments of the behavior of the analyte in MEKC were studied extensively by the author's group. MEKC has been established as one of the most popular separation modes in CE. This review describes how MEKC was developed and how it is useful as a method of analytical separation. © 2008 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 8: 291–301; 2008: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.20156     

Onset of non-linearity in zonal elution separators: the concept of effective analyte concentration

When an analyte injected in a zonal separation method (chromatography, capillary zone electrophoresis, field-flow fractionation) is not highly diluted in the carrier fluid, the retention ratio, R--or ratio of the cross-sectional average migration velocity of the analyte to that of the carrier fluid--depends on the local concentration, c, of the center of mass of the analyte zone, and the zone migration occurs in non-linear conditions. Because the zone broadens as it moves along the separator, R varies continuously from the inlet to the outlet of the separator. That concentration, c(eff), for which R(c(eff)) is equal to the length-averaged apparent retention ratio, R(app), is called effective concentration, and that distance, z(eff), from the separator inlet, for which c(z(eff)) is equal to c(eff), i.e. for which R(z(eff)) is equal to R(app), is called effective position. Assuming that near the onset the non-linear behavior, R(c), is a linear function, values of R(app), c(eff) and z(eff) have been computed in a wide range of operating conditions which are typical of situations encountered in capillary zone electrophoresis, liquid chromatography, or field-flow fractionation. Computations have been performed both in presence and in absence of the dispersion arising from the concentration dependence of the analyte migration rate (called thermodynamic dispersion in chromatography or electromigration dispersion in capillary zone electrophoresis). It is found that, whatever the range of analyte concentration covered from inlet to outlet of the separator, c(eff) is always close to two times the analyte concentration, c(out), at the outlet of the separator, and z(eff) between one-fourth and one-third of the separator length. As c(out) is easily determined from the peak recorded by a concentration-sensitive detector, a simple pragmatic expression is given for the estimation of c(eff). This effective concentration is the appropriate concentration to be used for comparing predictions of theoretical models of R(c) with experimental retention data. This is of particular interest for validating such models in field-flow fractionation.     

An Insight into Peak‐Splitting Phenomenon in On‐Column Concentration‐Micellar Electrokinetic Capillary Chromatography for Aqueous Sample Solution

Abstract

A detailed study was carried out to investigate the origin of the peak‐splitting phenomena in on‐column concentration‐micellar electrokinetic capillary chromatography for aqueous sample solution. The system studied was a basic phosphate and borax mixed buffer with sodium dodecyl sulfate (SDS) as micellar phase. Phenol, benzyl alcohol, phenyl ethanol, salicylic acid, and p‐hydroxy benzyl acid were selected as the analytes. Several factors that affect peak splitting were investigated. The injection time, SDS micellar concentration, hydrophobicity of the analytes, and analytes concentration were the most important factors. A hypothesis was proposed to explain the peak‐splitting phenomena. Several means to avoid peak‐splitting phenomena were proposed, such as controlling sample injection time and hydrophobicity of the analyte, decreasing SDS concentration and increasing sample concentration. However, the most practical method for avoiding peak splitting was to control the sample injection time.     

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.     

Separation selectivity patterns of fully charged achiral compounds in capillary electrophoresis with a neutral cyclodextrin

Based on the separation selectivity equation, related to the dimensionless parameters for fully charged achiral analytes using a neutral CD, the separation selectivity can be classified into seven patterns. With respect to CZE without CD, the presence of CD in the buffer may improve, or reduce, the separation selectivity with this effect being accompanied by the same or reversed electrophoretic mobility order for charged analytes. This can depend on the separation selectivity of the two analytes in free solution, the binding selectivity, the separation selectivity of analyte–CD complexes and the ratio of electrophoretic mobility of the analytes in free, and complexed forms. Using positional isomers of benzoic acids and phenoxy acids as test analytes and α‐CD as a selector, the observed separation selectivity shapes were found to be in excellent agreement with the predicted separation selectivities.     

Rapid and quantitative extraction and analysis of trace organics using directly coupled SFE-GC

Supercritical fluid extraction can be coupled with capillary gas chromatography (SFE-GC) using commercially-available on-column or split/splitless injection ports. While liquid solvent extractions require several hours or even days to perform, SFC-GC analyses can be completed in ≤ 1 hour including extraction, analyte concentration, and GC separation. SFE-GC yields chromatographic peak shapes that compare favorably with those obtained using conventional liquid solvent injections. Quantitative extraction and recovery of analytes is usually achieved in 10 minutes, and maximum sensitivity is obtained since the extracted analytes can be quantitatively transferred into the GC column for cryogenic focusing prior to GC analysis. SFE-GC analysis of a variety of organic pollutants from environmental solids and sorbent resins, and flavor and fragrance compounds from food products will be discussed.     

Improvement of reproducibility in capillary zone electrophoresis by sequential application of high voltage and pressure

In capillary zone electrophoresis (CZE) problems arise concerning the reproducibility of the measurements. This is due to variations in electroosmotic flow and analyte velocities. The discontinuous use of high voltage for the separation of analytes and the pressure for the transport to the detector offers the possibility to suppress the main causes for these phenomena. The procedure is demonstrated for the separation of 4 phenol derivatives. An increase in reproducibility is found for standard solutions from about 3% to 1%.