Peak broadening in capillary zone electrophoresis.

A review on peak broadening in capillary zone electrophoresis in free solutions is given which covers a selection of the literature published on this topic over the period mainly between 1992 and the beginning of 1997 (consisting of 71 publications). The contributions to peak dispersion from extracolumn effects (e.g. due to the finite length of the injection zone, or the aperture of the detector), from longitudinal diffusion, Joule heating, electromigration dispersion (concentration overload), a different path length of the solute ions, wall adsorption, laminar flow and the (longitudinally) homogeneous or nonhomogeneous electroosmotic flow are described. The latter may also occur when a longitudinally nonhomogeneous radial electric field is applied. Peak dispersion is depicted either by the plate-height model, or the concentration of the solute as a function of space and time is calculated either analytically or numerically by solving the equation of continuity with appropriate initial and boundary conditions and possibly completed by equations governing further quantities.

Influence of transference number on migration and deformation of concentration boundaries in capillary zone electrophoresis.

The change of the position and shape of a concentration boundary existing in the background electrolyte in capillary zone electrophoresis is treated theoretically. Such boundaries may occur when the ionic concentration, c, of the sample differs from that of the background electrolyte. Both mathematical treatment and numerical simulation lead to the result that when an (initially symmetrical) concentration boundary migrates in the capillary it becomes nonsymmetrical. The reason for this is the nonsymmetry of the function, which is obtained as the difference of the transference number (which is a function depending on c) and its linear approximation at the average value of the concentration. Numerical simulations for capillary zone electrophoresis with LiI or potassium benzorate as a hypothetical background electrolyte show the formation of asymmetrical plugs for samples of more diluted electrolyte solutions.

Dynamics of peak dispersion in capillary zone electrophoresis including wall adsorption: II. Exact analysis of unsteady linear adsorptive dispersion.

An exact analysis of the unsteady axial dispersion of an analyte, undergoing a linear adsorption at the column wall in capillary zone electrophoresis, is presented. A system of partial differential equations--in which the radial coordinate is one of the independent variables--is taken as a model for linear wall adsorption. It is shown that the dispersion is a sum of two terms, one which depends linearly on time and whose exact form is generally known, and a non-linear one. The most interesting result of this work is that it derives another system of differential equations, which this nonlinear term is to satisfy. It makes it possible to present a closed formula for the asymptotic value of the nonlinear term, i.e., its limit for large time. Its behavior for times close to zero is also studied.

Contribution of capillary coiling to zone dispersion in capillary zone electrophoresis.

A mathematical model of the deformation of the analyte zone in capillary zone electrophoresis (CZE) due to capillary coiling is presented and the influence of capillary coiling on the distribution of electric current density inside the capillary is described. The model gives a quantitative description of the potential contribution of capillary coiling to the total zone dispersion in CZE. The zone broadening caused by capillary coiling is calculated as the difference of migration distances of the particles migrating at the inner circumference of the capillary coil and those migrating at the outer circumference. This difference is shown to be directly proportional to the capillary diameter and to the number of capillary coils, i.e. indirectly proportional to the radius of the capillary coils into which the capillary of the given length is coiled. The contribution of capillary coiling to the total zone dispersion is compared with the contribution of the longitudinal diffusion of low and high molecular mass analytes and bioparticles. It is shown that, especially in the case of CZE separation of macromolecules and particles, capillary coiling can significantly decrease the separation efficiency. For that reason the small radius coiling of the capillary column in the CZE apparatuses should be avoided.

Dynamics of peak dispersion in capillary zone electrophoresis including wall adsorption I. Theoretical model and results of simulation.

A mathematical model is described for the simulation of peak profiles in capillary zone electrophoresis taking wall adsorption into account. It is based on such physico-chemical relations as mass balance equations, sorption rate equations and appropriate boundary conditions. The numerical solution of the model is carried out, allowing the depiction of the concentration profile of the sample in the capillary as a result of the dynamics of the processes involved, which depend on a number of input parameters: rate constants, adsorption isotherms, column dimensions, field strength etc. Four cases are discussed in detail, namely those with either slow or fast adsorption kinetics and linear or nonlinear isotherms.