Isotachophoresis at pH extremes: theory and experimental validation.

The evolution of an isotachophoresis (ITP) system in acidic or basic pH ranges can be quite different from that predicted by the existing theory. It was found theoretically and proved experimentally that the contribution of hydrogen or hydroxyl ion to conductivity of solution and/or its net charge changes the behavior of the ITP system, creating in the terminating electrolyte an additional zone close to the initial interfaces between electrolytes (leader and terminator). One boundary of the zone, being either sharp or dispersed, moves toward the leader; the other is always sharp and stationary and coincides with initial electrolytes' discontinuity. The latter can be registered in the presence of electroosmotic flow which delivers it to the detection point. In order to describe the dynamics of the ITP system at pH extremes an algorithm of analytical solution was developed, based on the revised Kohlrausch theory. Its predictions coincide well with computer simulations and experimental data. The results presented can help in a correct analysis of ITP data and explain some confusing phenomena which were considered to be artifacts.

On the solvent motion in electrophoretic systems.

A corrected model describing transport processes for multicomponent mixtures in electric field is proposed. This model is more consistent compared to the other models of this sort used for simulations of electrophoresis in the case of concentrated solutions. The main idea underlying the model is in accounting for the motion of a solvent. Usually the concentrations of solutes are considered small compared to that of a solvent and the equation describing its motion is not considered. This automatically leads to the violation of momentum balance and, hence, to the defects of the model itself. In the model presented in the paper by means of redefining the mass fluxes the balance of momentum is satisfied automatically while the equations governing the evolution of the system look more symmetric and simple. This model allows us to discover some fine effects in evolution of the mixture and clarify the essence of some conservation laws (conservation of the Kohlrausch function) arising in the simplified mathematical models describing electromigration phenomena.

On the measurements of electrophoretic mobilities by means of capillary isotachophoresis at a constant voltage.

A method for measuring electrophoretic mobilities by means of isotachophoresis (ITP) at a constant voltage as described by H. Carchon and E. Eggermont (Electrophoresis, 1982, 3, 263-274) is analyzed. An error made in this work, disregarding the pH shift arising at the initial discontinuity on the leader-terminator boundary, has been corrected. This method has been carefully studied and generalized for the presence of constant electroosmotic flow in a capillary. The limits of its applicability and the diffusionless ITP theory in general are discussed. A detailed study of the evolution of initial discontinuity (stationary boundary) showed some anomalies not reported previously, particularly non-monotonic concentration profiles in the vicinity of stationary boundaries. Moreover, in some cases, diffusion effects and the contribution of H+ ions can also strongly influence the behavior of moving boundaries. Computer modelling (confirmed by experimental data) showed that these effects could lead to the decay of the ITP train, despite the fact that the steady state diffusionless ITP theory predicts its stability.

Computer simulation of transient states in capillary zone electrophoresis and isotachophoresis.

Transient states in the evolution of electrophoretic systems comprising aqueous solutions of weak monovalent acids and bases are simulated. The mathematical model is based on the system of nonstationary partial differential equations, expressing the mass and charge conservation laws while assuming local chemical equilibrium. It was implemented using a high resolution finite-difference algorithm, which correctly predicted the behavior of the concentration, pH and conductivity fields at low computational expense. Both the regular and the irregular modes of separation in capillary zone electrophoresis and isotachophoresis are considered. It is shown that the results of separation, particularly zone order, strongly depend on pH distribution. Simulation data as well as simple analytical assessments may help to predict and correctly interpret the experimental results.