Modeling of the Impact of Ionic Strength on the Electroosmotic Flow in Capillary Electrophoresis with Uniform and Discontinuous Buffer Systems

Abstract

A new dynamic computer model permitting the combined simulation of the temporal behavior of electroosmosis and electrophoresis under constant voltage or current conditions and in a capillary which exhibits a pH-dependent surface charge has been constructed and applied to the description of capillary zone electrophoresis, isotachophoresis, and isoelectric focusing with electroosmotic zone displacement. Electroosmosis is calculated via use of a normalized wall titration curve (mobility vs pH). Two approaches employed for normalization of the experimentally determined wall titration data are discussed, one that considers the electroosmotic mobility to be inversely proportional to the square root of the ionic strength (method based on the Gouy-Chapman theory with the counterion layer thickness being equal to the Debye-Hückel length) and one that assumes the double-layer thickness to be the sum of a compact layer of fixed charges and the Debye-Hückel thickness and the existence of a wall adsorption equilibrium of the buffer cation other than the proton (method described by Salomon, K.; et al. J. Chromatogr. 1991, 559, 69). The first approach is shown to overestimate the magnitude of electroosmosis, whereas, with the more complex dependence between the electroosmotic mobility and ionic strength, qualitative agreement between experimental and simulation data is obtained. Using one set of electroosmosis input data, the new model is shown to provide detailed insight into the dynamics of electroosmosis in typical discontinuous buffer systems employed in capillary zone electrophoresis (in which the sample matrix provides the discontinuity), in capillary isotachophoresis, and in capillary isoelectric focusing.

Dynamic computer simulations of electrophoresis: A versatile research and teaching tool

“…For the system discussed here and having a buffer flow towards the cathode, the migrating system peak is detected first and the EOF marker shortly thereafter, provided the time interval is large enough such that the cationically migrating system peak had sufficient time to migrate out of the fluid element, which was initially occupied by the sample. This is illustrated with the emulated Generaluntreated fused-silica capillary [51,52]. The conductivity data presented in panel A of Fig.…”

“…Other codes included equilibria of solutes with paper surface [30,31] or channel walls [76][77][78], and one code dealt with the modeling of electrophoretic solute transport between two liquid phases [79]. Flow in GENTRANS and SIMUL5 is considered to be a plug flow with cross-sectional uniformity, which does not contribute to any boundary or zone dispersion [45,48,[50][51][52] whereas imposed flow in SPRESSO [69] or electroosmosis in a closed system [21] do broaden concentration gradients. GENTRANS also includes in situ calculation of electroosmosis using wall titration data as an input parameter [51,52,129].…”

“…Flow in GENTRANS and SIMUL5 is considered to be a plug flow with cross-sectional uniformity, which does not contribute to any boundary or zone dispersion [45,48,[50][51][52] whereas imposed flow in SPRESSO [69] or electroosmosis in a closed system [21] do broaden concentration gradients. GENTRANS also includes in situ calculation of electroosmosis using wall titration data as an input parameter [51,52,129]. For each column segment of length Dx, electroosmosis is calculated with the use of a pH and ionic strength-dependent electroosmotic mobility and the voltage gradient.…”

“…This is illustrated here with ZE, ITP, IEF and EKC separation examples. The data shown were produced with GENTRANS, the simulator based on the work of Bier, Mosher, Thormann, Breadmore and coworkers [1,37,38,44,[49][50][51][52][53][54]75].…”

“…New models with sophisticated integration schemes emerged and the number of applications increased. Models that are currently in use in various laboratories are the 1-D simulators GENTRANS of Mosher et al [44,49,[51][52][53][54][129][130][131][132] and SIMUL5 of Hruška et al [48] (available at http://www.natur.cuni.cz/ Gas), which succeeded SIMUL 4.0 [47]. Interesting new models are those of Bercovici et al [69] (available as the Stanford's Public Release Electrophoretic Separation Solver (SPRESSO), http://microfluidics.stanford.edu/spresso), the 2-D model of Shim et al [70,71,135,136] and the models of Yang and his collaborators [68,138].…”