Studies On Electrokinetics And Dynamics In Capillary Perfusive Electrochromatography And Liquid Chromatography

We have performed a systematic experimental study of electrokinetic mobilities (electroosmotic and electrophoretic mobilities ) depending on tile mobile phase ionic strength using different, well-defined porous and, for reference behaviour, nonporous silica-based spherical microparticles. One theoretical model has been established to explain the change of electroosmotic mobility with the mobile phase ionic strength on the basis of Rice-Whitehead equation. Bom perfusive flow effect and 'Zeta potential effect' contribute to the increase firstly and then the decrease of electroosmotic mobility for macroporous ODS particles, when the concentration of Tris buffer goes up. On the other hand, electrophoretic mobility results demonstrate a substantially different behaviour for the macroporous (i.e., permeable and conducting) particles with respect to nonporous (solid) spheres. It may be related to a macroporous particles dipole coefficient via the intraparticle void volume and, strongly depending on mobile phase ionic strength, to the actual magnitude of electroosmotic flow jetted through a particles interior, In contrast to the normal electrical double layer behaviour observed for the solid spheres (continuous decrease of mobility with ionic strength) these competitive contributions give rise to pronounced maxima in the mobility of the porous spheres. Our results are in qualitative agreement with the theoretical analysis of Miller et al. [J. Colloid Interface Sci. 1992, 153, 237] which, depending on the double layer interaction within a porous aggregate of solid spheres, predicts (significantly) lower or higher mobilities with respect to an impermeable and nonconducting particle.What's more, the chromatographic performance with respect to the column efficiency and dispersion in fixed beds of nonporous and macroporous particles has been studied in capillary electrochromatography (CEC). The existence of substantial electroosmotic intraparticle pore flow (perfusive electroosmosis) in columns packedwith the macroporous particles was found to reduce stagnant mobile mass transfer resistance and decrease the flow inhomogeneity over the whole column cross-section, leading to a significant improvement in column efficiency compared to capillary HPLC and normal CEC packed with particles of 60-80 A in pore diameter. The effect of electroosmotic perfusion on column efficiency and axial dispersion was shown to depend sensitively on the mobile phase ionic strength and a mean intraparticle pore diameter, thus, on electrical double layer interaction within the particles. Based on these data an optimum chromatographic performance in view of speed and efficiency can be achieved by straightforward adjustment of the electrolyte concentration and characteristic intraparticle pore size. Moreover, the effect of Joule heating due to higher electrical field on the retention of neutral and nonpolar compounds was modeled based on the approaches proposed by Bello et.al. and Davis and coworkers to explain the decrease of the retention factors with increasing the applied voltage and Tris buffer concentration in CEC. Lower retention of neutral and nonpolar compounds under CEC conditions than capillary HPLC can be attributed to Dynamic method in retention time measurement and Joule heating effect.Moreover, electrostatic exclusion effect (also called Donnan exclusion effect) on the retention of aromatic anionic species has also been investigated in capillary high-performance liquid chromaiography under reversed-phase conditions. Effect of Tris buffer concentration on electrostatic exclusion of stationary phase has been examined on three silica-based C18 stationary phases with different nominal pore sizes in the mobile phase of 80% (v/v) acetonitrile over a range of Tris buffer concentrations. The relationship between the "porosity", which refers to the fraction of the "passing-through" free space volume of anionic compound to the total column volume when it is eluted, and Ka has been illustrated. The change trend in chromatographic peak s...