Mobility, electro-osmotic ionic

Reversed-phase separations currently dominate in CEC. As a result, the vast majority of the mobile phases are mixtures of water and an organic solvent, typically acetonitrile or methanol. In addition to the modulation of the retention, the mobile phase in CEC also conducts electricity and must contain mobile ions. This is achieved by using aqueous mixtures of salts instead of pure water. The discussion in Sect. 2 of this chapter indicated that the electro osmotic flow is created by ionized functionalities. The extent of ionization of these functionalities that directly affects the flow rate depends on the pH value of the mobile phase. Therefore, the mobile phase must be buffered to a pH that is desired to achieve the optimal flow velocity. Obviously there are at least three parameters of the mobile phase that have to be controlled (i) percentage of the organic solvent, (ii) the ionic strength of the aqueous component, and (iii) its pH value. 

In MEKC, mainly anionic surface-active compounds, in particular SDS, are used. SDS and all other anionic surfactants have a net negative charge over a wide range of pH values, and therefore the micelles have a corresponding electrophoretic mobility toward the anode (opposite the direction of electro-osmotic flow). Anionic species do not interact with the negatively charged surface of the capillary, which is favorable in common CZE but especially in ACE. Therefore, SDS is the best-studied tenside in MEKC. Long-chain cationic ammonium species have also been employed for mainly anionic and neutral solutes (16). Bile salts as representatives of anionic surfactants have been used for the analysis of ionic and nonionic compounds and also for the separation of optical isomers (17-19). 

Separation is carried out by applying a high potential (10-30 kV) to a narrow (25-75 pm) fused silica capillary filled with a mobile phase. The mobile phase generally contains an aqueous component and must contain an electrolyte. Analytes migrate in the applied electric field at a rate dependent on their charge and ionic radius. Even neutral analytes migrate through the column due to electro-osmotic flow, which usually occurs towards the cathode. 

Although not a major separation parameter, as in gas chromatography, the operating temperature clearly affects migration behavior in capillary electrophoresis (CE). This alone should be reason to work under uniform, well-thermostated conditions. The temperature affects bulk viscosity, electro-osmotic flow, ionic mobilities, even pK values and buffer pH. 

As seen from Equation 1.7, the electro-osmotic flow depends on the dielectric constant and viscosity of pore fluid, as well as the surface charge of the solid matrix represented by the zeta potential (the electric potential at the junction between the fixed and mobile parts in the double layer). The zeta potential is a function of many parameters, including the types of clay minerals and ionic species that are present, as well as the pH, ionic strength, and temperature. If the cations and anions are evenly distributed, an equal and opposite flow occurs, causing the net flow to be zero. However, when the momentum transferred to the fluid in one direction exceeds the momentum of the fluid traveling in the other direction, electro-osmotic flow is produced. 

A theory of electro-osmotic phenomena suited to charged membranes has been developed by Schmid and co-workers [35-37]. It is assumed that an identical composition is maintained on the two sides of the membrane, so that there is no concentration gradient. The distribution of mobile species within the pores is also assumed to be uniform. The ionic fluxes are given by... 

When considering the transport of an electrolyte solution, one must consider the motion of mobile ionic species in addition to the considerations of traditional fluid mechanics. The transport of ions in a capillary results in an electrical current. In electro-osmotic flow, the total current is a result of three physical processes ... 

In CZE the transport phenomenon is based on the movement of ions in an electric field and on the electro-osmotic flow. The prediction of the mobilities of ions is mostly based on the prediction of the pH dependence of the ionic mobility The prediction of the electro-osmotic flow is very difficult because of the dependence on the surface characteristics whose actual condition is hardly ever known. For the prediction of the resolution it is interesting to note that the plate number N is dependent only on the voltage used, and not on the mobilities of the solutes and the given capillary. 

---