ABSTRACT
Gradient elution is routinely used in high-performance liquid chromatography (HPLC) to achieve the resolution of a mixture that could not be resolved using isocratic elution. Unlike isocratic elution, where the mobile phase composition remains constant throughout the experiment, in gradient elution the mobile phase composition changes with time. The change could occur continuously or in steps, i.e., "step gradient." In continuous gradient the analyst can pick one of three general shapes: linear, concave, or convex. Gradient elution in HPLC is achieved using two pumps, two solvents, and a solvent mixer. Step gradient is accomplished by periodic change of the mobile phase composition at predetermined times to achieve the resolution of a mixture. In capillary electrophoresis (CE) electroosmotic flow is used in place of a mechanical pump and controls the flow of the mobile phase, in most cases an aqueous buffer. Electroosmotic flow is controlled by many parameters, including buffer type, concentration, ionic strength, pH, and buffer modifiers such as organic solvents, surfactant and polymers, capillary surface chemistry, and applied voltage. Another approach to control electroosmotic flow is by applying an external electric field that can change the direction and rate of electroosmotic flow by external voltage.
