GRADIENT ELUTION IN CAPILLARY ELECTROCHROMATOGRAPHY

In analogy to pressure-driven gradient, techniques in high-performance liquid chromatography, a system has been developed for delivering ele ctroosmotically driven solvent gradients for capillary electrochromato graph (CEC). Dynamic gradients with submicroliter per minute now rates are generated by merging two electroosmotic flows that are regulated by computer-controlled voltages, These flows are delivered by two fuse d-silica capillary arms attached to a T-connector, where they mix and then now into a capillary column that has been electrokinetically pack ed with 3-mu m reversed-phase particles. The inlet of one capillary ar m is placed in a solution reservoir containing one mobile phase, and t he inlet of the other is placed in a second reservoir containing a sec ond mobile phase. Two independent computer-controlled, programmable, h igh-voltage power supplies (0-50 kV)-one providing an increasing ramp and the other providing a decreasing ramp-are used to apply variable h igh-voltage potentials to the mobile phase reservoirs to regulate the electroosmotic now in each arm. The ratio of the electroosmotic flow r ates between the two arms is changed with time according to the comput er-controlled voltages to deliver the required gradient profile to the separation column, Experiments were performed to confirm the composit ion of the mobile phase during a gradient run and to determine the cha nge of the composition in response to the programmed voltage profile. To demonstrate the performance of electroosmotically driven gradient e lution in CEC, a mixture of 16 polycyclic aromatic hydrocarbons was se parated in less than 90 min, This gradient technique is expected to be well-suited for generating not only solvent gradients II CEC but also other types of gradients, such as pH and ionic strength gradients, in capillary electrokinetic separations and analyses.