OPTIMIZATION IN SAMPLE STACKING FOR HIGH-PERFORMANCE CAPILLARY ELECTROPHORESIS

A simple model for the optimization of peak variance in sample stacking with a gravity-injected sample has been developed. In sample stacking, a long plug of low-concentration buffer containing a sample for separation is introduced into a column filled with a buffer of the same composition but of a higher concentration. The sample ions then migrate very rapidly under the enhanced applied electric field in the sample plug until they reach the concentration boundary. Once they cross the boundary, the ions slow down and stack into a narrow band. Theoretically, the peak width in sample stacking is proportional to the ratio, gamma, of buffer concentration in the original sample solution to that in the column. However, this difference in the concentrations inside the capillary column also generates an electroosmotic pressure originating at the concentration boundary. The laminar flow resulting from the electroosmotic pressure causes extra peak broadening. Sample stacking and laminar broadening work against each other to yield an optimal point relating to the sample buffer concentration, the separation buffer concentration, and the sample plug length. The predications of the model are in agreement with experimental results.