Numerical Study on Sample Stacking in Capillary Electrophoresis

Sample stacking in capillary electrophoresis is one of the effective techniques to concentrate sample species, thus improving their detection sensitivity. A 1-D mathematical model, including electrical potential distribution equation, buffer concentration equation, as well as sample electromigration and diffusion equation, is developed by proper simplifications and assumptions to study the sample-stacking process in capillary electrophoresis. These coupled governing equations are solved using finite-element method. The variations in the buffer concentration and the distribution of electrical field strength with time as well as the distribution of electrical potential in capillary during sample stacking were obtained. The sample stacking and the sample diffusion after stacking as well as the separation process of sample cations and anions are presented. It is found that the best stacking effect occurs near the entrance, where the species have not been separated well. With the increase in time, the stacking effect decreases whereas the distance between positively and negatively charged particles becomes larger, and the separation effect becomes better. The effect of buffer concentration ratio on sample stacking is also analyzed. It is found that the relationship between the sample-stacking effect and the buffer concentration ratio is not linear, and the maximum stacking effect is achieved within less time and less migration distance when the buffer concentration ratio is higher because of the stronger electrical field strength in sample plug region. It is anticipated that the numerical model developed in this study is helpful for the designing and optimization of sample stacking devices.