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 the detection sensitivity. A 1-D mathematical model, including the electrical potential distribution equation, the buffer concentration equation, as well as the sample electromigration and diffusion equation, is developed through proper simplifications and assumptions to study the sample stacking process in capillary electrophoresis. These coupled governing equations are solved using finite element method (FEM). The variations of the buffer concentration and the electrical field strength distribution with time as well as the electrical potential distribution in capillary during sample stacking are 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 development of time the stacking effect deteriorates while the distance between the 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 sample stacking effect and the buffer concentration ratio is not linear and the maximum stacking effect is achieved within less time and 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 paper is helpful for the design and optimization of sample stacking devices.