P. Cheng

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 strenthe 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.

Numerical analysis on the influence factors of sample stacking in capillary electrophoresis

Sample stacking in capillary electrophoresis can concentrate sample species through the electrical field strength gradient caused by the inhomogeneous buffer concentration field in capillary. The factors that affect the sample stacking process have been analyzed in detail by using a 1-D mathematic model. It was found from the simulation results that the electrical charge number and the electrical charge sign of sample particles can affect the electrophoretic velocity, which in turn has an important influence on the stacking process. The electrical potential can affect the migration captime of sample particles to reach detection window, and the initial length of sample plug has significant influence on the maximal sample concentration after stacking and the time to get the optimal stacking effect. The results obtained are helpful to the improvement of the sample stacking technique.

The Local and Average Heat Transfer Characteristic of Confined Jet Array Impingement Boiling of Aqueous Ethylene Glycol Solutions

Liquid Jet impingement cooling is deemed as one of the most promising high heat flux cooling technologies. Compared with single phase cooling, two-phase cooling has advantages of more uniform heating surface temperature, lower pressure drop and less mass flow rate. In this paper, a closed-loop experimental setup is built to study confined jet array impingement boiling of 43% mass concentration aqueous ethylene glycol solution. The rectangular heating surface made of thin metal film is 20 mm × 40 mm and with the thickness of 0.03 mm. The in-line jet array has the jet orifice diameter d = 1 mm, the dimensionless jet-to-target spacing H/d = 1, and the dimensionless jet-to-jet spacing S/d = 5. The experiments are performed at atmospheric pressure to explore the effects of jet impingement velocity and liquid subcooling. The tested jet velocity is 0.2, 0.31 and 0.5 m/s respectively, while the inlet subcooling is ranged from 36°C to 96°C. The results showed that wall temperature and even heat transfer mode at different locations of the heating surface are quite different, with the lowest temperature on the heating surface directly under the jets and the highest temperature on the heating surface under the center of four jets where the nucleation boiling incepts earliest and the critical heat flux (CHF) occurs. Increasing subcooling and jet velocity can delay the onset of nucleate boiling and enhance the critical heat flux dramatically. Wall temperature overshooting phenomenon can only be found on the heating surface under the center of four jets when the jet velocity is low and sub-cooling is high.Copyright