Research On The Influence Of Pressure Effect On Reproducibility Of Continuous Electrophoresis Detection In Microfluidic Chips

While performing continuous electrophoresis separation on microfluidic chips,along with separation keep on progress,pressure effects occur several anomalies to reproducibility of migration.Pressure-driven backflow that opposing EOF(electroosmotic flow) appears in microchannel,and reproducibility data on microfluidic chips are difficult to obtain,the RSD (relative standard deviation) of peak height and RSD of migration time is almost 20%and 25%respective.Through experimental observation,the existence of pressure-driven backflow is verified. It shows that opposing pressure induced flow in microchannels is presented during continuous electrophoresis separation,pressure-driven backflow is observed and characterized,hydraulic pressure and meniscus surface tension(Laplace pressure) is cited as the cause.The influence of hydraulic pressure and Laplace pressure to pressure-driven backflow was analyzed,it is found that the effect of induced hydrostatic pressure caused by unequal levels in solution reservoirs is not negligible,Laplace pressure is also considered as the dominating factor when continuous electrophoresis separation just start before long,while hydraulic pressure become main factor after several separation.It regards important that Laplace pressure induce pressure-driven backflow significantly during the whole continuous electrophoresis process.It shows that the finalresulting velocity is the superposition of EOF and pressure-driven backflow.Based on this,some chip designs are suggested to reduce the pressure-driven backflow, such as by reducing the hydraulic diameter of the microchannel,by using larger reservoirs to dramatically reduce hydrostatic pressure due to unequal levels in solution reservoirs,or by converting the hydrophobic properties of the reservoirs on the microchip into hydrophilic ones.The numerical simulation of EOF and opposing pressure induced flow inmicrochannel is presented,and the hydraulic diameter of the microchannel is optimized.One improved microchip is designed reduce these pressure-driven backflows.In the means of hot embossing process,the improved microchip is fabricated with cross-sections of 70μm×40μm and hydrophilic glass reservoirs.Before experiment,the high voltage power for electrophoresis is improved by using program technology of RS 232 serial port,which realizes the effective real time control between high voltage power and computer. After using laser induced fluorescence(LIF) detector and high voltage power to perform continuous electrophoresis separation on the improved microchip,obtained the RSD of peak height 3.8%and RSD of migration time 0.578%,which are of good reproducibility.