Microfluidic separation systems often use channels with turns to achieve long separation distance within a compact region. But the bends or curves degrade the performance of separation efficiency. To tackle this problem, channels are modeled with different types of 90 degree turns to reduce the "racetrack" effect. In this paper, the finite element method is used to numerically solve the equations governing electroosmotic flow and mass transport in a microchannel. The detailed results show that the sample plug broadening due to turns can be reduced when the outside of the turn is dented. It is also suggested that the band tilting can be corrected.