Abstract: Ion mobility coupling with mass spectrometer has become an important tool for analysis of complex mixtures. Generally, the type and composition of drift gas (e.g., N₂, He, CO₂) in an ion mobility spectrometer will impact on the separation of different types of analytes and transmission of ions with a wide mass range. U-shaped mobility analyzer (UMA) is a unique type of mobility device based on counter-flow mobility principle, and it can sort specific types of ions with high mobility resolving power under the co-effect of electric field and gas flow. UMA has independent drift gas channels which allows the drift gas in and out with high speed and accurate control, hence the on-line adjustment of the gas composition becomes possible. The effects of changing drift gas composition on sensitivity, separation capability and analysis speed were examined in this study. From the preliminary results, when using a mixture of drift gas, the mobility peak position (drift field strength) linearly correlated with the gas composition and met the predication of the Blanc′s law. With the constant drift gas velocity, the mobility resolution from He is lower than those from the heavier drift gas (N₂ or CO₂) based on our theory, but the ion transmission is much higher (better RF confinement in He) and the separation time can also be reduced. On the other hand, if keeping the peak position constant (adjusting drift gas velocity for different gas), the resolution from He can even theoretically surpass that from N₂ or CO₂ with the expense of sensitivity reduction. Therefore, optimum and balanced results for different applications (e. g. better separation for isomer analysis in N₂ or CO₂ with doping a specific ratio of He, fast and high sensitivity data acquisition for DDA or DIA operation in pure He) can be achieved by adjusting the drift gas composition. Finally, the possibility and advantages of dynamically controlling the gas composition were discussed, which may provide new perspectives for coupling with LC separation of very complex samples.