An ion mobility device having a first end for receiving ions and a second end for ejecting ions, the ion mobility device comprising: a plurality of electrodes arranged along an axis, the axis extending from the first end to the second end, the plurality of electrodes configured to receive a gas flow near the second end, wherein the plurality of electrodes form a channel of uniform cross section; a current source configured to: apply a first potential (E m1) to the plurality of electrodes to generate a first electric field during a trapping and equilibration time period, the first electric field being uniform along the length of the axis; apply a second potential (E m2) to the electrodes during an ejection time period to generate a second electric field, the second electric field being uniform along the length of the axis from the first end to the second end and being greater than the first electric field; such that, in use, during the trapping and equilibration time period, ions are separated along the axis based on their ion mobility to equilibrium positions where the force applied by the electric field balances the drag force applied by the gas flow; and during the ejection time period, ions are driven out the second end by the second electric field such that the separation achieved during the trapping and equilibration time period is maintained, characterized in that at least a subset of the plurality of electrodes create flow paths for the gas flow away from the channel and wherein the electrodes vary in length or outer diameter to provide flow paths with varying restriction on the gas flow to provide for an expansion of the gas flow from the second end to the first end such that a first gas velocity at the second end is greater than a second gas velocity at the first end. a plurality of electrodes arranged along an axis, the axis extending from the first end to the second end, the plurality of electrodes configured to receive a gas flow near the second end, wherein the plurality of electrodes form a channel of uniform cross section; a current source configured to: apply a first potential (E m1) to the plurality of electrodes to generate a first electric field during a trapping and equilibration time period, the first electric field being uniform along the length of the axis; apply a second potential (E m2) to the electrodes during an ejection time period to generate a second electric field, the second electric field being uniform along the length of the axis from the first end to the second end and being greater than the first electric field; such that, in use, during the trapping and equilibration time period, ions are separated along the axis based on their ion mobility to equilibrium positions where the force applied by the electric field balances the drag force applied by the gas flow; and apply a first potential (E m1) to the plurality of electrodes to generate a first electric field during a trapping and equilibration time period, the first electric field being uniform along the length of the axis; apply a second potential (E m2) to the electrodes during an ejection time period to generate a second electric field, the second electric field being uniform along the length of the axis from the first end to the second end and being greater than the first electric field; such that, in use, during the trapping and equilibration time period, ions are separated along the axis based on their ion mobility to equilibrium positions where the force applied by the electric field balances the drag force applied by the gas flow; and during the ejection time period, ions are driven out the second end by the second electric field such that the separation achieved during the trapping and equilibration time period is maintained,