A method comprising: separating a first population of ions according to a first physico-chemical property in a first separation device; and separating one or more groups of ions emerging from said first separation device in a drift tube and sampling each group of ions using said drift tube to determine the collision cross section of ions in each group of ions, wherein each group of ions corresponds to a range of said first physico-chemical property; wherein said drift tube is configured such that multiple groups of ions in said first population of ions can be sampled by said drift tube in a single cycle of separation of said first separation device, and the step of sampling each group of ions comprises: determining the mobility of ions in each group of ions by measuring their drift time through said drift tube; and determining the collision cross section of said ions in each group of ions using said determined mobility; wherein said first physico-chemical property is ion mobility; wherein said drift tube comprises a plurality of electrodes, and said method further comprises applying a constant DC voltage gradient to said plurality of electrodes to urge ions along said drift tube; wherein said first separation device comprises a plurality of electrodes and said method further comprises applying one or more transient DC voltages or potentials to at least some of said electrodes of said first separation device in order to urge ions along said separation device; and wherein said one or more transient DC voltages or potentials are swept, translated or sequentially applied along at least a portion of the axial length of said first separation device with a first velocity, so as to cause ions to be separated according to their ion mobility. separating a first population of ions according to a first physico-chemical property in a first separation device; and separating one or more groups of ions emerging from said first separation device in a drift tube and sampling each group of ions using said drift tube to determine the collision cross section of ions in each group of ions, wherein each group of ions corresponds to a range of said first physico-chemical property; wherein said drift tube is configured such that multiple groups of ions in said first population of ions can be sampled by said drift tube in a single cycle of separation of said first separation device, and the step of sampling each group of ions comprises: determining the mobility of ions in each group of ions by measuring their drift time through said drift tube; and determining the collision cross section of said ions in each group of ions using said determined mobility; wherein said first physico-chemical property is ion mobility; wherein said drift tube comprises a plurality of electrodes, and said method further comprises applying a constant DC voltage gradient to said plurality of electrodes to urge ions along said drift tube; wherein said first separation device comprises a plurality of electrodes and said method further comprises applying one or more transient DC voltages or potentials to at least some of said electrodes of said first separation device in order to urge ions along said separation device; and wherein said one or more transient DC voltages or potentials are swept, translated or sequentially applied along at least a portion of the axial length of said first separation device with a first velocity, so as to cause ions to be separated according to their ion mobility. determining the mobility of ions in each group of ions by measuring their drift time through said drift tube; and determining the collision cross section of said ions in each group of ions using said determined mobility; wherein said first physico-chemical property is ion mobility; wherein said drift tube comprises a plurality of electrodes, and said method further comprises applying a constant DC voltage gradient to said plurality of electrodes to urge ions along said drift tube; wherein said first separation device comprises a plurality of electrodes and said method further comprises applying one or more transient DC voltages or potentials to at least some of said electrodes of said first separation device in order to urge ions along said separation device; and wherein said one or more transient DC voltages or potentials are swept, translated or sequentially applied along at least a portion of the axial length of said first separation device with a first velocity, so as to cause ions to be separated according to their ion mobility.