An ion mobility spectrometer comprising: an ion source configured to produce ions, a drift tube defining a drift tube inlet configured to receive ions from the ion source and a drift tube outlet, the drift tube including a first ion gate positioned between the drift tube inlet and the drift tube outlet and partitioning the drift tube into a first drift tube region between the drift tube inlet and the first ion gate and a second drift tube region between the first ion gate and the drift tube outlet, the first ion gate responsive to a first control signal to inhibit passage of ions from the first drift tube region into the second drift tube region and to a second control signal to allow passage of ions from the first drift tube region into the second drift tube region, the drift tube configured to separate ions in time as a function of ion mobility between the drift tube inlet and the first ion gate and also between the first ion gate and the drift tube outlet, the drift tube defining a first ion activation region configured to selectively induce structural changes in at least some of the ions exiting the first drift tube region by one of: (i) fragmenting the at least some of the ions via collisions with the buffer gas; and (ii) inducing conformational changes in the at least some of the ions via collisions with the buffer gas without fragmenting the at least some of the ions, and the drift tube configured to separate the ions in time as a function of ion mobility in the first drift tube region, inducing the structural changes in at least some of the ions exiting the first drift tube region, and separating in time the ions exiting the first drift tube region, after inducing the structural changes in at least some of the ions exiting the first drift tube region, as a function of ion mobility in a second drift tube region, further comprising a source of buffer gas, wherein the drift tube is configured to receive buffer gas therein from the source of buffer gas, further comprising at least one voltage source coupled to the first ion activation region, the at least one voltage source configured to selectively establish an electric field in the first ion activation region that is sufficient to induce structural changes in the at least some of the ions. an ion source configured to produce ions, a drift tube defining a drift tube inlet configured to receive ions from the ion source and a drift tube outlet, the drift tube including a first ion gate positioned between the drift tube inlet and the drift tube outlet and partitioning the drift tube into a first drift tube region between the drift tube inlet and the first ion gate and a second drift tube region between the first ion gate and the drift tube outlet, the first ion gate responsive to a first control signal to inhibit passage of ions from the first drift tube region into the second drift tube region and to a second control signal to allow passage of ions from the first drift tube region into the second drift tube region, the drift tube configured to separate ions in time as a function of ion mobility between the drift tube inlet and the first ion gate and also between the first ion gate and the drift tube outlet, the drift tube defining a first ion activation region configured to selectively induce structural changes in at least some of the ions exiting the first drift tube region by one of: (i) fragmenting the at least some of the ions via collisions with the buffer gas; and (ii) inducing conformational changes in the at least some of the ions via collisions with the buffer gas without fragmenting the at least some of the ions, and the drift tube configured to separate the ions in time as a function of ion mobility in the first drift tube region, inducing the structural changes in at least some of the ions exiting the first drift tube region, and separating in time the ions exiting the first drift tube region, after inducing the structural changes in at least some of the ions exiting the first drift tube region, as a function of ion mobility in a second drift tube region, further comprising a source of buffer gas, wherein the drift tube is configured to receive buffer gas therein from the source of buffer gas, further comprising at least one voltage source coupled to the first ion activation region, the at least one voltage source configured to selectively establish an electric field in the first ion activation region that is sufficient to induce structural changes in the at least some of the ions.