A method is described to select ions based on their 
electrical mobility. Ions are subjected to at least one 
full cycle of a time-dependent 
electric field. Ions are separated in space, and a 
continuous flow of filtered ions with a 
narrow range of selected mobility ions is produced at the outlet of the device, as in Differential Mobility Analyzers (DMAs). Yet, no high fluid velocity field is required, avoiding limitations associated in DMAs to flow unsteadiness, 
compressibility and turbulent transition. Instead, separation relies on the 
use of time-dependent 
ion trajectories generated by the time-dependent electric fields. Unlike FAIMS, full separation according to the mobility takes place within one or a few characteristic times for field variation, rather than via many tiny separation steps over many periods of field variation producing separation according to the mobility non-linearities in the mobility. Unlike conventional pulsating 
ion mobility spectrometry, a steady 
ion flow is produced with high 
duty cycle. Different embodiments of the apparatus using the method of the present invention are also described comprising different geometrical configurations and different time-dependent electric profiles, each of them having specific advantages. In a first configuration proposed, a rotary 
electric field is combined with an axial steady field achieving 
high resolution and a continuous output throughout all the selected and undesired ions. In a second two-dimensional configuration, an oscillating field is combined with a perpendicular steady field. Much higher sample flow rates can be achieved, though the undesired ions will periodically be transferred. Configurations using more than 
one stage allow minimizing the undesired pulsed 
signal of non selected ions. In all those configurations, the trajectories of ions are time dependent and the selected ions are subjected to at least one 
full cycle of the variable 
electric field. Separation is based on 
synchronizing the period of the field with the 
flight time of an ion from an inlet to an outlet for a particular 
electrical mobility.