Method and apparatus to produce steady beams of mobility selected ions via time-dependent electric fields

摘要

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.