High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) is a technology that is capable of separating gas-phase ions at atmospheric pressure. In FAIMS the ions are introduced into an analyzer region across which an radio frequency (RF) waveform is applied such that the ions are alternately subjected to high and low electric fields. The waveform is asymmetric; the high field is applied for one time unit followed by an opposite-polarity low field component applied for twice as long. The field-dependent change in the mobility of the ions causes the ions to drift towards the walls of the analyzer region. Since the dependence of ion mobility on electric field strength is compound specific, this leads to a separation of the different types of ions one from the other, and is referred to as the FAIMS separation. In order to transmit an ion of interest through FAIMS, an appropriate DC voltage is applied to compensate for the drift of the ion of interest toward the analyzer wall. By varying the compensation voltage, different ions are selectably transmitted through the FAIMS device.
A number of different electrode geometries have been described for use with FAIMS, including concentric cylindrical electrodes with a domed inner electrode (d-FAIMS), concentric cylindrical electrodes in a side-to-side orientation, and stacked plate geometries with either flat or curved electrode plates. In the d-FAIMS geometry the ions are separated as they travel along the length of the electrodes. The ions become focused into a band extending around the inner electrode due to focusing fields that exists within the space between the inner and outer cylindrical electrodes. Advantageously the focusing effect extends around the domed terminus of the inner electrode, such that the ions are concentrated into a narrow beam prior to extraction. However, the d-FAIMS geometry tends to be rather bulky and ion residence times are relatively long.
The side-to-side geometry is more compact compared to the d-FAIMS geometry, since the ions travel circumferentially within the space between the inner and outer cylindrical electrodes. Unfortunately, typically there is no force to prevent the ions from spreading out laterally along the length of the electrodes as they travel through the ion separation region between the electrodes. Accordingly, ion transmission efficiency is low compared to the d-FAIMS geometry. One prior art approach has been to use a segmented outer or inner electrode, as described in U.S. Pat. No. 7,034,289 which issued on Apr. 25, 2006 in the name of Guevremont et al., the entire contents of which is herein incorporated by reference. The segmented electrode supports creation of a potential gradient along the lateral direction in a side-to-side FAIMS, for directing ions in a direction that is opposite the lateral spreading out behavior. Unfortunately, the complexity of the segmented electrode system, including associated voltage supplies and controllers, adds to the complexity and bulk of this otherwise compact design.
A similar problem is encountered in stacked plate geometry FAIMS devices. In this case, ions spread out laterally toward the edges of the plates as they travel through the ion separation region between the ion inlet and the ion outlet. One prior art solution to this problem involves modifying the end edges of a central electrode plate, so as to direct ions toward the central axis of the electrode plate immediately prior to the ions being extracted via the ion outlet orifice, as described in U.S. Pat. No. 6,806,466 which issued on Oct. 19, 2004 in the name of Guevremont et al., the entire contents of which is herein incorporated by reference. Unfortunately, the ions still spread out laterally during the time they spend within the ion separation region between the electrode plates. Accordingly, ion losses still occur as a result of collisions with a surface at the lateral boundaries of the ion separation region. There is no force that opposes the lateral spreading out of the ions within the ion separation region.
There is a need for FAIMS electrodes that achieve high ion transmission in a compact package.