Ion mobility spectrometers (IMS) are widely used for analyzing ionized compounds by their mobility, which is the function of ion charge, mass and shape. Typical IMS comprises an ion source for soft ionization of analyte compounds, an ion gate (typically Tyndal gate) to form short ion packets, a gas filled drift tube for ion separation in electrostatic fields, and a collector to measure time dependent signal. As a stand-alone analytical technique, IMS has low resolution (50-100). IMS was primarily considered as a low cost hand-held detector of toxic volatile compounds, with low detection limits that may be enhanced by specific ion molecular reactions with doping vapors. More recently IMS has been coupled with gas chromatography (GC), liquid chromatography (LC) and mass spectrometry (MS), where IMS brings an additional dimension of analytical separation. However, the straight forward coupling can cause strong signal losses in IMS due to ˜1% duty cycle of Tyndal ion gate and mismatch in gas pressures and ion cloud size between IMS and MS. If using scanning MS, like quadrupoles, there occurs a mismatch in time scales.
U.S. Pat. No. 5,200,614, incorporated herein by reference, discloses improvement of IMS sensitivity by trapping ions between gate pulses. U.S. Pat. No. 3,902,064, incorporated herein by reference, discloses a combination of IMS spectrometer with the downstream mass spectrometer for complimenting mobility measurements by ion mass measurements. Young, et. al. in paper J. Chem. Phys., v.53, No 11, pp. 4295-4302, incorporated herein by reference, discloses a combination of IMS spectrometer with the downstream orthogonally accelerating time-of-flight detector which is capable of fast recording of panoramic (all mass) spectra for higher speed and duty cycle of mass measurements. U.S. Pat. No. 5,905,258, incorporated herein by reference, discloses a combination of both features—an ion trap in-front of the IMS and orthogonal TOF past IMS, thus capitalizing on both advantages—higher duty cycle of IMS and MS.
U.S. Pat. No. 6,107,628, incorporated herein by reference, discloses an ion funnel device for converging ion flows at intermediate gas pressures. U.S. Pat. No. 6,818,890, incorporated herein by reference, discloses an ion funnel for ion confinement past IMS. Paper Anal. Chem., 2008, v.80, pp. 612-623, incorporated herein by reference, describes usage of the ion funnel device for both—for ion trapping prior to IMS and for ion confinement past the IMS. Details on the so-called hourglass ion funnel trap are also presented in Anal. Chem., 2007, v.79, pp. 7845-7852, incorporated herein by reference. The described method presents the ultimately sensitive IMS-MS of prior art, which still suffers several limitations. The number of trapped ions is limited by the space charge capacity of the ion trap and of IMS drift tube to 1E+7 charges per pulse. Both the hourglass gate and downstream ion funnel spread ion packets to about 200-400 us, which slows down the IMS speed, leads to long drift separation times of at or around 20-40 ms, requires constructing long (about 1 m long) IMS drift tubes, and limits the IMS charge throughput and the dynamic range.
WO2008112351, incorporated herein by reference, discloses a method of improving IMS dynamic range and space charge capacity by multiplexed coding of the ion trap which operates at much higher net frequency compared to conventional regime of single trap firing per IMS separation. However, the approach requires ion packets overlapping and is likely to cause confusions at data interpretation.
Summarizing the above, IMS and IMS-TOF of prior art are limited in their charge throughput, dynamic range and speed, which limits their combination with fast separation methods. Therefore, there is a need for improving IMS and IMS-TOF parameters.