Patent Publication Number: US-10788453-B2

Title: Closed path ion mobility spectrometer having a common ion inlet and outlet

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 15/606,478 filed May 26, 2017, which is a continuation-in-part of U.S. patent application Ser. No. 15/023,575, filed Mar. 21, 2016, which is a U.S. national phase of International Application No. PCT/US2014/056970, filed Sep. 23, 2014, which claims the benefit of, and priority to, U.S. Patent Application Ser. No. 61/882,891, filed Sep. 26, 2013, the disclosures of which are expressly incorporated herein by reference in their entireties. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with government support under GM090797 awarded by the National Institutes of Health. The United States Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This disclosure relates generally to the field of spectrometry, and more specifically to instruments for separating ions in time as a function of ion mobility. 
     BACKGROUND 
     Ion mobility spectrometers are analytical instruments used to investigate properties of charged particles by separating the charged particles, i.e., ions, in time as a function of ion mobility. In the typical ion mobility spectrometers, an electric drift field is established in a drift tube filled with a buffer gas, and as the ions move through the drift tube under the influence of the electric drift field the ions collide with the buffer gas and separate as a function their collision cross-sections such that more compact conformers reach the end of the drift tube faster than less compact conformers. Known drift tubes may be so-called single-pass drift tubes, i.e., linear or non-linear drift tubes through which ions traverse only once between ion inlets and outlets thereof, or so-called multiple-pass drift tubes, i.e., linear or closed-path drift tubes through which ions may traverse multiple times before exit. 
     SUMMARY 
     The present invention may comprise one or more of the features recited in the attached claims, and/or one or more of the following features and combinations thereof. In one aspect, a hybrid ion mobility spectrometer may comprise a single-pass drift tube having an ion inlet at one end and an ion outlet at an opposite end, the single-pass drift tube configured to separate in time ions entering the ion inlet thereof and traveling therethrough according to a first function of ion mobility, a multiple-pass drift tube having an ion inlet and an ion outlet each coupled to the single pass drift tube between the ion inlet of the single-pass drift tube and the ion outlet of the single-pass drift tube, the multiple-pass drift tube configured to separate in time ions entering the ion inlet of the multiple-pass drift tube and traveling one or more times therethrough according to the first or a second function of ion mobility, and at least one ion steering channel controllable to selectively pass ions traveling through the single-pass drift tube into the multiple-pass drift tube via the ion inlet of the multiple-pass drift tube and to selectively pass ions traveling through the multiple-pass drift tube into the single-pass drift tube via the ion outlet of the multiple-pass drift tube. 
     In another aspect, a hybrid ion mobility spectrometer may comprise a single-pass drift tube configured to separate in time ions traveling axially therethrough in a first direction of ion travel according to a first function of ion mobility, a closed-path, multiple-pass drift tube configured to separate in time ions traveling axially therethrough one or more times in a second direction of ion travel according to the first or a second function of ion mobility, the second direction of ion travel different from the first direction of ion travel, and an ion steering channel disposed in-line with single-pass drift tube and in-line with the multiple-pass drift tube, the ion steering channel selectively controllable to steer ions traveling therein from the single-pass drift tube into the multiple-pass drift tube, the ion steering channel further selectively controllable to steer ions traveling therein from the multiple-pass drift tube into the single-pass drift tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a simplified diagram of an embodiment of a hybrid ion mobility spectrometer. 
         FIG. 1B  is a simplified diagram of an alternate embodiment of a hybrid ion mobility spectrometer, 
         FIG. 1C  is a simplified diagram of another alternate embodiment of a hybrid ion mobility spectrometer. 
         FIG. 1D  is a simplified diagram of yet another alternate embodiment of a hybrid ion mobility spectrometer. 
         FIG. 1E  is a simplified diagram of the embodiment illustrated in  FIG. 1D  viewed orthogonally from the view illustrated in  FIG. 1D . 
         FIG. 1F  is a simplified diagram of an embodiment of the transition region of the hybrid ion mobility spectrometer illustrated in  FIGS. 1D and 1E . 
         FIG. 2  is a simplified diagram of an embodiment of a drift tube segment that may be used in any of the hybrid ion mobility spectrometers of  FIGS. 1A-1C . 
         FIG. 3  includes  FIGS. 3A and 3B  and is a simplified flowchart of an embodiment of a process for separating ions using any of the hybrid ion mobility spectrometers of  FIGS. 1A-1F . 
         FIG. 4A  is a simplified diagram of another embodiment of the transition region of the hybrid ion mobility spectrometer illustrated in  FIGS. 1D and 1E . 
         FIG. 4B  is a simplified diagram of still another embodiment of the transition region of the hybrid ion mobility spectrometer illustrated in  FIGS. 1D and 1E . 
         FIG. 5  is a bottom plan view of one of the planar members of either of the embodiments illustrated in  FIGS. 4A and 4B . 
         FIG. 6  is a simplified perspective view of an embodiment of an ion steering channel that may be implemented in any of the hybrid ion mobility spectrometers illustrated in  FIGS. 1A-1F . 
         FIG. 7A  is a simplified perspective diagram illustrating an example operating mode of the ion steering channel illustrated in  FIG. 6 . 
         FIG. 7B  is a simplified perspective diagram illustrating another example operating mode of the ion steering channel illustrated in  FIG. 6 . 
         FIG. 8  is a simplified elevational diagram illustrating an embodiment of an ion carpet that may be implemented in any of the hybrid ion mobility spectrometers illustrated in  FIGS. 1A-1F . 
         FIG. 9  is a simplified elevational diagram illustrating an embodiment of one of two planar members forming another embodiment of an ion steering channel that may be implemented in any of the hybrid ion mobility spectrometers illustrated in  FIGS. 1A-1F . 
         FIG. 10A  is a simplified elevational diagram illustrating an example operating mode of the ion steering channel partially illustrated in  FIG. 9 . 
         FIG. 10B  is a simplified perspective diagram illustrating another example operating mode of the ion steering channel partially illustrated in  FIG. 9 . 
         FIG. 11  is a simplified elevational diagram illustrating another embodiment of an ion carpet that may be implemented in any of the hybrid ion mobility spectrometers illustrated in  FIGS. 1A-1F . 
         FIG. 12A  is a simplified plan diagram illustrating an example combination the ion steering channel partially illustrated in  FIG. 9  and the ion carpet illustrated in  FIG. 11 , illustratively implemented in either of the hybrid ion mobility spectrometers illustrated in  FIGS. 1A and 1B . 
         FIG. 12B  is a simplified plan diagram illustrating another example combination the ion steering channel partially illustrated in  FIG. 9  and the ion carpet illustrated in  FIG. 11 , illustratively implemented in the hybrid ion mobility spectrometer illustrated in  FIG. 1A . 
         FIG. 13  is a simplified plan diagram illustrating an example combination the ion steering channel illustrated in  FIG. 6 , two of the ion carpets illustrated in  FIG. 8  and two of the ion carpets illustrated in  FIG. 11 , illustratively implemented in the hybrid ion mobility spectrometer illustrated in  FIGS. 1D and 1E . 
     
    
    
     DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to a number of illustrative embodiments shown in the attached drawing and specific language will be used to describe the same. 
     Referring to  FIG. 1A , a simplified diagram of an embodiment of a hybrid mobility spectrometer  10  is shown. The hybrid ion mobility spectrometer  10  illustratively includes a single-pass drift tube  12  through which ions can be separated in time according to a first function of ion mobility, and a multiple-pass drift tube  14 , coupled to the single-pass drift tube  12  between an ion inlet  16  and an ion outlet  20  of the single-pass drift tube  12 , through which ions can be separated in time according to a second function of ion mobility. The spectrometer  10  further illustratively includes a set of ion gates, e.g., G 1 -G 3 , each of which are controllable between open and closed positions, and the set of ion gates is illustratively controlled such that some or all of the ions traveling through the single-pass drift tube  12  may be selectively passed into the multiple-pass drift tube  14  via an ion inlet  36   1  of the multiple-pass drift tube  14 , and some or all of the ions traveling through the multiple-pass drift tube  14  may be selectively passed back into the single-pass drift tube  12  via an ion outlet  36   2  of the multiple-pass drift tube  14 , and the ions then exit the single-pass drift tube  12  via the ion outlet  20  thereof. In the embodiment illustrated in  FIG. 1A , an ion source  18  is coupled to the ion inlet  16  of the single-pass drift tube  12 , and an ion detector  22  is positioned to receive ions exiting the ion outlet  20  of the single-pass drift tube  12 . 
     The foregoing configuration of the ion mobility spectrometer  10  provides for the ability to pass all or a subset of ions in the single-pass drift tube  12  to the multiple-pass drift tube  14  for additional and/or alternate separation before the ions exit the outlet  20  of the single-pass drift tube  12 . Advantageously, because both drift tubes  12 ,  14  operate on ions generated from a single ion source  18  coupled to the ion inlet  16  of the single-pass drift tube  12 , such additional and/or alternate separation may thus be carried out using ions from the same sample. In one specific operational mode of the hybrid ion mobility spectrometer  10  illustrated in  FIG. 1A , for example, the set of ion gates, e.g., ion gates G 1 -G 3 , may be controlled such that ions generated by the ion source  18  are first confined to the single-pass drift tube  12 , and electric fields within the single-pass drift tube  12  are controlled such that ions generated at the ion source  18  travel, i.e., drift, through only the single-pass drift tube  12  where they separate in time as a first function of ion mobility defined by the various structural dimensions and operating parameters of the single-pass drift tube  12 . The resulting ion spectral information is then analyzed and if, for example, it is discovered that in the ion spectral information a subset, e.g., two or more, of ion intensity peaks in an ion mobility range of interest, e.g., in a particular range of drift times, are crowded together and cannot be satisfactorily resolved over the length of the single-pass drift tube  12 , ions are then generated a second time, or are continuously generated without interruption, and the set of ion gates, e.g., ion gates G 1 -G 3 , is controlled in one embodiment to pass or divert ions traveling through the single-pass drift tube  12  that are within the ion mobility range of interest into the multiple-pass drift tube  14 . The set of ion gates, e.g., ion gates G 1 -G 3 , is then controlled to confine the diverted ions within the multiple-pass drift tube  14  and electric fields within the multiple-pass drift tube  14  are controlled such that the diverted ions pass, i.e., drift, one or more times through, i.e., about, the multiple-pass drift tube  14  and separate in time according to a second function of ion mobility, which may or may not be the same as the first function of ion mobility, and which is defined by the structure and operating parameters of the multiple-pass drift tube  14 , and the set of ion gates, e.g., ion gates G 1 -G 3  may then be controlled to pass or divert some or all of the ions traveling through the multiple-pass drift tube  14  back into the single-pass drift tube  12  where they are then directed to the ion outlet  20  of the single-pass drift tube. In one alternate embodiment, the set of ion gates, e.g., ion gates G 1 -G 3 , may be controlled to pass or divert ions some or all of the ions traveling through the single-pass drift tube  12  into the multiple-pass drift tube  14 , and the electric fields within the multiple-pass drift tube  14 , along with the set of ion gates, e.g., ion gates G 1 -G 3 , may then controlled in a known manner to confine the diverted ions within the multiple-pass drift tube  14  so that the ions separate in time according to a second function of ion mobility in which only the diverted ions within the ion mobility range of interest pass one or more times through, i.e., about, the multiple-pass drift tube  14 . The set of ion gates, e.g., ion gates G 1 -G 3  may then be controlled to pass or divert some or all of the ions traveling through the multiple-pass drift tube  14  back into the single-pass drift tube  12  where they are then directed to the ion outlet  20  of the single-pass drift tube. In another alternate embodiment, the set of ion gates, e.g., ion gates G 1 -G 3 , may be controlled to pass or divert some or all of the ions traveling through the single-pass drift tube  12  into the multiple-pass drift tube  14 , and the electric fields within the multiple-pass drift tube  14 , along with the set of ion gates, e.g., ion gates G 1 -G 3 , may then controlled in a known manner to confine the diverted ions within the multiple-pass drift tube  14  so that the ions separate in time according to a second function of ion mobility, which may or may not be the same as the first function of ion mobility, and which is defined by the structure and operating parameters of the multiple-pass drift tube  14 . The ion gates, e.g., ion gates G 1 -G 3 , may then be controlled to pass or divert some or all of the ions traveling through the multiple-pass drift tube  14  back into the single-pass drift tube  12 , and one or more ion gates positioned within the drift tube section D 2  may be controlled to pass through the ion outlet  20  only ions within the ion mobility range of interest. 
     In some embodiments, one or more of the ion gates in the set of ion gates, e.g., G 1 -G 3 , may be controlled to one or more intermediate positions between the open and closed positions. In such embodiments, and according to another specific operating mode of the hybrid ion mobility spectrometer  10  illustrated in  FIG. 1A , for example, the set of ion gates, e.g., ion gates G 1 -G 3 , may be controlled to direct some of the ions traveling through the single-pass drift  12  into the multiple-pass drift tube  14  while also allowing others of the ions traveling through the single-pass drift tube  12  to travel completely through the single-pass drift tube  12 , e.g., to and through the outlet  20  thereof. In such an operating mode, ions supplied by the single or common ion source  18  to the inlet  16  of the single-pass drift tube  12  thus travel in parallel through the single-pass drift tube  12  and the combination of the single-pass drift tube  12  and the multiple-pass drift tube  14 , with some of the ions traveling directly through the single-pass drift tube  12  to and through the ion outlet  20  and others of the ions traveling through the single-pass drift tube  12 , to and through the multiple-pass drift tube  14 , then back to and through any remaining section(s) of the single-pass drift tube  12  and exiting the ion outlet  20  of the single-pass drift tube  12 . 
     In any case, further details relating to various structural embodiments of the hybrid ion mobility spectrometer briefly described above and the foregoing operation thereof are described below and/or illustrated in the attached drawings, although it will be understood that other structural embodiments and operational modes of the hybrid ion mobility spectrometer illustrated and described herein will occur to those skilled in the art and that such other structural embodiments and operational modes are contemplated by this disclosure. 
     Referring now specifically to  FIG. 1A , an embodiment of the hybrid ion mobility spectrometer  10  briefly described above is shown. As described above, the hybrid ion mobility spectrometer  10  includes an ion source  18  coupled to an ion inlet  16  defined at one end of a single-pass ion mobility spectrometer  12 , and an ion outlet  20  is defined at an opposite end of the single-pass ion mobility spectrometer  12 . A multiple-pass ion mobility spectrometer  14  is coupled to the single-pass ion mobility spectrometer  12  between the ion inlet  16  and the ion outlet  20  thereof such that ions may be selectively passed from the single-pass ion mobility spectrometer  12  to the multiple-pass ion mobility spectrometer  14  and vice versa. For purposes of this disclosure, the term “single-pass ion mobility spectrometer” means an ion mobility spectrometer, or portion thereof, through which ions pass a single time, and the term “multiple-pass ion mobility spectrometer” means an ion mobility spectrometer, or portion thereof, through which ions may pass multiple times. Neither such ion mobility spectrometer is limited to any particular shape or configuration, and the single-pass ion mobility spectrometer  12  and/or the multiple-pass ion mobility spectrometer  14  may be or include a linear, piecewise-linear and/or non-linear drift tube. 
     In the illustrated embodiment, the ion outlet  20  of the single-pass ion mobility spectrometer  12  is coupled to an ion detector  22  which is configured to detect, in a conventional manner, ions exiting the ion outlet  20  of the single-pass ion mobility spectrometer  12 . In alternate embodiments, one or more additional ion separation and/or ion analyzing apparatuses may be positioned between the ion outlet  20  of the single-pass ion mobility spectrometer  12  and the ion detector  22 , and in any such alternate embodiment one or more ion detectors  22  may be coupled to or integral with any of the additional ion separation and/or ion analyzing apparatuses, alternatively to or in addition to the single-pass ion mobility spectrometer  12 . 
     The ion source  18  may be any conventional ion source, examples of which include, but are not limited to, an electrospray ion source, a matrix-assisted laser desorption ion source (MALDI), or the like. Alternatively or additionally, the ion source  18  may include one or more conventional apparatuses to collect all or a subset of the generated ions (i.e., within a defined range of ion mobilities and/or within a defined range of ion mass-to-charge ratios) and/or to structurally modify, e.g., fragment and/or change the conformations of, some or all of the generated ions and/or to normalize or otherwise modify the charge states of one or more of the generated ions. Alternatively or additionally still, the ion source  18  may be or include one or more known apparatuses that separate ions and/or one or more isotopes thereof as a function of any molecular characteristic, e.g., ion mass-to-charge ratio, ion mobility, ion retention time, or the like. 
     In the embodiment illustrated in  FIG. 1A , the single-pass ion mobility spectrometer  12  is made up of three cascaded drift tube sections; a first drift tube section D 1  coupled to the ion source  18 , a transition drift tube section DT coupled to the first drift tube section D 1  and to the drift tube of the multiple-pass ion mobility spectrometer  14 , and a second drift tube section D 2  coupled to the transition drift tube section DT and, in the illustrated embodiment, to the ion detector  22 . In one embodiment, the first drift tube section D 1  is illustratively a linear drift tube section and includes a cascaded arrangement of any number, N, of conventional linear drift tube sub-sections  30   N  (three such drift tube sub-sections  30   1 ,  30   2  and  30   3  shown) and any number, M, of conventional linear drift tube funnels  32   M  (two such drift tube funnels  32   1  and  32   2  shown), wherein a different drift tube funnel  32  may be interposed between any number of cascaded drift tube sub-sections  30 . The second drift tube section D 2  is likewise illustratively a linear drift tube section and may likewise include a cascaded arrangement of any number, Q, of conventional drift tube sections  30   Q  (two such drift tube sub-sections  30   4  and  30   5  shown) and any number, R, of conventional drift tube funnels  32   R  (two such drift tube funnels  32   9  and  32   10  shown), wherein a different drift tube funnel  32  may be interposed between any number of cascaded drift tube sub-sections  30 . Alternatively, the second drift tube section D 2  may include only a single drift tube sub-section  30  or drift tube funnel  32  which is coupled at one end to the transition drift tube section DT and defines the ion outlet  20  of the single-pass ion mobility spectrometer  12  at its opposite end. Alternatively still, the second drift tube section D 2  may be omitted altogether and the ion outlet of the transition drift tube section DT may define the ion outlet  20  of the single-pass ion mobility spectrometer  12 . 
     The drift tube sub-sections  30  and the drift tube funnels  32  illustrated in  FIG. 1A  are illustratively linear components in that each drift tube sub-section  30  and each drift tube funnel  32  defines a linear ion drift tube axis therethrough between an ion inlet and ion outlet thereof. The resulting drift tube sections D 1  and D 2  shown in  FIG. 1A  therefore likewise linear drift tube sections, it will be understood that either or both of the drift tube sections D 1  and D 2  may alternatively be piecewise linear or non-linear, or include one or more piecewise linear or non-linear subsections. 
     In any case, the one or more drift tube funnels  32  are illustratively controlled in a conventional manner to radially focus ions inwardly toward a central ion drift axis defined through the drift tube funnel  32  from an ion inlet to an ion outlet thereof. Additionally, one or more of the ion funnels  32  and/or one or more of the drift tube sub-sections  30  may include one or more ion gates controllable in a conventional manner to selectively pass ions therethrough or block ions from passing therethrough. Alternatively or additionally, one or more of the ion funnels  32  may include one or more regions that is/are controllable in a conventional manner to modify the structures of some or all of the ions passing therethrough, e.g., via ion fragmentation and/or inducing conformational changes in the ions. Further details relating to illustrative embodiments of the drift tube sub-sections  30  and the drift tube funnels  32  shown in  FIG. 1A  and described above are described in U.S. Patent Pub. No. 2007/0114382 A1 and also in related U.S. Pat. No. 8,618,475, the disclosures of which are incorporated herein by reference. 
     The transition drift tube section DT in the embodiment illustrated in  FIG. 1A , is illustratively made up of a number, S, of curved drift tube sub-sections  34   S  (two such curved drift tube sub-sections  34   1  and  34   2  shown, with the curved drift tube sub-section  34   1  defining an ion inlet to the transition drift tube section DT and coupled to the ion outlet of the first drift tube section D 1 , and with the curved drift tube sub-section  34   2  defining an ion outlet of the transition drift tube section DT and coupled to the ion inlet of the second drift tube section D 2 ), a number, T, of the drift tube funnels  32   T  (three such drift tube funnels  32   3 ,  32   4  and  32   5  shown) and sub-sections of each of two curved, Y-shaped drift tube sections  36   1  and  36   2 . The multiple-pass ion mobility spectrometer  14 , in the embodiment illustrated in  FIG. 1A , is illustratively provided in the form of a closed-path drift tube made up of a number, U, of the curved drift tube sub-sections  34   U  (two such curved drift tube sub-sections  34   3  and  34   4  shown), remaining sub-sections of the two curved, Y-shaped drift tube sections  36   1  and  36   2 , and a number, V, of the drift tube funnels  32   V  (four such drift tube funnels  32   5 ,  32   6 ,  32   7  and  32   8  shown). It will be understood, however, that the multiple-pass drift tube  14  may alternatively not form a closed path but may nevertheless be configured to pass ions multiple times therethrough. 
     In the embodiment illustrated in  FIG. 1A , the sub-section or branch of the curved, Y-shaped drift tube section  36   1  that is coupled to the drift tube funnel  32   3  serves the dual function as part of the single-pass ion mobility spectrometer  12  and also as an ion inlet to the multiple-pass ion mobility spectrometer  14 , and the sub-section or branch of the curved, Y-shaped drift tube section  36   2  that is coupled to the drift tube funnel  32   4  likewise serves the dual function as part of the single-pass ion mobility spectrometer  12  and also as an ion outlet of the multiple-pass ion mobility spectrometer  14 . The drift tube funnel  32   5  is illustratively shared by the single-pass ion mobility spectrometer  12  and the multiple-pass ion mobility spectrometer  14  and therefore forms part of each. Further details relating to illustrative embodiments of the curved drift tube sub-sections  34 , the curved Y-shaped drift tube sections  36  and the closed-path configuration of the multiple-pass ion mobility spectrometer  14  shown in  FIG. 1A  and described above are described in U.S. Pat. No. 8,362,420, the disclosure of which is incorporated herein by reference. 
     The hybrid ion mobility spectrometer  10  illustrated in  FIG. 1A  includes three ion gates, G 1 -G 3 , each of which is controllable in a conventional manner to selectively allow ions to pass therethrough and to selectively block ions from passing therethrough. In one embodiment, the ion gates G 1 -G 3  are each provided in the form of a mesh or grid, and a DC potential applied thereto, or a DC differential applied between a mesh or grid and an adjacent ring, is controlled such that at one DC level or DC differential value ions pass through the ion gate and at a different DC level or DC differential value ions are blocked from passing through the ion gate. In alternate embodiments, the ion gate function of one or more of the ion gates G 1 -G 3  may be accomplished by selectively applying and varying the frequency and/or amplitude of an RF voltage to a non-meshed or gridded ring, in a conventional manner, to selectively allow passage or block passage of ions therethrough. In some embodiments, one or more of the gates G 1 -G 3  may be controlled with intermediate DC potentials and/or RF frequencies/amplitudes to pass therethrough only a portion of ions presented thereat, e.g., to allow passage through one or more of the ion gates G 1 -G 3  of only a percentage of ions that is less than 100% of the total number of ions traveling toward the one or more ion gates G 1 -G 3 . In any case, the three ion gates G 1 -G 3  are controllable, as will be described in detail below, to confine ions within the single-pass drift tube  12 , to confine ions within the multiple-pass drift tube  14 , to pass or divert at least some of the ions in the single-pass drift tube  12  into the multiple-pass drift tube  14  and/or to pass or divert at least some of the ions in the multiple-pass drift tube  14  back into the single-pass drift tube  12 . 
     In the embodiment illustrated in  FIG. 1A , a first one of the ion gates, G 1 , is illustratively positioned in the curved, Y-shaped drift tube section  36   2  at an interface of the sub-section of the Y-shaped drift tube section  36   2  that is coupled to the drift tube funnel  32   5  and the sub-section or branch of the Y-shaped drift tube section  36   2  that is coupled to the drift tube funnel  32   8 . A second one of the ion gates, G 2 , is illustratively positioned in the curved, Y-shaped drift tube section  36   2  at an interface of the sub-section of the Y-shaped drift tube section  36   2  that is coupled to the drift tube funnel  32   5  and the sub-section or branch of the Y-shaped drift tube section  36   2  that is coupled to the drift tube funnel  32   4 . A third one of the ion gate, G 3 , is illustratively positioned in the curved, Y-shaped drift tube section  36   1  at an interface of the sub-section of the Y-shaped drift tube section  36   1  that is coupled to the drift tube funnel  32   5  and the sub-section or branch of the Y-shaped drift tube section  36   1  that is coupled to the drift tube funnel  32   3 . It will be understood that the hybrid ion mobility spectrometer  10  may include more or fewer such ion gates, and that any such alternative embodiment of the hybrid ion mobility spectrometer is contemplated by this disclosure. 
     In one alternate embodiment of the hybrid ion mobility spectrometer  10 , one or more of the drift tube sub-sections  30 ,  34 ,  36  and/or one or more of the drift tube funnels  32  may be provided in the form of a two-part sub-section or funnel defining a first drift tube region having an ion inlet defining the ion inlet of the sub-section or funnel and an ion outlet coupled to an ion inlet of an ion elimination region having an ion outlet defining the ion outlet of the sub-section or funnel. Further details relating to the structure and various operational modes of such alternately configured drift tube sub-sections and/or funnels are described in co-pending U.S. Patent Application Pub. No. 2013/0292562, the disclosure of which is incorporated herein by reference. 
     In another alternate embodiment of the hybrid ion mobility spectrometer  10 , one or more of the drift tube funnels  32  and/or one or more of the drift tube sub-sections  30 ,  34 ,  36  may be provided in the form of a conventional drift tube sub-section  30  to which RF voltages may be applied to radially focus ions inwardly toward the ion drift path defined therethrough. One illustrative embodiment of such a drift tube sub-section  50  is shown in  FIG. 2 , and includes a series of identically-dimensioned, electrically insulating rings  56  each separating adjacent ones of a series of identically-dimensioned, electrically conductive rings  58 , with all such rings  56 ,  58  stacked and clamped together between an ion inlet  52  and an ion outlet  54  of the drift tube sub-section  50 . Illustratively, the last several, e.g., two, electrically insulating rings may be (but need not be) provided in the form of reduced-thickness rings  60  (e.g., approximately ½ of the thickness of the rings  56 ), the purpose which will be described below. 
     In the illustrated embodiment, an RF voltage source  70  produces two RF voltages, ϕ 1  and ϕ 2  each 180 degrees out of phase with respect to the other, with ϕ 1  applied via a separate capacitor, C 1 , to all odd (or even) numbered rings  58  and ϕ 2  applied via a separate capacitor, C 1 , to all even (or odd) numbered rings  58  such that ϕ 1  and ϕ 2  are applied alternately to the series of rings  58  in the stack. A DC potential is applied via series-connected resistors, R, to the rings  58  to create a substantially uniform electric drift field in the drift tube sub-section  50 , and ions drift through the drift tube sub-section  50  under the influence of the electric drift field. The frequencies and/or amplitudes of the RF voltages ϕ 1  and ϕ 2  are illustratively selected in a conventional manner to radially focus ions drifting through the drift tube sub-section  50  toward an ion drift axis defined centrally through the drift tube sub-section  50 . In embodiments in which the reduced-width, electrically insulating rings  60  are included, another RF voltage source  72  may be provided to produce an RF voltage ϕ 3  that is applied through a different capacitor, C 2 , to each of the electrically conductive rings  58  contacting one of the rings  60 . The frequency and/or amplitude of ϕ 3  is controlled in a conventional manner to selectively allow passage of ions through the electrically conductive rings  58  connected to ϕ 3  or block passage of ions therethrough to thereby provide an ion gating function. 
     The drift tube sub-sections  50  with the radial ion focusing feature described above may be used in place of one or more of the drift tube funnels  32  and/or in place of one or more of the drift tube sub-sections  30 ,  34 ,  36  illustrated in  FIG. 1A . Alternatively or additionally, the drift tube sub-sections  50  with or without the radial ion focusing feature but with the ion gating feature described above may be used in place of one or more of the ion gates G 1 -G 3  illustrated in  FIG. 1A . In any of the embodiments illustrated in the attached figures and described herein, either or both of the single-pass drift tube and the multiple-pass drift tube may be operated in a conventional traveling wave operating mode, i.e., one in which one or more oscillating, i.e., AC, electric fields are established within the various drift tube sections to cause the ions to separate as they drift through the respective drift tube. 
     Referring again to  FIG. 1A , a number of voltage sources  40  are electrically connected to various parts of the hybrid ion mobility spectrometer  10 , and the number of voltage sources  40  are selected and controlled to apply appropriate DC and/or AC voltages to the various parts and components of the hybrid ion mobility spectrometer  10  for operation thereof. For example, one or more of the voltage sources  40  is/are electrically connected to the ion source  18  to control the ion source  18  in a conventional manner to generate, collect and/or process ions as described above. One or more others of the voltage sources  40  is/are electrically connected to each drift tube sub-section  30 ,  34 ,  36  and each drift tube funnel  32  to establish an electric drift field therein through which ions traverse the single-pass drift tube  12  and the multiple-pass drift tube  14 . One or more others of the voltage sources  40  is/are electrically connected to the drift tube funnels  32  to radially focus ions inwardly toward the drift tube axis defined therethrough, and/or to control operation of one or more ion gates contained therein to pass or block ions, and/or to control one or more ion activation regions included in one or more of the funnels  32  to modify the structure of ions passing therethrough, e.g., via ion fragmentation and/or by inducing conformational changes in ions without fragmenting them. One or more others of the voltage sources  40  is/are electrically connected to each of the ion gates G 1 -G 3  and selectively controlled to pass or block ions as described above and as will be described in greater detail below with respect to the process illustrated in  FIG. 3 . In any case, the one or more voltage sources  40  are conventional and may be individually programmed for operation or controlled by a processor  42  (e.g., amplitude, frequency, timing of activation and/or deactivation, etc.) as shown by dashed-line representation. The processor  42  is, in any event, electrically connected to the ion detector, and the processor  42  includes a memory having instructions stored therein that are executable by the processor  42  to process ion detection signals produced by the ion detector  22  and produce corresponding ion mobility spectral information, e.g., as a function of ion drift time through the single-pass drift tube  12  and/or the multiple-pass drift tube  14 . 
     A gas source  44 , e.g., single buffer gas, combination of gases to form a buffer gas, one or a combination of other gases, etc., is fluidly coupled to the hybrid ion mobility spectrometer  10  via a fluid conduit  46 . In embodiments of the hybrid ion mobility spectrometer  10  constructed from open-ended sub-sections  30 ,  34 ,  36  and with or without open-ended drift tube funnels  32 , the resulting spectrometer  10  is a continuous cavity spectrometer, and the single gas source  44  may thus be used to fill the entire spectrometer  10 , including the single-pass drift tube  12  and the multiple-pass drift tube  14 . In alternative embodiments, two or more gas sources may be used, and the hybrid ion mobility spectrometer  10  may be partitioned in a conventional manner to confine the two or more gases to corresponding portions of the spectrometer  10 . The gas source  44  may be manually controlled, programmable for automatic control and/or controlled by the processor  42  as shown by dashed-line representation in  FIG. 1A . 
     Referring now to  FIG. 1B , an alternate embodiment of a hybrid ion mobility spectrometer  10 ′ is shown. The hybrid ion mobility spectrometer  10 ′ is identical in many respects to the hybrid ion mobility spectrometer  10  illustrated in  FIG. 1A  and described above. Like features are identified by like reference numbers, and a detailed description of common features between the two spectrometers  10  and  10 ′ will not be repeated here for brevity. It will be further understood that the various embodiments of the various components and aspects to the hybrid ion mobility spectrometer  10  described above apply equally to the hybrid ion mobility spectrometer  10 ′. 
     The hybrid ion mobility spectrometer  10 ′ illustrated in  FIG. 1B  differs from the hybrid ion mobility spectrometer  10  illustrated in  FIG. 1A  primarily in the construction of the single-pass drift tube  12 ′ and in the number and location of the various ion gates that are controlled to achieve operation of the spectrometer  10  as described above. In the embodiment illustrated in  FIG. 1B , for example, the drift tube funnel  32   3  is coupled at its ion inlet to one ion outlet branch of a Y-shaped drift tube sub-section  38   1  having another ion outlet branch coupled to an ion inlet of another drift tube funnel  32   11 , wherein both such ion outlet branches are coupled to a common ion inlet branch having an ion inlet coupled to an ion outlet of the drift tube funnel  32   2 . The drift tube funnel  32   4  is similarly coupled at its ion outlet to one ion inlet branch of another Y-shaped drift tube sub-section  38   2  having another ion inlet branch coupled to an ion outlet of the drift tube funnel  32   11 , wherein both such ion outlet branches are coupled to a common ion outlet branch having an ion outlet coupled to an ion outlet of the drift tube funnel  32   9 . In this embodiment, the single-pass drift tube  12 ′ is a linear drift tube made up of the linear drift tube segments D 1  and D 2  joined by a linear drift tube segment D 3  made up of the linear branches of the Y-shaped drift tube sub-sections  38   1 ,  38   2  and the drift tube funnel  32   11 . 
     The embodiment illustrated in  FIG. 1B  includes four ion gates G 1 -G 4  which are controllable, as will be described in detail below, to confine ions within the single-pass drift tube  12 ′, to confine ions within the multiple-pass drift tube  14 , to pass or divert at least some of the ions in the single-pass drift tube  12 ′ into the multiple-pass drift tube  14  and/or to pass or divert at least some of the ions in the multiple-pass drift tube  14  back into the single-pass drift tube  12 ′. A first one of the ion gates, G 1 , is illustratively positioned in the Y-shaped drift tube section  38   1  at an interface of the curved branch of the Y-shaped drift tube section  38   1  that is coupled to the drift tube funnel  32   1  and the branch of the Y-shaped drift tube section  38   1  that is coupled to the drift tube funnel  32   2 . A second one of the ion gates, G 2 , is illustratively positioned in the Y-shaped drift tube section  38   1  at an interface of the linear branch of the Y-shaped drift tube section  38   1  that is coupled to the drift tube funnel  32   11  and the branch of the Y-shaped drift tube section  38   1  that is coupled to the drift tube funnel  32   2 . A third one of the ion gates, G 3 , is illustratively positioned in the curved, Y-shaped drift tube section  36   2  at an interface of the sub-section of the Y-shaped drift tube section  36   2  that is coupled to the drift tube funnel  32   5  and the sub-section or branch of the Y-shaped drift tube section  36   2  that is coupled to the drift tube funnel  32   8 . A fourth one of the ion gates, G 4 , is illustratively positioned in the curved, Y-shaped drift tube section  36   2  at an interface of the sub-section of the Y-shaped drift tube section  36   2  that is coupled to the drift tube funnel  32   5  and the sub-section or branch of the Y-shaped drift tube section  36   2  that is coupled to the drift tube funnel  32   4 . It will be understood that the hybrid ion mobility spectrometer  10 ′ may include more or fewer such ion gates, and that any such alternative embodiment of the hybrid ion mobility spectrometer is contemplated by this disclosure. 
     Referring now to  FIG. 1C , another alternate embodiment of a hybrid ion mobility spectrometer  10 ″ is shown. The hybrid ion mobility spectrometer  10 ″ is also identical in many respects to the hybrid ion mobility spectrometer  10  illustrated in  FIG. 1A  and described above. Like features are identified by like reference numbers, and a detailed description of common features between the two spectrometers  10  and  10 ″ will not be repeated here for brevity. It will be further understood that the various embodiments of the various components and aspects to the hybrid ion mobility spectrometer  10  described above apply equally to the hybrid ion mobility spectrometer  10 ″. 
     The hybrid ion mobility spectrometer  10 ″ illustrated in  FIG. 1C  differs from the hybrid ion mobility spectrometer  10  illustrated in  FIG. 1A  primarily in the construction of each of the single-pass drift tube  12 ″ and the multiple-pass drift tube  14 ′, and also in the location of the various ion gates that are controlled to achieve operation of the spectrometer  10  as described above. In the embodiment illustrated in  FIG. 1C , for example, the drift tube funnel  32   2  is coupled at its ion outlet to an ion inlet of a drift tube sub-section  30   5 , and an ion outlet of the drift tube sub-section  30   5  is coupled to an ion inlet of the drift tube funnel  32   3  (corresponding to the drift tube funnel  32   9  in  FIG. 1A ). In this embodiment, the single-pass drift tube  12 ″ is thus a linear drift tube made up of the linear drift tube segments D 1  and D 2  joined by a linear drift tube segment DT made up of the drift tube sub-section  30   5 . The multiple-pass drift tube  14 ′ is, in the embodiment illustrated in  FIG. 1C , a closed-path drift tube made up of four curved drift tube sub-sections  34   1 - 34   4  each coupled between a different two of four drift tube funnels  32   5 - 32   8 . A drift tube section  30   8  has an ion inlet coupled to the drift tube sub-section  30   5  of the single-pass drift tube  12 ″ and an ion outlet coupled to an ion inlet of a drift tube section  35 . An ion outlet of the drift tube section  35  is coupled to the drift tube sub-section  34   2  of the multiple pass drift tube  14 ′. In some embodiments, such as that illustrated in  FIG. 1C , the drift tube section  35  may include an inlet/outlet, i.e. bi-directional, funnel which may be controlled in a conventional manner, e.g., via one or more voltage sources, to direct and focus ions moving from the single-pass drift tube  12 ″ into the multiple-pass drift tube  14 ′ via the drift tube sub-section  30   8 , and which may also be controlled in a conventional manner, e.g., via one or more voltage sources, to direct and focus ions moving from the multiple-drift tube  14 ′ into the single-pass drift tube  12 ″ via the drift tube sub-section  30   8 . In other embodiments, the bi-directional funnel may be replaced with another funnel structure or other mechanism (e.g., structure and/or energy source(s)), or omitted altogether. In any case, the drift tube sections  30   8 ,  35  form a T-connection between the single pass drift tube  12 ″ and the multiple-pass drift tube  14 ′. 
     The embodiment illustrated in  FIG. 1C  includes three ion gates G 1 -G 3  which are controllable, as will be described in detail below, to confine ions within the single-pass drift tube  12 ″, to confine ions within the multiple-pass drift tube  14 ′, to pass or divert at least some of the ions in the single-pass drift tube  12 ″ into the multiple-pass drift tube  14 ′ and/or to pass or divert at least some of the ions in the multiple-pass drift tube  14 ′ back into the single-pass drift tube  12 ″. A first one of the ion gates, G 1 , is illustratively positioned in the drift tube sub-section  30   5  at or just beyond the ion inlet of the drift tube section  30   8 . A second one of the ion gates, G 2 , is illustratively positioned in the drift tube section  30   8  at or just beyond the ion inlet thereof. A third one of the ion gates, G 3 , is illustratively positioned in the drift tube section  35  at or near the ion outlet thereof. It will be understood that the hybrid ion mobility spectrometer  10 ″ may include more or fewer such ion gates, and that any such alternative embodiment of the hybrid ion mobility spectrometer is contemplated by this disclosure. 
     Referring now to  FIGS. 1D-1F , yet another alternate embodiment of a hybrid ion mobility spectrometer  10 ′″ is shown. The hybrid ion mobility spectrometer  10 ′″ is also identical in many respects to the hybrid ion mobility spectrometers  10  and  10 ″ illustrated in  FIGS. 1A and 1C  respectively and described above. Like features are identified by like reference numbers, and a detailed description of common features between the spectrometers  10 ,  10 ″ and  10 ′″ will not be repeated here for brevity. It will be further understood that the various embodiments of the various components and aspects to the hybrid ion mobility spectrometer  10  and  10 ″ described above apply equally to the hybrid ion mobility spectrometer  10 ′″. 
     In one aspect, the hybrid ion mobility spectrometer  10 ′″ illustrated in  FIGS. 1D-1F  differs from the hybrid ion mobility spectrometer  10  and  10 ″ illustrated in  FIGS. 1A and 1C  respectively in that an ion travel axis  70  of the multiple-pass drift tube  14 ″, i.e., an axis defined, or parallel with an axis defined, centrally through the multiple-pass drift tube  14 ″ and along which ions travel through the multiple-pass drift tube  14 ″, lies in a plane that is different from the plane in which an ion travel axis  72  of the single-pass drift tube  12 ′″, i.e., an axis defined, or parallel with an axis defined, centrally through the single-pass drift tube  12 ′″ and along which ions travel through the single-pass drift tube  12 ′″, lies. In the illustrated embodiment, the planes in which the ion travel axes  70  and  72  lie are orthogonal, although it will be understood that this disclosure contemplates embodiments in which the two different planes in which the ion travel axes  70  and  72  lie are not orthogonal. 
     In another aspect, the hybrid ion mobility spectrometer  10 ′″ illustrated in  FIGS. 1D-1F  differs from the hybrid ion mobility spectrometer  10  and  10 ″ illustrated in  FIGS. 1A and 1C  respectively in that, in contrast to a drift tube transition section, DT, the hybrid ion mobility spectrometer  10 ′″ defines a transition region  80  (TR) as an interface between the single-pass drift tube  12 ′″ and the multiple-pass drift tube  14 ″. Referring specifically to  FIG. 1F , an example embodiment of the transition region  80  is illustrated. In this embodiment, the transition region  80  includes a first plate  82  defining an ion passage, e.g., opening,  90  therethrough, which represents an ion inlet to the transition region  80  positioned adjacent to the ion outlet of the drift tube funnel  32   2  (e.g., see  FIG. 1E ). Another plate  84  is positioned opposite to the plate  80  and defines another ion passage, e.g., opening,  92  therethrough (both shown by dashed-line representation in  FIG. 1F ), which represents an ion outlet of the transition region  80  positioned adjacent to the ion inlet of the drift tube funnel  32   3 . A third plate  86  is positioned between the plates  82  and  84  along one side thereof, and a fourth plate  88  is positioned between the plates  82  and  84  along another side thereof. The third and fourth plates  86 ,  88  each define an ion passage, e.g., opening, therethrough which represent an ion inlet/outlet to/of the transition region  80  with the opening defined through the plate  86  positioned adjacent to the ion inlet/outlet of the drift tube sub-section  34   1  and the opening defined through the plate  88  positioned adjacent to the ion inlet/outlet of the drift tube section  34   2 . One or more of the plates  82 ,  84 ,  86 ,  88  may illustratively be operated as an ion gate, such that the illustrated embodiment may include one or more of G 1 -G 4 . 
     As described briefly hereinabove, the ion gates G 1 -G 3  of the hybrid ion mobility spectrometers  10  and  10 ″ and the ion gates G 1 -G 4  of the hybrid ion mobility spectrometer  10 ′ and  10 ′″ are controllable to confine ions within the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″, to confine ions within the multiple-pass drift tube  14 ,  14 ′,  14 ″ to pass or divert at least some of the ions in the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ into the multiple-pass drift tube  14 ,  14 ′,  14 ″ and/or to pass or divert at least some of the ions in the multiple-pass drift tube  14 ,  14 ′,  14 ″ back into the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″. Referring now to  FIGS. 3A and 3B , a flowchart is shown illustrating a process  100  for controlling the hybrid ion mobility spectrometer  10 ,  10 ′,  10 ″,  10 ′″ according to a number of different operational modes of the hybrid ion mobility spectrometer  10 ,  10 ′,  10 ″,  10 ′″ in which the set of ion gates, e.g., ion gates G 1 -G 3  for the spectrometers  10 ,  10 ″ and ion gates G 1 -G 4  for the spectrometer  10 ′,  10 ′″, are controlled as described above. In one embodiment, some or all of the process  100  may be controlled by the processor  42  in accordance with instructions stored in a memory of the processor  42 . Alternatively or additionally, some or all of the process  100  may be controlled by programming one or more of the one or more voltage sources  40  in embodiments in which one or more of the voltage sources  40  are programmable. Some of the process  100  may be alternatively or additionally carried out manually. 
     In any case, the process  100  begins at step  102  where the ion source  18  is controlled in a conventional manner to generate ions, e.g., in embodiments in which the ion source  18  is or includes an ion generation structure for generating ions from a sample, or to otherwise supply ions, e.g., in embodiments in which the ion source  18  is another ion separation instrument and/or other ion processing instrument that does not itself generate ions but rather operates on ions generated elsewhere. Thereafter at step  104 , at least some of the generated or otherwise supplied ions are introduced into the ion inlet  16  of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″, e.g., by controlling a conventional ion gate positioned at the ion inlet  16  to pass ions therethrough and into the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″, by drawing generated ions into the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ using a static or dynamic electric field, or the like. At step  106 , an electric drift field is established within the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″, which may occur before or after step  104 . 
     In any case, the process  100  advances to step  108  where the ion gates, e.g., G 1 -G 3  in the case of the hybrid ion mobility spectrometer  10 ,  10 ″ and G 1 -G 4  in the case of the hybrid ion mobility spectrometer  10 ′,  10 ′″, are controlled to direct ions in the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ therethrough and through the ion outlet  20 , i.e., to confine ions within the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ such that the ions drift only through the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ from the ion inlet  16  to the ion outlet  20  thereof and not through the multiple-pass drift tube  14 ,  14 ′,  14 ″. In the single-pass drift tube  12  illustrated in  FIG. 1A , step  108  may be carried out by controlling G 1  to the closed or ion-blocking position and controlling G 2  and G 3  to the open or ion-passing position, such that ions entering the ion inlet  16  pass sequentially through D 1 , DT and D 2  of the single-pass drift tube  12 . In the single-pass drift tube  12 ′ illustrated in  FIG. 1B , step  108  may be carried out by controlling G 1  to the closed or ion-blocking position and controlling G 2  to the open or ion-passing position, such that ions entering the ion inlet  16  pass sequentially through D 1 , D 3  and D 2  of the single-pass drift tube  12 ′. In the single-pass drift tube  12 ″ illustrated in  FIG. 1C , step  108  may be carried out by controlling G 1  to the open or ion-passing position and controlling G 2  to the closed or ion-blocking position, such that ions entering the ion inlet  16  pass sequentially through D 1 , DT and D 2  of the single-pass drift tube  12 ″. In the single-pass drift tube  12 ′″ illustrated in  FIGS. 1D-1F , step  108  may be carried out by controlling G 1 , e.g., the opening  90  through the plate  82 , to the open or ion-passing position and controlling G 2 , e.g., the opening  92  through the plate  84 , to the open or ion-passing position, and likewise controlling the gates G 3 , G 4 , e.g., the openings through the plates  86 ,  88  respectively to the closed or ion blocking positions, such that ions entering the ion inlet  16  pass sequentially through D 1 , TR and D 2  of the single-pass drift tube  12 ′″. In each case, ions generated at or otherwise supplied by the ion source  18  travel, i.e., drift, through only the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ under the influence of the electric field established therein where they separate in time as a first function of ion mobility defined by the various structural dimensions and operating parameters of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″. 
     Following step  108 , the process  100  advances to step  110  where an ion mobility range of interest is determined based on at least some of the ions exiting the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″. As described above, it may be discovered upon analysis of ion spectral information resulting from the detection of ions exiting the ion outlet  20  of the single-pass drift tube  12 ,  12 ′,  12 ′,  12 ″″ pursuant to step  108  that a subset, e.g., two or more, of ion intensity peaks in a particular range of ion mobilities (or ion drift times) are crowded together and cannot be satisfactorily resolved over the length of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″. Such a range of ion mobilities may then be the ion mobility range of interest. In other cases, the ion mobility range of interest may be determined based on one or more alternate or additional criteria. In some cases, the ion mobility range of interest may be the same as that produced by the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″, and in other cases the ion mobility range of interest may be different as just described. Likewise, whereas the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ is generally operable to separate ions according to a first function of ion mobility and the multiple-pass drift tube  14 ,  14 ′,  14 ″ is generally operable to separate ions according to a second function of ion mobility, the first and second functions of ion mobility may be the same in some embodiments and different in others. 
     In one embodiment, the process  100  includes a step  112  as shown in dashed-line representation, and in this embodiment the process  100  advances from step  110  to step  114  wherein the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ is cleared of ions, e.g., by stopping the generation of ions by the ion source  18  and allowing the tube  12 ,  12 ′,  12 ″,  12 ′″ to clear. Thereafter at step  116 , the ion source  18  is controlled to begin generating ions again, and thereafter at step  118  at least some of the generated ions are introduced into the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ as described above with respect to step  104 . In alternate embodiments, the process  100  does not include step  112  and in some such embodiments the ion source  18  may be controlled to continually, periodically or intermittently generate ions while in other embodiments the ion source  18  may be started and then stopped, but ions need not be cleared from the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ before continuing to step  120 . 
     At step  120 , the set of ion gates, e.g., G 1 -G 3  in the case of the hybrid ion mobility spectrometer  10 ,  10 ″ and G 1 -G 4  in the case of the hybrid ion mobility spectrometer  10 ′,  10 ′″, is controlled to divert or pass some or all of the ions in or entering the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ into the multiple-pass drift tube  14 ,  14 ′,  14 ″, and at step  122  an electric field is established within the multiple-pass drift tube  14 ,  14 ′,  14 ″ to cause ions to drift through the multiple-pass drift tube  14 ,  14 ′,  14 ″. In the single-pass drift tube  12  illustrated in  FIG. 1A , step  120  may be carried out by controlling G 1  and G 3  to their open or ion-passing positions and controlling G 2  to the closed or ion-blocking position, such that ions entering the ion inlet  16  pass sequentially through D 1 , through part of DT and into the multiple-pass drift tube  14 . In the single-pass drift tube  12 ′ illustrated in  FIG. 1B , step  120  may be carried out by controlling G 1  and G 3  to their open or ion-passing positions, and controlling G 2  and G 4  to their closed or ion-blocking positions, such that ions entering the ion inlet  16  pass from D 1  directly into the multiple-pass drift tube  14 . In the single-pass drift tube  12 ″ illustrated in  FIG. 1C , step  120  may be carried out by controlling G 1  to the closed or ion-blocking position, and controlling G 2  and G 3  to their open or ion-passing positions with the electric drift field in the drift tube sections  30   8  and  35  controlled to pass ions moving through D 1  into the multiple-pass drift tube  14 ′. In the single-pass drift tube  12 ′″ illustrated in  FIGS. 1D-1F , step  120  may be carried out by controlling G 1 , e.g., the opening  90  through the plate  82 , to the open or ion-passing position, controlling G 2 , e.g., the opening  92  through the plate  84 , to the closed or ion-blocking position, and controlling the gates G 3  and/or G 4 , e.g., the openings through the plates  86 ,  88  respectively to the open or ion-passing positions, such that ions entering the ion inlet  16  pass from D 1  through TR and directly into the multiple-pass drift tube  14 ″. Ions generated at the ion source  18  thus travel, i.e., drift, through the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ under the influence of the electric field established therein where they separate in time as a first function of ion mobility defined by the various structural dimensions and operating parameters of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″, and after passage of some or all of such ions into the multiple-pass drift tube  14 ,  14 ′,  14 ″ the ions travel, i.e., drift, through the multiple-pass drift tube  14 ,  14 ′,  14 ″ under the influence of the electric field established therein where they separate in time as a second function of ion mobility defined by the various structural dimensions and operating parameters of the multiple-pass drift tube  14 ,  14 ′,  14 ″. The first and second functions of ion mobility may be the same in some embodiments and different in others. 
     At step  124 , the set of ion gates, e.g., G 1 -G 3  in the case of the hybrid ion mobility spectrometer  10 ,  10 ″ and G 1 -G 4  in the case of the hybrid ion mobility spectrometer  10 ′,  10 ′″, is controlled to cause ions within the multiple-pass drift tube  14 ,  14 ,  14 ″ to travel one or multiple times through or about the multiple-pass drift tube  14 ,  14 ′,  14 ″. The number of times the ions travel through or about the multiple-pass drift tube  14 ,  14 ′,  14 ″ will typically be dictated by the total length of the multiple-pass drift tube  14 ,  14 ′,  14 ″ needed to adequately resolve the ion peaks of interest, or by other additional or alternate criteria. In the single-pass drift tube  12  illustrated in  FIG. 1A , step  124  may be carried out by maintaining G 1  in its open or ion-passing position and G 2  in its closed or ion-blocking position, and controlling G 3  to its closed position such that the multiple-pass drift tube  14  is completely closed to the single-pass drift tube  12 . In the single-pass drift tube  12 ′ illustrated in  FIG. 1B , step  124  may be carried out by maintaining G 3  and in its open or ion-passing position and G 4  in its closed or ion-blocking position, and controlling G 1  to its closed or ion-blocking position such that the multiple-pass drift tube  14  is completely closed to the single-pass drift tube  12 ′. In the single-pass drift tube  12 ″ illustrated in  FIG. 1C , step  124  may be carried out by controlling G 2  and/or G 3  to closed or ion-blocking position, such that the multiple-pass drift tube  14 ′ is completely closed to the single-pass drift tube  12 ″. In the single-pass drift tube  12 ′″ illustrated in  FIGS. 1D-1F , step  124  may be carried out by controlling G 1 , G 2 , e.g., the openings through the plates  82 ,  84  respectively, to their closed or ion-blocking positions, and controlling G 3 , G 4 , e.g., the openings through the plates  86 ,  88  respectively, to their open or ion-passing positions, such that the multiple-pass drift tube  14 ″ is completely closed to the single-pass drift tube  12 ′″. The ions then travel, i.e., drift, through the multiple-pass drift tube  14 ,  14 ′,  14 ″ under the influence of the electric field established therein where they separate in time as a second function of ion mobility defined by the various structural dimensions and operating parameters of the multiple-pass drift tube  14 ,  14 ′,  14 ″. 
     In one embodiment, step  126  is included, and at step  126  the electric drift field established within the multiple-pass drift tube  14 ,  14 ′,  14 ″ is controlled to cause only ions within the ion mobility range of interest to travel through the multiple-pass drift tube  14 ,  14 ′,  14 ″. For example, the open/closed timing of the various ion gates (G 1 -G 3  or G 1 -G 4 ) may be controlled at step  120  to pass ions of all mobilities from the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ into the multiple-pass drift tube  14 ,  14 ′,  14 ″, and in such embodiments, electric fields within the sub-sections  34  and funnels  32  of the multiple-pass drift tube  14 ,  14 ′,  14 ″ are sequentially switched on and off in a conventional manner at a rate that allows only ions within the ion mobility range of interest to traverse the multiple-pass drift tube  14 ,  14 ′,  14 ″. In some alternate embodiments, the open/closed timing of the ion gates G 1 -G 3  (or G 1 -G 4 ) may be controlled at step  120  such that only ions within the ion mobility range of interest are passed from the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ into the multiple-pass drift tube  14 ,  14 ′,  14 ″, and in such embodiments step  126  may be carried out simply by controlling the application of the electric fields within the sub-sections  34  and funnels  32  of the multiple-pass drift tube  14 ,  14 ′,  14 ″ to pass ions of all ion mobilities or by sequentially switching such electric fields on and off at a rate that allows only ions within the ion mobility range of interest to continue to traverse the multiple-pass drift tube  14 ,  14 ′,  14 ″. 
     After the ions have traveled the multiple times through the multiple-pass drift tube  14 ,  14 ′,  14 ″ the set of ion gates, e.g., G 1 -G 3  in the case of the hybrid ion mobility spectrometer  10 ,  10 ″ and G 1 -G 4  in the case of the hybrid ion mobility spectrometer  10 ′,  10 ′″ is controlled at step  128  to pass at least some of the ions from the multiple-pass drift tube  14 ,  14 ′,  14 ″ back into the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″. In the single-pass drift tube  12  illustrated in  FIG. 1A , step  128  may be carried out by controlling G 1  to the closed or ion-blocking position and controlling G 2  to the open or ion-passing position, such that ions traveling through the multiple-pass drift tube  14  pass back into the single-pass drift tube  12 , i.e., sequentially via the Y-shaped drift tube segment  36   2 , the drift tube funnel  32   4  and the curved drift tube sub-section  34   2 . In the single-pass drift tube  12 ′ illustrated in  FIG. 1B , step  128  may be carried out by controlling G 3  to the closed or ion-blocking position and controlling G 4  to the open or ion-passing position, such that ions traveling through the multiple-pass drift tube  14  pass back into the single-pass drift tube  12 ′, i.e., sequentially via the Y-shaped drift tube segment  36   2 , the drift tube funnel  32   4  and the curved branch of the Y-shaped drift tube sub-section  38   2 . In the single-pass drift tube  12 ″ illustrated in  FIG. 1C , step  128  may be carried out by controlling G 1 , G 2  and G 3  to their open or ion-passing positions with the electric fields in the drift tube sections  30   8  and  35  set to direct ions from the drift tube  34   2  to the drift tube sub-section  30   5 . In the single-pass drift tube  12 ′″ illustrated in  FIGS. 1D-1F , step  128  may be carried out by controlling G 1  and either G 3  or G 4 , e.g., the openings through the plates  82  and  86  or  88  respectively, to their closed or ion-blocking positions, and controlling G 2  and the other of G 3  or G 4 , e.g., the openings through the plates  84  and  88  or  86  respectively, to their open or ion-passing positions, such that ions traveling through the multiple-pass drift tube  14 ″ pass back into the single-pass drift tube  12 ′″ via the transition region  80 . 
     In one embodiment, ions re-entering the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ travel, i.e., drift, through the remainder of the single-pass drift tube  12 ,  12 ′,  12 ″ toward and through the ion outlet  20  under the influence of the electric field established therein where they separate in time in D 2  according to the first function of ion mobility. In some alternate embodiments, the open/closed timing of the ion gates G 1 -G 3  (or G 1 -G 4 ) may be controlled at step  120  such that ions within all ion mobility ranges are passed from the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ into the multiple-pass drift tube  14 ,  14 ′,  14 ″, step  126  may be replaced by a step in which the open/closed timing of the ions gates G 1 -G 3  (or G 1 -G 4 ) are likewise controlled such that ions within all ion mobility ranges travel through the multiple-pass drift tube  14 ,  14 ′,  14 ″, step  128  may be modified to control the open/closed timing of the ion gates G 1 -G 3  (or G 1 -G 4 ) such that ions within all ion mobility ranges are passed from the multiple-pass drift tube  14 ,  14 ′,  14 ″ back into the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″, and one or more additional ion gates within D 2 , e.g., an ion gate positioned at the ion outlet  20 , may be controlled by selectively controlling the open/closed positions of the one or more additional ion gates, e.g., relative to an opening/closing of one or more upstream ion gates, such that only ions within the ion mobility range of interest exit the ion outlet  20  of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″. 
     Ions traveling through the ion outlet  20  are detected at step  130  by the ion detector, and thereafter at step  132  the detected ions are processed by the processor  42  to produce corresponding ion spectral information, e.g., as a function of ion drift time. 
     In an alternate embodiment, the process  100  illustratively includes a step  134  between steps  120  and  122  such that the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ and the multiple-pass drift tube  14 ,  14 ′,  14 ″ operate in parallel as described hereinabove. In one embodiment, step  134  may include steps  102 - 110  as illustrated in  FIG. 3A . In other embodiments, step  134  may include only steps  102 - 108 , and in still other embodiments in which ions are generated or otherwise supplied continually, intermittently or periodically step  134  may include only steps  104 - 108  or  104 - 110 . In other embodiments still, step  134  may include more, fewer and/or other steps than those just described. In any such embodiments, one or more of the ion gates in the set of ion gates, e.g., G 1 -G 3 , may be controlled to one or more intermediate positions between the open and closed positions to direct some of the ions traveling through the single-pass drift  12  into the multiple-pass drift tube  14  while also allowing others of the ions traveling through the single-pass drift tube  12  to travel completely through the single-pass drift tube  12 , e.g., to and through the outlet  20  thereof. In such a parallel operating mode, ions supplied by the single or common ion source  18  to the inlet  16  of the single-pass drift tube  12  thus travel in parallel through the single-pass drift tube  12  and also through the combination of the single-pass drift tube  12  and the multiple-pass drift tube  14 , with some of the ions traveling directly through the single-pass drift tube  12  to and through the ion outlet  20  and others of the ions traveling through the single-pass drift tube  12 , to and through the multiple-pass drift tube  14 , then back to and through any remaining section(s) of the single-pass drift tube  12  and exiting the ion outlet  20  of the single-pass drift tube  12 . 
     In the embodiments illustrated in  FIGS. 1A-1F , the variously located ion gates, e.g., G 1 -G 3 , are disclosed as being controllable in a conventional manner to selectively allow ions to pass therethrough or to selectively block ions from passing therethrough, and/or as being controllable in a conventional manner to selectively allow passage therethrough of only a portion of the ions traveling toward such gates. In some embodiments, one or more, or all, such ion grates are disclosed as being provided in the form of a mesh or grid suitably controlled by one or more conventional DC voltage sources. In some alternate embodiments, one or more, or all, such ion gates are disclosed as being provided in the form of a non-meshed, e.g., grid-less, structure defining an ion passageway therethrough and suitably controlled by one or more conventional DC voltage sources and/or RF voltage sources to selectively block or allow passage of ions therethrough. In still other alternate embodiments, e.g., as illustrated in  FIGS. 1D-1F  for example, any such ion gates are disclosed as being implemented in, or as part of, one or more ion inlet and/or outlet plates of an ion transition region  80  in which the one or more ion gates are controlled in a conventional manner to selectively control the direction of ion travel within the hybrid ion mobility spectrometer  10 ′″. 
     In each of the embodiments  10 ,  10 ′,  10 ″,  10 ′″ of the hybrid ion mobility spectrometers illustrated in  FIGS. 1A-1F  and described above, ion gates are positioned at specific locations therein and are controlled as described to selectively cause ions drifting in an axial direction along and through a drift tube or drift tube segment to either continue to travel in and along the same axial direction, or to divert or redirect the ions to travel in a different direction along and axially through a different drift tube or drift tube section. In some cases, such as with the ion gate pair G 1  and G 2  in the embodiment  10  illustrated in  FIG. 1A  and the gate pairs G 1 , G 2  and G 3 , G 4  in the embodiment  10 ′ illustrated in  FIG. 1B , the different directions of travel differ by an acute angle, and in other cases, such as with the ion gate pair G 1  and G 2  in the embodiment  10 ″ illustrated in  FIG. 1C  and the ion gates associated with one or more of the plates  82 ,  84 ,  86 ,  88  in the embodiment  10 ′″ illustrated in  FIGS. 1D-1F , the different directions of travel are transverse to each other, e.g., normal or approximately normal relative to each other. 
     In any of the hybrid ion mobility spectrometer embodiments  10 ,  10 ′,  10 ″,  10 ′″ illustrated in  FIGS. 1A-1F  and described above, one or more such ion gates and/or pairs of ion gates may illustratively be replaced by a single, gateless ion steering channel. Advantageously, and as will become apparent from the following description, such an ion steering channel does not operate, as the ion gate embodiments described above, to selectively block and allow passage of ions; rather, such an ion steering channel is controllable, e.g., via application of suitable DC voltages, to selectively steer or guide ions along different directions of ion travel to thereby direct or redirect ions into and through various drift tubes and/or drift tube sections of the spectrometer  10 ,  10 ′,  10 ″,  10 ′″. Ion loss is thus decreased by using one or more such ion steering channels since ions will not be blocked, and thus not lost as with conventional ion gates, and ion concentration in the hybrid ion mobility spectrometers  10 ,  10 ′,  10 ″,  10 ′″ will accordingly be increased. 
     An example embodiment of an ion steering or guiding structure  200  is illustrated in  FIG. 4A , and is provided in the form of an transition region, similar or identical to the transition region  80  illustrated in  FIG. 1F , in which an embodiment of an ion steering channel  210  is disposed in place of the ion gates G 1 -G 4 . For purposes of this disclosure, the transition region  200  and ion steering channel  210  will be described as replacing the transition region  80  illustrated in  FIG. 1E , although it will be understood that the transition region  200  and/or the ion steering channel  210  alone may be implemented as is, or in other forms, in one or more locations in any of the hybrid ion mobility spectrometers  10 ,  10 ′,  10 ″,  10 ′″ illustrated in  FIGS. 1A-1F  and described above. In the embodiment illustrated in  FIG. 4A , the transition region  200  illustratively includes a first plate  202  defining an ion passage, e.g., opening,  202 A therethrough, which represents an ion inlet to the transition region  200  positioned adjacent to, e.g., the ion outlet of the drift tube funnel  32   2  (e.g., see  FIG. 1E ). Another plate  208  opposite to and facing the plate  202  is spaced apart from the plate  202  and illustratively defines an ion passage  208 A, e.g., opening, therethrough identical or similar to the opening  202 A, which represents an ion outlet of the transition region  200  positioned adjacent to the ion inlet of the drift tube funnel  32   3 . A third plate  204  is positioned between the plate  202  and the plate  208  along one side of the transition region  200 , and a fourth plate  206  is positioned opposite the plate  204 . The third and fourth plates  204 ,  206  each define an ion passage, e.g., opening, therethrough which represents an ion inlet/outlet to/of the transition region  200  with the opening defined through the plate  204  positioned adjacent to the ion inlet/outlet of the drift tube sub-section  34   1  and the opening defined through the plate  206  positioned adjacent to the ion inlet/outlet of the drift tube section  34   2 . 
     Positioned within the transition region  200  illustrated in  FIG. 4A  is an embodiment of an ion steering channel  210  including planar structures  212 ,  214  spaced apart from each other by an ion directing channel  216  such that planes defined by each of the planar structures  212 ,  214  are parallel with each other so that the channel  216  is defined therebetween and such that the planes defined by each of the planar structures  212 ,  214  are generally normal or approximately normal to planes defined by each of the plates  202 ,  204 ,  206 ,  208 . As illustrated in  FIG. 4A , the ion steering channel  210  is positioned within the transition region  200  such that the channel  216  defined between the planar structures  212 ,  214  is illustratively aligned centrally or axially with the ion passages, e.g.,  202 A,  208 A, defined through the plates of the transition region  200  so that ions passing through the openings or ion passages defined through the plates of the transition region  200 , e.g., openings or ion passages  202 A,  208 A, either enter into or exit from the channel  216  in generally in a direction parallel to the planes defined by the opposed inner surfaces of the planar structures  212 ,  214 . An alternate embodiment of an ion steering or guiding structure  200 ′ is illustrated in  FIG. 4B , and in the embodiment depicted in  FIG. 4B  the ion steering or guiding structure  200 ′ illustratively includes the ion steering channel  210  as just described which is illustratively bounded on each side by opposing sidewalls  204 ′,  206 ′ each defining an ion passageway therethrough which is approximately sized identically or complementarily to the channel  216 . In some alternate embodiments, the opposing sidewalls  204 ′,  206 ′ may be replaced or supplemented with a similarly or identically configured front wall and/or a similarly or identically configured rear wall. 
     In some embodiments, the planar structures  212 ,  214  of the ion channel  210  are each illustratively provided in the form of a conventional circuit board having a plurality of electrically conductive surfaces or pads formed in a conventional manner on an inner, major surface thereof and each electrically connectable to a suitable voltage source, e.g., a DC or other voltage source. The electrically conductive surfaces or pads formed on the circuit board  212  are illustratively identical and complementary to those formed on the circuit board  214  such that the electrically conductive surfaces or pads formed on the circuit board  212  are juxtaposed with corresponding ones of the electrically conductive surfaces or pads formed on the circuit board  214  when the inner surfaces of the circuit boards  212 ,  214  are spaced apart to define the channel  216  as illustrated in  FIGS. 4A and 4B . The circuit boards  212 ,  214  may illustratively be conventional printed circuit boards (“PCB&#39;s”) or other conventional circuit boards formed of one or more conventional electrically insulating or non-conductive materials, e.g., fiber-reinforced or paper-reinforced epoxy resin, alumina, one or more ceramic materials, etc., and the electrically conductive surfaces or pads may be formed of one or more conventional electrically conductive materials, e.g., copper, aluminum and/or other metallic or non-metallic but electrically conductive materials. The circuit boards  212 ,  214  may illustratively use through-hole, surface-mounting and/or other structures and techniques for mounting electrical components and/or connecting electrical power sources thereto. 
     Referring now to  FIG. 5 , an example embodiment is shown of the inner, major surface  212 A one of the planar circuit boards  212  of  FIGS. 4A and 4B  upon which a pattern of 4 substantially identical and spaced apart electrically conductive pads  220 A- 220 D are formed. The inner, major surface  214 A of the planar circuit board  214  has an identical pattern of 4 electrically conductive pads  220 A′- 220 D′ formed thereon, and the electrically conductive pads  220 A- 220 D are juxtaposed over corresponding ones of the electrically conductive pads  220 A′- 220 D′ when the inner, major surface  212 A of the circuit board  212  is spaced apart from and generally parallel with the inner, major surface  214 A of the circuit board  214  so that the inner, major surfaces  212 A and  214 A define the channel  216  therebetween as illustrated in  FIG. 6 . In one embodiment, the distance, D P , of the channel or space  216  defined between the inner surfaces  212 A,  214 A of the circuit boards  212 ,  214  is approximately 5 cm, although in other embodiments the distance D P  may be greater or lesser than 5 cm, and it will be understood that the distance D P  will depend, at least in part, on the particular application in which the ion steering channel  210  is implemented. 
     Referring now to  FIG. 6 , the planar circuit board  212  is shown illustratively spaced apart from the planar circuit board  214  such that upstream edges  212 C and  214 C of the respective circuit boards  212 ,  214  are aligned, as are downstream edges  212 D,  214 D and opposing side edges  212 E,  214 E and  212 F,  214 F. The terms “upstream” and “downstream” illustratively refer to the direction of ion travel such that, in the embodiment illustrated in  FIG. 4A , for example, the aligned edges  212 C,  214 C are positioned in contact with or adjacent to the plate  202  such that the channel  216  is axially aligned with the opening  202 A, and the aligned edges  212 D,  214 D are positioned in contact with or adjacent to the plate  208  such that the channel  216  is axially aligned with the opening  208 A. In any case, the major surface  212 B of the planar circuit board  212  opposite the inner, major surface  212 A will illustratively be referred to as the outer surface of the planar circuit board  212 , and the major surface  214 B of the planar circuit board  214  opposite the inner, major surface  214 A will illustratively be referred to as the outer surface of the planar circuit board  214 . 
     In the embodiment illustrated in  FIG. 6 , a first DC voltage source DC 1  is electrically connected to each of the juxtaposed electrically conductive pads  220 A,  220 A′ such that the potential at both pads  220 A,  220 A′ is the potential produced by DC 1 , a second DC voltage source DC 2  is electrically connected to each of the juxtaposed electrically conductive pads  220 C,  220 C′ such that the potential at both pads  220 C,  220 C′ is the potential produced by DC 2 , a third DC voltage source DC 3  is electrically connected to each of the juxtaposed electrically conductive pads  220 B,  220 B′ such that the potential at both pads  220 B,  220 B′ is the potential produced by DC 3 , and a fourth DC voltage source DC 4  is electrically connected to each of the juxtaposed electrically conductive pads  220 D,  220 D′ such that the potential at both pads  220 D,  220 D′ is the potential produced by DC 4 . In some embodiments, one or more conventional electrical components, e.g., resistors or other components, may be interconnected between one or more of the voltage sources DC 1 -DC 4  and one or more of the corresponding electrically conductive pads  220 A- 220 D and  220 A′- 220 D′ and/or between two or more, and/or two or opposed pairs, of the electrically conductive pads  220 A- 220 D and  220 A′- 220 D′. In the illustrated embodiment, each of the DC voltage sources DC 1 -DC 4  is independently controlled, e.g., via manually or via the processor  42 , although in alternate embodiments two or more of the DC voltage sources DC 1 -DC 4  may be controlled together as a group. In any case, it will be understood that although the voltage sources DC 1 -DC 4  are illustrated and disclosed as being DC voltage sources, this disclosure contemplates other embodiments in which one or more of the voltage sources DC 1 -DC 4  is or includes an AC voltage source such as, for example, an RF voltage source suitably coupled, e.g., capacitively, via conventional electrical components to corresponding ones or pairs of the electrically conductive pads  220 A- 220 D and  220 A′- 220 D′. 
     Referring now to  FIGS. 7A and 7B , operation of the ion steering channel  210  illustrated in  FIG. 6  will be described as implemented in the form of the ion steering or guiding structure  200  or  200 ′ in place of the ion transition region  80  of the ion mobility spectrometer  10 ′″ illustrated in  FIGS. 1D-1F . It will be understood, however, that the ion steering channel  210  may alternatively or additionally be implemented in place of one or more ion gates in one or more of the other embodiments  10 ,  10 ′,  10 ″ of the ion mobility spectrometer described above. In any case, the DC voltage sources DC 1 -DC 4  are omitted in  FIGS. 7A and 7B  for clarity of illustration, and instead the various DC voltages produced thereby and applied to the connected pairs of electrically conductive pads  220 A/ 220 A′,  220 B/ 220 B′,  220 C/ 220 C′ and  220 D/ 220 D′ are represented graphically. Referring specifically to  FIG. 7A , the ion steering channel  210  is shown in a state in which a reference potential, V REF , is applied by each of DC 1  and DC 2  to the electrically conductive pad pairs  220 A/ 220 A′ and  220 C/ 220 C′ respectively, and a potential −XV, less than V REF , is applied by each of DC 3  and DC 4  to the electrically conductive pad pairs  220 B/ 220 B′ and  220 D/ 220 D′ respectively. Illustratively, V REF  may be any positive or negative voltage, or may be zero volts, e.g., ground potential, and −XV may be any voltage, positive, negative or zero voltage that is less than V REF  so as to establish an electric field E 1  which is parallel with the sides  212 E,  214 E and  212 F,  214 F of the circuit boards  212 ,  214  and which extends in a direction from and generally normal to the upstream edges  212 C,  214 C toward and normal to the downstream edges  212 D,  214 D of the circuit boards  212 ,  214  as depicted in  FIG. 7A . With the electric field, E 1 , established as illustrated in  FIG. 7A , ions  230  drifting through the drift tube segment  32   2  of the single-pass drift tube  12 ′″ enter the channel  216  between the upstream edges  212 C,  214 C and are steered or guided (or directed) by the electric field, E 1 , in a direction generally parallel with the ion travel axis  72  of the drift tube  12 ′″, and such ions thus pass into the drift tube segment  323  of the single-pass drift tube  12 ′″ as described above with respect to  FIGS. 1D-1F . 
     Referring now to  FIG. 7B , when it is desired to direct ions from the single-pass drift tube  12 ′″ into the multiple-pass drift tube  14 ″, as also described above with respect to  FIGS. 1D-1F , the reference potential, V REF , is applied by each of DC 2  and DC 4  to the electrically conductive pad pairs  220 C/ 220 C′ and  220 D/ 220 D′ respectively, and a potential −XV, less than V REF , is applied by each of DC 1  and DC 3  to the electrically conductive pad pairs  220 A/ 220 A′ and  220 B/ 220 B′ respectively, so as to establish an electric field E 2  which is parallel with the upstream edges  212 C,  214 C and the downstream edges  212 D,  214 D of the circuit boards  212 ,  214 , and which extends in a direction from and generally normal to the sides  212 F,  212 F toward and normal to the sides  212 E,  214 E of the circuit boards as depicted in  FIG. 7B . With the electric field, E 2 , established as illustrated in  FIG. 7B , ions  250  drifting through the drift tube segment  32   2  in the direction generally parallel with the ion travel axis  72  of the drift tube  12 ′″ and entering the channel  216  through the upstream edges  212 C,  214 C of the ion steering channel  210  will be steered or guided (or directed) by the electric field, E 2 , so as to be diverted to travel in a direction from and generally normal to the sides  212 F,  212 F toward and normal to the sides  212 E,  214 E of the circuit boards and thus into the drift tube segment  34   1  of the multiple-pass drift tube  14 ″ as described above with respect to  FIGS. 1D-1F . Thus, the ions drifting through the drift tube segment  32   2  in the direction generally parallel with the ion travel axis  72  of the drift tube  12 ′″ are diverted by E 2  to travel in a direction that is generally normal to the ion travel axis  72 . Thereafter, as long as the voltage sources DC 1 -DC 4  continue to be controlled so as to maintain the electric field E 2 , ions  270  drifting through the multiple-pass drift tube  14 ″ in a clockwise direction and generally parallel with the ion travel axis  70  of the multiple-pass drift tube  14 ″ (as viewed in  FIG. 1D ) will enter the channel  216  from the drift tube segment  34   2  at the sides  212 F,  214 F of the circuit boards  212 ,  214  and be steered or guided (or directed) by the electric field, E 2 , in a direction parallel with the ion travel axis  70  of the multiple-pass drift tube  14 ″ and thus back into the drift tube segment  34   1  as described above with respect to  FIGS. 1D-1F . 
     After a desired number of traversals by the ions through the multiple-pass drift tube  14 ″, the reference potential, V REF  or other suitable reference potential, is applied by each of DC 1  and DC 2  to the electrically conductive pad pairs  220 A/ 220 A′ and  220 C/ 220 C′ respectively, and the potential −XV or other suitable potential less than V REF , is applied by each of DC 3  and DC 4  to the electrically conductive pad pairs  220 B/ 220 B and  220 D/ 220 D′ respectively, so as to reestablish the electric field E 1 , or to establish another electric field in the same direction as E 1 , such that ions  280  drifting through the drift tube segment  34   2  of the multiple-pass drift tube  14 ″ and into the ion steering channel  210  will be steered or guided (or directed) by such an electric field in the downstream direction of the ion travel axis  72  of the single-pass drift tube  12 ′″ thus into the drift tube segment  32   3  of the single-pass drift tube  12 ′″ as described above with respect to  FIGS. 1D-1F . Thus, the ions  280  drifting through the drift tube segment  34   2  of the multiple-pass drift tube  14 ″ in the direction generally parallel with the ion travel axis  70  of the multiple-pass drift tube  14 ″ are diverted by E 2  to travel in a direction that is generally normal to the ion travel axis  70  as illustrated in  FIG. 7A . 
     In each of the embodiments  10 ,  10 ′,  10 ″,  10 ′″ of the hybrid ion mobility spectrometer illustrated in  FIGS. 1A-1F , ion funnels, e.g., in the form of ion funnels  32   1 - 32   11  and/or drift tube sections  50 , are illustratively implemented in and along the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ as well as in and along the multiple-pass drift tube  14 ,  14 ′,  14 ″. In some alternate embodiments, including any such embodiments which include one or more ion steering or guiding structures  200  and/or  200 ′ and/or ion steering channels  210 , one or more such ion funnels may be replaced by one or more conventional ion carpets of the general form illustrated in  FIG. 8 . Referring to  FIG. 8 , the illustrated ion carpet  300  is provided in the form of a planar structure  302 , e.g., a circuit board or other electrically insulating or non-conductive plate, having a ring structure  304  with multiple, axially aligned, electrically conductive rings with progressively decreasing ring diameters formed on one major surface  306 A thereof. In the embodiment illustrated in  FIG. 8 , the ring structure  304  includes a number of axially aligned, electrically conductive, concentric rings  308 A,  308 B,  308 C . . . formed on one major surface  306  thereof with each successive ring  308 A,  308 B,  308 C . . . reduced in diameter relative to the previous ring. Adjacent rings  308 A,  308 B,  308 C . . . are radially separated from each other by electrically insulating or non-conductive ring areas  310 A,  3106 ,  310 C . . . of the planar structure  302 . In embodiments in which the planar member  302  is a circuit board, the electrically conductive rings  308 A,  308 B,  308 C are illustratively applied in a conventional manner on and to the major surface  306 A of the circuit board  302  in the form of electrically conductive films or traces, e.g., copper, aluminum and/or other electrically conductive material. An ion passageway  312 , e.g., in the form of a through-hole, having a central axis in common with each of the number of electrically conductive rings  308 A,  308 B,  308 C . . . and electrically insulating or non-conductive areas  310 A,  3106 ,  310 C . . . of the planar structure  302  is defined through the planar structure  302 . In the illustrated embodiment, six such electrically conductive rings are shown, although it will be understood that the planar structure  302  may include more or fewer such rings. Moreover, it will be understood that whereas the electrically conductive rings  308 A,  308 B,  308 C . . . and electrically insulating or non-conductive areas  310 A,  3106 ,  310 C . . . of the planar structure  302  are illustrated in  FIG. 8  as being concentric structures, alternate embodiments are contemplated in which such structures are non-concentric but closed structures. In any case, a conventional AC voltage source, e.g., a conventional RF voltage source,  316  is coupled through a capacitor network to each of the electrically conductive rings  308 A,  308 B,  308 C . . . such that an RF voltage of a first phase, ϕ 1 , is applied to odd-numbered (or even-numbered) ones of the rings  308 A,  308 B,  308 C . . . and the same RF voltage of a second phase, ϕ 2 , is applied to even-numbered (or odd-numbered) ones of the rings  308 A,  308 B,  308 C . . . . In some embodiments, ϕ 1 −ϕ 2  (or ϕ 2 −ϕ 1 )=180 degrees such that an opposite phase RF voltage is applied to adjacent ones of the rings  308 A,  308 B,  308 C . . . of the ion carpet  300  as illustrated in  FIG. 8 . Generally, the ion carpet  300  illustratively operates functionally the same as ion funnels in that ions travelling toward the major surface  306 A of the ion carpet  300  are focused radially inwardly by the RF voltages applied to the rings  308 A,  308 B,  308 C . . . toward and through the central ion passageway  312 . Accordingly, the ion carpet  300  may be used in place of any ion funnel structure described in connection with any of the embodiments  10 ,  10 ′,  10 ″,  10 ′″ of the hybrid ion mobility spectrometer illustrated in  FIGS. 1A-1F . 
     Referring now to  FIG. 9 , a portion of another embodiment of an ion steering or guiding structure  400  is shown in the form of an electrically insulating or non-conductive planar member  410 , e.g., a printed circuit board or the like, having an inner, major surface  410 A upon which a number of electrically conductive pads are formed. The planar circuit board  410  illustratively includes an upstream edge  410 C, a downstream edge  410 D opposite the upstream edge  410 C and two opposing side edges  410 E,  410 F joining the upstream and downstream edges  410 C,  410 D, wherein the edges  410 C,  410 D,  410 E,  410 F surround the inner, major surface  410 A and an opposite outer, major surface  410 B (see, e.g.,  FIG. 14 ), and wherein the terms “upstream” and downstream” are as described above. In the illustrated embodiment, the planar circuit board has two sets of two substantially identical and spaced apart electrically conductive pads  402 A and  404 A are formed thereon. Along the upstream edge  410 C and spaced-apart between the side edges  410 E,  410 F a pair of square or rectangular electrically conductive pads  402 A,  404 A are formed on the inner, major surface  410 A of the planar circuit board  410 , and along the downstream edge  410 D and spaced-apart between the side edges  410 E,  410 F a pair of generally arcuate-shaped electrically conductive pads  402 B,  404 B are formed on the inner, major surface  410 A. Each square or rectangular pad  402 A,  404 A has a substantially planar upstream edge  402 A 1 ,  404 A 1  respectively that is substantially parallel with the upstream edge  410 C of the planar circuit board  410 , and a substantially planar downstream edge  402 A 2 ,  404 A 2  respectively that is substantially parallel with the planar upstream edge  402 A 1 ,  404 A 1  respectively. Each arcuate-shaped pad  402 B,  404 B, in contrast, has a generally planar upstream edge  40261 ,  404 B 1  respectively that is generally parallel with and spaced apart from the downstream edge  402 A 2 ,  404 A 2  of the respective square or rectangular pad  402 A,  404 A, and a generally planar downstream edge  402 B 2 ,  404 B 2  respectively that illustratively forms an acute angle with the respective upstream edge  402 B 1 ,  404 B 1  such that the arcuate-shaped pads  402 B,  404 B generally diverge from one another as the pads  402 B,  404 B extend generally toward the downstream edge  410 D of the planar circuit board  410 . Each of the actuate shaped pads  402 B,  404 B is flanked along opposing side edges by electrically conductive pads  402 C,  402 D and  404 C,  404 D respectively, each of which defines an arcuate-shaped side edge that faces a respectively arcuate-shaped side edge of the electrically conductive pads  402 B,  404 B such that all such opposing arcuate shaped side edges are spaced-apart equidistantly along the lengths of the opposing sides of the electrically conductive pads  402 B,  404 B. 
     Like the ion steering and guiding structure  210  illustrated in  FIG. 6 , the steering or guiding structure  400  illustratively includes a second planar circuit board  410  configured identically to the planar circuit board  410  just described and having identical spaced-apart electrically conductive pads  402 A- 402 D,  404 A- 404 D formed thereon. All such electrically conductive pads  402 A- 402 D,  404 A- 404 D of the two planar circuit boards  410  are juxtaposed over corresponding ones of the electrically conductive pads  402 A- 402 D,  404 A- 404 D when the inner, major surfaces  410 A of the two identical circuit boards  410  are spaced apart from and generally parallel with each other so that the inner, major surfaces  212 A and  214 A define a channel, e.g., similar or identical to the channel  216 , therebetween as illustrated in  FIG. 6  and described above. In the description that follows, it will be understood that an ion steering channel formed with the planar circuit board  410  illustrated in  FIG. 9  will necessarily include such a second, identically configured planar circuit board  410 , and that although only a single one of the planar circuit boards  410  is illustrated in  FIGS. 9 and 10A-10B , such a second planar circuit board  410  will be connected to voltage sources identically as shown and described with respect to the illustrated planar circuit board  410  and that the functional operation, e.g., relating to ion steering, of the ion steering channel formed with the illustrated planar circuit board  410  will generally take place in and through the channel formed between the illustrated circuit board  410  and such a second planar circuit board  410 . 
     In the embodiment illustrated in  FIG. 9 , a first DC voltage source DC 1  is electrically connected to each of the juxtaposed electrically conductive pads  402 A (of the illustrated planar circuit board  410  and a second, identically configured planar circuit board spaced apart therefrom) such that the potential at both pads  402 A is the potential produced by DC 1 , a second DC voltage source DC 2  is electrically connected to each of the juxtaposed electrically conductive pads  404 A such that the potential at both pads  404 A is the potential produced by DC 2 , a third DC voltage source DC 3  is electrically connected to each of the juxtaposed electrically conductive pads  402 B such that the potential at both pads  402 B is the potential produced by DC 3 , a fourth DC voltage source DC 4  is electrically connected to each of the juxtaposed electrically conductive pad pairs  402 C,  402 D such that the potential at both pairs of pads  402 C,  402 D is the potential produced by DC 4 , a fifth DC voltage source DC 5  is electrically connected to each of the juxtaposed electrically conductive pads  404 B such that the potential at both pads  404 B is the potential produced by DC 5 , and a sixth DC voltage source DC 6  is electrically connected to each of the juxtaposed electrically conductive pad pairs  404 C,  404 D such that the potential at both pairs of pads  404 C,  404 D is the potential produced by DC 6 . In some embodiments, one or more conventional electrical components, e.g., resistors or other components, may be interconnected between one or more of the voltage sources DC 1 -DC 6  and one or more of the corresponding electrically conductive pads  402 A- 402 D,  404 A- 404 D and/or between two or more, and/or two or opposed pairs, of the electrically conductive pads  402 A- 402 D,  404 A- 404 D. In the illustrated embodiment, each of the DC voltage sources DC 1 -DC 6  is independently controlled, e.g., via manually or via the processor  42 , although in alternate embodiments two or more of the DC voltage sources DC 1 -DC 6  may be controlled together as a group. In any case, it will be understood that although the voltage sources DC 1 -DC 6  are illustrated and disclosed as being DC voltage sources, this disclosure contemplates other embodiments in which one or more of the voltage sources DC 1 -DC 6  is or includes an AC voltage source such as, for example, an RF voltage source suitably coupled, e.g., capacitively, via conventional electrical components to corresponding ones or pairs of the electrically conductive pads  402 A- 402 D,  404 A- 404 D. 
     Referring now to  FIGS. 10A and 10B , operation of an ion steering channel  400 , formed by the planar circuit board  410  spaced apart from a second, identical planar circuit board  410  with corresponding ones of the electrically conductive pads  402 A- 402 D,  404 A- 404 D of each circuit board  410  juxtaposed with each other, will be described. The DC voltage sources DC 1 -DC 6  are omitted in  FIGS. 10A and 10B  for clarity of illustration, and instead the various DC voltages produced thereby and applied to the connected pairs of electrically conductive pads  402 A- 402 D,  404 A- 404 D are represented graphically. Referring specifically now to  FIG. 10A , the ion steering channel  400  is shown in a state in which a reference potential, VREF, is applied by DC 1  to the electrically conductive pad pairs  402 A and by DC 4  to the electrically conductive pad pairs  402 C and  402 D, and a potential −XV, less than VREF, is applied by DC 3  to the electrically conductive pad pairs  402 B. Illustratively, VREF may be any positive or negative voltage, or may be zero volts, e.g., ground potential, and −XV may be any voltage, positive, negative or zero voltage that is less than VREF so as to establish an electric field E 1  which is normal to the planar edges  402 A 1 ,  402 A 2 ,  402 B 1 ,  402 B 2  of the electrically conductive pad pairs  402 A,  402 B respectively and which thus follows the arcuate shape of the electrically conductive pad pairs  402 B as shown. Because the potential applied by DC 4  to the electrically conductive pad pairs  402 C and  402 D is also VREF, additional electric fields, ES, are thus established normal to the arcuate side edges of the electrically conductive pad pairs  402 B in the direction of the electrically conductive pad pairs  402 B to thereby confine ions to the arcuate path defined by the electrically conductive pad pairs  402 B such that the ions entering the ion steering channel  400  at and in a direction normal to the upstream edges  410 C travel linearly across the electrically conductive pad pairs  402 A and then across the electrically conductive pad pairs  402 B generally in the arcuate direction defined by the arcuate electric field E 1 . 
     Referring specifically now to  FIG. 10B , the ion steering channel  400  is shown in a state in which a reference potential, VREF, is applied by DC 2  to the electrically conductive pad pairs  404 A and by DC 6  to the electrically conductive pad pairs  404 C and  404 D, and a potential −XV, less than VREF, is applied by DC 5  to the electrically conductive pad pairs  404 B. Illustratively, VREF may be any positive or negative voltage, or may be zero volts, e.g., ground potential, and −XV may be any voltage, positive, negative or zero voltage that is less than VREF so as to establish an electric field E 2  which is normal to the planar edges  404 A 1 ,  404 A 2 ,  404 B 1 ,  404 B 2  of the electrically conductive pad pairs  404 A,  404 B respectively and which thus follows the arcuate shape of the electrically conductive pad pairs  404 B as shown. Because the potential applied by DC 6  to the electrically conductive pad pairs  404 C and  404 D is also VREF, additional electric fields, ES, are thus established normal to the arcuate side edges of the electrically conductive pad pairs  404 B in the direction of the electrically conductive pad pairs  404 B to thereby confine ions to the arcuate path defined by the electrically conductive pad pairs  404 B such that the ions entering the ion steering channel  400  at and in a direction normal to the upstream edges  410 C travel linearly across the electrically conductive pad pairs  404 A and then across the electrically conductive pad pairs  404 B generally in the arcuate direction defined by the arcuate electric field E 2 . 
     In the embodiment shown in  FIG. 10A , a potential +YV is illustratively applied by DC 2 , DC 5  and DC 6  to the electrically conductive pad pairs  404 A,  404 B and  404 C/ 404 D respectively, and in the embodiment shown in  FIG. 10B  the potential +YV is illustratively applied by DC 1 , DC 3  and DC 4  to the electrically conductive pad pairs  402 A,  402 B and  402 C/ 402 D respectively. Illustratively, +YV is selected to establish a repulsive electric field ER of sufficient strength to confine ions entering the ion steering channel  400  at and in a direction normal to the upstream edges  410 C to the electrically conductive pad pairs  402 A and  404 B respectively so that such ions will not be lost and will be directed by the electric fields E 1  and E 2  respectively to follow the arcuate path established thereby. In some alternate embodiments, DC 2 , DC 5  and DC 6  may instead apply VREF to the electrically conductive pad pairs  404 A,  404 B and  404 C/ 404 D respectively and/or DC 1 , DC 3  and DC 4  may instead apply VREF to the electrically conductive pad pairs  402 A,  402 B and  402 C/ 402 D respectively. 
     As illustrated in  FIGS. 10A and 10B , the ion steering channel  400  is selectively operable, via control of the voltage sources DC 1 -DC 6 , to direct ions entering the ion steering channel  400  at and in a direction normal to the upstream edges  410 C along the arcuate path established by E 1  or E 2 , wherein the ion outlets of the arcuate paths diverge from each other. In some embodiments, the ion steering channel  400  may be used in place of the ion gate pairs in any of the diverging drift tube segments of either of the embodiments  10 ,  10 ′ illustrated in  FIGS. 1A and 1B , e.g., in place of the ion gate pair G 1 , G 2  in the drift tube segment  36   2  of the hybrid ion mobility spectrometer  10  illustrated in  FIG. 1A , in place of the ion gate pair G 1 , G 2  in the drift tube segment  38   1  of the hybrid ion mobility spectrometer  10 ′ illustrated in  FIG. 1B  and/or in place of the ion gate pair G 3 , G 4  in the drift tube segment  36   2  of the hybrid ion mobility spectrometer  10 ′. Alternatively or additionally, the ion steering channel  400  illustrated in  FIGS. 9-10B  may be rotated 180 degrees such that ions enter the ion steering channel  400  at and in a direction normal to the edges  410 D and the rotated ion steering channel  400  may be used in place of the ion gate G 3  in the drift tube segment  36   1  of the hybrid ion mobility spectrometer  10  illustrated in  FIG. 1A . In any such embodiment, the electrically conductive arcuate pad pairs  402 B- 402 D,  404 B- 404 D are illustratively shaped such that the curvatures of such pad pairs match and align with the curvatures of the diverging paths of the drift tube segments  36   2 ,  38   1  and/or of the converging paths of the drift tube segment  36   1 . 
     In some embodiments in which either of the ion steering channel  210  and/or the ion steering channel  400  is implemented, it may be desirable to, under some operating conditions, selectively confine ion travel or passage to only one side or the other of the channel  210 ,  400  and, under other operating conditions, to allow for ion travel through both sides of the channel  210 ,  400 . As used herein, “through one side of the channel” will be understood to mean ion travel through the channel  210 ,  400  along one axially or transversely aligned set of pairs of electrically conductive pads, e.g., axially along the aligned set of the electrically conductive pad pairs  402 A and  402 B under the influence of the electric field E 1  as illustrated in  FIG. 10A , axially along the aligned set of the electrically conductive pad pairs  404 A and  404 B under the influence of the electric field E 2  as illustrated in  FIG. 10B , transversely along the aligned set of electrically conductive pad pairs  220 C,  220 C′ and  220 A,  220 A′ as depicted by the dashed line  280  in  FIG. 7A , and axially along the aligned set of electrically conductive pad pairs  220 C,  220 C′ and  220 D,  220 D′ as depicted by the solid line  250  in  FIG. 7B . Similarly, “through both sides of the channel” will be understood to mean ion travel through the channel  210 ,  400  along both axially or transversely aligned sets of pairs of electrically conductive pads, e.g., axially along the aligned sets of electrically conductive pad pairs  220 A,  220 A′ and  220 B,  220 B′ and, at the same time, axially along the aligned sets of electrically conductive pad pairs  220 C,  220 C′ and  220 D,  220 D′ as depicted by the solid line  230  in  FIG. 7A , and transversely along the aligned sets of electrically conductive pad pairs  220 C,  220 C′ and  220 A,  220 A′ and, at the same time, transversely along the aligned sets of electrically conductive pad pairs  220 D,  220 D′ and  220 B,  220 B′ as depicted by the dashed line  270  in  FIG. 7B . 
     In some such embodiments, selective guidance or steering of ion travel or passage through only one side or the other, or through both sides, of an ion steering channel  210 ,  400  is accomplished with a dual-passage or ion carpet, one example embodiment of which is illustrated by example in  FIG. 11 . Referring now to  FIG. 11 , the illustrated dual-passage ion carpet  500  is provided in the form of a planar structure  502 , e.g., a circuit board or other electrically insulating or non-conductive plate, having side-by-side ring structure  504 A,  504 B each with multiple, axially aligned and progressively smaller electrically conductive rings formed on one major surface  506 A thereof. In the illustrated embodiment, the planar circuit board  502  illustratively has a top edge  502 A, a bottom edge  502 B opposite the top edge  502 A with the top and bottom edges  502 A,  502 B joined by opposing edges  502 C,  502 D. The major surface  506 A of the circuit board is defined between the edges  502 A- 502 D on one side of the circuit board  502 , and another major surface  506 B (see, e.g.,  FIGS. 12A-13 ) is defined between the edges  502 A- 502 D opposite the major surface  506 A. 
     In the embodiment illustrated in  FIG. 11 , the ring structure  504 A includes a number of axially aligned, electrically conductive, elliptical rings  508 A,  508 B,  508 C . . . formed on the major surface  506 A of the circuit board  502  with each successive ring  508 A,  508 B,  508 C . . . reduced in length of both major and minor axes relative to the previous ring. Adjacent rings  508 A,  508 B,  508 C . . . are radially separated from each other by electrically insulating or non-conductive ring areas  510 A,  510 B,  510 C . . . of the planar structure  502 . An ion passageway  512 , e.g., in the form of a through-hole, having a central axis in common with each of the number of electrically conductive rings  508 A,  508 B,  508 C . . . and electrically insulating or non-conductive areas  510 A,  510 B,  510 C . . . of the planar structure  502  is defined through the planar structure  502 . Illustratively, the ring structure  504 A is positioned on the major surface  506 A of the circuit board  502  such that the ion passageway  512  aligns with the channel  216  or space defined between the opposed circuit boards of an ion steering channel  210 ,  400  and is also centrally aligned with axially or transversely aligned with sets of pairs of electrically conductive pads formed on one side, e.g., a left side, of the ion steering channel  210 ,  400  when the ion carpet  500  is operatively mounted to or operatively positioned adjacent to the ion steering channel  210 ,  400 , e.g., such that the ion passageway  512  bisects or approximately bisects the channel  216  and bisects or approximately bisects the axially or transversely aligned sets of pairs of electrically conductive pads on one side of the ion steering channel. In the illustrated embodiment, 13 such electrically conductive rings are shown, although it will be understood that the planar structure  502  may include more or fewer such rings. Moreover, it will be understood that whereas the electrically conductive rings  508 A,  508 B,  508 C . . . and electrically insulating or non-conductive areas  510 A,  5106 ,  510 C . . . of the planar structure  502  are illustrated in  FIG. 11  as being elliptical structures, alternate embodiments are contemplated in which such structures are concentric or other closed structures. In any case, a conventional AC voltage source  516 , e.g., a conventional RF voltage source, is operatively coupled through a capacitor network to each of the electrically conductive rings  508 A,  508 B,  508 C . . . such that an RF voltage of a first phase, ϕ 1 , is applied to odd-numbered (or even-numbered) ones of the rings  508 A,  508 B,  508 C . . . and the same RF voltage of a second phase, ϕ 2 , is applied to even-numbered (or odd-numbered) ones of the rings  508 A,  508 B,  508 C . . . . In some embodiments, ϕ 1 −ϕ 2  (or ϕ 2 −ϕ 1 )=180 degrees such that an opposite phase RF voltage is applied to adjacent ones of the rings  508 A,  508 B,  508 C . . . of the ion carpet  500 . 
     The ring structure  504 B of the ion carpet  500  likewise includes a number of axially aligned, electrically conductive, elliptical rings  528 A,  528 B,  528 C . . . formed on the major surface  506 A of the circuit board  502  with each successive ring  528 A,  528 B,  528 C . . . reduced in length of both major and minor axes relative to the previous ring. Adjacent rings  528 A,  528 B,  528 C . . . are radially separated from each other by electrically insulating or non-conductive ring areas  530 A,  530 B,  530 C . . . of the planar structure  502 . An ion passageway  532 , e.g., in the form of a through-hole, having a central axis in common with each of the number of electrically conductive rings  528 A,  528 B,  528 C . . . and electrically insulating or non-conductive areas  530 A,  530 B,  530 C . . . of the planar structure  502  is defined through the planar structure  502 . Illustratively, the ring structure  504 B is positioned on the major surface  506 A of the circuit board  502  such that the ion passageway  532  aligns with the channel  216  or space defined between the opposed circuit boards of an ion steering channel  210 ,  400  and is also centrally aligned with axially or transversely aligned with sets of pairs of electrically conductive pads formed on the other side (relative to the ion passageway  512 ), e.g., a right side, of the ion steering channel  210 ,  400  when the ion carpet  500  is operatively mounted to or operatively positioned adjacent to the ion steering channel  210 ,  400 , e.g., such that the ion passageway  532  bisects or approximately bisects the channel  216  and bisects or approximately bisects the axially or transversely aligned sets of pairs of electrically conductive pads on the side of the ion steering channel opposite that with which the ion passageway  512  is aligned. In the illustrated embodiment the ring structure  504 B is identical to the ring structure  504 A such that it includes  13  electrically conductive and elliptically shaped rings, although it will be understood that the ring structure  504 B may be different from the ring structure  504 A by having more or fewer such rings and/or by having rings of other shapes. In embodiments in which the planar member  502  is a circuit board, the electrically conductive rings  508 A,  508 B,  508 C . . . and  528 A,  528 B,  528 C . . . are illustratively applied in a conventional manner on and to the major surface  506 A of the circuit board  502  in the form of electrically conductive films or traces, e.g., copper, aluminum and/or other electrically conductive material. 
     In any case, a conventional AC voltage source  536 , e.g., a conventional RF voltage source, is operatively coupled through a capacitor network to each of the electrically conductive rings  528 A,  528 B,  528 C . . . such that an RF voltage of a first phase, ϕ 1 , is applied to odd-numbered (or even-numbered) ones of the rings  528 A,  528 B,  528 C . . . and the same RF voltage of a second phase, ϕ 2 , is applied to even-numbered (or odd-numbered) ones of the rings  528 A,  528 B,  528 C . . . . In some embodiments, ϕ 1 −ϕ 2  (or ϕ 2 −ϕ 1 )=180 degrees such that an opposite phase RF voltage is applied to adjacent ones of the rings  528 A,  528 B,  528 C . . . of the ion carpet  500 . 
     Generally, the ring structures  504 A,  504 B of the ion carpet  500  are selectively controllable, together or each independently of the other, to operate as described above with respect to the ion carpet  300  illustrated in  FIG. 8 . In some embodiments and/or under some operating conditions in which it is desired to focus and pass ions through both ion passageways  512 ,  532 , the ring structures  504 A and  504 B are controlled via the voltage sources  516  and  532  such that some of the ions travelling toward the major surface  506 A of the ion carpet  500  are focused radially inwardly by the RF voltages applied by the voltage source  516  to the ring structure  504 A toward and through the central ion passageway  512  and others of the ions travelling toward the major surface  506 A of the ion carpet  500  are focused radially inwardly by the RF voltages applied by the voltage source  536  to the ring structure  504 B toward and through the central ion passageway  532 . In other embodiments and/or under other operating conditions in which it is desired to focus and pass ions only through the ion passageway  512 , the ring structure  504 A is controlled via the voltage source  516  such that ions travelling toward the major surface  506 A of the ion carpet  500  are focused radially inwardly by the RF voltages applied by the voltage source  516  to the ring structure  504 A toward and through the central ion passageway  512 , and the voltage source  536  is either not activated so that ions traveling toward the major surface  506 A of the ion carpet  500  are not radially focused relative to the ring structure  504 B and therefore generally do not pass through the central ion passageway  532 , or is activated and controlled in a manner that does not cause ions traveling toward the major surface  506 A of the ion carpet to be radially focused relative to the ring structure  504 B and therefore generally does not provide for the passage of ions through the central ion passageway  532 . In still other embodiments and/or under still other operating conditions in which it is desired to focus and pass ions only through the ion passageway  532 , the ring structure  504 B is controlled via the voltage source  536  such that ions travelling toward the major surface  506 A of the ion carpet  500  are focused radially inwardly by the RF voltages applied by the voltage source  536  to the ring structure  504 B toward and through the central ion passageway  532 , and the voltage source  516  is either not activated so that ions traveling toward the major surface  506 A of the ion carpet  500  are not radially focused relative to the ring structure  504 A and therefore generally do not pass through the central ion passageway  512 , or is activated and controlled in a manner that does not cause ions traveling toward the major surface  506 A of the ion carpet to be radially focused relative to the ring structure  504 A and therefore generally does not provide for the passage of ions through the central ion passageway  512 . Accordingly, the ion carpet  500  may be used in combination with any of the ion gates G 1 -G 4  and/or with any ion steering channel  210 ,  400  implemented in any of the embodiments  10 ,  10 ′,  10 ″,  10 ′″ of the hybrid ion mobility spectrometer illustrated in  FIGS. 1A-1F . 
     Referring now to  FIG. 12A , an example embodiment is shown of a portion of the hybrid ion mobility spectrometer  10  or  10 ′ in which the ion funnel  32   5  and the ion gates G 1 , G 2  ( FIG. 1A ) or G 3 , G 4  ( FIG. 1B ) positioned in the drift tube section  36   2  are replaced by an ion carpet  500  coupled to the upstream end of an ion steering channel  400 . The ion carpet  500  is illustratively positioned such that the central ion passageway  512  of the ring structure  504 A bisects or approximately bisects the space between the planar circuit boards  410  and axially bisects or approximately bisects the electrically conductive pad pairs  404 A at the upstream edge  410 C of the ion steering channel  400 , and that the central ion passageway  532  of the ring structure  504 B bisects or approximately bisects the space between the planar circuit boards  410  and axially bisects or approximately bisects the electrically conductive pad pairs  402 A at the upstream edge  410 C of the ion steering channel  400 . Although not shown in  FIG. 12A , it will be understood that the voltage sources DC 1 -DC 6  are operatively connected to the electrically conductive pad pairs  402 A- 402 D,  404 A- 404 D as illustrated in  FIG. 9  and described above. 
     When it is desired to direct ions from the drift tube section  36   1  into the upper arm of the drift tube section  36   2 , the voltage sources  516  and  536  are controlled, as described above, so that ions drifting through the drift tube section  36   1  toward the major surface  506 A of the ion carpet  500  pass through only the ion passageway  512  defined centrally through the ring structure  504 A. The voltage sources DC 1 -DC 4  and DC 6  are set to VREF and DC 5  is set to −XV so as to establish the electric field E 2  between the electrically conductive pad pairs  404 A and  404 B as illustrated in  FIG. 10B . Thus as ions drift through the drift tube section  36   1  toward the major surface  506 A of the ion carpet  500 , such ions are radially focused by the ring structure  504 A and pass through the ion passageway  512  thereof where such ions are then steered or guided by the electric field E 2  toward and into the upper arm of the drift tube section  36   2 . 
     When it is desired to direct ions from the drift tube section  36   1  into the lower arm of the drift tube section  36   2 , the voltage sources  516  and  536  are controlled, as described above, so that ions drifting through the drift tube section  36   1  toward the major surface  506 A of the ion carpet  500  pass through only the ion passageway  532  defined centrally through the ring structure  504 B. The voltage sources DC 1 , DC 2  and DC 4 -DC 6  are set to VREF and DC 3  is set to −XV so as to establish the electric field E 1  between the electrically conductive pad pairs  402 A and  402 B as illustrated in  FIG. 10A . Thus as ions drift through the drift tube section  36   1  toward the major surface  506 A of the ion carpet  500 , such ions are radially focused by the ring structure  504 B and pass through the ion passageway  532  where such ions are then steered or guided by the electric field E 1  toward and into the lower arm of the drift tube section  36   2 . 
     Referring now to  FIG. 12B , an example embodiment is shown of a portion of the hybrid ion mobility spectrometer  10  in which the ion funnel  32   5  and the ion gate G 3  positioned in the drift tube section  36   1  are replaced by an ion carpet  500  coupled to the downstream end of an ion steering channel  400  rotated 180 degrees relative to the configuration illustrated in  FIGS. 9-10A . The ion carpet  500  is illustratively positioned such that the central ion passageway  512  of the ring structure  504 A bisects or approximately bisects the space between the planar circuit boards  410  and axially bisects or approximately bisects the electrically conductive pad pairs  404 A at the (now) downstream edge  410 C of the ion steering channel  400 , and that the central ion passageway  532  of the ring structure  504 B bisects or approximately bisects the space between the planar circuit boards  410  and axially bisects or approximately bisects the electrically conductive pad pairs  402 A at the (now) downstream edge  410 C of the ion steering channel  400 . Although not shown in  FIG. 12B , it will be understood that the voltage sources DC 1 -DC 6  are operatively connected to the electrically conductive pad pairs  402 A- 402 D,  404 A- 404 D as illustrated in  FIG. 9  and described above. 
     When it is desired to direct ions from the upper arm of the drift tube section  36   1  into the drift tube section  36   2 , the voltage sources DC 1  and DC 3 -DC 6  are set to VREF and DC 2  is set to −XV so as to establish an electric field E 3  between the electrically conductive pad pairs  404 B,  404 C,  404 D and  404 B in the direction of the ion carpet  500 . The voltage sources  516  and  536  are controlled, as described above, so that ions being steered or guided by the electric field E 3  toward the major surface  506 A of the ion carpet  500  are radially focused by the ring structure  504 A and pass through only the ion passageway  512  defined centrally through the ring structure  504 A. Thus as ions drift through and along the upper arm of the drift tube section  36   1  toward the ion steering channel  400 , such ions are steered or guided by the electric field E 3  toward the ring structure  504 A defined on the major surface  506 A of the ion carpet  500 , and such ions are then radially focused by the ring structure  504 A and pass through the ion passageway  512  thereof and into the drift tube section  36   2 . 
     When it is desired to direct ions from the lower arm of the drift tube section  36   1  into the drift tube section  36   2 , the voltage sources DC 2 -DC 6  are set to VREF and DC 1  is set to −XV so as to establish an electric field E 4  between the electrically conductive pad pairs  402 B,  402 C,  402 D and  402 B in the direction of the ion carpet  500 . The voltage sources  516  and  536  are controlled, as described above, so that ions being steered or guided by the electric field E 4  toward the major surface  506 A of the ion carpet  500  are radially focused by the ring structure  504 B and pass through only the ion passageway  532  defined centrally through the ring structure  504 B. Thus as ions drift through and along the lower arm of the drift tube section  36   1  toward the ion steering channel  400 , such ions are steered or guided by the electric field E 4  toward the ring structure  504 B defined on the major surface  506 A of the ion carpet  500 , and such ions are then radially focused by the ring structure  504 B and pass through the ion passageway  532  thereof and into the drift tube section  36   2 . 
     In some alternate embodiments of the structure illustrated in  FIG. 12B , the ion carpet  500  may be replaced by an ion carpet  300  positioned in the upper arm of the drift tube section  36   1  and spaced apart from or positioned adjacent to the ion entrance end of the electrically conductive pad pairs  404 B, and another ion carpet  300  positioned in the lower arm of the drift tube section  36   1  and spaced apart from or positioned adjacent to the ion entrance end of the electrically conductive pad pairs  402 B. Such ion carpets  300  may illustratively be controlled similarly as just described with respect to the ion carpet  500  to selectively guide ions from the upper or lower arms of the drift tube section  36   1  into the drift tube section  36   2 . 
     In other alternate embodiments of the hybrid ion mobility spectrometer  10  illustrated in  FIG. 1A , some the structures illustrated in  FIGS. 12A and 12B  may be combined to replace the ion funnel  32   5  and the ion gates G 1 -G 3 . In one example, the ion steering channels  400  illustrated in  FIGS. 12A and 12B  may be positioned adjacent to each other with an ion carpet  500  positioned therebetween. In another example, the ion steering channels  400  illustrated in  FIGS. 12A and 12B  may be positioned adjacent to each other, e.g., in cascaded relationship, and individual ion carpets  300  may be positioned in the upper and lower arms of the drift tube segment  36   1  and/or in the upper and lower arms of the drift tube segment  36   2 . Those skilled in the art will recognize other combinations of one or more ion steering channels  400  and one or more ion carpets  300  and/or one or more ion carpets  500  that may replace one or more corresponding ion funnels, ion gates and/or ion gate combinations in any of the hybrid ion mobility spectrometer embodiments  10 ,  10 ′,  10 ″,  10 ′″ described herein and/or in any other conventional ion separation instrument, and it will be understood that this disclosure contemplates any such other combinations. 
     Referring now to  FIG. 13 , an example embodiment is shown of a portion of the hybrid ion mobility spectrometer  10 ′″ in which the ion transition region  80  is replaced by an ion transition region  600  including a combination of an ion steering channel  210 , two ion carpets  500   1 ,  500   2  and two ion carpets  300   1 ,  300   2 . The ion steering channel  210  is illustratively arranged as illustrated in  FIG. 6  and with the upstream edges  212 C,  214 C of the circuit boards  212 ,  214  at or adjacent the ion outlet end of the drift tube section  30   4 , with the downstream edges  212 D,  214 D at or adjacent to the ion inlet end of the drift tube section  30   6 , with the side edges  212 E,  214 E at or adjacent to the ion inlet end of the drift tube section  34   1  and with the side edges  212 F,  214 F at or adjacent to the ion outlet end of the drift tube section  34   2  (only the edges  212 C,  212 D,  212 E,  212 F of the circuit board  212  depicted in the top plan view illustrated in  FIG. 13 ). An ion carpet  500   1  is illustratively positioned at or adjacent to the upstream edges  212 C,  214 C of the ion steering channel  210  such that the major surface  506 A thereof having the ring structures  504 A,  504 B formed thereon faces away from the ion steering channel  210 , and another ion carpet  500   2  is illustratively positioned at or adjacent to the side edges  212 F,  214 F of the ion steering channel  210  such that the major surface  506 A thereof having the ring structures  504 A,  504 B formed therein faces away from the ion steering channel  210 . An ion carpet  300   1  is illustratively positioned in the drift tube section  30   6  with the major surface  306 A thereof having the ring structure  304  formed thereon facing the ion steering channel  210  and illustratively spaced apart from the downstream edges  212 D,  214 D by a distance D 3 , and another ion carpet  300   2  is illustratively positioned in the drift tube section  34   1  with the major surface  306 A thereof having the ring structure  304  formed thereon facing the ion steering channel  210  and illustratively spaced apart from the side edges  212 E,  214 E by a distance D 4 . The ion carpets  300   1 ,  300   2  are each illustratively positioned such that the central passageway  312  of the ring structure  304  bisects or approximately bisects the channel  216  of the ion steering channel  210  and centrally bisects or approximately bisects the circuit boards  212 ,  214 . The ion carpets  500   1 ,  500   2  are illustratively positioned such that the central ion passageways  512 ,  532  of the ring structures  504 A,  504 B respectively bisect or approximately bisect the channel  216  of the ion steering channel  210 . The ion carpet  500   1  is further illustratively positioned such that the central ion passageway  512  of the ring structure  504 A bisects or approximately bisects the aligned electrically conductive pad pairs  220 A,  220 A′ and  220 B,  200 B′ at the upstream edge  212 C,  214 C of the ion steering channel  210 , and that the central ion passageway  532  of the ring structure  504 B bisects or approximately bisects the aligned electrically conductive pad pairs  220 C,  220 C′ and  220 D,  200 D′ at the upstream edge  212 C,  214 C of the ion steering channel  210  (only the electrically conductive pads  220 A- 220 D of the circuit board  212  are depicted in dashed-line in the top plan view illustrated in  FIG. 13 ). The ion carpet  500   2  is further illustratively positioned such that the central ion passageway  512  of the ring structure  504 A bisects or approximately bisects the aligned electrically conductive pad pairs  220 C,  220 C′ and  220 A,  200 A′ at the side edge  212 F,  214 F of the ion steering channel  210 , and that the central ion passageway  532  of the ring structure  504 B bisects or approximately bisects the aligned electrically conductive pad pairs  220 D,  220 D′ and  220 B,  200 B′ at the side edge  212 E,  214 E of the ion steering channel  210 . Although not shown in  FIG. 13 , it will be understood that the voltage sources DC 1 -DC 4  are operatively connected to the electrically conductive pad pairs  220 A/ 220 A′,  2206 / 2206 ′,  220 C/ 220 C′ and  220 D/ 220 D′ as illustrated in  FIG. 6  and described above. 
     When it is desired to axially pass ions from the drift tube section  30   4  into the drift tube section  30   6 , the voltage sources DC 1  and DC 2  are set to VREF and the voltage sources DC 3  and DC 4  are set to −XV so as to establish the electric field E 1  between the aligned electrically conductive pad pairs  220 A,  220 A′ and  220 B,  220 B′ and between the aligned electrically conductive pad pairs  220 C,  220 C′ and  220 D,  220 D′ in the axial direction of the drift tube sections  30   4  and  30   6 . The voltage sources  516  and  536  for the ion carpet  500   1  are controlled, as described above, so that ions drifting through the drift tube section  30   1  toward the major surface  506 A of the ion carpet  500   1  are radially focused by both ring structures  504 A and  504 B and therefore pass through both of the ion passageways  512 ,  532  defined centrally through the ring structures  504 A,  504 B respectively. Thus as ions drift through and along the drift tube section  30   4 , such ions are radially focused by the ring structures  504 A,  504 B of the ion carpet  500   1  and pass in separate focused ion streams through the ion passageways  512 ,  532  respectively whereupon the focused ion stream exiting the ion passageway  512  is steered or guided by the electric field E 1  established between the electrically conductive pad pairs  220 A,  220 A′ and  220 B,  220 B′ into the drift tube section  30   6  and the focused ion stream exiting the ion passageway  532  is likewise steered or guided by the electric field E 1  established between the electrically conductive pad pairs  220 C,  220 C′ and  220 D,  220 D′ into the drift tube section  30   6 . As the two separate focused ion streams drift through the distance D 3  of the drift tube section  30   6  and toward the ion carpet  300   2 , they are radially focused by the ring structure  304  of the ion carpet  300   2  into a single, combined ion stream to pass through the ion passageway  312  of the ion carpet  300   2  which ion passageway  312  is illustratively axially aligned with the ion travel axis  72  of the single-pass drift tube  12 ′″ illustrated in  FIG. 1E  and described above. Illustratively, the distance D 3  is selected so allow the two separate focused ion streams exiting the ion steering channel  210  to be combined and radially focused into a single focused ion stream to pass through the passageway  312  of the ion carpet  300   2 . 
     When it is desired to transversely pass ions from the drift tube section  30   4  of the single-pass drift tube  12 ′″ into the drift tube section  34   1  of the multiple-pass drift tube  14 ″ illustrated in  FIGS. 1D and 1E , the voltage sources DC 2  and DC 4  are set to VREF and the voltage sources DC 1  and DC 3  are set to −XV so as to establish the electric field E 2  between the aligned electrically conductive pad pairs  220 C,  220 C′ and  220 A,  220 A′ and between the aligned electrically conductive pad pairs  220 D,  220 D′ and  220 B,  220 B′ in the axial direction of the drift tube sections  34   1  and  34   2 . The voltage sources  516  and  536  for the ion carpet  500   1  are controlled, as described above, so that ions drifting through the drift tube section  30   1  toward the major surface  506 A of the ion carpet  500   1  are radially focused only by the ring structure  504 B and therefore pass only through the ion passageways  532  defined centrally through the ring structure  504 B. As ions drift through and along the drift tube section  30   4  and are radially focused only by the ring structure  504 B of the ion carpet  500   1 , such ions pass in a single focused ion stream through the ion passageway  532  in a direction generally parallel with the ion travel axis  72  of the single-pass drift tube  12 ′″ toward the aligned electrically conductive pads  220 C,  220 C′ and  220 D,  220 D′. As the focused ion stream exits the ion passageway  532 , however, it is steered or guided by the electric field E 2  established between the electrically conductive pad pairs  220 C,  220 C′ and  220 A,  220 A′ and between the electrically conductive pad pairs  220 D,  220 D′ and  220 B,  220 B′ to change directions by approximately 90 degrees so as to pass into the drift tube section  34   1  of the multiple-pass drift tube  14 ″. As the redirected ions drift through the distance D 4  of the drift tube section  34   1  and toward the ion carpet  300   1 , they are radially focused by the ring structure  304  of the ion carpet  300   1  so as to pass through the ion passageway  312  of the ion carpet  300   1  which ion passageway  312  is illustratively axially aligned with the ion travel axis  70  of the multiple-pass drift tube  14 ″ illustrated in  FIGS. 1E and 1F  and described above. Illustratively, the distance D 4  is selected so allow the two separate focused ion streams exiting the ion steering channel  210  to be combined and radially focused into a single focused ion stream to pass through the passageway  312  of the ion carpet  300   1 . 
     As described above, the multiple-pass drift tube  14 ″ is illustratively controlled to allow ions to travel therethrough any number of times, and in so doing the voltage sources DC 1  and DC 2  are the voltage sources DC 2  and DC 4  are maintained at VREF and the voltage sources DC 1  and DC 3  are maintained at −XV so as to maintain the electric field E 2  between the aligned electrically conductive pad pairs  220 C,  220 C′ and  220 A,  220 A′ and between the aligned electrically conductive pad pairs  220 D,  220 D′ and  220 B,  220 B′ in the axial direction of the drift tube sections  34   1  and  34   2 . The voltage sources  516  and  536  for the ion carpet  500   2  are controlled, as described above, so that ions drifting through the drift tube section  34   2  toward the major surface  506 A of the ion carpet  500   2  are radially focused by both ring structures  504 A and  504 B and therefore pass through both of the ion passageways  512 ,  532  defined centrally through the ring structures  504 A,  504 B respectively. Thus as ions drift through and along the drift tube section  34   2 , such ions are radially focused by the ring structures  504 A,  504 B of the ion carpet  500   2  and pass in separate focused ion streams through the ion passageways  512 ,  532  respectively whereupon the focused ion stream exiting the ion passageway  512  is steered or guided by the electric field E 2  established between the electrically conductive pad pairs  220 C,  220 C′ and  220 A,  220 A′ into the drift tube section  34   1  and the focused ion stream exiting the ion passageway  532  is likewise steered or guided by the electric field E 2  established between the electrically conductive pad pairs  220 D,  220 D′ and  220 E,  220 E′ into the drift tube section  34   1 . As the two separate focused ion streams drift through the distance D 4  of the drift tube section  34   1  and toward the ion carpet  300   1 , they are radially focused by the ring structure  304  of the ion carpet  300   1  into a single, combined ion stream to pass through the ion passageway  312  of the ion carpet  300   1  which passageway  312  is illustratively axially aligned with the ion travel axis  70  of the multiple-pass drift tube  14 ″ illustrated in  FIG. 1D  and described above. 
     When it is desired to transversely pass ions from the drift tube section  34   2  of the multiple-pass drift tube  14 ″ into the drift tube section  30   6  of the single-pass drift tube  12 ′″ illustrated in  FIGS. 1D and 1E , e.g., after ions have circulated about the multiple-pass drift tube  14 ′ a desired number of times, the voltage sources DC 1  and DC 2  are set to VREF and the voltage sources DC 3  and DC 4  are set to −XV so as to reestablish the electric field E 1  between the aligned electrically conductive pad pairs  220 A,  220 A′ and  220 B,  220 B′ and between the aligned electrically conductive pad pairs  220 C,  220 C′ and  220 D,  220 D′ in the axial direction of the drift tube sections  30   4  and  30   6 . The voltage sources  516  and  536  for the ion carpet  500   2  are illustratively controlled, as described above, so that ions drifting through the drift tube section  34   2  toward the major surface  506 A of the ion carpet  500   2  are radially focused only by the ring structure  504 A and therefore pass only through the ion passageway  516  defined centrally through the ring structure  504 A. As ions drift through and along the drift tube section  34   2  and are radially focused only by the ring structure  504 A of the ion carpet  500   2 , such ions pass in a single focused ion stream through the ion passageway  512  in a direction generally parallel with the ion travel axis  70  of the multiple-pass drift tube  14 ″ toward the aligned electrically conductive pads  220 C,  220 C′ and  220 A,  220 A′. As the focused ion stream exits the ion passageway  512 , however, it is steered or guided by the electric field E 1  established between the electrically conductive pad pairs  220 C,  220 C′ and  220 D,  220 D′ and between the electrically conductive pad pairs  220 A,  220 A′ and  220 B,  220 B′ to change directions by approximately 90 degrees so as to pass into the drift tube section  30   6  of the single-pass drift tube  12 ′″. As the redirected ions drift through the distance D 3  of the drift tube section  30   6  and toward the ion carpet  300   2 , they are radially focused by the ring structure  304  of the ion carpet  300   2  so as to pass through the ion passageway  312  of the ion carpet  300   2  which ion passageway  312  is, as described above, axially aligned with the ion travel axis  72  of the single-pass drift tube  12 ′″. 
     It will be understood that the embodiment illustrated in  FIG. 13  represents only one non-limiting combination of the ion steering channel  210  and the ion carpets  300  and  500  implemented in the hybrid ion mobility spectrometer illustrated in  FIGS. 1D and 1E , and that that other combinations which may include more or fewer ion carpets  300  and/or  500 , which may include additional ion steering channels  210  and/or which may include one or more of ion steering channels  400  are contemplated by this disclosure. As one specific and non-limiting example, the embodiment  600  illustrated in  FIG. 13  may alternatively include one or more ion carpets  300  in place of either or both of the ion carpets  500 . As another non-limiting example, one or more of the ion carpets  300 ,  500  may be omitted in favor one or more of the ion funnels illustrated in  FIGS. 1D and 1E . Those skilled in the art will recognize other combinations of one or more ion steering channels  212  and one or more ion carpets  300  and/or one or more ion carpets  500  that may replace one or more corresponding ion funnels, ion gates and/or ion gate combinations in any of the hybrid ion mobility spectrometer embodiments  10 ,  10 ′,  10 ″,  10 ′″ described herein and/or in any other conventional ion separation instrument, and it will be understood that this disclosure contemplates any such other combinations. 
     While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments thereof have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. For example, in some alternate embodiments, one or more conventional ion analytical instruments may be substituted for either or both of the ion source  18  and the ion detector  22  such that alternate and/or additional ion separation, ion conformation alteration, ion processing and/or ion analysis may be carried out on ions prior to entering and/or after exiting the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″. Alternatively or additionally, one or more conventional ion analytical instruments may be positioned within or interposed along either or both of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ and the multiple-pass drift tube  14 ,  14 ′,  14 ″ such that alternate and/or additional ion separation, ion conformation alteration, ion processing and/or ion analysis may be carried out within or along the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ and/or the multiple-pass drift tube  14 ,  14 ′,  14 ″. In any case, examples of such conventional ion analytical instruments that precede the ion inlet  16  of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″, that follow the ion outlet  20  of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ and/or that are positioned within or interposed along the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ and/or the multiple-pass drift tube  14 ,  14 ′,  14 ″ may include, but are not limited to, one or more drift tubes identical to or different from the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ and/or the multiple-pass drift tube  14 ,  14 ′,  14 ″, one or more mass analyzers and/or mass spectrometers, one or more liquid and/or gas chromatographs, one or mass filters (e.g., one or more multiple-pole mass filters), one or more collision cells and/or other ion fragmentation devices or regions, one or more ion activation regions in which an electric field is established that is high enough to alter the conformation of one or more ions but not high enough to fragment ions, or the like. It will be further understood that in embodiments that include two or more such conventional ion analytical instruments together, such two or more conventional ion analytical instruments may be positioned in parallel relative to each other, in series relative to each other (i.e., cascaded) or any combination of series and parallel. 
     Additionally or alternatively, those skilled in the art will recognize that the multiple-pass drift tube  14  illustrated in any of  FIGS. 1A-1B  can, in some embodiments, be provided in the form of two or more series-connected and/or parallel-connected multiple-pass drift tubes. Alternatively or additionally still, such one or more multiple-pass drift tubes  14  can be augmented by one or more single-pass drift tubes  12  and/or by one or more conventional analytical instruments of the type described by example in the previous paragraph. 
     As another example, it will be understood that while the various embodiments of the hybrid ion mobility spectrometer  10 ,  10 ′,  10 ″,  10 ′″ illustrated and described herein include a multiple-pass drift tube  14 ,  14 ′,  14 ″ coupled to a single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ between an ion inlet  16  and an ion outlet  20  of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″, this disclosure contemplates alternative embodiments in which the multiple-pass drift tube  14 ,  14 ′,  14 ″ or other suitable multiple-pass drift tube is positioned upstream of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″, i.e., prior to the ion inlet  16  and/or downstream of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″, i.e., following the ion outlet  20 . 
     As still another example, operation of the ion gates G 1 -G 3  or G 1 -G 4  has been described herein in which such ion gates G 1 -G 3  or G 1 -G 4  are controlled to block or allow passage therethrough of some or all ions from a preceding, e.g. upstream, stage or section of the hybrid ion mobility spectrometer  10 ,  10 ′,  10 ″,  10 ′″. It will be understood that this disclosure contemplates embodiments in which any one or more of the gates G 1 -G 3  or G 1 -G 4  may be controlled to intermediate positions, i.e., between their open and closed positions, to allow pass therethrough of only a fraction of the ions at any one or more times. This would allow, for example, operation of the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ to be carried out simultaneously with the operation of the multiple-pass drift tube  14 ,  14 ′,  14 ″ such that ions exiting more quickly from the single-pass drift tube  12 ,  12 ′,  12 ″,  12 ′″ can be analyzed prior to analyzing ions exiting the multiple-pass drift tube.