Abstract:
A method and an apparatus for selectively transmitting ions a FAIMS analyzer is disclosed. An ion diverter is included within a FAIMS analyzer for affecting the trajectories of ions after separation to direct the ions in a known fashion. The ion diverter is optically a gas flow source or an electrode for generating an electrical field to alter ion flow.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an apparatus and method for separating ions, more particularly the present invention relates to an apparatus and method for separating ions based on the ion focusing principles of high field asymmetric waveform ion mobility spectrometry (FAIMS).  
         BACKGROUND OF THE INVENTION  
         [0002]    High sensitivity and amenability to miniaturization for field-portable applications have helped to make ion mobility spectrometry (IMS) an important technique for the detection of many compounds, including narcotics, explosives, and chemical warfare agents as described, for example, by G. Eiceman and Z. Karpas in their book entitled “Ion Mobility Spectrometry” (CRC, Boca Raton, 1994). In IMS, gas-phase ion mobilities are determined using a drift tube with a constant electric field. Ions are gated into the drift tube and are subsequently separated in dependence upon differences in their drift velocity. The ion drift velocity is proportional to the electric field strength at low electric field strength, for example 200 V/cm, and the mobility, K, which is determined from experimentation, is independent of the applied electric field. Additionally, in IMS the ions travel through a bath gas that is at sufficiently high pressure such that the ions rapidly reach constant velocity when driven by the force of an electric field that is constant both in time and location. This is to be clearly distinguished from those techniques, most of which are related to mass spectrometry, in which the gas pressure is sufficiently low that, if under the influence of a constant electric field, the ions continue to accelerate.  
           [0003]    E. A. Mason and E. W. McDaniel in their book entitled “Transport Properties of Ions in Gases” (Wiley, New York, 1988) teach that at high electric field strength, for instance fields stronger than approximately 5,000 V/cm, the ion drift velocity is no longer directly proportional to the applied field, and K becomes dependent upon the applied electric field. At high electric field strength, K is better represented by K h , a non-constant high field mobility term. The dependence of K h  on the applied electric field has been the basis for the development of high field asymmetric waveform ion mobility spectrometry (FAIMS), a term used by the inventors throughout this disclosure, and also referred to as transverse field compensation ion mobility spectrometry, or field ion spectrometry. Ions are separated in FAIMS on the basis of a difference in the mobility of an ion at high field strength, K h , relative to the mobility of the ion at low field strength, K. In other words, the ions are separated because of the compound dependent behavior of K h  as a function of the applied electric field strength. FAIMS offers a new tool for atmospheric pressure gas-phase ion studies since it is the change in ion mobility, and not the absolute ion mobility, that is being monitored.  
           [0004]    The principles of operation of FAIMS using flat plate electrodes have been described by I. A. Buryakov, E. V. Krylov, E. G. Nazarov and U. Kh. Rasulev in a paper published in the International Journal of Mass Spectrometry and Ion Processes; volume 128 (1993), pp. 143-148, the contents of which are herein incorporated by reference. The mobility of a given ion under the influence of an electric field is expressed by: K h =K(1+f(E)), where K h  is the mobility of an ion at high electrical field strength, K is the coefficient of ion mobility at low electric field strength and f(E) describes the functional dependence of the ion mobility on the electric field strength. Ions are classified into one of three broad categories on the basis of a change in ion mobility as a function of the strength of an applied electric field, specifically: the mobility of type A ions increases with increasing electric field strength; the mobility of type C ions decreases; and, the mobility of type B ions increases initially before decreasing at yet higher field strength. The separation of ions in FAIMS is based upon these changes in mobility at high electric field strength. Consider an ion, for example a type A ion, which is being carried by a gas stream between two spaced-apart parallel plate electrodes of a FAIMS device. The space between the plates defines an analyzer region in which the separation of ions occurs. The net motion of the ion between the plates is the sum of a horizontal x-axis component due to the flowing stream of gas and a transverse y-axis component due to the electric field between the parallel plate electrodes. The term “net motion” refers to the overall translation that the ion, for instance said type A ion, experiences, even when this translational motion has a more rapid oscillation superimposed upon it. Often, a first plate is maintained at ground potential while the second plate has an asymmetric waveform, V(t), applied to it. The asymmetric waveform V(t) is composed of a repeating pattern including a high voltage component, V 1 , lasting for a short period of time t 2  and a lower voltage component, V 2 , of opposite polarity, lasting a longer period of time t 1 . The waveform is synthesized such that the integrated voltage-time product, and thus the field-time product, applied to the plate during each complete cycle of the waveform is zero, for instance V 1  t 2  +V 2 t 1 =0; for example +2000 V for 10 μs followed by −1000 V for 20 μs. The peak voltage during the shorter, high voltage portion of the waveform is called the “dispersion voltage” or DV in this disclosure.  
           [0005]    During the high voltage portion of the waveform, the electric field causes the ion to move with a transverse y-axis velocity component v 1 =K h E high , where E high  is the applied field, and K h  is the high field ion mobility under ambient electric field, pressure and temperature conditions. The distance traveled is d 1 =v 1 t 2 =K h E high t 2 , where t 2  is the time period of the applied high voltage. During the longer duration, opposite polarity, low voltage portion of the asymmetric waveform, the y-axis velocity component of the ion is v 2 =KE low , where K is the low field ion mobility under ambient pressure and temperature conditions. The distance traveled is d 2 =v 2 t 1 =KE low t 1 . Since the asymmetric waveform ensures that (V 1  t 2 )+(V 2  t 1 )=0, the field-time products E high t 2  and E low t 1  are equal in magnitude. Thus, if K h  and K are identical, d 1  and d 2  are equal, and the ion is returned to its original position along the y-axis during the negative cycle of the waveform, as would be expected if both portions of the waveform were low voltage. If at E high  the mobility K h &gt;K, the ion experiences a net displacement from its original position relative to the y-axis. For example, positive ions of type A travel farther during the positive portion of the waveform, for instance d 1 &gt;d 2 , and the type A ion migrates away from the second plate. Similarly, positive ions of type C migrate towards the second plate.  
           [0006]    If a positive ion of type A is migrating away from the second plate, a constant negative dc voltage can be applied to the second plate to reverse, or to “compensate” for, this transverse drift. This dc voltage, called the “compensation voltage” or CV in this disclosure, prevents the ion from migrating towards either the second or the first plate. If ions derived from two compounds respond differently to the applied high strength electric fields, the ratio of K h  to K may be different for each compound. Consequently, the magnitude of the CV necessary to prevent the drift of the ion toward either plate is also different for each compound. Thus, when a mixture including several species of ions is being analyzed by FAIMS, only one species of ion is selectively transmitted for a given combination of CV and DV. The remaining species of ions, for instance those ions that are other than selectively transmitted through FAIMS, drift towards one of the parallel plate electrodes of FAIMS and are neutralized. Of course, the speed at which the remaining species of ions move towards the electrodes of FAIMS depends upon the degree to which their high field mobility properties differ from those of the ions that are selectively transmitted under the prevailing conditions of CV and DV.  
           [0007]    An instrument operating according to the FAIMS principle as described previously is an ion filter, capable of selective transmission of only those ions with the appropriate ratio of K h  to K. In one type of experiment using FAIMS devices, the applied CV is scanned with time, for instance the CV is slowly ramped or optionally the CV is stepped from one voltage to a next voltage, and a resulting intensity of transmitted ions is measured. In this way a CV spectrum showing the total ion current as a function of CV, is obtained. It is a significant limitation of early FAIMS devices, which used electrometer detectors, that the identity of peaks appearing in the CV spectrum are other than unambiguously confirmed solely on the basis of the CV of transmission of a species of ion. This limitation is due to the unpredictable, compound-specific dependence of K h  on the electric field strength. In other words, a peak in the CV spectrum is easily assigned to a compound erroneously, since there is no way to predict or even to estimate in advance, for example from the structure of an ion, where that ion should appear in a CV spectrum. In other words, additional information is necessary in order to improve the likelihood of assigning correctly each of the peaks&#39; in the CV spectrum. For example, subsequent mass spectrometric analysis of the selectively transmitted ions greatly improves the accuracy of peak assignments of the CV spectrum.  
           [0008]    In U.S. Pat. No. 5,420,424 which issued on May 30 1995, B. L. Carnahan and A. S. Tarassove disclose an improved FAIMS electrode geometry in which the flat plates that are used to separate the ions are replaced with concentric cylinders, the contents of which are herein incorporated by reference. The concentric cylinder design has several advantages, including higher sensitivity compared to the flat plate configuration, as was discussed by R. W. Purves, R. Guevremont, S. Day, C. W. Pipich, and M. S. Matyjaszczyk in a paper published in Reviews of Scientific Instruments; volume 69 (1998), pp 4094-4105. The higher sensitivity of the cylindrical FAIMS is due to a two-dimensional atmospheric pressure ion focusing effect that occurs in the analyzer region between the concentric cylindrical electrodes. When no electrical voltages are applied to the cylinders, the radial distribution of ions should be approximately uniform across the FAIMS analyzer. During application of DV and CV, however, the radial distribution of ions is not uniform across the annular space of the FAIMS analyzer region. Advantageously, with the application of an appropriate DV and CV for an ion of interest, those ions become focused into a band between the electrodes and the rate of loss of ions, as a result of collisions with the FAIMS electrodes, is reduced. The efficiency of transmission of the ions of interest through the analyzer region of FAIMS is thereby improved as a result of this two-dimensional ion focusing effect.  
           [0009]    The focussing of ions by the use of asymmetric waveforms has been discussed above. For completeness, the behavior of those ions that are not focussed within the analyzer region of a cylindrical geometry FAIMS is described here, briefly. As discussed previously, those ions having high field ion mobility properties that are other than suitable for focussing under a given set of DV, CV and geometric conditions will drift toward one or another wall of the FAIMS device. The rapidity with which these ions move towards the wall depends on the degree to which their K h /K ratio differs from that of the ion that is transmitted selectively under the prevailing conditions. At the very extreme, ions of completely the wrong property, for instance a type A ion versus a type C ion, are lost to the walls of the FAIMS device very rapidly.  
           [0010]    The loss of ions in FAIMS devices should be considered one more way. If an ion of type A is focussed, for example at DV 2500 volts, CV −11 volts in a given geometry, it would seem reasonable to expect that the ion is also focussed if the polarity of DV and CV are reversed, for instance DV of −2500 volts and CV of +11 volts. This, however, is not observed and in fact the reversal of polarity in this manner creates a mirror image effect of the ion-focussing behavior of FAIMS. The result of such polarity reversal is that the ions are not focussed, but rather are extremely rapidly rejected from the device,. The mirror image of a focussing valley, is a hill-shaped potential surface. The ions slide to the center of the bottom of a focussing potential valley (2 or 3-dimensions), but slide off of the top of a hill-shaped surface, and hit the wall of an electrode. This is the reason for the existence, in the cylindrical geometry FAIMS, of the independent “modes” called 1 and 2. Such a FAIMS instrument is operated in one of four possible modes: P1, P2, N1, and N2. The “P” and “N” describe the ion polarity, positive (P) and negative (N). The waveform with positive DV, where DV describes the peak voltage of the high voltage portion of the asymmetric waveform, yields spectra of type P1 and N2, whereas the reversed polarity negative DV, waveform yields P2 and N1. The discussion thus far has considered positive ions but, in general, the same principles apply to negative ions equally.  
           [0011]    A further improvement to the cylindrical FAIMS design is realized by providing a curved surface terminus of the inner electrode. The curved surface terminus is continuous with the cylindrical shape of the inner electrode and is aligned co-axially with an ion-outlet orifice of the FAIMS analyzer region. The application of an asymmetric waveform to the inner electrode results in the normal ion-focussing behavior described above, except that the ion-focussing action extends around the generally spherically shaped terminus of the inner electrode. This means that the selectively transmitted ions cannot escape from the region around the terminus of the inner electrode. This only occurs if the voltages applied to the inner electrode are the appropriate combination of CV and DV as described in the discussion above relating to 2-dimensional focussing. If the CV and DV are suitable for the focussing of an ion in the FAIMS analyzer region, and the physical geometry of the inner surface of the outer electrode does not disturb this balance, the ions will collect within a three-dimensional region of space near the terminus. Several contradictory forces are acting on the ions in this region near the terminus of the inner electrode. The force of the carrier gas flow tends to influence the ion cloud to travel towards the ion-outlet orifice, which advantageously also prevents the trapped ions from migrating in a reverse direction, back towards the ionization source. Additionally, the ions that get too close to the inner electrode are pushed back away from the inner electrode, and those near the outer electrode migrate back towards the inner electrode, due to the focusing action of the applied electric fields. When all forces acting upon the ions are balanced, the ions are effectively captured in every direction, either by forces of the flowing gas, or by the focussing effect of the electric fields of the FAIMS mechanism. This is an example of a three-dimensional atmospheric pressure ion trap, as disclosed in a copending PCT application in the name of R. Guevremont and R. Purves, the contents of which are herein incorporated by reference.  
           [0012]    Ion focusing and ion trapping requires electric fields that are other than constant in space, normally occurring in a geometrical configuration of FAIMS in which the electrodes are curved, and/or are not parallel to each other. For example, a non-constant in space electric field is created using electrodes that are cylinders or a part thereof; electrodes that are spheres or a part thereof; electrodes that are elliptical spheres or a part thereof; and, electrodes that are conical or a part thereof. Optionally, various combinations of these electrode shapes are used.  
           [0013]    As discussed above, one previous limitation of the cylindrical FAIMS technology is that the identity of the peaks appearing in the CV spectra are not unambiguously confirmed due to the unpredictable changes in K h  at high electric field strengths. Thus, one way to extend the capability of instruments based on the FAIMS concept is to provide a way to determine the make-up of the CV spectra more accurately, such as by introducing ions from the FAIMS device into a mass spectrometer for mass-to-charge (m/z) analysis. Advantageously, the ion focusing property of cylindrical FAIMS devices acts to enhance the efficiency for transporting ions from the analyzer region of a FAIMS device into an external sampling orifice, for instance an inlet of a mass spectrometer. This improved efficiency of transporting ions into the inlet of the mass spectrometer is optionally maximized by using a 3-dimensional trapping version of FAIMS operated in nearly trapping conditions. Under near-trapping conditions, the ions that have accumulated in the three-dimensional region of space near the spherical terminus of the inner electrode are caused to leak from this region, being pulled by a flow of gas towards the ion-outlet orifice. The ions that leak out from this region do so as a narrow, approximately collimated beam, which is pulled by the gas flow through the ion-outlet orifice and into a small orifice leading into the vacuum system of a mass spectrometer.  
           [0014]    Additionally, the resolution of a FAIMS device is defined in terms of the extent to which ions having similar mobility properties as a function of electric field strength are separated under a set of predetermined operating conditions. Thus, a high-resolution FAIMS device transmits selectively a relatively small range of different ion species having similar mobility properties, whereas a low-resolution FAIMS device transmits selectively a relatively large range of different ion species having similar mobility properties. The resolution of FAIMS in a cylindrical geometry FAIMS is compromised relative to the resolution in a parallel plate geometry FAIMS because the cylindrical geometry FAIMS has the capability of focusing ions. This focusing action means that ions of a wider range of mobility characteristics are simultaneously focused in the analyzer region of the cylindrical geometry FAIMS. A cylindrical geometry FAIMS with narrow electrodes has the strongest focusing action, but the lowest resolution for separation of ions. As the radii of curvature are increased, the focusing action becomes weaker, and the ability of FAIMS to simultaneously focus ions of similar high-field mobility characteristics is similarly decreased. This means that the resolution of FAIMS increases as the radii of the electrodes are increased, with parallel plate geometry FAIMS having the maximum attainable resolution.  
           [0015]    Note that, while the above discussion refers to the ions as being “captured” or “trapped”, in fact, the ions are subject to continuous ‘diffusion’. Diffusion always acts contrary to focussing and trapping. The ions always require an electrical, or gas flow force to reverse the process of diffusion. Thus, although the ions are focused into an imaginary cylindrical zone in space with almost zero thickness, or within a 3-dimensional ion trap, in reality it is well known that the ions are actually dispersed in the vicinity of this idealized zone in space because of diffusion. This is important, and should be recognized as a global feature superimposed upon all of the ion motions discussed in this disclosure. This means that, for example, a 3-dimensional ion trap actually has real spatial width, and ions continuously leak from the 3-dimensional ion trap, for several physical, and chemical reasons. Of course, the ions occupy a smaller physical region of space if the trapping potential well is deeper.  
           [0016]    It is a limitation of the prior art FAIMS devices operating in a trapping or near trapping mode that the ions are difficult to extract from the FAIMS analyzer once they have been separated. Typically, the flow of carrier gas is used to prevent the ions from being attracted to one of the electrodes, and further to carry ions entrained therein out of the trapping region for detection. Of course, the flow of carrier gas is optimized for separation of ions during the time that they are resident within the analyzer region, and not for extracting the ions subsequent to their separation. In some cases, therefore, it is possible that the carrier gas flow rate will be other than sufficient to extract the selectively transmitted ions from the focusing region near the ion outlet. It would be advantageous to provide a method and a system for increasing the efficiency of ion extraction from the FAIMS analyzer by diverting the ion flow substantially away from the focusing region.  
         OBJECT OF THE INVENTION  
         [0017]    In order to overcome these and other limitations of the prior art, it is an object of the present invention to provide a high field ion mobility spectrometer for separating ions in which trajectories of ions are affected by an ion diverter for directing the ions in a known fashion.  
         SUMMARY OF THE INVENTION  
         [0018]    In accordance with the invention there is provided an apparatus for separating ions, comprising:  
           [0019]    a high field asymmetric waveform ion mobility spectrometer including:  
           [0020]    two electrodes at least one of which is for receiving an asymmetric waveform electrical signal and for producing a field between the electrodes;  
           [0021]    an ion inlet;  
           [0022]    an analyzer region in communication with the ion inlet and defined by at least  
           [0023]    a first ion flow path between the two electrodes; and,  
           [0024]    an ion diverter separate from the two electrodes for diverting the ions from the ion flow path in a known fashion.  
           [0025]    In accordance with the invention there is provided a method for separating ions, comprising the steps of:  
           [0026]    a) providing an asymmetric waveform and a direct-current compensation voltage to an electrode to form an electric field, the field for effecting a difference in net displacement between ions in a time of one cycle of the applied asymmetric waveform for effecting a first separation of the ions by forming a subset thereof;  
           [0027]    b) producing ions within an ionization source;  
           [0028]    c) transporting said produced ions through the electric field along at least a first ion flow path in a direction approximately transverse to the electric field; and,  
           [0029]    d) diverting the selectively transmitted ions relative to the ion flow path absent the step of diverting in a predetermined fashion after separation.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 shows three possible examples of changes in ion mobility as a function of the strength of an electric field;  
         [0031]    [0031]FIG. 2 a  illustrates the trajectory of an ion between two parallel plate electrodes under the influence of the electrical potential V(t);  
         [0032]    [0032]FIG. 2 b  shows an asymmetric wavefonn described by V(t);  
         [0033]    [0033]FIG. 3 shows a simplified block diagram of an analyzer region of a parallel plate FAIMS device according to the prior art;  
         [0034]    [0034]FIG. 4 a  shows a simplified block diagram of an analyzer region of an improved FAIMS device with at least an ion diverting device, as operating in a first mode, according to a first embodiment of the present invention;  
         [0035]    [0035]FIG. 4 b  shows a simplified block diagram of an analyzer region of an improved FAIMS device with at least an ion diverting device, as operating in a second mode, according to a first embodiment of the present invention;  
         [0036]    [0036]FIG. 4 c  shows a simplified block diagram of an analyzer region of an improved FAIMS device with at least an ion diverting device, as operating in a first mode, according to a second embodiment of the present invention;  
         [0037]    [0037]FIG. 4 d  shows a simplified block diagram of an analyzer region of an improved FAIMS device with at least an ion diverting device, as operating in a second mode, according to a second embodiment of the present invention;  
         [0038]    [0038]FIG. 4 e  shows a simplified block diagram of an analyzer region of an improved FAIMS device with at least an ion diverting device, as operating in a first mode, according to a third embodiment of the present invention;  
         [0039]    [0039]FIG. 4 f  shows a simplified block diagram of an analyzer region of an improved FAIMS device with at least an ion diverting device, as operating in a second mode, according to a third embodiment of the present invention;  
         [0040]    [0040]FIG. 5 shows a simplified block diagram of a modified analyzer region of an improved FAIMS device with at least an ion diverting device according to a fourth embodiment of the present invention;  
         [0041]    [0041]FIG. 6 shows a simplified block diagram of another modified analyzer region of an improved FAIMS device with at least an ion diverting device according to a fifth embodiment of the present invention;  
         [0042]    [0042]FIG. 7 shows a simplified block diagram of another cylindrical geometry FAIMS device with a ion diverting device according to a sixth embodiment of the present invention;  
         [0043]    [0043]FIG. 8 shows a simplified block diagram of another cylindrical geometry FAIMS device with a ion diverting device according to a seventh embodiment of the present invention;  
         [0044]    [0044]FIG. 9 shows a simplified block diagram of a cylindrical geometry FAIMS device with another ion diverting device according to a seventh embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0045]    Referring to FIG. 1, shown are three possible examples of the change in ion mobility properties with increasing electric field strength, as was discussed previously. The separation of ions in FAIMS is based upon a difference in these mobility properties for a first ion relative to a second ion. For instance, a first type A ion having a low field mobility K 1,low  is other than separated in a FAIMS device from a second type A ion having a second different low field mobility K 2,low , if under the influence of high electric field strength, the ratio K 1,high /K 1,low  is equal to the ratio K 2,high /K 2,low . Interestingly, however, this same separation is achieved using conventional ion mobility spectrometry, which is based on a difference in ion mobilities at low applied electric field strength.  
         [0046]    Referring to FIG. 2 a , shown is a schematic diagram illustrating the mechanism of ion separation according to the FAIMS principle. An ion  1 , for instance a positively charged type A ion, is carried by a gas stream  2  flowing between two spaced apart parallel plate electrodes  3  and  4 . One of the plates  4  is maintained at ground potential, while the other plate  3  has an asymmetric waveform described by V(t), applied to it. The peak voltage applied during the waveform is called the dispersion voltage (DV), as is shown in FIG. 2 b . Referring still to FIG. 2 b , the waveform is synthesized so that the electric fields during the two periods of time t high  and t low  are not equal. If K h  and K are identical at high and low fields, the ion  1  is returned to its original position at the end of one cycle of the waveform. However, under conditions of sufficiently high electric fields, K h  is greater than K and the distances traveled during t high  and t low  are no longer identical. Within an analyzer region defined by a space  16  between the first and second spaced apart electrode plates,  3  and  4 , respectively, the ion  1  experiences a net displacement from its original position relative to the plates  3  and  4  as illustrated by the dashed line  5  in FIG. 2 a.    
         [0047]    If a type A ion is migrating away from the upper plate  3 , a constant negative dc compensation voltage CV is applied to plate  3  to reverse or “compensate” for this offset drift. Thus, the ion  1  does not travel toward either plate. If two species of ions respond differently to the applied high electric field, for instance the ratios of K h  to K are not identical, the compensation voltages necessary to prevent their drift toward either plate are similarly different. To analyze a mixture of ions, the compensation voltage is, for example, scanned to transmit each of the components of a mixture in turn. This produces a compensation voltage spectrum, or CV spectrum.  
         [0048]    Referring to FIG. 3, a simplified block diagram of a parallel plate FAIMS device according to the prior art is shown generally at  10 . The analyzer region is defined by a space  16  between two flat, parallel plate electrodes  3  and  4 , and between an ion-inlet electrode  6  having an ion-inlet orifice  19  and an ion-outlet electrode  8  having an ion-outlet orifice  21 . The electrodes  3  and  4  are connected to an electrical controller  7  such that, in use, an asymmetric waveform and a superimposed dc compensation voltage is applied to electrode  3 . Typically, the electrode  4  is maintained at a same dc voltage relative to each of the ion-inlet electrode  6  and the ion-outlet electrode  8 . In this example, the asymmetric waveform and CV are set so that a particular species of positively charged ion (not shown) is transmitted through the analyzer region within space  16  between the plates  3  and  4 , for instance the CV is negative, and the waveform has positive polarity. The “net” ion trajectory through the analyzer region is indicated in FIG. 3 by dotted line  18 . In general, the powered electrode  3  is attracting the positive ion toward itself due to the negative dc bias, as indicated in FIG. 3 by the arrowheads of the electric force lines that are directed toward electrode  3 . Fortunately, within the analyzer region  16  the effect of the asymmetric waveform is to push the ion away from the electrode  3 , as is indicated in FIG. 3 by the arrowheads of the electric force lines that are directed away from electrode  3 . As long as the electric fields are strong, and as long as the fields stay constant in strength, a balanced condition that is necessary to allow the ion to pass through the analyzer region  16  is maintained. This balanced condition is shown schematically in FIG. 3 as a series of double-headed electric force lines, comprising a DV and CV component, which are selected for transmitting ions having specific high field mobility properties.  
         [0049]    Of course, the fields are not strong everywhere around the powered electrode  3 . On a side  3   a  of the powered electrode  3  that opposes the second electrode  4 , and on the end edges of the electrode facing one of the ion-inlet electrode  6  and the ion-outlet electrode  8 , the fields are strong and the balanced condition exists. However, where the electric field strength changes, such as occurs on a side  3   b  of the powered electrode  3  at the end edges of the electrode facing one of the ion-inlet electrode  6  and the ion-outlet electrode  8 , the ion path change rapidly, resulting in a dramatic redirection of the ion stream. This redirection lacks the balanced conditions that the ion stream experiences between the plates  3  and  4 . This means that on the back side  3   b  of the powered electrode  3  the ion will impact onto the metal surface, pulled by the negative polarity of the applied CV. Although the ion maintains a stable trajectory along side  3   a  where the opposing electrical forces are balanced, upon exiting space  16  the ion follows a curved path towards the back side  3   b  of electrode  3 . The negative dc bias applied to electrode  3  creates a potential hillside for the ion to slide down. The carrier gas flow is other than able to prevent this downward slide unless the CV is very low or the gas flow is very high. Even if impact with the plate  3  is avoided, many ion paths do not proceed toward the ion-outlet orifice  21  of the device  10 , the ions being lost to a collision with a different part of the FAIMS apparatus.  
         [0050]    Referring to FIGS. 4 a  and  4   b , a first preferred embodiment of the present invention is shown generally at  120 . Additionally, FIGS. 4 c  to  4   f  show different modes of operation of a same electrode geometry as shown FIG. 4 a , wherein different combinations of applied voltages are described. Therefore, reference numerals indicating elements of the drawings identical to those elements previously described with reference to FIG. 4 a  have been omitted from FIGS. 4 b  to  4   f  in the interest of clarity and brevity. Of course, an ion-inlet electrode  131  and an ion-outlet electrode  132 , described with reference to FIG. 4 a  below, have similarly been omitted from FIGS. 4 b  to  4   f , but are understood to have a crucial role in producing the strong electric fields that are described subsequently for each mode of operation with reference to FIGS. 4 b  to  4   f . Further, ions are shown schematically in FIGS. 4 a  to  4   f  (not to scale) as circles in which a ‘+’ sign appears to indicate ion species of positive polarity charge, and as circles in which a ‘−’ sign appears to indicate ion species of negative polarity charge. Circles having a dark boarder are used in some cases, for instance to distinguish between two ions having a same charge but having different mobility properties as a function of electric field strength.  
         [0051]    Referring again to FIG. 4 a , the analyzer region includes a first electrode  121 , a second electrode  122 , a third electrode  123 , a fourth electrode  124  and a fifth electrode  125  in a substantially uniformly spaced-apart stacked arrangement. Thus, two spaces  126   a  and  127   a  are disposed on a first side of electrode  123  and two different spaces  126   b  and  127   b  are disposed on the opposite side of electrode  123 . In a most basic version of the present embodiment, the electrodes shown in FIG. 4 a  are flat parallel plates with square ends. In a first mode of operation a CV and DV is applied to the third electrode  123 , while the second electrode  122  and the fourth electrode  124  are maintained at ground potential or some other dc potential. In this case, ions are drawn to the third electrode  123  due to the dc bias, and are carried by the uniform gas flow predominantly through spaces  127   a  and  127   b . Since the electrodes are flat parallel plates the electric fields within each space  127   a  and  127   b  are constant, such that ion focusing does not occur. Additionally the electric fields are the same within each space, such that a same ion is selectively transmitted through both spaces  127   a  and  127   b . Of course, the electrodes are mounted in an insulating support, which is omitted for clarity in FIG. 4 a . Each space  126   a ,  127   a ,  127   b  and  126   b  defines a separate ion flow path that is closed on four sides such that it is other than possible for ions to move from one space to the other space. Further, a physical barrier (not shown) is provided along the outer surfaces of electrodes  121  and  125  for preventing the flow of carrier gases through spaces other than  126   a ,  127   a ,  127   b  and  126   b.    
         [0052]    Typically, electrode  123  is connected to an electrical controller (not shown) such that, in use, an asymmetric waveform and a superimposed first dc voltage, wherein the superimposed first dc voltage is other than the compensation voltage, is applied to the electrode  123 . The electrodes  122  and  124  are connected to at an electrical controller (not shown), such that, in use, electrodes  122  and  124  are maintained at a predetermined second dc voltage or at a ground potential. The ion-inlet electrode  131 , having an ion-inlet orifice  135  therethrough, and an ion-outlet electrode  132 , having an ion-outlet orifice  136  therethrough, are also maintained at predetermined dc voltages by power supplies (not shown). The CV is the difference between the superimposed first dc voltage applied to the electrode  123  and the second dc voltage applied to the electrodes  122  and  124 . Those ions having appropriate mobility properties for a particular combination of DV and CV are selectively transmitted through the analyzer region, for instance within space  127   a  between the electrodes  122  and  123 , and within space  127   b  between the electrodes  123  and  124 . For example, the selective transmission of an analyte ion through the FAIMS analyzer region may require the electrode  123  to be biased 5 volts lower than electrodes  122  and  124 , for instance the CV is negative 5 volts, and for the waveform to be of positive polarity, for example 2500 volts. The electrodes  121  and  125  are connected to at least a dc voltage controller, for instance two separate dc voltage controllers (not shown), such that, in use, electrodes  121  and  125  are maintained at a third predetermined second dc voltage or at ground potential.  
         [0053]    A ‘net’ trajectory for a selectively transmitted ion through the FAIMS analyzer region is shown diagrammatically in FIG. 4 a  at dotted lines  133  and  134 . In general, the powered electrode  123  is attracting the ions toward itself due to the negative dc bias relative to electrodes  122  and  124 , as indicated in FIG. 4 a  by the arrowheads of the electric force lines that are directed toward electrode  123 . Fortunately, within the FAIMS analyzer region the effect of the asymmetric waveform is to push the ion away from the electrode  123 , as is indicated in FIG. 4 a  by the arrowheads of the electric force lines that are directed away from electrode  123 . As long as the electric fields are strong, and as long as the fields stay constant in strength, a balanced condition that is necessary to allow the ion to pass through the analyzer region is maintained. This balanced condition is shown schematically in FIG. 4 a  as a series of double-headed electric force lines, comprising a DV and CV component that are selected for transmitting ions having specific high-field mobility properties. This balanced condition extends completely around the inlet end of the electrode  123  facing the ion-inlet electrode  131   a  and completely around the outlet end of the electrode  123  facing the ion-outlet electrode  132   a.    
         [0054]    Unlike the prior art parallel plate FAIMS, the electric fields extend on both sides of the third electrode  123  symmetrically within the analyzer region, such that the ion continues to “see” the same balancing electric forces and will continue along a stable trajectory to exit the analyzer. The electrical forces for selectively transmitting the ion remain balanced beyond the physical limit of the electrodes because the two sides of the powered third electrode  123  are symmetrical. For instance a metal conductive surface of electrodes  122  and  124  is located a same distance from each surface of the powered third electrode  123 . Under these conditions, even slowly flowing gas will tend to keep the ions positioned near the trailing edge of the electrode, in a position close to the ion-outlet electrode  132 . Further, if the third electrode  123  of the system shown generally at  120  in FIG. 4 a  is narrow relative to the spaces  127   a  and  127   b  between the electrodes, then the specific shape of the corners at the edges of the electrode plates will other than critically influence the ion trajectory. For instance rounded or squared corners behave more or less the same in terms of the resulting fields that the ion will experience in this region. This is because the electric fields tend to ‘smooth’ themselves out over a distance away from a corner of the electrode, such that effectively the fields around the electrode look exactly the same as if it was rounded once you move more than some distance away. If the electrode is thick, for example more than approximately 20% of the thickness of the spaces, then the shape is important. Also, if the ion trajectory is very close to the third electrode  123 , a contour at an edge of the electrode has more influence on the path of travel than when the ions are further away from the third electrode  123 .  
         [0055]    Advantageously, when these balanced condition extend around the inlet edge of the electrode  123 , collisions between the ions entering through the ion-inlet orifice  135  in the ion-inlet electrode  131  and the leading edge of electrode  123  are minimized. The ions are prevented from approaching the electrode  123  by the effective repelling force that is created by the asymmetric waveform. Similarly, at the opposite end of the electrode  123  the balanced condition tends to pull the ions towards the electrode  123  as they pass by the outlet end of the electrode  123 , giving the ‘near-trapping’ conditions shown by the ion trajectory shown at dotted lines  133  and  134  in FIG. 4 b . The ions would otherwise be trapped at the outlet edge of electrode  123 , for example the ions are unable to move in any direction, absent a gas flow that is sufficiently strong to carry the ions to the ion-exit orifice. Of course, the dc voltage applied to the ion-exit electrode  132  is adjusted to help pull the ions away from the trailing edge of electrode  123  in a controlled fashion. Alternatively, the ions are detected by electrometric means (not shown) external to the analyzer region. As previously described, electrodes  121  and  125  are connected to at least a dc controller, such that a dc bias is optionally applied to the first and fifth electrodes  121  and  125  for diverting the ions. For instance in FIG. 4 a  positively charged ions are selectively transmitted through spaces  127   a  and  127   b  and collected at electrodes  121  and  125 , which have a negative dc bias applied. Alternatively, a positive dc bias is applied to electrodes  121  and  125  for focusing the positively charged ions into a narrow beam exiting the analyzer region for highly efficient extraction, as shown for a second mode of operation in FIG. 4 b . Of course in FIG. 4 b , the ion-outlet electrode  123  is additionally provided with an orifice for transmitting ions to a detector.  
         [0056]    In a second preferred embodiment of the present invention, a same combination of CV and DV are applied to the second electrode  122 , and to the fourth electrode  124  while the first electrode  121 , the third electrode  123  and the fifth electrode  125  are maintained at ground potential as shown at FIGS. 4 c  and  4   d . Alternatively the first, third and fifth electrodes are maintained at some other dc potential. In this mode the constant electric fields within the spaces  126   a ,  127   a ,  127   b  and  126   b  are identical, such that a same ion species is selectively transmitted through each of the four spaces. Advantageously, the ions will be distributed along four analyzer regions instead of only two, which reduces the space-charge induced ion-ion repulsion and minimizes ion losses in the analyzer region. Ion focusing occurs at the outlet edge of each powered electrode  122  and  124 , as was previously discussed with reference to FIG. 4 b.    
         [0057]    In an alternate mode of operation for the second preferred embodiment, a different combination of CV and DV are applied to the second electrode  122 , and to the fourth electrode  124  while the first electrode  121 , the third electrode  123  and the fifth electrode  125  are maintained at ground potential as shown in FIG. 4 d . Alternatively the first, third and fifth electrodes are maintained at some other dc potential. FIG. 4 d  illustrates a mode of operation wherein a positive polarity waveform and negative CV are applied to the second electrode  122 , whereas a negative polarity waveform and positive CV are applied to the fourth electrode  124 . As shown in FIG. 4 d , positive ions are selectively transmitted through spaces  126   a  and  127   a , whereas negative ions are selectively transmitted through spaces  127   b  and  126   b . This is referred to as a multi-mode parallel plate FAIMS. The current mode of operation selectively transmits a same species of positive ion within spaces  126   a  and  127   a , since the electric fields are identical within the spaces  126   a  and  127   a . Similarly, a same species of negative ion is transmitted within spaces  126   b  and  127   b , since the electric fields are identical within the spaces  126   b  and  127   b.    
         [0058]    As shown in FIGS. 4 e  and  4   f  for a third embodiment of the present invention, when the third electrode  123  is electrically insulated from the remaining electrodes, a dc potential is optionally applied to the third electrode  123  for diverting the ions in a predetermined manner. In FIG. 4 e  an example is shown wherein a same DV and CV combination are applied to the second and fourth electrodes  122  and  124  for selectively transmitting a same positive ion species. Then, a positive dc potential applied to the ion diverter third electrode  123  will cause all ion trajectories to diverge away from the central axis of the device. Alternatively, a negative dc potential applied to the ion diverter third electrode  123  will cause all ion trajectories to diverge towards the central axis of the device, for instance the positive ions will be focused into a narrow beam coaxial with the center axis of the device. If a more negative dc potential is applied, then the positive ions are optionally collected and detected at the third electrode  123 . Of course, the electric fields within spaces  126   a  and  127   a  are different, because on one side of the powered electrode  122  the compensation voltage is determined relative to a ground potential, whereas on the opposing side the compensation voltage is determined relative to a predetermined applied dc potential. Consequently, positive ions are transmitted through each space, however, a first species of positive ion is transmitted through spaces  126   a  and  126   b , and a second different species of positive ion is transmitted through spaces  127   a  and  127   b.    
         [0059]    In FIG. 4 f  an alternative mode of operation of the third embodiment is shown, wherein a different DV and CV combination is applied separately to the second and fourth electrodes  122  and  124 . For instance, a positive polarity waveform and negative CV are applied to the second electrode  122 , whereas a negative polarity waveform and positive CV are applied to the fourth electrode  124 . As shown in FIG. 4 f , positive ions are selectively transmitted through spaces  126   a  and  127   a , whereas negative ions are selectively transmitted through spaces  127   b  and  126   b . Operated in the mode illustrated in FIG. 4 f , the FAIMS analyzer functions as a four-mode FAIMS device, characterized in that a first species of positive ion is transmitted through space  126   a , a second different species of positive ion is transmitted through space  127   a , a first species of negative ion is transmitted through space  127   b , and a second different species of negative ion is transmitted through space  126   b . Of course, in practice it is difficult to control conditions appropriate for the selective transmission of four different ion species, nevertheless it is possible in principle to selectively transmit one to four ion species in parallel using the third embodiment of the present invention.  
         [0060]    Still referring to FIG. 4 f , a positive dc potential applied to the ion diverter third electrode  123  causes the positive ions that are selectively transmitted through spaces  126   a  and  127   a  to diverge away from the third electrode  123  of the device, whereas the negative ions that are selectively transmitted through spaces  127   b  and  126   b  will see an attractive force and be diverted towards the third electrode  123  of the device. Alternatively, a negative dc potential applied to the ion diverter third electrode  123  causes the positive ions that are selectively transmitted through spaces  126   a  and  127   a  to diverge towards the third electrode  123  of the device, whereas the negative ions that are selectively transmitted through spaces  127   b  and  126   b  will see an repulsive force and be diverted away from the third electrode  123  of the device. Of course, by applying a more positive dc bias toward electrode  123  it is possible to collect negative ions at electrode  123 , with the positive ions being diverted more strongly. Conversely, by applying a more negative dc bias toward electrode  123  it is possible to collect positive ions at electrode  123 , with the negative ions being diverted more strongly.  
         [0061]    Although the preferred embodiment of the present invention as described with reference to FIGS. 4 c  to  4   f  includes an electrode  123  for diverting ions, optionally other ion diverting means are used. For example, a slit-shaped orifice including a gas jet is optionally provided in place of electrode  123  for providing a flow of gas for diverting the ions. The ion diverting gas flow augments the uniform gas flow that, in use, is moving through the analyzer region for carrying the ions in a direction transverse to the applied electric fields. Advantageously, the ion diverting gas flow is used to push ions from the analyzer region through an outlet for subsequent analysis or collection. Further advantageously, the ion diverting gas flow diverts positively charged ions and negatively charged ions in a same direction, either away from the ion diverting means or towards the ion diverting means.  
         [0062]    The embodiments described with reference to FIGS. 4 a  to  4   f  have employed flat parallel plate electrodes with square end edges. Optionally, the first to fifth electrode plates  121  to  125 , respectively, are parallel flat-plate electrodes having a leading and a trailing edge, with respect to a direction of ion flow through the analyzer region when in use, that are rounded in cross section. The radius of curvature of the smooth curve provided at the leading and trailing edges of each electrode  121  to  125  are appropriate to focus and trap the ions at leading and trailing edges, and of course the electrode plates are thick enough to accommodate the radius of curvature. Of course, in the case where each of the first through fifth electrodes  121  to  125  are provided with leading and trailing edges that are rounded in cross sections, ion focusing will occur only at those electrodes to which a DV and superimposed CV are applied. This focussing effect with flat-plate electrodes was disclosed in a copending PCT application in the name of R. Guevremont and R. Purves  
         [0063]    Optionally, in addition to providing a curved cross section at the leading and trailing edges of the electrode plates, at least one of the leading edge and the trailing edge are further shaped with a concave smooth curve that is directed away from the direction of ion flow. The concave smooth curve at the at least an edge of the electrode plates is for producing electric fields that are shaped to direct the flow of ions generally inwardly towards the center of the electrode plate. Advantageously the ions are focused into a narrow beam before entering the flat plate analyzer region, which minimizes ion losses during separation. Further advantageously, the efficiency of ion extraction is improved at the trailing edge of the plate by focusing the ions further into a narrow beam prior to their extraction, thus maximizing ion transmission and minimizing overall losses.  
         [0064]    Further optionally the first through fifth electrode plates  121 ,  122 ,  123 ,  124  and  125  are curved, wherein the first through fifth curved electrode plates are referred to as  151 ,  152 ,  153 ,  154  and  155 , respectively, for a fourth embodiment of the present invention as shown in FIG. 5 a . An ion-inlet electrode (not shown) and an ion-outlet electrode (not shown) are additionally provided at the ion-inlet and the ion-outlet edges, respectively, of the electrode plates  151 ,  152 ,  153 ,  154  and  155 . The ion-inlet electrode and the ion-outlet electrode having an ion-inlet orifice and an ion-outlet orifice, respectively, each orifice being aligned with electrode plate  153 . The ion-inlet electrode and the ion-outlet electrode play a crucial role in producing the high strength electric fields near the ion-inlet and ion-outlet edges, respectively, of the curved electrode plates.  
         [0065]    Of course, such a curved electrode geometry produces non-constant electric fields, such that in the fourth embodiment of the present invention a focusing region exists within each space  156   a ,  157   a ,  157   b  and  156   b . Further, the field produced on one side of a powered electrode plate is other than identical to the field that is produced on the opposite side of the powered electrode plate. Interestingly, for the mode of operation described with reference to FIG. 4 a  or  4   b  in which the parallel plate electrodes are replaced by curved electrode plates, two different species of ions are typically transmitted; a first ion species within space  157   a  and a second ion species within space  157   b . Of course, for the mode of operation described with reference to FIG. 4 d ,  4   e  or  4   f  in which the electrodes are curved electrode plates, four different ion species are typically transmitted; a first ion species within space  156   a , a second ion species within space  157   a , a third ion species within space  157   b , and a fourth ion species within space  156   b . FIG. 5 b  shows the electrode plate  153  is optionally shaped with curved ends for directing ions generally inwardly toward the center of the electrode edge. Further optionally, each of the other electrode plates  151 ,  152 ,  154  and  155  are also shaped for directing the ion trajectories.  
         [0066]    Referring to FIGS. 6 a  and  6   b , a fifth embodiment of the present invention having a lens shaped third electrode  163  is described. As discussed previously, curved electrode geometry produces non-constant electric fields, such that in the fifth embodiment of the present invention a focusing region exists within each space  166   a ,  167   a ,  167   b  and  166   b . When an appropriate combination of DV and CV is applied to the electrode  163 , identical electric fields that are non-constant in space are produced within spaces  167   a  and  167   b , such that one species of ion is selectively transmitted. Alternatively, a combination of DV and CV is applied to electrodes  162  and  164 . If a same combination of DV and CV is applied to both electrodes  162  and  164 , then identical non-constant electric fields are produced within spaces  166   a  and  166   b , and different identical non-constant electric fields are produced in spaces  167   a  and  167   b . In this example a first species of ion is selectively transmitted through spaces  166   a  and  166   b , and a second different species of ion is selectively transmitted through spaces  167   a  and  167   b . Alternatively, if a different combination of DV and CV, for example the polarities of each potential is reversed, then different, non-constant electrical fields are produced within each space  166   a ,  166   b ,  167   a  and  167   b . In this example, a different species of ion is selectively transmitted through each different space,  166   a ,  166   b ,  167   a  and  167   b . FIG. 6 b  shows the electrode plate  163  is optionally shaped with curved ends for maximizing ion transmission.  
         [0067]    Referring again to FIGS. 4 a  to  4   f , two-dimensional ion focusing does not occur within the spaces  126   a ,  127   a ,  127   b  and  126   b  when the electrodes  121 ,  122 ,  123 ,  124  and  125  are flat, parallel plate electrodes. Thus, while it is assumed that only one ion species is selectively transmitted for a given combination of DV and CV, in fact a subset of ions are transmitted, wherein each ion species of the subset of ions has an approximately same appropriate mobility properties. Of course, two-dimensional ion focusing does occur within the spaces  156   a ,  157   a ,  157   b  and  156   b  between the curved electrode plates  151 ,  152 ,  153 ,  154  and  155 , shown in FIG. 5 a . In this latter case, while a subset of ions having appropriate mobility properties are transmitted also, the two-dimensional ion focussing effect that exits between the curved electrode plates reduces significantly the range of appropriate mobility properties. Advantageously, a subset of ions including fewer different ion species are transmitted through an analyzer region between curved electrode plates, relative to the subset of ions that are transmitted through an analyzer region between flat, parallel plate electrodes.  
         [0068]    Of course, the analyzer regions according to the first, second and third embodiments of the present invention, as described with reference to FIGS. 4 a  to  4   f , have a rotational axis of symmetry that is coaxial with the third electrode  123  and within the plane of the drawing. Rotation about this rotational axis of symmetry leads to a concentric cylinder FAIMS device, in which electrodes  121  and  125  form a continuous outer cylindrical surface  171 , electrodes  122  and  124  form a continuous inner cylindrical surface  172  and electrode  123  forms an ion diverter  173  that is coaxially aligned with the outer and inner concentric cylinder electrodes  171  and  172 , respectively. Of course, a single annular analyzer region  174  is defined by the space between the outer cylindrical electrode  171  and the inner cylindrical electrode  172 . This is a sixth embodiment of the invention and will be described with reference to FIG. 7. The CV and DV are applied to the inner cylindrical electrode  172  in the example that is illustrated in FIG. 7; however, the CV and DV is alternatively applied to the outer cylindrical electrode  171 . Since the same DV and CV must be applied to a cylindrical electrode, only one species of ion is transmitted through the analyzer region at one time. The ion diverter  173 , in the form of a probe electrode, is shown at ground potential in FIG. 7, however in practice the probe electrode  173  is biased at negative or positive dc. In the case of positive ions being transmitted, a negative dc bias applied between the probe electrode  173  and the inner cylindrical electrode  172  will divert ions toward the probe electrode  173 . If the negative dc bias is strong enough, ions will impact the probe electrode and are optionally detected. Alternatively, if a positive dc bias applied between the probe electrode  173  and the inner cylindrical electrode  172 , the positively charged ions will be diverted away from the probe electrode  173 . With an appropriate negative dc bias, ions will be focused into a narrow beam substantially axially aligned with the probe electrode for extraction from the analyzer region through an ion outlet.  
         [0069]    Alternatively, the device shown generally at  170  in FIG. 7 has an ion diverter  173  in the form of an orifice having a gas nozzle for directing a jet of gas along the central axis of the inner electrode  172 . The ion diverting gas jet pushes ions away from the terminus of the inner cylinder for extraction through an ion-outlet orifice (not shown) in an ion-outlet electrode (not shown).  
         [0070]    Referring to FIG. 8 shown is a simplified block diagram of a cylindrical geometry FAIMS device with an ion diverting device according to a seventh embodiment of the present invention. The seventh embodiment is very similar to the sixth embodiment, except the inner cylindrical electrode  208  is provided with a curved surface terminus  207 , and the inner surface of the outer electrode  204  is shaped to maintain a substantially uniform distance to the inner cylindrical electrode  208  near the curved surface terminus  207 . This geometry of FAIMS is referred to as a dome-FAIMS or dFAIMS. Shown also in FIG. 8 is an ion diverter  210  in the form of a probe electrode whose outer surface is continuous with the outer surface of the inner electrode  208  only at a small region  209  near the tip of the inner electrode. The ion diverter  210  is coaxially aligned with the inner cylindrical electrode  208  and the outer cylindrical electrode  204 , and with an ion-outlet orifice  217  in the outer cylindrical electrode  204 . It should be noted that the ion diverter  210  is easily removed from the FAIMS apparatus when so desired. Optionally, the ion diverter  210  is operated at a same voltage as the inner electrode  208 , such that the electric fields near the terminus  207  are other than perturbed by the ion diverter  210 .  
         [0071]    The device called dFAIMS is typically used in two fashions of operation. First, it is used for 3-dimensional trapping, since the ions that are swept along the inner electrode  208  through space  206  arrive at the tip of the dome and are unable to proceed further because of the trapping action that extends along the sides of the inner electrode and around the tip of the electrode. This has been discussed in greater detail above with respect to FIG. 4 a . Secondly, the device is optionally operated in continuous flow mode, if the electrode voltages are such that the stream of ions, which travel along the side of the inner electrode  208  through space  206 , escape from the zone near the tip of the electrode. The ions tend to travel along the curved spherical surface of the inner electrode towards the central axis of the inner electrode  208 , following the focusing fields, and are extracted as a narrow beam of ions.  
         [0072]    The dFAIMS is improved by the addition of a probe electrode, the ion diverter  210  in FIG. 8, through the center of the inner electrode  208 . The purpose of this electrode is to modify the electric fields near the terminus of the dome  207  of the inner electrode  208 . The objectives are two-fold. First, the ions that accumulate near the terminus of  207  the inner electrode  208  are ejected or forced away from the inner electrode  208  by the repulsive forces of an electric field applied via a voltage on the probe electrode  210  relative to the inner electrode  208 . Secondly, under some circumstances it is advantageous to pull the ions out of the trapping region towards this probe electrode  210 . Ultimately the probe electrode  210  is used, for example, to collect a sample of the ions that collide with the surface of the probe electrode  210  that is exposed and substantially continuous with the inner electrode  208  at the tip of the domed surface  207  of the inner electrode  208 .  
         [0073]    The probe electrode  210  is supported and aligned by insulating materials, and extends to the surface  209 , which is substantially continuous with the surface of the inner electrode  208 . The asymmetric waveform that is applied to the inner electrode  208  through a connection screw (not shown) is also applied to the probe electrode  210 . The probe electrode  210  does not contact the inner electrode  208 , and a set of insulators (not shown) serve to suspend the probe electrode  210  away from the surfaces of the inner electrode  208 . An additional electronic source (not shown) for applying a small dc bias between the probe electrode  210  and the inner electrode  208  is also provided.  
         [0074]    Still referring to FIG. 8 there is a small space between the short cylindrical rod  226  and the short cylindrical hole  227  drilled into the center of the inner electrode  208 . Although this space is important for electrical insulation purposes, the space also serves as a conduit for an (optional) flow of gas. The gas flow serves several potential purposes. First, a gas flow traveling from the analyzer region into the channel between rod  226  and hole  227  serves to ensure that no contaminants are added to the analyzer region via this channel. If the gas flows into the channel, the flow will augment the existing flow along the analyzer region  206 , and reduce the residence time of the ions inside the FAIMS, and bring the ions more quickly to the tip of the inner electrode  208 . The dimensions of the conduit between rod  226  and  227  are optionally small or wide. The removal of rod  226 , as shown in FIG. 9, to permit the hole  227  to be used only for the flow of gas is feasible with minimum modifications to this version of FAIMS.  
         [0075]    A second optional use for the gas flow through the hole  227  is also envisioned. The ions that are trapped in the region  216  are optionally reacted chemically with a gas that exits from the hole  227  in the tip of the inner electrode  208 . An example of a gas that reacts with an ion is carbon dioxide, which is known to form complexes with some types of ions. This new complex has different mobility properties compared to the bare ion, permitting that ion to leave the trapping region  216  for detection, even if another non-reactive ion resides in the trapping region  216 . Similarly the reactant gas is optionally used to reject some unwanted ion by forming a complex whose properties of mobility at low and high fields are no longer appropriate for the storage of this ion under the prevailing conditions of CV and DV, thereby increasing the relative number of the of ions of interest within the trapping region.  
         [0076]    A third purpose for a gas flow out of the hole  227  in the inner electrode  208  is easily visualized. This gas flow is used to assist in ejecting ions out of the trapping region  216 , if the flow along the analyzer region  206  is not sufficiently high for this purpose. The flow along the analyzer region  206  must in practice be set for the optimum separation of ions along the annular space between the outer electrode  204  and the inner electrode  208 . This flow along the analyzer region  206  may not be the optimum flow necessary to move ions out of the dFAIMS at the tip of the inner electrode  208 . A gas flow into or out of the hole  227  in the inner electrode  208  will serve to permit optimum flows both in the analyzer region  206  and out of the hole  217  in the outer electrode  204  which leads to an optional ion detection system (not shown).  
         [0077]    Of course, numerous other embodiments could be envisioned, without departing significantly from the teachings of the present invention.