Patent Publication Number: US-7714282-B2

Title: Apparatus and method for forming a gas composition gradient between FAIMS electrodes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a National Stage application under 35 U.S.C. §371 of PCT Application No. PCT/CA2006/000227, filed 17 Feb. 2006, entitled “APPARATUS AND METHOD FOR FORMTNG A GAS COMPOSITION GRADIENT BETWEEN FAIMS ELECTRODES”, which claims the priority benefit of U.S. Provisional Patent Application No. 60/653,484, filed 17 Feb. 2005, entitled “APPARATUS AND METHOD FOR FORMING A GAS COMPOSITION GRADIENT BETWEEN FAIMS ELECTRODES”, which applications are incorporated herein by reference in their entireties. 
   This application claims benefit from U.S. Provisional application 60/653,484 filed Feb. 17, 2005, the entire contents of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The instant invention relates generally to High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS). In particular, the instant invention relates to a method and apparatus for providing a gradient in the gas composition within the carrier gas in a FAIMS analyzer region. 
   BACKGROUND OF THE INVENTION 
   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), the entire contents of which is incorporated herein by reference. In IMS, gas-phase ion mobilities are determined using a drift tube with a constant electric field. Ions are separated in the drift tube on the basis of differences in their drift velocities. At low electric field strength, for example 200 V/cm, the drift velocity of an ion is proportional to the applied electric field strength, 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 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. 
   E. A. Mason and E. W. McDaniel in their book entitled “Transport Properties of Ions in Gases” (Wiley, New York, 1988), the entire contents of which is incorporated herein by reference, 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 electric field, and 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). 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 due to the compound dependent behavior of K H  as a function of the applied electric field strength. 
   In general, a device for separating ions according to the FAIMS principle has an analyzer region that is defined by a space between first and second spaced-apart electrodes. The first electrode is maintained at a selected dc voltage, often at ground potential, while the second electrode 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 H , lasting for a short period of time t H  and a lower voltage component, V L , of opposite polarity, lasting a longer period of time t L . The waveform is synthesized such that the integrated voltage-time product, and thus the field-time product, applied to the second electrode during each complete cycle of the waveform is zero, for instance V H  t H +V L  t L =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, which is identically referred to as the applied asymmetric waveform voltage. 
   Generally, the ions that are to be separated are entrained in a stream of gas flowing through the FAIMS analyzer region, for example between a pair of horizontally oriented, spaced-apart electrodes. Accordingly, the net motion of an ion within the analyzer region is the sum of a horizontal x-axis component due to the stream of gas and a transverse y-axis component due to the applied electric field. During the high voltage portion of the waveform, an ion moves with a y-axis velocity component given by v H =K H E H , where E H  is the applied field, and K H  is the high field ion mobility under operating electric field, pressure and temperature conditions. The distance traveled by the ion during the high voltage portion of the waveform is given by d H =v H t H =K H E H t H , where t H  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 L =KE L , where K is the low field ion mobility under operating pressure and temperature conditions. The distance traveled is d L =v L t L =KE L t L . Since the asymmetric waveform ensures that (V H  t H )+(V L  t L )=0, the field-time products E H t H  and E L t L  are equal in magnitude. Thus, if K H  and K are identical, d H  and d L  are equal, and the ion is returned to its original position along the y-axis during the negative cycle of the waveform. If at E H  the mobility K H &gt;K, the ion experiences a net displacement from its original position relative to the y-axis. For example, if a positive ion travels farther during the positive portion of the waveform, for instance d H &gt;d L , then the ion migrates away from the second electrode and eventually will be neutralized at the first electrode. 
   In order to reverse the transverse drift of the positive ion in the above example, a constant negative dc voltage is applied to the second electrode. The difference between the dc voltage that is applied to the first electrode and the dc voltage that is applied to the second electrode is called the “compensation voltage” (CV). The CV prevents the ion from migrating toward either the second or the first electrode. 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 that is necessary to prevent the drift of the ion toward either electrode is also different for each compound. Thus, when a mixture including several species of ions, each with a unique K H /K ratio, is being analyzed by FAIMS, only one species of ion is selectively transmitted to a detector for a given combination of CV and DV. In one type of FAIMS experiment, 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. 
   Numerous ionization sources, including atmospheric pressure ionization sources, have been described for use with FAIMS. Some examples of ionization sources include MALDI, ESI, nanoelectrospray, picoelectrospray, APCI, laser desorption chemical ionization, photoionization, corona discharge, as non-limiting examples. In addition, detection of ions using several types of detectors, including mass spectrometry is known. Other examples of post-FAIMS ion processing tools include FAIMS, IMS, ion funnels, as some non-limiting examples. The above-mentioned ionization sources and detectors optionally are further assembled into various tandem arrangements, including ESI-FAIMS-funnel-IMS-funnel-MS, or ESI-FAIMS-FAIMS trap-IMS-funnel-MS, as two very complex but non-limiting examples of tandem instruments with practical importance in chemical analysis. 
   In an analytical instrument that includes (1) an atmospheric pressure ionization source, such as for example electrospray ionization (ESI), (2) an atmospheric pressure gas phase ion separator, such as for example high-field asymmetric waveform ion mobility spectrometer (FAIMS) and (3) a detection system, such as for example mass spectrometry, (MS) it is advantageous to provide each with independent control of some of the operating conditions including temperature, operating gas pressure, and operating gas composition. In these regards, the ion source, FAIMS and mass spectrometer have significantly different requirements for optimum performance. 
   The performance of FAIMS for separation of ions may be dependent on temperature. For example an elevation in temperature may cause peaks in a CV spectrum to widen because of an increase in ion diffusion. Under this condition two ions that are separated at room temperature fail to be separated at 100° C., for example. Similarly, two ions that fail to separate at room temperature are separated at 10° C. with cooled FAIMS electrodes, for example. 
   Furthermore, the efficiency of transmission of ions through FAIMS is a function of temperature. For example, some types of ions are subject to thermal dissociation and therefore are more efficiently transmitted through FAIMS in a cool bath gas. 
   Furthermore, the separation of ions is a function of the composition of the carrier gas. Some mixtures of gases, including nitrogen plus helium, and helium plus carbon dioxide, as some non-limiting examples, are known to significantly affect the compensation voltage of the transmission of some ions. These mixtures of gases optionally are controlled and selected to separate ions which otherwise are not separated in any one pure type of carrier gas. Prior U.S. Pat. No. 6,774,360 describes the method and apparatus for improvements in separation and sensitivity in FAIMS, and is included herein by reference. Related patent publications WO 03/067237 and WO 03/067242 describe detection of traces of gases in FAIMS using the shift of CV of a monitor ion, and also are included herein by reference. The CV of the monitor ion shifts because the presence of the trace gas changes the carrier gas composition and therefore changes the optimum conditions for the transmission of the monitor ion. 
   Furthermore, the separation of ions and the efficiency of ion transmission in the FAIMS analyzer are a function of many mechanical electrode dimensions and a function of many aspects of the voltages and experimental conditions used in FAIMS. For example, the resolution of the separation in FAIMS is a function of the diameters of the electrodes, the width of the analyzer region between the electrodes, the length of time that the ions reside within the analyzer region, the longitudinal location of the inner electrode (domed type electrodes), the frequency of the applied asymmetric waveform, the shape of the asymmetric waveform (square vs two or more superimposed sinusoidal waves), the peak voltage of the asymmetric waveform (DV), as some non-limiting examples. A skilled user of FAIMS adjusts these parameters, and others, to achieve separations. 
   Accordingly, it would be advantageous to provide control of a number of non-mechanical experimental parameters that impact on the separation of ions, including the temperature of one or both FAIMS electrodes, the pressure of the carrier gas in the FAIMS analyzer, the temperature gradient across the analyzer region of FAIMS, and the composition of the gas mixture used as the carrier gas in the FAIMS analyzer, as some non-limiting examples. These parameters optionally are adjusted independently, or in conjunction with each other, to achieve the performance that is desired. 
   A method and apparatus for control of the temperature of the ionization source of FAIMS has been described in U.S. Pat. No. 5,736,739 and is incorporated herein by reference. The methods and apparatus for independent control of temperatures and pressures of the ion sources and FAIMS systems was first introduced in previously filed U.S. provisional applications 60/536,707 and 60/572,116 which are incorporated by reference herein. In these filings it was shown to be advantageous to design cylindrical FAIMS and parallel plate FAIMS with independent control of temperatures of the two electrodes to permit adjustment of the two electrodes to be at different temperatures, and at temperatures that differ from the average temperature of the carrier gas. Appropriate selection of these temperatures produces temperature gradients in the gas across the analyzer region, to beneficially influence the ion transmission efficiency and the separation of ions during their passage through the analyzer region. 
   Certain mixtures of carrier gases are known to significantly impact on the performance of FAIMS. Examples of reports in the scientific literature describing this impact include a paper by Barnett, D. A.; Purves, R. W.; Ells, B. Guevremont, R., entitled “ Separation of ortho -,  meta -,  and para - phthalic acids by high - field asymmetric wavefrom ion mobility spectrometry using mixed carrier gases , ” in J. Mass Spectrom. 2000, 35, 976-980 and a paper authored by Shvartsburg, A.; Tang, K.; Smith, R. D., entitled “ Understanding and designing field asymmetric waveform ion mobility separations in gas mixtures , ” in Analytical Chemistry 2004, 76, 7366-7374, the entire contents of both of which are incorporated herein by reference. For example, additions of carbon dioxide (1% to 20% by volume) to a carrier gas of nitrogen increases the CV of transmission and the efficiency of transmission for many low-mass ions, as a non-limiting example. 
   SUMMARY OF THE INVENTION 
   It is an object of at least one of the embodiments of the instant invention to affect ion transmission and ion separation by forming gradients in the composition of the mixture of gas that serves as the carrier gas. 
   It is a further object of at least one of the embodiments of the instant invention to deliver two or more gases together in a laminar flow between the FAIMS electrodes, to minimize turbulence and mechanical mixing of the gases. 
   According to an aspect of the instant invention there is provided an apparatus for separating ions, comprising: a first electrode and a second electrode disposed one relative to the other in a spaced-apart facing arrangement for defining an analyzer region therebetween, the analyzer region including a first end and a second end and having a length extending between the first end and the second end; a first gas inlet in fluid communication with the analyzer region, for providing a flow of a carrier gas of a first composition; a second gas inlet in fluid communication with the analyzer region, for providing a flow of a carrier gas of a second composition; and, a gas-flow directing element in fluid communication with the first gas inlet and in fluid communication with the second gas inlet, for receiving the flow of the carrier gas of the first composition and the flow of the carrier gas of the second composition, and for providing within a portion of the analyzer region a carrier gas flow having a composition that is non-uniform in space. 
   According to an aspect of the instant invention, provided is a method of separating ions, comprising: providing a high field asymmetric waveform ion mobility spectrometry (FAIMS) analyzer region for separating ions; providing a flow of a carrier gas within a portion of the FAIMS analyzer region, the flow of carrier gas having a composition that is non-uniform in space along a direction transverse to the flow of the carrier gas; introducing ions into the FAIMS analyzer region; providing electric field conditions within the FAIMS analyzer region for selectively transmitting a subset of the ions through the FAIMS analyzer region; and, selectively transmitting the subset of ions along an average ion flow path through the FAIMS analyzer region. 
   According to an aspect of the instant invention, provided is a method of separating ions, comprising: providing a high field asymmetric waveform ion mobility spectrometry (FAIMS) analyzer region for separating ions, the FAIMS analyzer region comprising an ion origin end that is in fluid communication with an ionization source, and an ion exit end that is in fluid communication with an ion detecting device, a length of the FAIMS analyzer region defined along a direction between the ion origin end and the ion detection end; providing a flow of a first gas into a gas inlet region of the FAIMS analyzer region; and, providing separately a flow of a second gas into the gas inlet region of the FAIMS analyzer region, and absent forming a homogeneous carrier gas flow including the first gas and the second gas within the gas inlet region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention will now be described in conjunction with the following drawings, in which similar reference numerals designate similar items: 
       FIG. 1  is a simplified block diagram of a chemical analysis system showing a tandem arrangement including an ion source, a FAIMS, and a mass spectrometer; 
       FIG. 2  is a simplified block diagram of a chemical analysis system comprising a tandem arrangement including an ion source, a FAIMS, and a mass spectrometer, supporting independent temperature, pressure, and gas composition control of a source region, a FAIMS region, and a mass spectrometer region; 
       FIG. 3  is a simplified block diagram of a chemical analysis system comprising a tandem arrangement including an ion source, a FAIMS, and a mass spectrometer, further incorporated into a drug discovery and drug production environment; 
       FIG. 4  is a simplified block diagram of a chemical analyzer comprising a tandem arrangement including an ion source, a FAIMS, and a mass spectrometer, further incorporated within a sampling system to provide detection of chemicals; 
       FIG. 5  is a parallel plate FAIMS provided with two gases of differing composition; 
       FIG. 6  illustrates the gradient of the composition of the gas between FAIMS electrodes; 
       FIG. 7  illustrates the motion of ions in two regions of differing gas composition between FAIMS electrodes; 
       FIG. 8  illustrates the focusing of ions while being transported from an ion inlet to an ion outlet, in the presence of a gradient of gas composition between the FAIMS electrodes; 
       FIG. 9  is a cylindrical geometry side-to-side electrode, suitable for operation using a gradient in gas composition; 
       FIG. 10  is a cylindrical geometry FAIMS with a domed inner electrode, suitable for operation using a gradient in gas composition; 
       FIG. 11  is a segmented cylindrical FAIMS electrode suitable for operation using gradients of gas composition, which may be combined with longitudinal fields generated by voltages applied to the segments; 
       FIG. 12  is a segmented cylindrical FAIMS electrode system with internal ionization, suitable for operation with gradients in the gas composition, which can be combined with longitudinal fields generated by voltages applied to the segments; 
       FIG. 13  is a simplified flow diagram of a method of separating ions according to an embodiment of the instant invention; and, 
       FIG. 14  is a simplified flow diagram of another method of separating ions according to an embodiment of the instant invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and the scope of the invention. Thus, the present invention is not intended to be limited to the embodiments disclosed, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
   Throughout much of the following discussion it is assumed that the FAIMS electrodes operate at atmospheric pressure, but operating at pressures below and at pressures exceeding ambient atmospheric pressure conditions also are envisaged. 
   Because ion separation and ion transmission in a FAIMS system is susceptible to changes in temperature, it is desirable to operate at a selected temperature. For example, a rise in temperature leads to a decrease in the number density of the gas (N, molecules per cc) and therefore the operating electric field (E/N) increases. Similarly an increase in gas pressure increases N and therefore decrease the effective E/N conditions. In order that experiments give consistent results when repeated, the temperatures and pressures preferably are maintained at selected values within known tolerance limits. 
   It is also beneficial that the physical conditions in the analyzer region of FAIMS do not significantly change the CV of the transmission of the ion of interest while it is passing through the analyzer region, to a degree that prevents the transmission of the ion of interest. For example, if the conditions differ substantially as the ions are carried through FAIMS, those ions that are initially being successfully transmitted near the ion inlet region may be lost to the electrode walls at a later time during their passage through the FAIMS analyzer region. This occurs if the conditions near the inlet are in a balanced condition for the selected ion, and the ion is being transmitted near the inlet, but at a location elsewhere in the analyzer region the conditions are sufficiently different that the same ion migrates to the electrode walls and is neutralized. Temperature, pressure and spacing between the electrodes are among the physical conditions, assuming constant applied voltages, affecting the CV of transmission of an ion. For example, a substantial difference in the electrode spacing near the ion inlet and near the ion outlet results in the field E/N near the inlet and near the outlets being different from each other. Moderate changes are beneficial to improve ion separation in certain instances, but larger changes that the ion experiences for longer periods of time result in complete loss of ion transmission. Additionally, the physical conditions may be beneficially varied in specific locations within the FAIMS analyzer region, for example the field E/N is stronger near the inner electrode than near the outer electrode. Such local variations are beneficial so long as the overall conditions are not sufficiently changed so as to result in complete loss of the ions. The magnitudes of the total changes in physical conditions, and of the local changes in physical conditions, are established by experimental measurements, and the conditions adjusted to achieve the ion transmission sensitivity and the ion separation required. 
   In cylindrical and spherical geometry FAIMS it is known that an ion focusing mechanism is a result of the gradient of E/N that forms between the inner and outer electrode. The ion focusing causes the ion cloud to be constrained in the vicinity of an optimal, radial location between the electrodes, and therefore assists in minimization of ion loss to the electrode walls. In FAIMS having electrode geometry in which the electric field strength (E/N) changes across the analyzer region between electrodes, this gradient of E/N is responsible for the focusing mechanism. Electrodes with cylindrical and spherical geometry are some non-limiting examples wherein the field, E/N, changes strength along the radial direction between the FAIMS electrodes. At appropriate conditions of applied waveform and compensation voltage, as well as pressure, temperature, gas composition etc. as some non-limiting examples, the ion cloud is focused in the analyzer region, an effect that is beneficial by minimization of ion loss via collision with the electrode walls. The value of E/N is modified by voltages applied to the electrodes, and by the temperature and the pressure of the gas between the electrodes. Moreover, a gradient of E/N is formed when gradients of the temperature and the pressure of the gas are formed. For example a gradient of E/N is produced when a voltage difference is applied between two electrodes across a region where the gas adjacent to a first electrode is at higher temperature than the region adjacent to the second electrode, and where the temperature in the gas between the electrodes varies gradually between these two temperatures. In this example the value of N, which is the number density of the gas, varies with temperature and thereby changing the value of E/N as a function of the temperature. The gradients of E/N induced by temperature gradients in the gas between FAIMS electrodes are optionally used to beneficially modify the focusing properties of both cylindrical and parallel plate versions of FAIMS. 
   The parallel plate version of FAIMS is known to lack any focusing properties, away from the edges of the plates, in the absence of temperature gradients between the electrodes. A beneficial focusing occurs when temperature conditions between the electrodes serve to mimic the E/N gradient found in cylindrical geometry FAIMS. The transmission of ions at a fixed CV requires control of the temperature of the gas and the electrodes, such that the CV conditions for transmission of a selected ion do not change excessively during the time it takes for an ion to pass between the electrodes It is beneficial that the CV of transmission is constant throughout the device, while simultaneously affecting the temperature of the gas between the electrodes to create conditions for focusing of the ion cloud. In a second approach the value of N is changed by causing the pressure to change in regions between the electrodes, for example using an acoustic transducer as a non-limiting example. Other means of modifying N in the space between the electrodes using lasers, for example, are envisioned. 
     FIG. 1  is a simplified block diagram of a chemical analyzer showing a tandem arrangement including an ion source  2 , a FAIMS  4 , and a detection system  6 . In the specific and non-limiting example of  FIG. 1 , an electrospray ionizer is shown. However, many other suitable ion sources are known, including nano-electrospray, pico-electrospray systems, photoionization sources, atmospheric pressure MALDI, radioactivity based sources, corona discharge sources, and other rf-based capacitive and/or inductively coupled discharge sources, as a few non-limiting examples. Depending on the mechanism and design of the ionizer, the ionizer operates on samples presented as gases, streams of liquids, liquids on solid support, or solids, to name a few non-limiting examples. The components  2 ,  4 , and  6  that are shown at  FIG. 1  are all at room temperature, but it is advantageous to set operational variables, including temperature, pressure, gas composition etc. to values that are best suited for the analysis in which the chemical analyzer is operating. In addition, the chemical analyzer is optionally operated in conjunction with other sample preparation and separation systems including autosamplers, robotic sample handling systems, gas chromatographs, liquid chromatographs, and capillary electrophoresis, as some non-limiting examples. In summary, since FAIMS is integrated into the chemical analysis system between the ionization source and the detection system, all other peripheral systems that are commonly used in a chemical analysis system continue to be operative. The FAIMS generally does not limit the scope of other peripheral instruments, nor the type of chemical analysis that can be performed by the generalized chemical analysis system shown in  FIG. 1 . The detection system  6  optionally is one of an electrometric ion current sensor, a mass spectrometer, an optical sensor, and an ion processing device including further FAIMS, ion-trapping FAIMS, IMS, ion funnels as some non-limiting examples. The detection system  6  optionally is a tandem arrangement of these devices, for example trapping FAIMS-funnel-IMS-funnel-MS, as a non-limiting example. 
     FIG. 2  is a simplified block diagram showing a tandem arrangement including an ion source  12 , a FAIMS  14 , and a mass spectrometer  16 , supporting independent temperature, pressure and gas composition control of a source region  18 , a FAIMS region  20 , and a mass spectrometer region  22 . While the control of gas composition is emphasized throughout this document, it is to be understood that operation at gas pressures higher than and lower than atmospheric pressure is also envisaged and operation at temperatures above and below room temperature is also envisaged. For example the ion source  12  operates optionally at twice atmospheric pressure provided that an appropriate chamber (not shown) surrounds the source region  18 , and FAIMS  14  operates optionally at 0.3 of an atmosphere provided that an appropriate chamber (not shown) surrounds the FAIMS region  20  and appropriate apertures (not shown) separate the source region  18  and the FAIMS region  20 . Of course, any mention of specific operating pressures and/or temperatures is given by way of non-limiting example only. 
   Referring now to  FIG. 3 , shown is a chemical analysis system  33  that includes sub-systems comprising chemical sample processor  30 , ionization system  31 , and an ion analyzer  32 . Ion analyzer  32  optionally comprises one or more sub-systems, individually or in tandem arrangement, including FAIMS, drift tube ion mobility spectrometry, mass spectrometry, etc. As a first non-limiting example, the ion analyzer  32  comprises a tandem arrangement of FAIMS and a mass spectrometer similar to the system shown in  FIG. 2 . As a second non-limiting example, the ion analyzer  32  comprises a tandem arrangement of FAIMS coupled to a drift tube mobility analyzer coupled in turn to a time-of-flight mass spectrometer. 
   Still referring to  FIG. 3 , the chemical analysis system  33  is situated in a central chemical laboratory  39 . Arrows  37  and  38 , and other arrows not enumerated, represent the exchange of samples and data between subdivisions  34 ,  35  and  36  of the organization that utilizes the services of the chemical analysis laboratory  39 . Some non-limiting examples of the subdivisions are shown in  FIG. 3 . For instance, a first subdivision  34  is responsible to ensure quality control in a pharmaceutical production factory. A second subdivision  35  is engaged in drug discovery, and provides samples related to drug interactions with chemical entities in living organisms, the chemical entities including enzymes, proteins, DNA, RNA, cell walls, sub-cellular entities including mitochondria, as some non-limiting examples. A third subdivision  36  is engaged in pharmico-kinetics and provides samples indicative of the efficacy of drug products and the formation of secondary chemical species resulting from drug metabolism. This diagram is intended to be a non-limiting example of the wide applicability of chemical analysis technology within organizations. Those applications that were previously operative using chemical tools including LC, ESI, MALDI, and mass spectrometry may be significantly improved by including FAIMS with the gas composition gradient as described herein. 
     FIG. 4  illustrates a chemical analysis system that is suitable for monitoring chemicals in locations other than an analytical chemistry laboratory. A mobile chemical analyzer  43  is provided with a sample flow  45  into a sample inlet conduit  44 . The sample is delivered to the chemical analysis system  33 . The chemical analysis system  33  includes sub-systems that may include a chemical sample processor  30 , ionization system  31 , and an ion analyzer  32 . The ion analyzer  32  optionally includes one or more further sub-systems including a FAIMS analyzer, a drift tube ion mobility spectrometer, or a mass spectrometer. The subsystems are assembled in one of a plurality of different ways, depending upon specific requirements. In a first non-limiting example, the ion analyzer  32  comprises a tandem arrangement of FAIMS and a mass spectrometer similar to the system shown in  FIG. 1  and  FIG. 2 . As a second non-limiting example, the ion analyzer  32  comprises a tandem arrangement of FAIMS coupled to a drift tube mobility analyzer coupled in turn to a time-of-flight mass spectrometer. 
   Still referring to  FIG. 4 , the chemical analyzer  43  is designed to detect chemical substances provided through sample flow  45 . The sample flow optionally is one of a gas, liquid, or a solid, or a combination including solid particles suspended in a flow of gas, or liquid droplets suspended in a gas, or solid particles suspended in a liquid, as some non-limiting examples. The chemical analyzer  43  detects the presence one or more targeted compounds to indicate the presence of one or more substances including explosives, narcotics, contraband materials, biological substances including bacteria or spores or virus, chemical poisons, biological poisons, bio-terror or chemical weapons, as some non-limiting examples, and provides information to a communication system  46 , such as for instance a human interface including an alarm or computer network to further transmit information, as some non-limiting examples. 
     FIG. 5  illustrates one possible version of a flat plate geometry of FAIMS that is operated in a tandem arrangement with an electrospray ionization source and a mass spectrometer, for ion mass analysis and detection. The flat plate geometry of FAIMS includes an upper electrode  65  and a lower electrode  63 , which define an analyzer region  68  therebetween. In this example, ions are formed from a flow of liquid sample in an ionization region  54  adjacent to an electrospray needle  52  that is held at high potential relative to a curtain plate  57 . Some of the ions thus formed pass through the curtain plate aperture  55  against a counter-flow of curtain gas  58  supplied to the region  53  between the curtain plate  57  and the upper FAIMS electrode  65 . The region  53  between the curtain plate  57  and the upper FAIMS electrode  65  is enclosed by insulating material so that the curtain gas  58  exits only through the curtain plate aperture  55  and/or through the ion inlet  64  to the FAIMS analyzer region  68 . Power supply  51  is provided for applying a voltage on curtain plate  57 , so as to establish a voltage difference between curtain plate  57  and upper FAIMS electrode  65  for directing ions toward ion inlet  64 . Optionally, the gas flow though ion inlet  64  is low, and ions pass through the ion inlet  64  into a first end of analyzer region  68  under the influence of the electric fields formed by the voltage difference between the curtain plate  57  and the upper FAIMS electrode  65 . Further optionally, a portion of curtain gas flow  58  passes through the ion inlet  64  and helps carry ions into the first end of analyzer region  68  where the flow of gas through ion inlet  64  combines with the flow  61   a  and  61   b  to transport the ions to the ion outlet  66 . 
   Still referring to  FIG. 5 , the ions that enter the analyzer region  68  through ion inlet  64  are carried along the analyzer region by a flow of gas  61   a  and  61   b , where the composition of gas flow  61  a optionally differs from the composition of gas flow  61   b . During transport along the analyzer region  68  the ions are separated according to the FAIMS mechanism. The high voltage rf frequency asymmetric waveform is applied to lower electrode  63  from power supply  90 . The voltage on upper electrode  65  is provided by power supply  91 . The voltages and width of the analyzer region  68 , as well as other operational variables including gas composition, gas pressure, gas temperature, gradient in temperature of the gas across the analyzer region  68 , are selected to permit a subset of the ions provided from the ionization source to be transmitted to the ion outlet  66 , and subsequently to an ion detection system  56 , such as for instance a mass spectrometer ion inlet system as a non-limiting example. 
   Still referring to  FIG. 5 , a gas-flow directing element is provided in the form of a plurality of plate structures  70 . Each plate structure of the plurality of plate structures is a flat plate including a first major surface and a second major surface along opposite sites thereof, for instance upper and lower plate surfaces, respectively, in  FIG. 5 . Each plate structure further includes a first edge surface and a second edge surface along opposite ends thereof, for instance left and right edge surfaces in  FIG. 5 . The plurality of plate structures  70  is disposed in a stacked, spaced-apart arrangement such that the first major surface of each plate structure faces the second major surface of an adjacent plate structure. The plurality of plate structures  70  is supported by electrically insulating material  71 , so as to form a stack extending between the upper electrode  65  and the lower electrode  63 . Accordingly, the plurality of plate structures defines a plurality of generally uniform gas-passage spaces  73  in an alternating arrangement with the plurality of plate structures. Each gas-passage space has a height along a stacking direction that is small relative to a length along a gas-flow direction. As shown in  FIG. 5 , the first edge surfaces of some of the plate structures is juxtaposed with a first gas inlet  74   a , and the first edge surfaces of other of the plate structures is juxtaposed with a second gas inlet  74   b . The plurality of plate structures  70  helps to form the gas flow  61   a  into a low-turbulence laminar flow that smoothly flows adjacent to upper electrode  65 . The stack of plates  70  also helps to form the gas flow  61   b  into a comparable laminar flow adjacent to lower electrode  63 . Optionally, the gas flow  61   a  differs in composition from that of the gas flow  61   b . Since the two streams of gas diffuse into each other, and mix at the interface between the flows, a gradient of gas composition is formed, where the composition of the gas is similar to that of the gas flow  61   a  near upper electrode  65 , and similar to the gas flow  61   b  near the lower electrode  63 , but forms intermediate mixtures in the region midway between the upper electrode  65  and the lower electrode  63 . The stream of ions that is carried from the ion inlet  64  to the ion outlet  66  is selectively located in a region of the gas composition gradient that has a gas composition suitable for focusing of the ions in dependence on operating conditions of voltage (DV and CV), temperature, pressure, analyzer gap width, as some non-limiting examples. The gas composition gradient provides a new tool for improving the efficiency of ion transmission, by providing a region towards which ions preferentially migrate, and therefore minimizing their collisions with the electrodes. 
     FIG. 6  is an expanded view of a portion of  FIG. 5 , illustrating the region of the FAIMS analyzer  68  between upper electrode  65  and lower electrode  63 . The stack of plate structures  70  serves to smooth the flows of gas flow  61   a  and gas flow  61   b  to travel parallel to the electrodes, with minimum turbulence or vortex motions. Curve  83  illustrates the percentage of gas flow  61   a  that contributes to the composition of the gas across the analyzer region  68 . Dashed line  82  represents the zero composition limit. For example, at region  80  the gas composition is largely similar to gas flow  61   a . The curve  83  approaches the dashed line  82  near the lower electrode  63  and indicates that the composition of the gas near the lower electrode  63  has very small contribution from the gas flow  61   a . Similarly, dotted line  81  indicating percentage contribution from gas flow  61   b , reaches a maximum near the lower electrode  63  and approaches the dashed line  82  near the upper electrode  65 . The curves  83  and  81  are hand-drawn approximations to the changes in composition to be used for illustrative purposes, whereas the actual composition gradient may be more abrupt, or more gradual than these curve indicate. 
     FIG. 7  illustrates the analyzer region  117  between a first FAIMS electrode  100  and a second FAIMS electrode  101 . A first curve  102  represents the percentage of a first gas forming a gradient of composition in the gas across the analyzer region  117 . A second curve  105  represents the percentage of a second gas forming a gradient of composition in the gas across the analyzer region  117 . The dashed line  108  indicates zero contribution of the gas to the mixture. Near the first electrode  100  the curve  102  is at a maximum value indicating a high percentage of the mixture is composed of this first gas. The curve  102  is near the dashed line  108  adjacent the second electrode  101 , and therefore is at lower percentage contribution to the mixture adjacent the second electrode  101 . An asymmetric waveform (peak voltage DV) and a dc compensation voltage (CV) are applied to the electrodes, in the usual fashion for separation of ions in the FAIMS mechanism. For purposes of discussion, it is assumed that a first ion  110  has high field mobility properties in the gas near the first electrode  100 , such that the ion drifts away from the first electrode  100  in a direction towards the second electrode  101 . If the gas composition at all locations in the analyzer region is constant, in the same composition as the gas composition near electrode  100 , the ion is expected to drift completely across the analyzer region  117  and collide with the second electrode  101 , assuming flat parallel plate electrode-geometry in this example. By selection of a second gas with a differing composition, wherein the percentage composition of the second gas is indicated by curve  105 , a second ion  111  of the same type located near the second electrode  101  drifts away from electrode  101  in a direction towards electrode  100 . Since, under the same electric field, temperature, gas pressure, conditions the ion near the first electrode  100  drifts towards the middle of the analyzer region  117 , and the same type of ion located near the second electrode  101  also drifts towards the middle of the analyzer region  117 , this type of ion accumulates and is focused away from the electrodes. This focusing mechanism minimizes collision of this type of ion with the electrodes, and permits higher transmission efficiency of this ion through the FAIMS device. Moreover, other types of ions, which do not have the appropriate behavior of ion mobility at high field relative to low field as does the ion shown in  FIG. 7 , are expected to be lost through collision with the electrodes. Ions with mobility properties very similar to the ion shown in  FIG. 7  may also be focused in these conditions, but may focus at slightly differing distance from one or the other electrode than the ion shown in  FIG. 7 . 
   Still referring to  FIG. 7 , the selection of the types of ions that are transmitted between electrodes that have a gradient of gas composition is controlled by a multitude of FAIMS operating parameters, including voltages such as DV and CV, physical geometry including width of the gap between the electrodes and the curvature of the electrodes (not shown in  FIG. 7 ), and operational variables including the selection of the types of gases, temperature, temperature gradient, gas pressure, as some examples of the variables important to ion behavior in a FAIMS electrode. 
   Still referring to  FIG. 7 , an example of a non-limiting condition under which this effect occurs is discussed for illustrative purposes. In this example the type of ion and the type of gases is selected to illustrate the effect shown in  FIG. 7 , but is not selected to indicate that this is the only example to which this gradient of gas composition is applicable. It is known from the literature that the mobility of chloride anion increases in nitrogen whereas the mobility of this ion decreases in helium. If an asymmetric waveform with a negative polarity is applied to the first electrode  100  (electrode  101  at ground potential), and the electric fields at the peak of the waveform exceed about 50 Td, then a chloride anion is expected to migrate away from the first electrode  100  when the gas in the analyzer region  117  is nitrogen. This occurs because in nitrogen the mobility of chloride is higher when the first electrode is at the negative maximum voltage, than when the same electrode is at the maximum positive voltage during application of the asymmetric waveform (recall that DV is negative in this example). Under the same voltage conditions, but with helium gas in the analyzer region  117  the chloride anion drifts towards the first electrode  101 . In this example, at a CV of zero volts, a gradient of gas composition having nitrogen near the first electrode  100  and helium near the second electrode  101 , causes the chloride ion to behave in the manner discussed-above. When located near the first electrode  100  the chloride ion is contained in a gas primarily composed of nitrogen, and the chloride anion migrates away from the first electrode  100 . When near the second electrode  101  the chloride ion is contained in a gas primarily composed of helium, and the chloride anion migrates away from the second electrode  101 . When the gradient of gas composition is maintained along the ion pathway, the likelihood that the chloride anion will collide with one of the electrodes is decreased significantly. In this example, the ability of helium to diffuse is very high, and the mixture will gradually become uniform as the nitrogen and the helium form a mixture. 
   Still referring to  FIG. 7 , many other combinations of gases are expected to have a longer lifetime without complete mixing, than does nitrogen/helium. Mixing is less rapid using carbon dioxide as a first gas, and nitrogen as a second gas, as a further non-limiting example. Further optionally, the two gases discussed above are each a premixed gas prior to delivery to the FAIMS analyzer. It is known that certain mixtures of gases have very significant deviations from Blanc&#39;s law behavior and some ions therefore have very high CV&#39;s in these mixtures. As another non-limiting example, the first gas is a premixed binary combination of 50% nitrogen and 50% carbon dioxide, and the second gas is premixed binary combination of 5% sulfurhexafluoride and 95% carbon dioxide. In each case the first and second gases are selected to provide focusing of an ion of interest, at appropriate voltage and operating conditions, this focusing being promoted by the gradient in the composition of the gas in the analyzer region of FAIMS. 
     FIG. 8  illustrates the focusing of ions in a region within the analyzer region  68  between the upper electrode  65  and the lower electrode  63 . Voltages are applied to the electrodes by power supplies  91  and  90  respectively. Ions  122  are introduced through ion inlet  64 , preferably without introduction of a significant quantity of gas. Ions leave the analyzer through ion outlet  66 . A gradient of gas composition is formed by a smooth flow of a first gas  61  a that is flowing parallel to, and at the same velocity as a flow of a second gas  61   b . The diffusion of gases  61   a  and  61   b  into each other forms a gradient across the analyzer region  68 . The ions  122  that pass into the analyzer region  68  are confined to a limited region indicated by the dashed lines  121  and  123 . In some cases, the gradient in composition changes with time, and therefore in this illustration the dashed lines  121  and  123  are shown not to be parallel to each other. 
   Still referring to  FIG. 8 , other types of ions, having mobility properties unlike those of ion  122 , are lost by collision with the electrodes  63  and  65 . Some other ions, having mobility properties similar to those of the ion shown in  FIG. 8 , are also transmitted through the device, but may be focused at a location different than that between the lines  121  and  123  that are shown in  FIG. 8 . The gradient of gas composition cannot be maintained indefinitely because of mixing and diffusion. Furthermore, if the ions are carried out through the ion outlet  66  by the flow of gas, this gas composition gradient is modified in location and in gradient steepness as the gases approach the ion outlet  66 . Some part of the flow of gas, or all of the flow of gas, in the analyzer is optionally used to carry the ions out through the ion outlet  66 . 
     FIG. 9  illustrates a cylindrical geometry FAIMS of the side-to-side type, having an ion inlet  131  and an ion outlet  135 . Gases are provided to the analyzer region  132  to form a gradient of composition across the analyzer region  132 . In a first optional approach a first gas is provided through a first set of not illustrated holes in the outer electrode  134  and a second gas is provided through a second set of not illustrated holes in the inner electrode  130 . In a second optional approach a first gas is provided through the ion inlet  131 , and the second gas is provided through a set of not illustrated holes in the inner electrode  130 . The gradient of gas composition in the analyzer region  132  results in focusing of the ions to a radial distance indicated by the dashed line  137 , which is shown only for illustrative purposes. From the complex mixture that may be provided into the ion inlet  131 , only those ions, whose mobility properties are appropriate at the voltage and operational parameters of FAIMS, are transmitted and these selected ions leave FAIMS through the ion outlet  135 . 
   Referring now to  FIG. 10 , shown is a longitudinal cross-sectional view of an electrospray ion source  150  disposed in fluid communication with an ion inlet  152  of a FAIMS  154 , the FAIMS  154  being mounted in and supported by an insulating material  156 . According to  FIG. 10 , the inner electrode  158  and the outer electrode  160  are supported in a spaced-apart arrangement by an insulating material  156  with high dielectric strength to prevent electrical discharge. Some non-limiting examples of suitable materials for use as the insulating material  156  include Teflon™, and PEEK. A passageway  162  for introducing a curtain gas is shown by dashed lines in  FIG. 10 . Gas delivery ports  902   a  and  902   b  (shown as dashed lines) provide two gases of differing composition to the analyzer region  153  between the outer electrode  160  and the inner electrode  158 . As a result of the gradient of gas composition, one or more ion focusing regions  903  surround the inner electrode  158 , and assist in transmitting ions between the ion inlet  152  and the ion outlet  174 . 
   Still referring to  FIG. 10 , a first gas and a second gas are provided through gas delivery ports  902   a  and  902   b , respectively. The gases are distributed around the circumference of the inner electrode  158  by channels  905   a  and  905   b  behind a gas-flow directing element in the form of an array of plates  904 , which ensures that the gases are flowing uniformly and parallel to the surfaces of the electrodes, so as to provide a stable and long-lived gradient of gas composition. The array of plates  904  comprises a plurality of axially aligned, cylindrical plate structures that are disposed in a radially spaced-apart arrangement. Stated differently, each cylindrical plate structure includes a convexly curved outer surface and a concavely curved inner surface that are joined by a first edge surface and by a second edge surface. The plurality of cylindrical plate structures are nested such that the convexly curved outer surface of each cylindrical plate structure faces the concavely curved inner surface of an adjacent cylindrical plate structure, so as to define a plurality of generally uniform annular gas-passage spaces in an alternating arrangement with the plurality of cylindrical plate structures. Furthermore, the plurality of cylindrical plate structures is disposed such that the first edge surface of some of the cylindrical plate structures is juxtaposed with gas delivery port  902   a , and the first edge surface of other of the cylindrical plate structures is juxtaposed with gas delivery port  902   b . The array of plates  904  directs the first gas to flow approximately parallel to, and adjacent to, the outer electrode  160 . Similarly, the array of plates  904  directs the second gas to flow approximately parallel to, and adjacent to, the inner electrode  158 . Preferably, the first gas and the second gas flow through the array of plates  904 , traveling at approximately equal velocity, so as to minimize formation of turbulence and eddies in the gas flow. 
   In  FIG. 10 , the ions are formed near the tip of an electrospray needle  164  and drift towards a curtain plate  166 . The curtain gas, introduced below the curtain plate  166  via the passageway  162 , divides into two flows, the majority of which exits through an aperture  168  in the curtain plate  166 , to prevent neutrals and droplets from entering the curtain plate aperture  168 . Ions are driven against this gas by a voltage gradient between the needle  164  and the curtain plate  166 . A field generated in the desolvation region  172  between the curtain plate  166  and the FAIMS outer electrode  160  pushes ions that pass through the aperture  168  in the curtain plate  166  towards the ion inlet  152  of FAIMS  154 . A small portion of the curtain gas flows into the ion inlet  152 . The gases forming the composition gradient carry the ions along the length of the FAIMS electrodes to the ion outlet  174 , and into a mass spectrometer  170 . Those ions with appropriate mobility properties are focused in the region indicated by the dashed line  903  and are transmitted, whereas other ions with different mobility properties collide with the electrodes are lost. 
   Referring now to  FIG. 11 , shown is a simplified view of a cylindrical segmented FAIMS  1100 . The segmented inner electrode  199  is composed of a series of segments  1111 ,  1112 ,  1113  as well as further segments not enumerated, and the outer segmented electrode  198  is similarly subdivided into segments  1101 ,  1102 ,  1103  and further segments not enumerated. The inner segmented electrode  199  and the outer segmented electrode  198  are spaced apart by not-shown insulating support members. The segments comprising the segmented inner electrode  199  are electrically isolated from each other to permit application of independent voltages to each segment. Preferably the segments are close together, so it is expected that high voltage differences between the adjacent segments may cause electrical discharges between the segments. Preferably therefore, voltage differences between adjacent segments are low enough to avoid discharge. 
   Still referring to  FIG. 11 , the segments comprising the segmented inner electrode  199  and the segmented outer electrode  198  are spaced apart from each other by not-shown insulators. Preferably, the segments are closely spaced and the insulators separating the segments are not ‘visible’ to the ion flow. The collision of an ion with an insulating material produces an electric charge on the insulating material, because by definition the insulator cannot carry away the electricity. The electric charge is not controlled, and produces unpredictable electrostatic fields around the charged insulating surface. This means that preferably the not-shown insulator between segments  1111  and  1112  (and other similar pairs) is recessed below the outer surfaces of the segments  1111 ,  1112 ,  1113  and other segments that comprise the annular analyzer region  197 . It is preferable that the ions  1151 ,  1152  and other ions that are flowing along the annular analyzer region  197  avoid collision with the not-shown insulation material that separates segments  1111  and  1112 , and other similar pairs of segments, from each other. In this example the not-shown insulating material separating each pair of segments comprising both segmented inner electrode  199  and outer segmented electrode  198  is sufficiently below the surfaces of the segments that face into the analyzer region  197 , that the electrostatic charge build up that might occur on the surfaces of the insulating material because of collisions with ions has minimum effect on the overall electric fields in the analyzer region  197 . 
   Still referring to  FIG. 11 , a flow of gas  1150 , shown as solid headed arrows flows in the annular analyzer region  197  between the segmented inner and outer electrodes  199  and  198 , respectively. A not-shown ion source provides ions to the annular analyzer region  197 , where the ions are caused to move by electric fields generated by application of voltages to the segments comprising the inner and outer segmented electrodes. In the example shown in  FIG. 11  the ions  1151 ,  1152  and other ions not enumerated are transported by electric fields in a direction contrary to the flow of gas  1150 . The voltages applied to consecutive segments is selected in this example to produce an electric field gradient that causes ions  1151 ,  1152  and other ions to be moved in the direction shown by the open headed arrows, while the gas  1150  flows in a direction shown by the closed headed arrows. Voltages are applied to the segments by electric power supplies  1120 ,  1121  and  1122 . Connections to every segment of the inner segmented electrode  199  and outer segmented electrode  198  are not shown. The bundle of connections  1130  provides voltages from power supply  1121  to the segments  1111 ,  1112 ,  1113  and the other segments of the inner segmented electrode  199 . In this example the voltage applied consists of a radio-frequency (rf) ac component added to a de voltage, where the rf component is equal in every segment, but the dc voltage may differ amongst the segments of the inner segmented electrode  199 . Similarly a bundle of connectors  1141 ,  1142 ,  1143  and others not shown, provide voltages from outer bias power supply  1122  to the segments  1101 ,  1102 ,  1103  and other segments of the outer segmented electrode  198 . In this example, the voltages applied to the outer segmented electrode  198  differ amongst the segments, and in this case rf voltage is not applied to any parts of the outer segmented electrode  198 . 
   Still referring to  FIG. 11 , the rf voltage applied to the inner segmented electrode  199  is an asymmetric waveform produced by waveform generator voltage supply  1120  and delivered to power supply  1121  through connector  1153 . The power supply  1121  provides a dc voltage offset, superimposed on the asymmetric waveform, to each segment of the segmented inner electrode  199 , routed to each segment by an independent conductor comprising the bundle of connections  1130 . 
   Still referring to  FIG. 11 , in use the series of segments are used to propel the ions along the length of the device, in a way that optionally is independent of the flow of gas, for example. Many optional arrangements of waveforms can be applied to the series of segments to capture the ions among certain segments, or to form a series of traveling waves. Advantageously, this device optionally is operated using a gradient in the composition of the gas in the analyzer region  197 . The gradient in gas composition is optionally formed in a manner analogous to that shown in  FIG. 10 , each gas delivered to a region surrounding the circumference of the inner electrode  199 . Optionally the gas is delivered to a region that is constrained by a gas diffuser that allows the gas to equilibrate at constant pressure at all circumferential locations, and therefore to flow out of the diffuser at constant flow rates at every circumferential location. The gas is then passed amongst an array of plates, as a non-limiting example, to further smooth the flow and to direct the gas flow to be parallel to the electrodes. In  FIG. 11  the gas optionally flows in either direction along the analyzer, since the ions are propelled by the longitudinal fields generated by the segments of the electrodes. 
   Still referring to  FIG. 11 , the cloud of ions is constrained within certain radial locations by the gradient in gas composition, but simultaneously forced to move along the length of the device by control of the dc voltage offsets applied to the individual segment pairs, for example the pair of segments  1101  and  1111 , the pair of segments  1102  and  1112 , and so on throughout the device. In a non-limiting example the dc level of segments  1101  and  1111  is 10 volts, and the dc level of segments  1102  and  1112  is 9 volts, and the dc level of segments  1103  and  1113  is 8 volts, and so on along the electrodes. For example a sinusoidal voltage is applied to the inner electrodes to produce a 10 volt p-p superimposed on the dc level of each inner electrode. Continuing this example, the dc level of inner electrode  1111  is 10 volts, plus a sinusoidal wave that carries the voltage 5 volts more positive (up to +15 V) and 5 volts more negative (down to +5 V) than the dc value of 10 volts. Similarly, the dc level of inner electrode  1112  is 9 volts, now with an added a sinusoidal wave that carries the voltage 5 volts more positive (to +14 V) and 5 volts more negative (i.e. to +4 V) than the de value of 9 volts. Under these dc levels amongst the segments, a positive ion is caused to drift from right to left in  FIG. 11 . In this non-limiting example the series of segments are arranged to produce a uniform longitudinal drift along the annular tube. If a pulse of ions is introduced at the not illustrated inlet, the ions are separated in the manner of conventional drift tube ion mobility spectrometry, namely the highest mobility ions traversing the device more quickly than the lowest mobility ions. This device, because of the added benefit of the gradient in gas composition that helps to promote ion focusing, is characterized by very good ion transmission efficiency. This transmission efficiency beneficially increases ion focusing above that inherent in cylindrical geometry FAIMS, since FAIMS in cylindrical geometry also focuses the ions within limited radial locations in the annular region between the inner electrode  199  and the outer electrode  198 . 
     FIG. 12  is a cylindrical geometry FAIMS  200 , with a segmented inner electrode  224  including segments  224   a  to  224   h  and outer electrode  208  including segments  208   a  to  208   h . Short segments  224   b  to  224   g  are spaced apart in a radial direction from similar length segments  208   b  to  208   g , respectively. Ions are produced by ionizer  202 , which optionally is one of an electrospray ionization source, a corona discharge ionization source, and an atmospheric pressure chemical ionization source as some non-limiting examples. The ionizer  202  is mounted in an insulating member  204  that also serves to support a short inner cylinder  206  and a long outer cylinder  208   a . Flows of two types of carrier gases of differing composition pass through a pair of passageways  210   a  and  210   b  shown by dashed lines in insulating member  204 . A flow of sampler gas flows through passageway  212  shown by dashed lines in insulating member  204 . The carrier gases enter pressure equalization chambers  214   a  and  214   b , and the sampler gas enters a separate equalization chamber  216 . Diffusers  218  and  220  serve to restrict the carrier and sampler gases, respectively, and to allow these gases to flow uniformly around the circumference of the electrodes. The two types of carrier gas pass separately through the diffuser  218 , and combine after the diffuser to flow in a smooth laminar flow along the annular space between the short inner cylinder  206  and the long outer cylinder  208   a . Optionally, the two types of carrier gas is passed amongst an array of plates similar to the array of plates  904  described supra with reference to  FIG. 10 , as a non-limiting example, after the diffuser  218  to further smooth the flow and to direct the gas flow to be parallel to the electrodes. Similarly the sampler gas passes through the diffuser  220 , and flows in a smooth laminar flow along the annular space between the ionizer  202  and the short inner cylinder  206 . The sampler gas flows through the inner passage  222  within the inner electrode  224 . 
   Still referring to  FIG. 12 , the ions produced by ionization source  202  are accelerated away from the source  202  in an outwardly radial direction by a voltage difference between the ionization source  202  and the short inner cylinder  206 . Some ions pass through a gap  226  between the short inner cylinder  206  and the first segment of the inner cylinder  224   a . Those ions that pass through the gap  226  may be entrained by the carrier gas and carried along the analyzer region  228 , which is the annular space between the segmented inner cylinder  224  and the long segmented outer cylinder  208 . The ions for which the gradient of gas composition, temperature, pressure, the applied waveform voltage and the compensation voltage are appropriate, pass along the analyzer region  228 , and are carried by the carrier gas out of the FAIMS  200  through ion outlet  230 . Optionally, the ions are analyzed further by mass spectrometry, or by other types of ion mobility spectrometers, further FAIMS devices etc., or are detected using ion detection technologies including amperometric or photometric as some non-limiting examples. 
   Still referring to  FIG. 12  an asymmetric waveform and compensation voltage may be applied to the inner electrode  224 . Bias voltages are applied to the short inner electrode  206  and the long outer electrode  208 . The segments that comprise the inner electrode  224  and the long outer electrode  208  are at the same potential, or optionally are at potentials that permit measurement of the low-field mobility of the ions that are successfully transmitted at the asymmetric waveform voltage and the compensation voltage under the ambient conditions of gas composition (and gradient), gas pressure, and gas temperature. 
   Still referring to  FIG. 12 , it is preferable that a portion of the carrier gas that flows into the passageway  210  and through diffuser  218  enters the inner passage  222  within the inner electrode  224  by flowing radially inward through the gap  226 . This inward flow of carrier gas helps to desolvate ions from ionization source  202  that are flowing outward through gap  226 . This countercurrent of flowing gas helps to desolvate the ions and also prevents neutrals coming from the ionization source from entering the analyzer region  228 . The neutrals produced from the sample, but not ionized by the ionizer  202 , flow with the sampler gas along the inner passage  222  within the inner electrode  224  and out of sample outlet port  232 . Preferably a not illustrated gas pump assists in pulling the sampler gas out of port  232 , and assists in pulling a desolvating portion of carrier gas inward radially through the gap  226 . 
   Still referring to  FIG. 12 , the number of segments of the inner electrode  224  and of the outer electrode  208  may be larger or fewer than shown in this figure. Further discussions assume that the electrodes are divided into a large number of segments. The cylindrical arrangement of the inner and outer coaxially arranged electrodes shown in  FIGS. 11 and 12  give rise to an ion focusing in the annular analyzer region between the inner and outer electrodes, for an ion transmitted at the selected asymmetric waveform (DV) and the selected compensation voltage (CV), and for the particular gradients of gas composition and temperature that may be employed. This focusing helps to prevent ions from colliding with the inner and outer electrodes. The application of differing bias voltages on the segments of the segmented FAIMS shown in  FIGS. 11 and 12  makes it possible to transport these ions along the length of the device. Ions are therefore selected on the basis of their high-field mobility behavior (to pass FAIMS at the selected DV and CV) as well as by their transport time through the device as selected by appropriate voltages and arrangements of voltages applied to the segments of the inner and outer electrodes. 
   Referring now to  FIG. 13 , shown is a simplified flow diagram of a method of separating ions according to an embodiment of the instant invention. At step  1300  a high field asymmetric waveform ion mobility spectrometry (FAIMS) analyzer region is provided for separating ions. At step  1302  a flow of a carrier gas is provided within a portion of the FAIMS analyzer region. The flow of carrier gas has a composition that is non-uniform in space along a direction transverse to the flow of the carrier gas. At step  1304  ions are introduced into the FAIMS analyzer region. At step  1306  electric field conditions are provided within the FAIMS analyzer region for selectively transmitting a subset of the ions through the FAIMS analyzer region. At step  1308  the subset of ions is selectively transmitting along an average ion flow path through the FAIMS analyzer region. 
   Referring now to  FIG. 14 , shown is a simplified flow diagram of another method of separating ions according to an embodiment of the instant invention. At step  1400  a high field asymmetric waveform ion mobility spectrometry (FAIMS) analyzer region is provided for separating ions. In particular, the FAIMS analyzer region comprising an ion origin end that is in fluid communication with an ionization source, and an ion exit end that is in fluid communication with an ion detecting device, a length of the FAIMS analyzer region defined along a direction between the ion origin end and the ion detection end. At step  1402  a flow of a first gas is provided into a gas inlet region of the FAIMS analyzer region. At step  1404  a flow of a second gas is provided separately into the gas inlet region of the FAIMS analyzer region. In particular, the flow of the second gas is provided absent forming a homogeneous carrier gas flow including the first gas and the second gas within the gas inlet region. 
   Numerous other embodiments may be envisaged without departing from the spirit and scope of the invention.