Patent Publication Number: US-7714284-B2

Title: Methods and apparatus for enhanced sample identification based on combined analytical techniques

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/738,967, filed on Dec. 13, 2003, which is a continuation-in-part of U.S. patent application Ser. No. 10/187,464, filed on Jun. 28, 2002, which is a continuation-in-part of U.S. patent application Ser. No. 09/896,536, filed on Jun. 30, 2001 and which claims the benefit of and priority to U.S. Provisional Application No. 60/340,894, filed on Oct. 30, 2001, U.S. Provisional Application No. 60/334,685, filed on Nov. 15, 2001. U.S. Provisional Application No. 60/340,904, filed on Dec. 12, 2001, U.S. Provisional Application No. 60/342,588, filed on Dec. 20, 2001, U.S. Provisional Application No. 60/351,043, filed on Jan. 23, 2002. This application claims the benefit of and priority to U.S. Provisional Application No. 60/536,182, filed on Jan. 13, 2004, entitled “DMS-IMS Chemical Identification System.”The entire teachings of the above referenced applications are incorporated herein by reference. 
     This application also incorporates by reference the entire contents of the following co-pending U.S. patent applications: U.S. Ser. No. 10/215,251, filed on 7 Aug. 2002; U.S. Ser. No. 10/462,206, filed on 13 Jun. 2003; U.S. Ser. No. 10/684,332, filed on 10 Oct. 2003; U.S. Ser. No. 10/734,499, filed on 12 Dec. 2003; U.S. Ser. No. 10/797,466, filed on 10 Mar. 2004; U.S. Ser. No. 10/821,812, filed on 8 Apr. 2004; U.S. Ser. No. 10/824,674, filed on 14 Apr. 2004; U.S. Ser. No. 10/836,432, filed on 30 Apr. 2004; U.S. Ser. No. 10/840,829, filed on 7 May 2004; U.S. Ser. No. 10/866,645, filed on 10 Jun. 2004; U.S. Ser. No. 10/887,016, filed on 8 Jul. 2004; U.S. Ser. No. 10/894,861, filed on 19 Jul. 2004; U.S. Ser. No. 10/903,497, filed on 30 Jul 2004; U.S. Ser. No. 10/916,249, filed on 10 Aug. 2004; U.S. Ser. No. 10/932,986, filed on 2 Sep. 2004; U.S. Ser. No. 10/943,523, filed on 17 Sep. 2004; U.S. Ser. No. 10/981,001, filed on 4 Nov. 2004; U.S. Ser. No. 10/998,344, filed 24 Nov. 2004; and U.S. Ser. No. 11/015,413, filed on Dec. 17, 2004. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to mobility-based systems, methods and devices for analyzing samples. More particularly, in various embodiments, the invention relates to improving the detection capability of ion mobility based systems using DMS in combination with other detection techniques, such as IMS detection techniques, to analyze the constituents of a sample. 
     BACKGROUND 
     There are a number of different circumstances in which it is desirable to perform analysis to identify compounds in a sample. Such samples may be taken directly from the environment or they may be provided by front end specialized devices to separate or prepare compounds before analysis. There exists, a demand for low cost, compact, low-power, accurate, easy to use, and reliable devices capable of detecting compounds in a sample. 
     One class of known analyzers are mass spectrometers (MS). Mass spectrometers are generally recognized as being the most accurate type of analyzers for compound identification. However, mass spectrometers are quite expensive, easily exceeding a cost of $100,000 or more and are physically large enough to become difficult to deploy everywhere the public might be exposed to dangerous chemicals. Mass spectrometers also suffer from other shortcomings such as the need to operate at relatively low pressures, resulting in complex support systems. They also need a highly trained operator to tend to and interpret the results. Accordingly, mass spectrometers are generally difficult to use outside of laboratories. 
     A class of chemical analysis instruments more suitable for field operation is known as Field Asymmetric Ion Mobility Spectrometers (FAIMS) or Differential Mobility Spectrometers (DMS), and also known as Radio Frequency Ion Mobility Spectrometers (RFIMS) among other names. Hereinafter, FAIMS, DMS, and RFIMS, are referred to collectively as DMS. This type of spectrometer subjects an ionized fluid (e.g., gas, liquid or vaper) sample to a varying high-low asymmetric electric field and filters ions based on their field mobility. 
     The sample flows through a filter field which allows selected ion species to pass through, according to a compensation voltage (Vcomp) applied to filter electrodes, and specifically those ions that exhibit particular mobility responses to the filter field. An ion detector then collects ion intensity/abundancy data for the detected ions. The intensity data exhibits attributes, such as “peaks” at particular compensation voltages. 
     A typical DMS device includes a pair of electrodes in a drift tube. An asymmetric RF field is applied to the electrodes across the ion flow path. The asymmetric RF field, as shown in  FIG. 1 , alternates between a high or “peak” field strength and a low field strength. The field varies over a particular time period (T), frequency (f) and duty cycle (d). The field strength E varies with an applied field voltage (Vrf) and the size of the gap between the electrodes. Ions pass through the gap between the electrodes when their net transverse displacement per period of the asymmetric field is zero. In contrast, ions that undergo a net displacement eventually undergo collisional neutralization on one of the electrodes. In a given RF field, a displaced ion can be restored to the center of the gap (i.e. compensated, with no net displacement for that ion) by superimposing a low strength direct current (dc) electric field (e.g., by applying Vcomp across the filter electrodes) on the RF. Ions with differing displacement (owing to characteristic dependence of mobility in the particular field) pass through the gap at differing characteristic compensation voltages. By applying a substantially constant Vcomp, the system can be made to function as a continuous ion filter. Alternatively, scanning Vcomp obtains a spectral measurement for a sample. A recorded image of the spectral scan of the sample is sometimes referred to as a “mobility scan” or as an “ionogram.” 
     Examples of mobility scans based on the output from a DMS device are shown in  FIGS. 2A and 2B . The compounds for which scans are depicted are acetone and an isomer of xylene (o-xylene). The scan of  FIG. 2A  resulted from a single compound, acetone, being independently applied to the DMS analyzer. The illustrated plot is typical of the observed response of the DMS device, with an intensity of detected ions dependent on Vcomp. For example, the acetone sample exhibits a peak intensity response at a Vcomp of approximately −2 Vdc. 
       FIG. 2B  illustrates the results when analyzing a mixture of acetone and o-xylene. The combined response shows two peaks in approximately the same region as for the independent case. The compounds in the mixture can be detected by comparing the response against a library, for example, of stored known responses for independently analyzed compounds, or libraries of mixtures. Thus, the scans for independently analyzed compounds, such as the scan of  FIG. 2A  for acetone, can be stored in a computer system, and when compound responses such as that in  FIG. 2B  are observed, the relative locations of the peaks can be compared against the stored responses in the library to determine the constitution of the compound. 
     A specific RF field voltage and field compensation voltage Vcomp permits only ion species having a particular ion mobility characteristic to pass through the filter to the detector. By noting the RF level and compensation voltage and the corresponding detected signal, various ion species can be identified, as well as their relative concentrations (as seen in the peak characteristics). 
     Consider a plot of ion mobility dependence on Vrf, as shown in  FIG. 3 . This figure shows ion intensity/abundancy versus RF field strength for three examples of ions, with field dependent mobility (expressed as the coefficient of high field mobility, α) shown for species at greater, equal to and less than zero. The velocity of an ion can be measured in an electric field (E) low enough so that velocity (v) is proportional to the electrical field as v=KE, through a coefficient (K) called the coefficient of mobility. K can be shown to be related to the ion species and gas molecular interaction properties. This coefficient of mobility is considered to be a unique parameter that enables the identification of different ion species and is determined by, ion properties such as charge, size, and mass as well as the collision frequency and energy obtained by ions between collisions. 
     When the ratio of E/N, where N is gas density, is small, K is constant in value, but at increasing E/N values, the coefficient of mobility begins to vary. The effect of the electric field can be expressed approximately as K(E)=K( 0 )[1+α(E)], where K( 0 ) is a low voltage coefficient of mobility, and α is a specific parameter showing the electric field dependence of mobility for a specific ion. 
     Thus, as shown in  FIG. 3 , at relatively low electric field strengths, for example, of less than approximately 10,000 V/cm, multiple ions may have the same mobility. However, as the electric field strengths increase, the different species diverge in their response such that their mobility varies as a function of the applied electric field. This shows that ion mobility is independent of applied RF field voltage at relatively low RF field strengths, but is field-dependent at higher RF field strengths. 
       FIGS. 2A and 2B  demonstrate that species can have a unique behavior in high fields according to mobility characteristics. The ions passing through the filter are detected downstream. The detection signal intensity can be plotted as a characteristic detection peak for a given RF field voltage and field compensation voltage Vcomp. Peak intensity, location, and shape are typically used for species identification. 
     However, a problem occurs in that the peaks, as seen in the typical DMS spectra, are generally broad in width. Therefore, compounds exhibiting intensity peaks at similar compensation voltages may be difficult to separate from each another. Consequently, there may be particular conditions under which two different chemicals generate indistinguishable scans for a particular Vcomp and a particular RF field voltage, or for other combinations of filter field/flow channel parameters. In such a case, it is may not be possible to differentiate between the two different compounds. Another problem may occur when two or more chemical species have the same or almost the same ion mobility characteristic for a particular set of field/flow channel parameters. This is most likely to happen in the low electric field regime (referred to herein as Ion Mobility Spectrometry or IMS), where many existing ion mobility spectrometer systems operate. Therefore, if two or more chemical species have the same or almost the same mobility characteristic, then their spectroscopic peaks will overlap, and identification and quantification of individual species will be difficult or impossible. 
       FIG. 4  is a graph of Vcomp versus Vrf according to an illustrative embodiment of the invention, but also highlighting the above described prior art drawback. More particularly,  FIG. 4  depicts a graph of Vcomp versus Vrf for four compounds: lutidine; cyclohexane; benzene; and dimethyl-methl-phosphonate (DMMP). Each curve shows the location of detected ion intensity peaks, such as those circled at  100 , at the various (Vrf, Vcomp) locations, which in total provide the peak characteristics for each particular compound. As shown, there is a region  100  in which the intensity peaks and mobility curves for DMMP and cyclohexane overlap with each other. As can be seen, operating in a Vrf region of from approximately 2,500 Vpeak to approximately 2,650 Vpeak, at a Vcomp of about −6 Vdc to about −8 Vdc, one would find it virtually impossible to discriminate between the two compounds based on a single Vcomp scan at a single Vrf. Specifically, in a conventional spectral scan approach that plots intensity/abundance versus Vcomp over a range of Vcomp for a single Vrf plots the overlapping peaks as a single peak. 
     Another drawback of conventional mobility based ion detection systems is that they are susceptible to competitive ionization, such as atmospheric pressure competitive ionization (APCI). APCI occurs when one compound is preferentially ionized over another compound. If a desired compound is not ionized into an ion species, a mobility-based detector will not identify or detect the presence of that compound. Systems have been developed that remove compounds from a sample that preferentially ionize to enable a desired compound to then be ionized and detected. For example, a gas chromatograph (GC) has been employed as a front end for a DMS to pre-separate a sample into its constituent compounds before detection. However, GCs are generally slow, and add complexity and expense to mobility-based detection systems. Also, conventional mobility based ion detection systems are not sensitive enough to detect very small amounts of chemical or biological agents which may pose a health risk to humans. 
     A further drawback of mobility based ion detection systems is that these systems often employ one type of ion mobility detection technique. While one ion mobility detection technique may provide adequate identification for certain types of ion species and/or sample constituent, other ion mobility detection techniques may be better suited for the identification of other types of ion species and/or sample constituents. 
     Accordingly, there is a need for improved ion mobility based compound identification using a combination of detection techniques such as DMS in combination with IMS detection. 
     SUMMARY OF THE INVENTION 
     The invention addresses the deficiencies of the prior art by providing, in various embodiments, improved mobility based systems, devices and methods for analyzing constituents in a sample. More particularly, in various embodiments, the invention provides for improved sample analysis by employing multiple detection techniques, such as combined IMS and DMS techniques. 
     Sample analysis may be enhanced by combining DMS techniques with sample detection using another type of device such as IMS, TOF IMS, MS, electrochemical detector, or the like. In one illustrative embodiment of the invention, DMS detection is combined with IMS detection to enhance sample identification. 
     IMS technology uses the coefficient of mobility (K) to identify chemical constituents of a sample by measuring the different values of mobility associated with different sample constituent ion species. The coefficient of mobility K may be expressed as: K(E)=K( 0 )[1−α(E)]. 
     Because a conventional TOF IMS operates at low field conditions, a TOF IMS may be employed to plot and determine the K( 0 ) of a particular ion species. Because a DMS alternately operates at high and low field conditions, a DMS may be employed to plot and determine the alpha parameter α(E) of a particular ion species. Thus, by using a DMS in combination with a TOF IMS, the coefficient of mobility K(E) for a particular ion species may be plotted over a range of electric field strengths and, thereby, provide enhanced ion species identification based on the derived coefficient of mobility over a range of field strengths. 
     Also, by detecting a select ion species using multiple detection techniques, improved analysis may be achieved where one detection technique, e.g., DMS, provides better ion species differentiation and identification than another detection technique, e.g., TOF IMS, and visa versa. 
     In one embodiment of the invention, a system for identifying a constituent in a sample includes a first analyzer for measuring an differential field mobility characteristic as a function of a varying RF electric field strength for the sample to determine an ion mobility signature for the sample. The system also includes a second analyzer measuring low field ion mobility coefficient for the sample and a processor for determining a total coefficient of mobility for the sample based at least in part on the ion mobility signature and the low field mobility coefficient of the sample, and for identifying the constituent based at least in part on the total coefficient of mobility for the sample. The first analyzer may include a DMS while the second analyzer may include an IMS. 
     In another embodiment, the second analyzer employs a modulated electric field voltage for measuring the low field ion mobility coefficient for the sample. 
     In certain embodiments, the first analyzer includes detectors for determining the ion mobility signature for the sample for both negative and positive mode ions while the second analyzer includes one or more collectors for measuring the low field ion mobility coefficient for the sample for both the negative and positive mode ions. The system also includes a processor that determines the total coefficient of mobility for both the positive and negative mode ions. 
     In a further embodiment of the invention, a system for identifying a constituent in a sample includes a DMS analyzer for measuring a first ion mobility characteristic for the sample and a first IMS analyzer for measuring a second ion mobility characteristic for the sample. The first and second ion mobility characteristics may be either or both positive and negative mode characteristics. The system also includes a processor for identifying the constituent based at least in part on at least one of the first and second ion mobility characteristics. 
     In one embodiment, the processor identifies the constituent based at least in part on a combination of both the first and second ion mobility characteristics. In another embodiment, the processor selects, based at least in part on a mass of the sample, either the first or the second ion mobility characteristic for use in identifying the constituent. In certain embodiments, the DMS includes a detector that operates as a shutter for gating ions into the first IMS analyzer. In other embodiments, the system includes an outlet for exhausting neutral molecules from the DMS analyzer without introducing the neutral molecules into the first IMS analyzer. 
     In another embodiment, the system includes a second IMS analyzer for measuring a third ion mobility characteristic. In this case, the processor identifies the constituent based at least in part on the first, second and third ion mobility characteristics. The second ion mobility characteristic may be a positive mode characteristic while the third ion mobility characteristic may be a negative mode characteristic. 
     In certain embodiments, the first and second analyzers measure the first and second ion mobility characteristics concurrently. 
     In a further embodiment, a system for identifying a constituent in a sample includes an analyzer operable in a first mode for measuring an differential field mobility characteristic as a function of a varying RF electric field strength for the sample to determine an ion mobility signature for the sample, and operable in a second mode for measuring low field ion mobility coefficient for the sample. The system also includes a processor for determining a total coefficient of mobility for the sample based at least in part on the ion mobility signature and the low field mobility coefficient of the sample, and for identifying the constituent based at least in part on the total coefficient of mobility for the sample. The first mode may be a DMS mode and the second mode may be an IMS mode. 
     As discussed above, atmospheric pressure competitive ionization (APCI) may cause compounds with the highest proton affinity (PA) and/or highest electron affinity (EA) to capture preferentially or take up the charge from an ionization source. If there is a limited amount of charge available, for example, in a compact DMS system with limited power resources, the amount of available charge may not be sufficient to charge or ionize all of the molecules in a sample matrix. Thus, if only some of the molecules in a sample matrix are ionized, only that limited amount of molecules may be detected, resulting in erroneous analysis of a chemical matrix. Furthermore, certain compounds may not be ionized due to APCI, resulting in no detection of these compounds. 
     According to one aspect, the invention pre-separates certain ion species of a sample to reduce, and in some cases, eliminate the problem of competitive ionization within ion based mobility detection analyzers. The invention includes embodiments that eliminate or mitigate the effects of competitive ionization by separating ion species before sample detection to prevent one ion species from consuming the charge intended to be used to ionize another ion species. 
     According to one embodiment, neutrals, i.e., molecules of a sample that are not ionized, are mixed with a new supply of charge, e.g., reactant ions or a plasma field, to enable further APCI reactions to occur. The newly created ions may then be removed for analysis or simply discarded. This process may be repeated until a desired compound type is ionized and detected using an analyzer. 
     In one implementation, a sample matrix is exposed to an ionization source to cause particular compounds in the sample to be ionized, the ionized compounds to be removed, and the residual neutrals to be re-circulated. The ionization source may be, for example, an UV source, laser, plasma source, soft X-ray source, or reactant ions. Repeated interrogation of chemical compounds in a sample based on competitive ionization and the reaction of residual and/or un-reacted neutrals provides a comprehensive measure of the chemical composition of the sample, without the need for traditional GC techniques. 
     The process of competitive ionization and the removal of product ions may be repeated, enabling incremental isolation of product ions and neutrals. Additionally, chemical ionization may be employed to inject fresh charge using reactant ions. 
     According to one aspect, the invention ionizes sample molecules to cause a subset of the sample molecules to combine to form first product ions. It then separates the first product ions from a first un-ionized group of sample molecules. Next, it ionizes a subset of the first un-ionized group of sample molecules to form second product ions, and separates the second product ions from a second un-ionized group of sample molecules. 
     In one embodiment, the invention flows the first un-ionized group of sample molecules and the first product ions through a first field to separate the first product ions from the first un-ionized group of sample molecules. According to one implementation of this embodiment, the inventions flows the second un-ionized group of sample molecules and the second product ions through a second field to separate the second product ions from the second un-ionized group of sample molecules. In some implementations, the first and second fields are the same field. However, in other implementations, the first and second fields are different fields. 
     In an alternative embodiment, the invention employs a mechanical separation for separating the first product ions from the first un-ionized group of sample molecules. According to another alternative embodiment, the invention employs a chemical process for separating the first product ions from the first un-ionized group of sample molecules. 
     According to another embodiment, the invention, subsequent to extracting the second product ions, ionizes a subset of the second un-ionized group of sample molecules to form third product ions, and separates the third product ions from a third un-ionized group of sample molecules. 
     The invention employs various approaches for ionizing the sample molecules. In some instances, the invention mixes the first reactant ions with the sample molecules to form the first product ions. The invention may also mix the second reactant ions with the first un-ionized group of sample molecules to form the second product ions. According to one feature, the invention controls an effluent flow to control contact time between the first reactant ions and the sample molecules. According to another feature, the invention injects the first reactant ions into a flow of the sample molecules to mix the sample molecules to with first reactant ions. 
     According to one approach, the invention exposes the sample molecules to a first ionization source to form the first product ions, and re-circulates the first un-ionized group of sample molecules to expose them to the first ion source to form the second product ions. According to an alternative approach, the invention exposes the sample molecules to a first ion source to form the first product ions, and flows the first un-ionized group of sample molecules to expose them to a second ion source to form the second product ions. 
     According to one embodiment, the invention flows the sample molecules along a first flow path past a first ionization source to form the first product ions and then directs the first product ions along a second flow path to separate the first product ions from the first un-ionized group of sample molecules. The invention may further flow the first un-ionized group of sample molecules past a second ionization source in the first flow path to form the second product ions and then direct the second product ions into the second flow channel to separate the second product ions from a second un-ionized group of sample molecules. The invention may further flow the second un-ionized group of sample molecules past a third ionization source in the first flow channel to form the third product ions and then direct the third product ions into the second flow channel to separate the third product ions from a third un-ionized group of sample molecules. 
     The invention employs various approaches to directing product ions. In some instances, the directing includes attracting the first product ions into the second flow channel. In other instances, the directing includes deflecting the first product ions into the second flow channel. The directing may also include directing the first product ions into the second flow channel via an opening in a barrier between the first and second flow channels. In certain instances, the first flow path includes a substantially cylindrical portion while the second flow channel is substantially enclosed. Alternatively, the second flow path may be substantially unenclosed. 
     In certain embodiments, the invention mixes the sample molecules with one or more dopants to improve separation of the first product ions from the first un-ionized group of sample molecules. The dopants may include any one or combination of methylene bromide (CH 2 Br 2 ), methylene chloride (CH 2 Cl 2 ), chloroform (CHCl 3 ), water (H 2 O), methanol (CH 3 OH), and isopropanol. 
     According to another aspect, the invention ionizes sample molecules to cause a subset of the sample molecules to combine to form first product ions and separates the first product ions from a first un-ionized group of sample molecules. Subsequent to separating the first product ions, the invention ionizes a subset of the first un-ionized group of sample molecules to form second product ions and separates the second product ions from a second un-ionized group of sample molecules. Then, the invention analyzes the sample based at least in part on the first and second product ions. 
     In one embodiment, the invention flows the first and second product ions to the first analyzer and processes the information from the first analyzer about the first and second product ions to perform an analysis of the sample. In an alternative embodiment, the invention flows the first product ions to a first analyzer, flows the second product ions to a second analyzer, and processes the information from the first and second analyzers about the first and second product ions to perform an analysis of the sample. The first and second analyzers may be in series or parallel with each other. In certain instances, the invention analyzes the sample based at least in part on at least one of the first and second groups of un-ionized sample molecules. 
     In another embodiment, the invention directs the first product ions from a first flow channel into an analyzer flow channel and causes a flow from the analyzer flow channel toward a first flow channel containing the first product ions and the first group of un-ionized sample molecules. The flow is directed from the analyzer to inhibit the first un-ionized groups of sample molecules from entering the analyzer flow channel. 
     In another embodiment, a system for pre-separating a sample includes a first ionizer for ionizing sample molecules to cause a subset of the sample molecules to combine to form first product ions and a first separator for separating the first product ions from a first un-ionized group of sample molecules. The system also includes a second ionizer for ionizing a subset of the first un-ionized group of sample molecules to form second product ions and a second separator for separating the second product ions from a second un-ionized group of sample molecules. The first and second ionizers may be the same ionizer or different ionizers. Also, the first and second separators may be the same separator or different separators. 
     In another embodiment, a compact DMS system includes a sample pre-separation unit for pre-separating product ions from un-ionized sample molecules, a filter unit for passing particular ones of the product ions, and a detection unit for detecting the particular ones of the product ions passed by the filter unit. 
     In addition to being used for analysis, the invention may be used for selectively cleaning and/or conditioning samples, e.g., for removing selected molecules from a sample stream. For example, certain semiconductor industry or other process control applications require ultra pure or clean gasses. In these processes, water molecules are considered a contaminant in a gas stream of Nitrogen or Argon. In certain embodiments of the invention, water within a gas sample may be preferentially ionized and then removed from the gas stream while purified Argon or Nitrogen are then used in a low pressure chemical vapor deposition or for another semiconductor application. 
     While current mobility based analyzers such as DMS, IMS, and MS systems are sensitive, there is a need to detect concentrations in ranges lower than parts-per-trillion (ppt). For instance, a very small number of anthrax spores may cause significant health effects. However, existing analyzers may not be sensitive enough to detect the charge generated by such a small number of spores. One technique for overcoming this limitation involves concentrating and/or amplifying the number of molecules of a sample, in time, to enable an analyzer to produce a larger signal for detection. 
     In embodiment, the invention ionizes the molecules of a sample and then filters the ionized sample to pass particular ion species of a sample constituent to a detector. The invention mixes the constituent from the detector with additional molecules of the sample and then ionizes the mixture of the constituent and the additional molecules of the sample. The invention then filters the ionized mixture to pass a concentration of the particular ion species of the constituent to the detector. The preceding steps of mixing, ionizing, and filtering may be repeated until a desired concentration of the particular ion species of the constituent is achieved and detected. 
     In other embodiments, the invention provides improved sample collection, filtration, detection, measurement, identification and/or analysis (collectively “analysis”) using, for example: dispersion characteristics; sample fragmentation; and/or sample processing variations, such as and without limitation, variations in flow channel/filter field conditions. Such conditions may include, any spectral changes, including, without limitation changes in: pressure; temperature; humidity; field strength, duty cycle, and/or frequency; field voltage amplitude, frequency and/or duty cycle; detector bias voltage magnitude and/or polarity; and/or filter field compensation voltage magnitude and/or polarity. 
     In one practice, the invention employs one or more of the above to provide a library of spectral signatures for a plurality of known species, and identifies unknown species by comparing at least a portion of a spectral signature for the unknown species to at least a portion of one or more of the spectral signatures stored in the library. The spectral signature is a compilation of spectral information for a particular species. The spectral information may include, without limitation, spectral peak amplitude; spectral peak width; spectral peak slope; spectral peak spacing; spectral peak quantity; relative shifts in spectral peaks due, for example, to changes in processing conditions; spectral discontinuities; Vrf versus Vcomp characteristics or any other characteristics of any of the above described conditions plotted against any one or more other above described conditions. 
     According to one aspect, the invention provides improved ion-based systems, methods and devices for analyzing samples by varying a first sample processing condition over a first plurality of values, and one or more second sample processing conditions over a second plurality of values to determine spectral information for a sample. In one particular embodiment, the invention scans a field compensation voltage Vcomp over a range of values for one or more Vrf values to generate a spectral representation at each of the one or more Vrf values. 
     According to one feature, the invention adjusts a third sample processing condition to narrow the widths of the resulting spectral peaks of the determined ion spectral information. Such width reduction reduces spectral peak overlap for samples having similar mobility characteristics, improves resolution of an ion mobility-based analyzer, and thus, provides more accurate discrimination between sample species. In one configuration, the third sample processing condition includes pressure in a sample flow channel, and the invention reduces the pressure in the sample flow channel to decrease the width of the spectral peaks. 
     According to another feature, the invention adjusts a third sample processing condition to change a location of the resulting spectral peaks of the determined ion spectral information, relative to a Vcomp at which they occur. Since peaks of differing species may shift differently, such shifts can provide improved discrimination between peaks of species having similar mobility characteristics. In one configuration, the third sample processing condition includes Vrf, and the invention applies more than two field voltages Vrf to provide peak shifting information for species identification. 
     According to another feature, the invention adjusts a third sample processing condition to provide spectral information regarding both positive and negative ions of the sample. More particularly, in one configuration, the invention provides both negative and a positive bias voltages to multiple detector electrodes concurrently or to a single detector electrode alternatively to provide both negative and positive mode scans. Since compounds that have similar ion mobility characteristics relative to one mode may have differing ion mobility characteristics relative to the other mode, adjusting the polarity of a bias voltage to detector electrodes can further improve sample analysis. 
     In a further embodiment, the invention employs various n-dimensional representations of ion spectral information, to enhance the quality of spectral signatures, improve differentiation between species having similar ion mobility characteristics, and thus, improve identification accuracy, specifically, and sample analysis, generally. By way of example, in one configuration, the invention scans Vcomp for &gt;2 field voltages Vrf, to capture additionally, for example, spectral peak shift information. The invention then generates an n-dimensional representation of the spectral information that aggregates the spectral information captured by scanning Vcomp at each Vrf. In one example, the n-dimensional representation is a two-dimensional plot of Vrf versus Vcomp aggregating the spectral information captured by scanning Vcomp at each of the &gt;Vrf field voltages. In a further example, the aggregated representation is a three-dimensional representation aggregating the spectral information captured from scanning Vcomp at the &gt;2 Vrf field voltages. 
     According to one approach, the three-dimensional representation is a plot of ion intensity as a function of Vrf and Vcomp. According to one implementation, Vcomp and Vrf are represented in special coordinates, such as x- and y-coordinates, and variations in ion intensity at the (Vcomp,Vrf) coordinates is represented in variations of any color-related feature, including without limitation, variations in gray scale, color saturation, or color at those coordinates. Such color-related representations provide easily recognized distinctions between species that were difficult or impossible to distinguish between, without the n-dimensional aggregation of the invention. 
     In a related implementation, a curve circumscribing the color-related differences may be generated and the color-related differences themselves may be discarded. In this way, the invention can provide a two-dimensional representation of the spectral peaks, for example, on a Vcomp versus Vrf grid, while still incorporating the spectral information captured by scanning Vcomp over a plurality of Vrf values. In another alternative implementation, Vcomp, Vrf, and ion intensity are mapped into a three-dimensional (x,y,z) spatial representation. 
     According to a related embodiment, any or all of the spectral information may be represented in n-dimensional space as a function of any or all of the processing variations to create &gt;3 dimensional spectral signatures for both known and unknown species. Conventional n-dimensional cluster matching techniques may then be employed for identifying the unknown species. 
     In any of the above described n-dimensional representations, any or all of the spectral information represented may be incorporated into the spectral signatures for known species and stored in the library of such signatures. Conventional pattern recognition techniques may be employed to correspond at least portions of the spectral signatures from unknown species with at least portions of the signatures from known samples stored in the library to identify the unknown species. In other implementations, both the library of signatures and the captured signatures from the unknown species are represented as mathematical descriptions, and any suitable approach for making comparisons between such mathematical descriptions may be employed to identify the unknown species. 
     According to another embodiment, the invention employs fragmentation to improve DMS analysis. Fragmentation includes breaking large molecules of samples into smaller molecules, molecule clusters, components, and/or base elements. The fragments may then be individually analyzed, in series and/or in parallel to generate more spectral information for the sample than would be otherwise available without fragmentation. Fragmentation may be achieved, for example and without limitation, by using any one or a combination of a chemical reaction, a high energy field strength, high Vrf, heating, laser light, colliding the sample molecules with other molecules, soft x-ray, electromagnetic waves, or the like. According to one feature, the invention incorporates any or all of the above described spectral information for the fragment spectral peaks into the spectral signature. According to a further feature, the invention incorporates the point (e.g. the temperature, pressure, field strength, Vrf, colliding molecule mass, colliding molecule velocity, laser intensity, laser frequency, x-ray intensity etc.) into the spectral signature. 
     According to other aspects, the invention provides various serial and parallel combinations of ion-based analyzers employing features, including those summarized above. In additional aspects, the invention provides various compact, handheld, lightweight and low power based analyzers, for example, for detecting chemical warfare agents (CWAs), Toxic Industrial Compounds (TICs), and/or Toxic Industrial Materials (TIMs). 
     The invention will now be described with reference to various illustrative embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, advantages, and illustrative embodiments of the invention will now be described with references to the following drawings in which like reference designations refer to the same parts throughout the different views. These drawings are not necessarily to scale, emphasis instead being placed upon illustrating principles of the invention. 
         FIG. 1  is a graph depicting an asymmetric field having a peak RF, time period, and duty cycle. 
         FIGS. 2A and 2B  are graphs showing ion abundance (intensity) versus applied field compensation voltage for acetone alone and for a combination of ortho-xylene and acetone, respectively, as detected in a field asymmetric ion mobility spectrometer. 
         FIG. 3  is a graph of ion mobility versus electric field strength for three different compounds in a differential mobility spectrometer (DMS). 
         FIG. 4  is a graph of Vrf versus Vcomp indicating intensity peak locations according to an illustrative embodiment of the invention and conceptualizing drawbacks of prior art approaches. 
         FIG. 5  is a conceptual diagram of a DMS according to an illustrative embodiment of the invention. 
         FIG. 6  is a graph of ion intensity versus field compensation voltage for positive mode spectra for a sample containing various amounts of ethyl mercaptan as measured in a DMS. 
         FIG. 7  is a graph of ion intensity versus compensation voltage for negative mode spectra of a sample containing various amounts of ethyl mercaptan. 
         FIG. 8  is a graph of ion intensity versus field compensation voltage illustrating negative mode separation between monomer and reactant ion peak (RIP) detections for sulfur hexafluoride (SF6). 
         FIG. 9  is a graph of ion intensity versus field compensation voltage illustrating the positive mode separation between monomer and reactant ion peak (RIP) detections for sulfur hexafluoride (SF6). 
         FIG. 10  is a graph of ion intensity versus field compensation voltage illustrating a DMS response at various RF voltage levels in the negative ion mode and also showing the RIP detected in absence of SF6. 
         FIG. 11  is a graph of ion intensity versus field compensation voltage illustrating a DMS response in the positive ion mode where the SF6 peak is not isolated from the RIP. 
         FIG. 12  is graph of ion intensity (abundance) versus field compensation voltage illustrating an ability to improve discrimination between detected ion species by observing ion spectral peak shifts corresponding to a change in field strength. 
         FIGS. 13A and 13B  are graphs of ion intensity (abundance) versus field compensation voltage illustrating an ability to improve discrimination between detected ion species by observing ion spectral peak shifts due to reducing field strength. 
         FIGS. 14A and 14B  are graphs of ion intensity at multiple field strengths versus field compensation voltage, showing the affect of changes in compensation voltage on specific spectra, and show the divergent behavior of monomer, cluster, and reactant ion peak (RIP) detections with changes in field strength and field compensation voltage. 
         FIG. 15A  is a three-dimensional color dispersion plot illustrating detection of methyl salicylate over a range of field voltages and field compensation voltages with varying ion intensity represented in varying color according to an illustrative embodiment of the invention. 
         FIG. 15B  is a two-dimensional graph of ion intensity versus field compensation voltage for methyl salicylate at a single field voltage. 
         FIG. 16A  is a three-dimensional color dispersion plot illustrating detection of DMMP over a range of field voltages and field compensation voltages with varying ion intensity represented in varying color according to an illustrative embodiment of the invention. 
         FIG. 16B  is a two-dimensional graph of ion intensity versus field compensation voltage for DMMP at a single field voltage. 
         FIG. 17  is a three-dimensional color dispersion plot illustrating detection of DIMP over a range of field voltages and field compensation voltage with varying ion intensity represented in varying color according to an illustrative embodiment of the invention. 
         FIG. 18  is a two-dimensional graph of ion intensity versus field compensation voltage for DIMP at a single field voltage. 
         FIG. 19  is a graph of ion intensity at a plurality of field voltages versus field compensation voltage illustrating the effects of changes in field conditions on location of individual detection peaks and the ability to separate the detection. 
         FIG. 20A  is a graph of ion intensity versus field compensation voltage illustrating the separation of detection peaks at different compensation voltages between light and heavy molecules according to an illustrative embodiment of the invention. 
         FIG. 20B  is a graph of ion intensity versus field compensation voltage showing the increase in number of peaks detected after sample fragmentation according to an illustrative embodiment of the invention. 
         FIG. 21  is a conceptual diagram of a DMS system using fragmentation operating in parallel with a DMS system not using fragmentation to improve sample analysis according to an illustrative embodiment of the invention. 
         FIG. 22  is a conceptual diagram of a DMS system not using fragmentation operating in series with a DMS system using fragmentation to improve sample analysis according to an illustrative embodiment of the invention. 
         FIG. 23A  is a graph of ion intensity versus field compensation voltage showing peak detection for the DMS system of  FIG. 22  not using fragmentation. 
         FIG. 23B  is a graph of ion intensity versus field compensation voltage showing peak detection for the DMS system of  FIG. 22  using fragmentation. 
         FIG. 24  is a conceptual block diagram of a DMS system including a fragmentation region according to an illustrative embodiment of the invention. 
         FIG. 25  is a three-dimensional color dispersion plot illustrating detection of agent GA according to an illustrative embodiment of the invention. 
         FIGS. 26A-26H  are two-dimensional graphs of ion intensity versus field compensation voltage at particular field voltages, the two-dimensional graphs being of the type combinable into the three-dimensional color dispersion plot of  FIG. 25 , according to an illustrative embodiment of the invention. 
         FIGS. 27A and 27B  are graphs of ion intensity at a plurality of pressures versus field compensation voltage according to an illustrative embodiment of the invention. 
         FIGS. 28A and 28B  are graphs of ion intensity versus pressure showing a quantifiable effect on positive and negative background spectra, respectively, caused by a decrease in pressure according to an illustrative embodiment of the invention. 
         FIGS. 29A and 29B  are graphs of ion intensity at a plurality of pressures versus field compensation voltage showing the effect of varying pressure on negative and positive tert-butylmercaptan or tert-butylithiol (TBM) spectra, respectively, according to an illustrative embodiment of the invention. 
         FIGS. 30A and 30B  are graphs of ion intensity versus pressure showing the effect of varying pressure on negative and positive TBM ion peak parameters, respectively, according to an illustrative embodiment of the invention. 
         FIG. 31  is a graph that shows the effect of reduced pressure on analyte peaks for chemical warfare agents such as DMMP, DIMP, and MS. 
         FIGS. 32A-32D  are graphs of ion intensity versus field compensation voltage showing improved detection resolution for agent GF at reduced pressures according to an illustrative embodiment of the invention. 
         FIG. 33  is a three-dimensional color dispersion plot illustrating detection of positive ions of 0.005 mg/m 3  DIMP at about 0.65 atm and over a range of field voltages and field compensation voltages with varying intensity depicted by varying colors. 
         FIG. 34  is a three-dimensional color dispersion plot illustrating detection of positive ions of 0.005 mg/m 3  DIMP at about 0.5 atm and over a range of field voltages and field compensation voltages with varying intensity depicted by varying colors. 
         FIG. 35  is a graph that shows positive (left) and negative (right) three-dimensional color dispersion plots for 0.85 mg/m 3  agent GB with a relative humidity (RH)=87 in a DMS system operating at 0.5 atm and for a fragmented sample. 
         FIGS. 36A and 36B  are graphs that show a plot of compensation versus field strength of detected monomer and cluster ion peaks for a family of ketones according to an illustrative embodiment. 
         FIGS. 37 and 38  are tables, each including a collection of detection data for a group of monomer and dimers (clusters) of eight ketones respectively, that were used to generate the curves in the graphs of  FIGS. 36A and 36B . 
         FIGS. 39A and 39B  are graphs of a ratio of field strength to gas density (E/N) versus field compensation voltage that illustrate the results of calculating normalized alpha parameter curves. 
         FIG. 40A  is a flow diagram of an exemplary sequence of steps of a computer process used to acquire data concerning a particular chemical ion species. 
         FIG. 40B  shows a diagram of a data structure for a library of stored compound data measurement information. 
         FIG. 40C  is a flow diagram of a series of steps that may be applied to perform a chemical recognition. 
         FIG. 40D  is a flow diagram of a series of steps that may be added to the data acquisition and chemical recognition processes using alpha curve fitting. 
         FIG. 40E  shows a diagram of a more complex data structure. 
         FIG. 40F  is a flow diagram of a sequence of processes that may be used to distinguish monomer and cluster peak responses. 
         FIG. 40G  is a flow diagram of a process showing the combination of monomer and cluster scoring. 
         FIG. 41  is a conceptual diagram of a compact DMS analyzer system  1400  used to detect and identify chemical warfare agents (CWAs), Toxic Industrial Compounds (TICs) and Toxic Industrial Materials (TIMs) which may be released in warfare or terrorist situations according to an illustrative embodiment of the invention. 
         FIG. 42  is a graph of multiple plots showing experimental results for a series of warfare agent simulants selectively mixed with 1% headspace of AFFF. 
         FIG. 43  is a three-dimensional color dispersion plot of the detection of positive ions of agent GA over a range of field voltages and field compensation voltages with varying intensity represented in varying color according to an illustrative embodiment of the invention. 
         FIG. 44  is a conceptual block diagram of a chemical and/or biological agent detection system using an ion mobility analyzer system, membrane, and recirculation system according to an illustrative embodiment of the invention. 
         FIG. 45  is a conceptual block diagram of a chemical and/or biological agent detection system configured for reduced pressure analysis according to an illustrative embodiment of the invention. 
         FIG. 46  is a conceptual block diagram of a chemical and/or biological agent detection system using a cylindrical DMS analyzer system, recirculation system, and multiple flow channels according to an illustrative embodiment of the invention. 
         FIGS. 47-53  are conceptual block diagrams respectively of chemical and/or biological agent detection systems using various configurations of a DMS analyzer system, recirculation system, and other components according to an illustrative embodiment of the invention. 
         FIG. 54A  is a conceptual diagram showing a pre-separation process of a sample matrix according to an illustrative embodiment of the invention. 
         FIG. 54B  is a conceptual diagram showing a pre-separation process of a sample matrix according to another illustrative embodiment of the invention. 
         FIG. 55  is a conceptual block diagram of a sample pre-separation system using a first and second ionization region and first and second deflector regions to separate a sample matrix according to an illustrative embodiment of the invention. 
         FIG. 56A  is a conceptual diagram of a sample pre-separation process where a sample matrix may be re-circulated multiple times to interact with an ionization source such as reactant ions to sequentially remove differing compound product ions according to an illustrative embodiment of the invention. 
         FIG. 56B  is a conceptual diagram of a sample pre-separation process where a sample may be re-circulated multiple times to interact with an ionization source, such as an electric or magnetic field, to sequentially remove differing compound ions according to an illustrative embodiment of the invention. 
         FIG. 57  is a conceptual block diagram of a sample pre-separation system capable of re-circulating a sample through an ionization region multiple times to sequentially remove differing compound ions having differing proton or electron affinities according to an illustrative embodiment of the invention. 
         FIG. 58A  is a conceptual diagram of a sample pre-separation system where selected ions are intermixed with a sample to enable the pre-separation of ions having a particular proton or electronic affinity according to an illustrative embodiment of the invention. 
         FIG. 58B  is a conceptual diagram of a sample pre-separation system where selected ions, having been filtered and pre-selected, are then intermixed with a sample to enable the pre-separation of ions having a particular proton or electronic affinity according to an illustrative embodiment of the invention. 
         FIG. 59A  is a conceptual diagram of a sample pre-separation system including two flow channels and multiple (and optionally different) ionization sources for selective ion separation from a sample matrix according to an illustrative embodiment of the invention. 
         FIG. 59B  is a conceptual diagram of a sample pre-separation system having two flow channels and multiple (and optionally different) ionization sources for selective ion separation from a sample matrix where at least one of the ionization sources is a plasma ionization source. 
         FIG. 60  is a graph of ionization energies required for various NOx ion species to form either positive or negative ions by direct photo ionization in air. 
         FIG. 61A  is a graph of relative intensity versus mass units showing the mass-spectra to positive NOx ion NO. 
         FIG. 61B  is a graph of relative ion intensity versus mass units showing the mass-spectra for positive NOx ion NO 2 . 
         FIG. 61C  is a graph of ion intensity versus field compensation voltage showing the ion intensity peaks for NO and NO 2 . 
         FIG. 62  is a conceptual diagram of a cylindrical sample pre-separation system including an integrated cylindrical DMS or other analyzer according to an illustrative embodiment of the invention. 
         FIG. 63  is a conceptual block diagram of a sample pre-separation system capable of mixing dopants with a sample matrix in a controlled manner before or after the reactant ions are added according to an illustrative embodiment of the invention. 
         FIG. 64  is a conceptual diagram of an array of logic circuits including an “or” flow circuit and an “and” flow circuit used to cause multiple and different ions to interact and form a desired reactant ion species according to an illustrative embodiment of the invention. 
         FIG. 65  is a conceptual diagram of a sample pre-separation and analysis system using multiple ionization zones and multiple analyzers to analyze various ions of a sample matrix according to an illustrative embodiment of the invention. 
         FIG. 66  is a conceptual diagram of a sample pre-separation and analysis system using multiple ionization zones and DMS analyzers, including a DMS with a drift tube and ion filter region arbitrarily curved, according to an illustrative embodiment of the invention. 
         FIG. 67  is a conceptual diagram of a sample pre-separation and analysis system employing multiple ionization zones and analyzers along with a filtered gas source to control pressure within the analyzers according to an illustrative embodiment of the invention. 
         FIG. 68  is a flow diagram of a sample analysis process including sample re-circulation and pre-separation according to an illustrative embodiment of the invention. 
         FIG. 69  is a flow diagram of a process showing the analysis of a sample matrix composed of multiple molecule species according to an illustrative embodiment of the invention. 
         FIG. 70  is a conceptual diagram of a sample pre-separation (neutrals removal) system where the neutral molecules are removed from the ionized molecules rather than removing the ions from the neutral gas stream. 
         FIG. 71  is a conceptual diagram of a sample pre-separation system employing an ionization region, DMS filter, deflector, pump, and valve to selectively filter an ion species for analysis according to an illustrative embodiment of the invention. 
         FIG. 72  is a conceptual diagram of a sample pre-separation system employing an ionization region, ion guiding region, DMS ion filter, positive and negative ion deflectors, optional analyzers, flow generator, selective concentrator and valve for ion species analysis according to an illustrative embodiment of the invention. 
         FIG. 73A  is a conceptual diagram of a sample amplification system employing a DMS filter, detector and neutralizer, and recirculation loop for selected ion species analysis according to an illustrative embodiment of the invention. 
         FIG. 73B  is a conceptual diagram of a sample amplification system employing a DMS filter, detector, ionization source, deflector, and an optional DMS with a re-circulation channel for selected ion species analysis according to an illustrative embodiment of the invention. 
         FIG. 74  is a conceptual diagram of a sample amplification and analysis system employing a re-circulation channel according to an illustrative embodiment of the invention. 
         FIG. 75  is a flow diagram of a process for amplifying a selected ion species using an analyzer, such as a DMS analyzer, according to an illustrative embodiment of the invention. 
         FIG. 76  is a graph of ion intensity versus drift time in a conventional IMS for ions of benzene, acetone, and toluene respectively. 
         FIG. 77  is a graph of ion intensity versus field compensation voltage in a DMS for acetone, acetone 0-xylene, acetone m-xylene, acetone-toluene, and acetone-benzene respectively. 
         FIG. 78  is a graph of ion intensity versus field compensation voltage in a DMS for ions of DEMP and DEEP respectively. 
         FIG. 79  is a graph of ion intensity versus drift time in a conventional IMS for DEMP and DEEP respectively. 
         FIG. 80  is a graph of field compensation voltage versus mass in a DMS and drift time versus mass in an IMS illustrating the effect of ion mass on the type of detection method performed. 
         FIG. 81  A is a graph of the alpha parameter versus electric field strength for two ion species with similar alpha parameters. 
         FIG. 81B  is a graph of the coefficient of mobility versus electric field strength for two ion species having similar alpha parameters but different low field mobility parameters. 
         FIG. 82A  is a graph of the alpha parameter versus electric field strength for two ion species with different alpha parameters. 
         FIG. 82B  is a graph of the coefficient of mobility versus electric field strength for two ion species with similar low field mobility parameters but different alpha parameters. 
         FIG. 83  is a conceptual diagram of a DMS-IMS detection system according to an illustrative embodiment of the invention. 
         FIG. 84  is a conceptual diagram of a DMS-IMS detection system using a shutterless IMS according to an illustrative embodiment of the invention. 
         FIG. 85  is a conceptual diagram of a DMS-IMS detection system where the IMS is connected to the DMS in manner that reduces the introduction of neutral molecules into the IMS according to another illustrative embodiment of the invention. 
         FIG. 86  is a conceptual diagram of a DMS-IMS detection system using a shutterless IMS that is connected to the DMS in a manner that reduces the introduction of neutral molecules into the IMS according to an illustrative embodiment of the invention. 
         FIG. 87  is a conceptual diagram of a DMS-IMS detection system using two IMS detectors according to an illustrative embodiment of the invention. 
         FIG. 88  is a conceptual diagram of a DMS-IMS detection system using two shutterless IMS detectors according the an illustrative embodiment of the invention. 
         FIG. 89  is a conceptual diagram of a DMS-IMS detection system that supports a DMS mode and an IMS mode according to an illustrative embodiment of the invention. 
         FIG. 90  is a conceptual diagram of a DMS-IMS detection system where IMS and DMS detection occur concurrently and/or near simultaneously according to an illustrative embodiment of the invention. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     As described above in summary, the invention is generally directed to systems, methods and devices for providing improved detection, measurement, discrimination and analysis (collectively “analysis”) of compounds. The compounds analyzed may include any compound, both organic and inorganic, and without limitation elements, chemicals, and biologicals. In particular illustrative embodiments, the invention is directed to improved ion mobility-based compound analysis. Particular features of the invention include using multiple combined analytical techniques to improve compound analysis. By way of example, in various illustrative embodiments, the invention combines Field Asymmetric Ion Mobility Spectrometers (FAIMS), also known as Differential Mobility Spectrometers (DMS) or Radio Frequency Ion Mobility Spectrometers (RFIMS) among other names (collectively DMS) with ion mobility spectrometry (IMS), time of flight (TOF) IMS, gas chromatography (GC), Fourier transform infrared (FTIR) spectroscopy, mass spectrometry (MS), and liquid chromatography mass spectrometry (LCMS) techniques. According to other illustrative embodiments, the invention employs dispersion plots, sample fragmentation and/or pressure controls to improve discrimination between compounds having similar or overlapping ion mobility characteristics. 
       FIG. 5  is a block diagram of a DMS system  10  of the type that may employ the invention. The system  10  includes a flow section  15  and a processor section  40 . The flow section  15  includes a flow channel  11  extending from a flow inlet  12  to a flow outlet  13 . Opposing filter electrodes  20  and  21  are located within the flow channel  11 . Detector electrodes  26  and  30  are also located within the flow channel  11 . The processor section  40  includes an RF voltage generator  42  for providing an RF field voltage to the filter electrodes  20  and  21 , and direct current (dc) voltage generator  44  for providing a dc compensation voltage Vcomp to the filter electrodes  20  and  21 . The processor section  40  also includes a processor  46  for controlling the voltage generators  42  and  44 , and for processing inputs from the ion detectors  28  and  30  by way of the amplifiers  36  and  38  the A/D converter  48 . The processor section  40  also provides a display  49  for providing analysis information to a user. One feature of the system  10  is that it may be contained in a hand held unit weighing less than about one pound. 
     In operation, a sample S enters the flow channel  11  at the flow channel inlet  12 . The sample S may, for example, be drawn in from the environment or received from a front end device, such as another DMS, an IMS, TOFIMS, GC, FTIR, MS, or LCMS. The sample S may be mixed with an effluent, such as a gas, liquid or vapor. In the instant example, a carrier gas CG is employed to flow the sample S through the flow channel  11 . Upon entering the flow channel  11 , the sample S flows into an ionization region  14 . The sample is ionized by an ionization source  16  as it flows through the ionization region  14 , creating a set of ionized molecules  17 +,  17 −, with some neutral molecules  17   n , of various chemical species in the sample S. This may include, for example, monomer ions and cluster ions. Such clusters may be created when a monomer combines with water molecules or other background molecules, and the combination is ionized. 
     The carrier gas CG then carries the ionized sample S into the ion filter field  18  located between the opposing filter electrodes  20  and  21  of the ion filter  24 . Filtering proceeds based on differences in mobility in the filter field  18  of the various ions included in the sample S. Ion mobility is influenced, for example, by ion size, shape, mass and charge. The field generator  42  applies an asymmetric field voltage Vrf across the filter electrodes  20  and  21  to cause the field strength within the filter field  18  to alternate between high and low field strengths. The ions  17 +,  17 − and  17   n  move in response to the field, based on their mobility characteristics. Typically, an ion&#39;s mobility in the high field strength condition differs from its mobility in the low field strength condition. This mobility difference produces a net transverse displacement of the ions as they travel longitudinally through the filter  24 . The transverse displacement defines an ion trajectory for each of the sample S ions. 
     As described above, the voltage generator  44 , under the control of the processor  46 , applies a dc compensation voltage Vcomp across the electrodes  20  and  21 . The compensation voltage Vcomp causes particular ion species to be returned toward the center of the flow path  14 , and thus enables them to exit the filter field  18 , without colliding with either of the filter electrodes  20  or  21  and without being neutralized. Other species, for which the applied Vcomp is not sufficient ultimately collide with the filter electrodes  20  and  21  and are neutralized. The neutralized ions are purged, for example, by the carrier gas CG, or by heating the flow path  11 . 
     The illustrative system  10  of  FIG. 5  also can discriminate between ions based on differences in polarity, as is the case with the ions  17 −and  17 +. According to one feature, the system  10  of  FIG. 5  can be operated to concurrently, or in some instances, substantially simultaneously detect both positive and negative ions in the sample S. This feature enables identification of two compounds concurrently, or in some instances, substantially simultaneously. This feature also enables concurrent or substantially simultaneous detection of two modes of a single compound. 
     In operation, the two species of ions  17 + and  17 −, enter the detection region  25 , where further separation occurs followed by their intensity determination. In an illustrative embodiment, the electrode  28  of the detector  26  may be positively biased to attract the ions  17 − and repel the ions  17 +. Alternatively, the electrode  30  of the detector  26  may be biased negatively to attract the ions  17 + while repelling the ions  17 −. The signals generated by the ions collecting at the detector electrodes  28  and  30  are amplified by respective amplifiers  36  and  38  and provided to the processor  46  by way of the A/D converter  48 . According to one feature, the processor  46  compares the digitized signals from the A/D converter  48 , with a library of ion intensity curves for known compounds stored in the memory  47 , to identify compounds in the sample S. The results of the comparison operation can then be provided to an appropriate output device, such as the display  49 , or may be provided to an external destination by way of an interface  56 . 
     According to a further illustrative embodiment, the system  10  is calibrated prior to employing it for analyzing a sample. More particularly, the library of ion intensity curves for known species of ions at particular Vcomp and Vrf settings is created and stored in the memory  47 . According to one feature, once the system  100  is calibrated, it may be used continuously, without need for further calibration. However, it is also within the scope of the invention to calibrate the system  10  using the reactant ion peak (RIP) or a dopant peak, for example. 
     According to various illustrative embodiments, field strength within the filter field  18  resulting from an applied field voltage Vrf may have values ranging from about 1,000 V/cm to about 30,000 V/cm, or higher. The frequency of Vrf may have values ranging from about 1 to about 20 megahertz (MHz), with the higher frequencies having an approximately 30 percent duty cycle. 
     It should be noted that the system  10  may be tuned by employing any suitable operating values of, for example, Vrf, Vcomp, field strength, Vrf duty cycle, Vrf wavelength and Vrf frequency. Additionally, as described in further detail below, to improve analysis, the system  10  may be tuned by varying values of other flow channel conditions, such as and without limitation, temperature, pressure, humidity, flow rate, doping and carrier gas CG composition. As also described below in more detail, multiple scans of the sample S taken, for example, by recirculating the sample S and/or processing the sample in parallel and/or in series with one or more additional DMS, IMS, TOFIMS, FTIMS, GC, FTIR, MS, or LCMS, at differing flow channel and/or filter field conditions may be employed to improve analysis of the sample S. 
     According to one illustrative embodiment, the processor  46  causes the voltage generator  44  to scan or sweep a range of field compensation voltages Vcomp for a particular RF field strength as controlled by the applied Vrf to obtain a first spectrum for the sample S. Then, Vrf is set to a different level and the Vcomp is once again scanned to establish a second spectrum for the sample S. This information can be compared to a library of spectral scans in a similar fashion as that described above to identify a compound in a sample. 
     If a particular combination of peaks in a spectral scan is known to indicate the presence of a particular compound, data representing the multiple peaks can be stored and future detection data can be compared against this stored data. For example, under controlled filter field conditions, such as at a raised field strength, a clustered compound may become de-clustered. The detection results in a signature of peaks that can be used to identify the source compound being detected even as detected in a single scan. 
     According to one illustrative application, the invention is used for detecting sulfur-containing compounds in a hydrocarbon background. In one example, negative and positive ions are separately detected. The detected data enables a quantitative measurement of concentration of these sulfur-containing compounds, independent of the hydrocarbon background. 
     In another illustrative application, the invention is used for detecting trace amounts (parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt)) of mercaptan in varying and even high hydrocarbon backgrounds. The system  10  of  FIG. 5  is also able to characterize hydrocarbon gas backgrounds. For example, the invention is capable of detecting mercaptans, such as ethyl mercaptan in a methane background, and is also capable of detecting a gas, such as methane, in a mercaptan background. 
     In this practice of the invention, where mercaptans were detected in hydrocarbon background, the asymmetric voltage applied to the ion filter electrodes ranged from about 900 to about 1.5 kV (high field condition), and a low voltage of about −400 to about −500 V (low field condition). The frequency ranged from about 1 to about 2 MHz, and the high frequency had an approximate 30% duty cycle, although other operating ranges may be employed. In one embodiment, the detector electrodes were biased at +5 v and −5 v. With this arrangement, the mercaptans can be detected by the negative mode (−5 v) detector and the hydrocarbon gases can be detected by the positive mode (+5 v) detector. 
     The system  10  employs various conventional components. By way of example, the amplifiers  36  and  38  may be Analog Devices model 459 amplifiers. Additionally, the A/D converter may be included on a National Instruments circuit component (model 6024E) for digitizing and storing the scans, and may include software for displaying the results as spectra, topographic plots, dispersion plots or graphs of ion intensity versus time. Alternatively, such software may be stored in the memory  47  and may control the processor  46 . The ionization source may be, for example, a plasma, laser, radioactive, UV lamp, or any other suitable ionization source. 
     According to one illustrative embodiment, Vrf is applied across the filter electrodes  20  and  21 . However in some configurations, Vrf is applied to one filter electrode, e.g., electrode  20 , and the other electrode, e.g., electrode  22 , is tied to ground. Vcomp is then applied to one of the filter electrodes  20  and  21 , or alternatively, across the filter electrodes  20  and  21 , according to the ions species to be passed. According to another feature, the detector electrodes  28  and  30  are biased with a floating bias, such as with the electrode  28  being biased at −5 Vdc and the electrode  30  being biased at +5 Vdc, leads to good performance for detection of mercaptans in hydrocarbon or air backgrounds. 
       FIG. 6  is a graph of ion intensity versus field compensation voltage for “positive mode” spectra for a sample containing varying amounts of ethyl mercaptan as measured in a DMS system of the type depicted at  10  in  FIG. 5 . For positive mode detection, the detector electrode  28  is negatively biased and attracts positive methane ions  17   m + for detection.  FIG. 7  is a graph of ion intensity versus compensation voltage for “negative mode” spectra of a sample containing various amounts of ethyl mercaptan. For negative mode detection, the detector electrode  30  is positively biased and attracts the negative mercaptan ions  17   m − for detection. As can by seen from  FIGS. 6 and 7 , the mercaptan signatures are captured independent of the air-hydrocarbon carrier gas CG background, at various dosage levels and the detected sample peaks are fully isolated from the background. As can be seen in  FIG. 6 , the reactant ion peak (RIP) is isolated; and as shown in  FIG. 7 , the background (sample # 9 ) is flat. 
     As mentioned above, the detector electrodes  28  and  30  can be oppositely biased to enable concurrent, or in some configurations, substantially simultaneous detection of both positive and negative ions. Even in a sample such as mercaptan, which when ionized may have predominantly negative ions, detecting both positive and negative ions provides improved analysis accuracy over a single mode detection approach. This, in turn, improves identification accuracy and confidence, and reduces the likelihood of false positives and false negatives. 
     For example, Sulfur hexafluoride (SF6) can be well detected in the negative mode. However, the response in the positive mode, while alone not definitive, has a profile, and thus in combination with the negative mode, is confirmative and provides a lower likelihood of a false detection. According to one feature, the invention can detect SF6 in single mode (e.g., only negative mode detection) or dual mode (both negative and positive mode detection), seriatim, concurrently, or simultaneously. 
     SF6 gas is used in atmospheric tracer applications to monitor air flow, as a tracer for leak detection in pipes to point detect sources of leaks, in power plants to isolate switches to reduce, or prevent breakdown of the switches, among other uses. Isolation and detection of SF6 is often found to be a difficult proposition. 
     According to one illustrative application, a system of the invention is employed to detect SF6 in air. According to a further illustrative embodiment, the invention provides a portable, battery powered unit for the detection of SF6 with a sensitivity of about 1×10 −9  atm cc/sec SF6 (0.01 PPM). In this illustrative embodiment, the invention may be used, for example, in the power industry to ensure the leak tightness of High Voltage Switchgear and in the laboratory for testing fume hoods to the ASHREA 110 specification. Other applications include torpedo head, pipework systems, and air bag integrity testing. The high sensitivity, rugged design and ease of use and set up of the invention are advantageous for many applications that involve the detection of SF6. 
       FIG. 8  is a graph of ion intensity (y-axis) versus Vcomp (x-axis) for negative mode detection of SF6 according to an illustrative embodiment of the invention. As can be seen, application of the invention provides a distinct peak for the SF6, separate from the reactant ion peak.  FIG. 9  provides a similar plot for SF6 for positive mode detection. As can be seen, for positive mode detection, there is no significant difference between the signal  51  without the SF6 present and the signal  53  with the SF6 present.  FIG. 10  shows a plot of intensity (y-axis) versus Vcomp (x-axis) for SF6 at three different field voltages Vrf (shown at  57 ,  58  and  61  for negative mode detection along with the RIP  55  detected in absence of SF6.  FIG. 11  shows a similar plot to that of  FIG. 10 , for positive mode detection. As would be expected, the positive mode detection curves  69 ,  71  and  73 , substantially track their corresponding RIP curves  63 ,  65  and  67 , respectively. As mentioned above with respect  FIG. 16 , while alone this is not definitive, it is an expected detection and therefore may be used as confirmative when combined with a definitive SF6 negative mode detection. 
     According to another feature, the above described library data for known ion species intensity signatures for known device characteristics may be accessed for either single mode or simultaneous positive and negative mode detections. By comparison with historical detection data for the device, these peaks can be more clearly identified as the tell-tale spectra of the mercaptan. Both spectra give an indication of the mercaptan, qualitatively and quantitatively. Although the advantages of the simultaneous positive and negative mode detection is described above with respect to mercaptan, they may be employed to the analysis of any sample, and are especially useful with real-time analysis of complex samples, such as ones containing mercaptans and hydrocarbon gas, which have similar ion mobility characteristics, and are therefore, difficult to discriminate between. 
     The foregoing demonstrates favorably obtaining multiple detection data from a single mobility scan for identification of detected ion species in a sample. This innovation is useful in many applications. Notwithstanding this valuable innovation, a still higher level of confidence and further reduced false positives may be obtained by (1) obtaining multiple detection data from multiple ion mobility scans, and (2) further processing such data to extract device independent attributes, such as a mobility coefficient, α. 
     According to one illustrative “multiple scan” embodiment, ions are identified based not on a single set of field conditions, but instead on multiple ion intensity scans taken at at least two and possibly additional numbers of field conditions (e.g., at at least two field measurement points). Detections are correlated with the Vrf and Vcomp, at the at least two different field conditions, to characterize a given detected compound. Because multiple detection data are associated with a given ion species of interest, more accurate detections can be made. Comparison with stored data results in reliable identification of detected compounds. 
     Strategies for identifying detected ions based on data in spectral peaks or in mobility curves include: curve matching, peak fitting, deconvolution (for overlapping peaks), multi-dimensional mapping, for example, employing three-dimensional representations, including (x,y,z, etc.) spatial coordinate systems and/or (x,y, etc.) coordinate systems, with z- or other values represented by color variations. These techniques enable identification of detected ion species based peaks in a single scan, including simultaneous positive and negative mode detections, and also in multiple scans. The goal is the same: analysis of multiple detection data that can be used to definitively identify, detect, measure or otherwise analyze the species of a detected ion. 
     As described above, different ion species of chemicals exhibit different mobility as a function of the compensated applied Vrf. Thus, by applying a set of different Vrf voltages and measuring the Vcomp at the ion abundance peak locations, for example, as detected by the detector  26  of  FIG. 1 , for the various compounds, a family of measurement points characteristic of a compound can be developed. This family of points can then be plotted to determine the ion mobility curve signature for specific species as a function of Vrf and Vcomp, for example, as shown in  FIG. 4 . As also described above, such data can be stored and compared with data from scans of unknown compounds to identify the unknown compounds. While some comparison approaches perform curve matching, other approaches determine an ion intensity for a particular ion species for two nearby field strength and Vcomp conditions. The slope between the two data points is calculated and employed as a signature for the particular ion species. The selection of measurement points and the number of measurement points may be adjusted for the specificity required for a particular application. The minimum number of measurement points is two, which at least identifies an aspect (such as slope) of the characteristic curve for a compound, given the known field values. 
     Although performing slope and/or curve matching for an individual or for multiple scans, where a single filter field/flow channel condition is varied, may provide sufficiently accurate results for some applications, one illustrative embodiment of the invention recognizes that multiple scans taken while varying multiple filter field and/or flow channel conditions can provide improved results. By way of example, according to one illustrative embodiment, the invention steps Vrf through a plurality of values and scans Vcomp at each of the plurality of Vrf values to generate unique sets of data, which better distinguish between compounds and, thus, provide more accurate identification of detected compounds. This approach can be employed to create a data store of more accurate ion mobility signatures for compounds of interest. 
     According to one illustrative embodiment, the invention incorporates information regarding shifts in an ion abundance peak for a particular ion species at multiple filter field/flow channel conditions into the spectral signature for a compound. More specifically, at a particular Vrf (Vrf 1 ) an ion abundance peak may be detected at a particular Vcomp (Vcomp 1 ). However, the ion abundance peak may shift to be detected at a second Vcomp (Vcomp) for a second Vrf (Vrf 2 ). One illustrative embodiment of the invention recognizes that, in many instances, the ion peak shift from Vcomp 1  to Vcomp in response to varying Vrf from Vrf 1  to Vrf 2  is indicative of a particular ion species. Similar measurements of unknown compounds can be compared against this portion of the spectral signature to aid in identification of the unknown compound. 
       FIG. 12  depicts an example illustrating the above described ion abundance spectral shift due to a change in Vrf from 1400 Vpeak to 1450 Vpeak over a scanned Vcomp. In  FIG. 12 , the peaks  110 - 1 ,  110 - 2 ,  110 - 3 , and  110 - 4  occur at a particular field compensation voltages Vcomp, for Vrf at 1400 Vpeak (corresponding to a field strength of 28,000 V/cm), but shift to be located at different compensation voltages in response to Vrf being changed to 1450 Vpeak (corresponding to a field strength of 29,000 V/cm). As can be seen from  FIG. 12 , even small changes in a field condition, such as a change in Vrf, can cause a measurable ion peak shift, and can thus provide significant additional information to the ion spectral signature. In the specific example of  FIG. 12 , the shift in ion peak due to the change in Vrf is employed when making a comparison to ion spectral signatures for known compounds to identify an unknown compound. 
       FIGS. 13A and 13B  show an experimental example illustrating how ion spectral peak shifting can be employed to identify an unknown species. In  FIGS. 13A and 13B , in a field strength of about 24000 V/cm, peaks for three different isomers of xylene in a sample, p-, o-, and m-, were detected. In  FIG. 13A , the peaks for p- and o- are indistinguishable, while the peak for m- is well defined. To further evaluate the sample, a second detection ( FIG. 13B ) was performed at a lower field strength of 18000 V/cm. As can be seen in  FIG. 13B , the peak shift due to the change in field strength causes the three different isomers p-, o-, and m- of xylene to be more clearly distinguishable, and thus more accurately identified. As can be seen from  FIGS. 13A and 13B , better discrimination between species is not always a result of applying a higher field strength. More particularly, in this example, the p- and o- xylene isomers become more distinguishable at a reduced field strength. 
     According to another illustrative embodiment and as mentioned above, the invention generates detection data over a range of applied filter field/flow channel conditions. For example,  FIGS. 14A and 14B  show the effect of changes in field strength on the location of detection peaks at different Vcomp levels for hexanone and octanone, as detected in a DMS system of the type depicted at  10  in  FIG. 1 . The curves are offset on the vertical axis, with the offset increasing as electric field strength increases. While various operating ranges are possible, as an illustration,  FIGS. 14A and 14B  may be understood as presenting peak Vrf between a low of about 620 Vpeak (lowermost plot in each) and a high of around 1450 Vpeak (uppermost plot in each). Several attributes are noted in this series of responses. For example, referring specifically to the hexanone plot of  FIG. 14A , a monomer peak of  601 - 1  of particular interest is somewhat obscured in the lowest field strength condition. However, at the highest applied field strength, the peak  601 -m corresponding to hexanone is clearly discemable from the other peaks. 
     Several phenomena have occurred with the increase in increasing applied field strength. First, a reactant ion peak (RIP)  605 - 1  is relatively dominant in the low field strength detection. However, as electric field strength is increased, the RIP  605 -m shifts to the left at a more rapid rate than the monomer ion peak  601 -m of interest. This is because the α parameter for the mobility coefficient for the reactant ion species is different than the α parameter for the monomer ion of interest. 
     In addition, the relative amplitude of the RIP  605  decreases markedly with the increase in the electric field strength. Thus, RIP  605 -m is observed at much lower amplitude and well separated from the monomer peak  601 -m of interest at a specific field condition. While the monomer peaks  601  also shift, they do not shift by the same amount, or by as much. Thus, by analyzing the compound over a range of applied field conditions, a condition can be discovered at which the RIP  605  shifts away from or off the scale of other observed peak voltages. In some cases, this allows easier detection of the monomer ion peak  601  of interest. 
     Similar behavior is observed in the monomer peaks  610 - 1 ,  610 - . . . ,  610 -n observed for octanone and the resulting reactant ion peaks  615 - 1  to  615 -m. This information can thus be used to identify a species by comparing a family of response curves to a stored family of known response curves. 
     Another observed effect shown in both  FIGS. 14A and 14B  is that a group of cluster ions  608  and  610  are seen. The cluster ions  608  represent clusters of chemical materials in the sample. Typical cluster ions, having a heavier chemical weight, have peaks that are shifted differently from monomer ion peaks of interest. In this example, the cluster peaks shift in a direction away from the direction of shift of the monomer peaks with increasing applied field strength. This characteristic feature of cluster ions, observed with this sample, can also be stored and utilized in recognizing the hexanone and/or octonone ions. The curves shown in  FIGS. 14A and 14B  are but one example of how applying a range of field/flow channel conditions to detect a given sample can be utilized to an advantage. 
     As mentioned above briefly, according to one illustrative embodiment, the invention employs multi-dimensional compound signatures for comparison with multi-dimensional representations of unknown compounds to identify and more generally analyze the unknown compounds. Such multi-dimensional representations may arise, for example, from plotting ion abundancy as a function of a plurality of varying filter field/flow channel conditions. Such conditions may include, without limitation, Vrf, Vcomp, filter field strength, Vrf duty cycle, Vrf wavelength and Vrf frequency; temperature, pressure, humidity, flow rate, doping and carrier gas CG composition. Multi-dimensional representations may also result from taking multiple scans of the sample S taken, for example, by recirculating the sample S and/or processing the sample S in parallel and/or in series with one or more additional DMS, IMS, TOFIMS, GC, FTIR, MS, or LCMS, at the same or differing flow channel/filter field conditions. The multi-dimensional representation, according to one illustrative embodiment, is a three-dimensional dispersion plot, employing x- and y-spatial coordinates, with a z-coordinate being represented by a variation in color. 
       FIG. 15A  shows a three-dimensional color dispersion plot  620  depicting detection of methyl salicylate over a range of field voltages Vrf (y-axis) and field compensation voltages Vcomp (x-axis), with varying ion intensity (abundance) represented in varying colors, according to an illustrative embodiment of the invention. Although, particular color coordination may vary, the dispersion plot of  FIG. 15A  represents the highest ion intensity in blue with yellow representing the lowest. The three-dimensional color dispersion plot  620  represents an aggregation of data from a plurality of two-dimensional graphs, such as that shown in  FIG. 15B . More specifically,  FIG. 15B  shows a plot  622  of ion intensity (y-axis) versus Vcomp (x-axis) at a particular Vrf for methyl salicylate. A plurality, illustratively more than two, of such graphs taken at a plurality, illustratively more than two, of field voltages Vrf are aggregated to provide the color plot  620  of  FIG. 11A . Aggregating a plurality of scans taken at a plurality of filter field voltages Vrf (and thus, field strengths) provides a more discriminating scan than a single scan taken at a single Vrf. One reason for this is that the aggregated scans incorporate the above discussed peak shifting that occurs due to the changes in Vrf. As can be seen, the three-dimensional representation of  FIG. 15A  provides three signature peaks  621 ,  623 , and  625 , as opposed to the two peaks  627  and  629  of  FIG. 15B . 
     The effect of the increased resolution provided by employing dispersion plots, is even more evident, when trying to distinguish between compounds having similar ion mobility characteristics. By way of example,  FIGS. 16A and 16B  show positive mode plots  624  and  626  for DMMP, while  FIGS. 17 and 18  show positive mode plots  628  and  630  for DIMP. More specifically,  FIGS. 16B and 18 , plot ion intensity (y-axis) versus Vcomp (x-axis) at a particular Vrf for DMMP and DIMP, respectively. As shown, both  FIGS. 16B and 18  included three peaks of similar magnitude, located at a approximately the same field compensation voltages, and similarly spaced apart. Distinguishing between DMMP and DIMP, based solely on the individual plots  626  and  630  of  FIGS. 16B and 18  is at best unreliable, and at worst impossible. However, referring to  FIGS. 16A and 17 , the three-dimensional plots  624  and  636  are easily visually distinguishable. 
     More particularly, the DMMP color plot  624  of  FIG. 16A  shows three clear peaks  638 ,  639  and  640 , while the DIMP color plot  628  shows four clear peaks  631 ,  632 ,  634  and  636 . While the peaks  638 ,  639  and  640  nearly overlay the peaks  631 ,  634  and  636 , the fourth blue peak  632  for DIMP, which is lacking for DMMP, easily distinguishes the DMMP scan from the DIMP scan. Also, the branches  634  and  636  of the color plot  628  are closer together than the branches  638  and  640  of the color plot  624 . Additionally, the color distribution (e.g., saturation) throughout the branches of the three-dimensional color plot  624  is not the same as the color distribution throughout the branches of the plot  628 . As in the case of previously discussed signature scans, three-dimensional signature scans of the type depicted in  FIGS. 15A-18  may be stored in a library for known compounds. At least portions of one or more of the stored scans may be compared with at least portions of similar scans of unknown species to identify and generally analyze the unknown species. Any suitable pattern matching approach, including conventional pattern matching approaches, may be employed for such comparison. 
     It should be noted that although the above discussed dispersion plots of  FIGS. 15A ,  16 A and  17  employ color changes to indicate intensity, changes in any color-related feature, such as changes in color saturation, gray scale or black and white may be employed instead or in combination. Additionally, in a further illustrative embodiment, the invention generates a curve circumscribing the intensity peaks, and the color-related information may be discarded. By way of example, in this illustrative embodiment, the outlines, for example, for the intensity peaks  632 ,  634  and  636  would remain, without the color-related information. Removing the color-related information provides a two-dimensional dispersion representation of, for example, Vrf versus Vcomp that also takes into account the spectral information gained from aggregating a plurality of Vcomp scans at a plurality of Vrf values. Any or all of this two-dimensional information may be incorporated into the above discussed signature information. 
     As described above, various illustrative comparison approaches may employ pattern matching using, for example, the above described two- and/or three-dimensional dispersion plots. However, in other illustrative embodiments, the information provided by the dispersion plots is stored in the library as mathematical relationships, and suitable conventional approaches for comparing such mathematical relationships are employed to identify the unknown species. 
     According to another illustrative embodiment, Vcomp may be plotted on the x-axis, Vrf on the y-axis, and ion intensity on the z-axis. Thus, instead of showing ion intensity as color, saturation, gray scale or black and white variations, as in the three-dimensional color plots  620 ,  624 , and  628 , ion intensity may be depicted/conceptualized in a topographical manner. Multi-dimensional signature representations of this sort may also be stored in the library of known species and used in the same fashion as the above described ion mobility signatures. In other embodiments of the invention, more than three dimensions may be employed, for example, plotting spectral data as clusters in n-dimensional space and employing known cluster matching algorithms. 
     A processor, such as the processor  46  of  FIG. 5 , may be programmed in a conventional fashion to automatically step an analyzer, such as the system  10 , through a range of field voltages Vrf and a scanned Vcomp, and provide the data to a display or other system for processing and generation of a three-dimensional dispersion plot. 
     Another analysis improving effect can be observed with the application of relatively high field strengths. Specifically, complex ion groupings can be fragmented, for example, by applying a high field strength to the sample. Sample fragmentation is a useful technique for enhancing species separation, detection, and identification. Fragmentation includes a process in which large molecules of samples are broken up into smaller molecules, components, or fragments prior to sample detection. This enables the components of the group to be individually detected and more generally analyzed. 
       FIG. 19  is an example of such an effect on a mercaptan sample. In particular, a range of background voltages (from 620-1450 Vpeak) were applied to an ethyl mercaptan spectra in which a general shift of ion peak behavior can be seen as electric field conditions are strengthened. However, a fragmentation condition can also be observed. Specifically, at lower applied field conditions, strong single peak is observed, such as at  701 - 1 . However, as electric field strength is increased, multiple peaks  701 -n,  702 , . . .  710  are observed in a spectra. By observing and recording the peak locations, not only at the low voltage field conditions, but also at a range of field conditions, this fragmentation behavior can be further exploited to better identify compounds. According to one feature, data indicating the peak RF voltage at which fragmentation occurs is incorporated into the stored spectral signatures for the known samples. According to another feature, the locations of the fragment peaks are also or instead incorporated into the stored spectral signatures for further use for matching detection data with known data. 
       FIG. 20A  is a graph  712  of ion intensity (y-axis) versus field compensation voltage Vcomp (x-axis) illustrating the separation of detection peaks at different compensation voltages between light and heavy molecules according to an illustrative embodiment of the invention. The graph  712  shows that light molecules associated with the RIP background peak  714  may be identified at an arbitrary −30 Vdc compensation voltage, while heavier molecules tend to be clustered and form a peak  716  at about 0 Vdc compensation. By fragmenting a sample of heavy molecules and detecting the fragments using, for example, a DMS or IMS system, a plurality of ion intensity peaks, each associated with a fragment, may be used to create a unique signature of the sample to enable subsequent identification of that sample. Fragmentation of a sample may be achieved, for example and without limitation, by using any one or a combination of a chemical reaction, a high energy field at high strength, high field voltage, heating, laser light, colliding the sample molecules with other molecules, soft x-ray, or the like. 
       FIG. 20B  is a graph  718  of ion intensity (y-axis) versus field compensation voltage (x-axis) showing the increase in number of peaks detected after sample fragmentation according to an illustrative embodiment of the invention. The graph  718  shows that fragments are lighter, and therefore, have lower mass and higher associated compensation voltages, resulting in improved resolution of and differentiation between the fragments. Also, the graph  718  shows an increased number of peaks  720  associated with the fragmented sample, which increases the collective data that may be used to fingerprint the compound. The additional detection data enable a more accurate identification of the detected species, such as by comparing the signature detected with a set of signatures in a look up table and by other techniques disclosed herein. 
       FIG. 21  is a conceptual block diagram of a dual channel detection system  748  including a first DMS system  722  using fragmentation and forming a first channel operating in parallel with a second DMS system  724  not using fragmentation and forming a second channel to improve sample analysis according to an illustrative embodiment of the invention. As shown, the DMS system  724  includes a sample inlet  726 , ionization region  728 , ion source  730 , analyzer region  732 , and outlet  734 . Similarly, the DMS system  722  includes a sample inlet  736 , ionization region  738 , ion source  740 , analyzer region  742 , and outlet  744 . The DMS system  722 , however, also includes a fragmentation energy source  746  within the ionization region  738 . The analyzer regions  732  and  742 , respectively, include a DMS filter and detector to enable detection and identification of samples. In operation, the dual channel detection system  748  operates DMS systems  722  and  724  concurrently, simultaneously or alternatively. With respect to the DMS system  724 , a sample S is introduced into ionization region  728  via the sample inlet  726 . The ionization source  730  may then ionize the sample S into positive and/or negative ions that are then delivered to the analyzer region  732 . The analyzer region  732  performs filtering and detection of the sample which then exits the DMS system  724  via the outlet  734 . The DMS system  722  operates in a similar manner as the DMS system  724 , but with an additional fragmentation source  746 . Thus, when the sample S enters ionization region  738  of DMS system  724 , the fragmentation source  746  breaks up/fragments the sample S molecules into lighter, less massive molecules. These lighter molecules are then delivered to analyzer region  742  for filtering and detection. 
     Thus, the dual channel detection system  748  using DMS systems  722  and  724  may improve sample analysis by substantially simultaneously analyzing a sample S and its fragments to create a more complete signature of the sample. Alternatively, the dual channel detection system  748  may selectively compare the fragmentation spectra, depending on the sample species to be detected and the need for better discrimination from other interferants or compounds. 
       FIG. 22  is a conceptual diagram of a DMS system  750 , not using fragmentation, and operating in series with a DMS system  752  using fragmentation to improve sample analysis according to an illustrative embodiment of the invention. The combination of the DMS systems  750  and  752  form a serial detection system  754 . As shown, the serial detection system  754  includes a sample inlet  756 , the DMS system  750 , the DMS system  752 , and an outlet  758 . The DMS system  750  includes an ionization region  760 , ion source  762 , ion filter  764 , and detector  766 . The DMS system  752  includes an ionization region  768 , ion source  770 , fragmentation source  772 , ion filter  774 , and detector  776 . 
     In operation, a sample S is introduced into the serial detection system  754  via the sample inlet  756 . The DMS system  750  ionizes the sample S using the ionization source  762  within the ionization region  760 . Then, the ionized sample S is delivered to the ion filter  764 . The ion filter  764  applies a combination of field and field compensation voltage to the sample S to allow selected ion species to reach and be detected by the detector  766 . 
       FIG. 23A  is a graph  778  of ion intensity (y-axis) versus Vcomp (x-axis) showing peak detection for the DMS system  750 . As shown previously, when no fragmentation occurs, the relatively heavy sample molecules cluster to form a peak  780  at Vcomp=approximately 0 Vdc. 
     After analysis by the DMS system  750 , the sample S is delivered to the DMS system  752 , where the sample S is ionized by an ionization source  770 , and also fragmented by the fragmentation source  772 . The fragmentation source  772  may be a radioactive source, a high energy voltage source or the like with enough energy to break up the relatively large sample molecule into a plurality of fragment molecules, fragments, components, or atoms. Then, the fragments are delivered to the ion filter  774  whereupon a combination of filter field voltages Vrf and field compensation voltages Vcomp applied a plurality of filter field conditions to the fragments to filter them before detection by the detector  776 . 
       FIG. 23B  is a graph  782  of ion intensity versus compensation voltage showing peak detection for the DMS system  752  of  FIG. 22  using fragmentation. As shown previously, when fragmentation occurs, the relatively lighter fragments form a plurality of ion intensity peaks  784  at various distinct field compensation voltages Vcomp. 
     Thus, the serial detection system  754  using the DMS systems  750  and  752  may improve sample analysis by serially detecting a sample S and its fragments to create a more complete signature or fingerprint of the sample. Alternatively, the serial detection system  754  may selectively compare the fragmentation spectra depending on the sample species to be detected and the need for better discrimination from other interferants or compounds. 
       FIG. 24  is a conceptual block diagram of a DMS system  786  including a fragmentation region  792  according to an illustrative embodiment of the invention. As shown, the DMS system  786  includes a sample introduction region  788 , ionization region  790 , fragmentation region  792 , fragmentation source  806 , fragmentation effluent inlet  794 , transport effluent inlet  796 , ion filter  798 , detector  800 , and controller  812 . An ionization source  802  may optionally be located within the fragmentation region  792 . An ionization source  804  may optionally be located within ion filter  798 . 
     In operation, a sample S is introduced into sample introduction region  788 . The sample introduction region  788  may perform pre-separation of the sample S to reduce the amount of interferants or unwanted compounds. The ionization source  808  then ionizes the sample S in the ionization region  790 . Once the sample S is delivered to the fragmentation region  792 , the fragmentation source  806  fragments the relatively heavy molecules of the sample S into a plurality of lighter fragments. Alternatively, a fragmentation gas including fragmentation molecules may be introduced into fragmentation region  792  via fragmentation gas inlet  794 . The fragmentation gas molecules, upon colliding with the sample S molecules, cause a portion of the sample S molecules to break up into sample S fragments. 
     After fragmentation, a transport effluent, such as a carrier gas CG may be introduced via the transport effluent inlet  796  to deliver the sample S fragments to the ion filter  798 . After filtering, the fragments are then detected by the detector  800 . The ionization source  802  may optionally be located in the fragmentation region  792 . Furthermore, as in the case of all of the previously described illustrative embodiments, the fragmentation source  806  may function additionally as a ionization source. The ionization source  804  may optionally be located in the ion filter  798 . Furthermore, the ion filter  798  may also act as either a fragmentation source  810  or an ionization source  804 . 
     It should be noted that although the previously described embodiments refer to separate ionization and fragmentation sources, in other illustrative embodiments, a single source may attend to both fragmentation and ionization. Additionally, any of the previously described fragmentation approaches may be employed in addition to or in replacement of the fragmentation sources of  FIGS. 21 ,  22  and  24 . The controller  821  may switch fragmentation on and off as needed by activating or deactivating the fragmentation source  806  or by introducing or not introducing a fragmentation effluent via fragmentation effluent inlet  794 . 
     The foregoing fragmentation techniques and system implementing these fragmentation techniques may be used to enhance the detection of a sample S, such as without limitation, Sarin gas, also known as:
         GB   Zarin   Phosphonofluoridic acid, methyl-, isopropyl ester   Phosphonofluoridic acid, methyl-, 1-methylethyl ester   Isopropyl methylphosphonofluoridate   Isopropyl ester of methylphosphonofluoridic acid   Methylisoproposfluorophosphine oxide   Isopropyl Methylfluorophosphonate   0-Isopropyl Methylisopropoxfluorophosphine oxide   0-Isopropyl Methylphosphonofluoridate   Methylfluorophosphonic acid, isopropyl ester   Isoproposymethylphosphonyl fluoride       

     Sarin, a colorless and odorless gas, has a lethal dose of 0.5 milligram for an adult. It is 26 times more deadly than cyanide gas and is 20 times more lethal than potassium cyanide. Just 0.01 milligram per kilogram of body weight in a pinprick sized droplet will kill a human. 
       FIG. 25  is a three-dimensional color dispersion plot  814  of the type described above with respect to  FIGS. 15A-18  and illustrating detection of agent GA over a range of field voltages Vrf and field compensation voltages Vcomp with varying ion intensity presented in varying color according to an illustrative embodiment of the invention. The color dispersion plot  814  includes branches  816 ,  818 ,  820 , and  822  that represent the detection of fragments of agent GA using, for example, DMS system  786  having a Ni 63  ionization source for fragmentation of the GA sample at 0.14 ng/l. The branch  840  represents an original peak before fragmentation. 
       FIGS. 26A-26H  depict two-dimensional graphs  824 ,  826 ,  828 ,  830 ,  832 ,  834 ,  836 , and  838  of ion intensity (y-axis) versus Vcomp (x-axis), each at a particular Vrf. As described above with respect to  FIGS. 15A-18 , the two-dimensional graphs  824 ,  826 ,  828 ,  830 ,  832 ,  834 ,  836 , and  838  are aggregated into the three-dimensional color dispersion plot  814  of  FIG. 25 . As discussed previously, the color dispersion plot  814  improves the analysis process of a particular species such as agent GA or GB, for example, because it takes into account peak shifts due to changes in Vrf, and because the color nature of the three-dimensional dispersion plot  814  makes more evident the signature behavior of particular ion species in relation to other ion species, especially after fragmentation. 
     As described above with respect to  FIGS. 15A ,  16 A, and  17 , the dispersion plot of  FIG. 25 , may employ color saturation, gray scale variations, black and white variations and/or peak outlines in place of the color variations depicted. 
     The fragmentation techniques described herein are not limited to DMS systems and may be employed with other mobility-based detection systems such as ion mobility spectrometry (IMS), time of flight (TOF) IMS, Fourier Transform (FT) IMS, gas chromatography (GC), Fourier transform infrared (FTIR) spectroscopy, mass spectrometry (MS), liquid chromatography mass spectrometry (LCMS), surface acoustic wave (SAW) sensors, and the like. 
     Another technique for improving ion species detection, identification and analysis generally is operating the mobility-based detection system, such as any of the systems described herein, below atmospheric pressure. By operation below atmospheric pressure, the separation between ion intensity detection peaks is increased and the width of the peaks is narrowed. This provides improved resolution, resulting in improved system discrimination and sensitivity. By operating, for example a DMS system at various pressure conditions, the change in ion species behavior with respect to pressure may be measured and used as another characteristic for identifying ion species. According to various illustrative embodiments, the invention performs ion scans at pressures between about 0.2 and about 0.9 atmospheres, less than about 0.3 atmospheres, less than about 0.4 atmospheres, less than about 0.5 atmospheres, less than about 0.6 atmospheres, less than about 0.7 atmospheres, or less than about 0.8 atmospheres. 
       FIG. 27A  is a graph  840  of background (RIP) ion intensity versus field compensation voltage at a plurality of pressures for a DMS system in positive ion detection mode according to an illustrative embodiment of the invention. The graph  840  shows that the field voltage may be adjusted to maintain the ion intensity peak within the same compensation voltage position as the pressure within a DMS system is adjusted. More specifically, according to the graph  840 , as the pressure decreases, the field voltage decreases to maintain the ion intensity peak for a species at the same compensation voltage. Furthermore, changes in pressure at lower pressures result in the need for greater changes in field voltage to maintain a constant compensation voltage. For example, when reducing the pressure by approximately 100 mmHg from 760 mmHg to 655 mmHg, the reduction in field voltage is approximately 40 Vpeak from about 1050 Vpeak to about 1010 Vpeak. For approximately the same pressure reduction from 655 mmHg to 556 mmHg, the reduction in Vrf is approximately 90 volts from about 1100 Vpeak to about 920 Vpeak. Thus, the field voltage decrease is approximately twice as great for changes in pressure in the 600 mmHg range, which indicates that the resolution is improved at reduced pressure. 
       FIG. 27B  is a graph  842  of background (RIP) ion intensity versus field compensation voltage at a plurality of pressures for a DMS system in negative ion detection mode according to an illustrative embodiment of the invention. Like positive mode graph  840 , the graph  842  shows that, in negative detection mode, the field voltage may be adjusted to maintain the ion intensity peak within the same compensation voltage position as the pressure within a DMS system is adjusted. 
     As shown by comparing the graph  840  with the graph  842 , there is an offset in the ion intensity peak between the positive mode ion intensity peaks of graph  840  and negative mode ion intensity peaks of graph  842  at the same pressure and field voltage. This offset may indicate a difference in the alpha parameter between positive and negative mode detection for an ion species. The alpha parameter is discussed in further detail below. The DMS flow rate is approximately 300 cc/min in graphs  840  and  842 . 
       FIGS. 28A and 28B  depict graphs  844  and  846 , respectively, of ion intensity (y-axis) versus pressure (x-axis) showing a quantifiable effect on positive and negative background spectra, respectively, caused by a decrease in pressure according to an illustrative embodiment of the invention. More specifically, the graph  844  shows that field voltage is decreased by about 50% when pressure is decreased to about 0.3 atmosphere (atm). The graph  846  also shows a similar field voltage decrease of about 50% when pressure is decreased to about 0.3 atm. 
       FIGS. 29A and 29B  depict graphs  848  and  850 , respectively, showing ion intensity (y-axis) versus field compensation voltage (x-axis) for a plurality of pressures and showing the effect of varying pressure on negative and positive tert-butylmercaptan and tert-butylithiol (TBM) spectra, respectively. While the graphs  848  and  850  show that field voltage decreases as pressures decrease for a particular field compensation voltage, the graphs  848  and  850  also show that the ion intensity peak positions for TBM spectra shift in the opposite direction as the ion intensity peak shifts for the background (RIP) spectra of graphs  840  and  842 . Furthermore, the level of change of the ion intensity peaks in graphs  848  and  850  for TBM spectra is less than the level of change of the ion intensity peaks in graphs  840  and  842  for background spectra. 
       FIGS. 30A and 30B  depict graphs  852  and  854  showing ion intensity (y-axis) versus pressure (x-axis) and showing the effect of varying pressure on negative and positive TBM ion peak parameters, respectively. More specifically, the graph  852  shows that the ion intensity peak remains relatively constant as the pressure is varied for negative ion spectra. The graph  854  shows that the ion intensity peak remains relatively constant with the level decreasing slightly at a lower pressure for positive spectra. Because changes in pressure impact the background (RIP) and analyte spectra differently, pressure may be manipulated, regulated, or otherwise controlled in such a manner as to improve the ability of a DMS system to detect and identify ion species with better resolution while minimizing the negative effects of background spectra interference. 
     In certain embodiments, it may be desirable to maintain uniform detection results by maintaining a constant ratio of electric field strength to gas density N or pressure P where the ratio is expressed as E/N or E/P. Thus, when the gas operating pressure within a DMS system is decreased, the field voltage is correspondingly lowered to maintain a constant E/N or E/P. This reduction in field voltage results in a reduction in power consumption which, in turn, results in smaller, lighter weight, and lower cost detection systems. 
       FIG. 31  is a graph  856  showing the effect of reduced pressure on analyte peaks for chemical warfare agents, such as DMMP, DIMP, and MS. The top graph  857  shows the ion intensity results at atmospheric pressure, while the bottom two graphs  859  and  861  show the results at 0.65 and 0.5 atm, respectively. At 1 atm with field voltage at Vrf=about 1000 Vpeak, the top spectra shows the overlap  858  of monomer and dimmer cluster peaks for DIMP over a range of about 10 Vdc field compensation voltage. But at 0.65 atm and Vrf=about 800 Vpeak, the monomer peak  860  and cluster peak  862  are separated with the monomer peak  860  at Vcomp=about −3 Vdc and cluster peak  862  at Vcomp=about +1 volt. At 0.5 atm and Vrf=about 650 Vpeak, the DIMP monomer peak  864  and DIMP cluster peak  866  are each narrower with the peaks  864  and  866  at Vcomp=about −2.5 Vdc and about +1 Vdc, respectively. The narrower peaks  864  and  866  at 0.5 atm result in higher resolution for a DMS system. 
       FIGS. 32A-32D  depict graphs  868 ,  870 ,  872 , and  874 , respectively, showing ion intensity (y-axis) versus Vcomp (x-axis). The graphs  868 ,  870 ,  872  and  874  show improved detection resolution for agent GF at reduced pressures, according to an illustrative embodiment of the invention. The graphs  868  and  870  show the ion intensity spectra of agent GF at Vrf of 1500 and 1000 Vpeak, respectively, at 1 atm. The graphs  872  and  874  show the ion intensity spectra of agent GF at Vrf of 1000 and 750 Vpeak, respectively, at 0.5 atm. According to the graph  870 , the monomer and dimer peaks overlap at peak  876  at Vrf=about 1000 Vpeak. According to the graph  868 , however, the monomer peak  878  and dimer peak  880  are separated at Vrf=about 1500 Vpeak. Thus, DMS system resolution may be increased by increasing the field voltage (Vrf). 
     In the graph  872 , the DMS system pressure is reduced to about 0.5 atm with Vrf at about 1000 Vpeak. The graph  872  shows the monomer peak  882  clearly isolated from any dimer peak, because the cluster or dimer RIP peaks are off-scale of the graph  872 . In the graph  874 , the field voltage Vrf is reduced to about 750 Vpeak, with a system pressure at about 0.5 atm. The graph  874  shows clear separation of the GF monomer peak  884  from the dimer peaks  886  and RIP peak  888 . Thus, GF may be detected and identified by the signature peaks illustrated in graph  874  in a DMS system utilizing reduced pressure, reduced field voltage, and, therefore, reduced power. 
     As described above, three-dimensional color dispersion plots may be used to significantly enhance the ability of a DMS system to detect and identify ion species of interest by allowing a user or pattern recognition program to match the color patterns against a library of similar color pattern for known compounds. 
       FIG. 33  is a three-dimensional color dispersion plot  890  depicting intensity of positive ions of 0.005 mg/m 3  DIMP at about 0.65 atm and over a range of field strengths, gas densities (E/N) and field compensation voltages Vcomp. As shown, gas density is plotted on the x-axis, Vcomp is plotted on the y-axis, and variations in intensity depicted by variations in color. The plot  890  includes several prominent branches  892 ,  894 , and  896 . 
       FIG. 34  plots the same information as  FIG. 33 , except as obtained at a decreased pressure of about 0.50 atm. As shown in plot  898 , the reduction in pressure in relation to plot  890  results in significantly more prominent branches  900 ,  902 , and  904 , thus providing enhanced resolution. 
       FIG. 35  is a graph depicting positive ( 906 ) and negative ( 908 ) mode three-dimensional color dispersion plots for about 0.85 mg/m 3  of agent GB RIP, at a relative humidity (RH)=about 87%, in a DMS system operating at about 0.5 atm for a fragmented sample. The negative mode plot  908  shows only a single strong RIP branch  909 , while the positive mode plot  906  shows two strong trace analyte peaks  901  and  903  to the right of the heavy background RIP branch  905 . Thus, plotting three-dimensional graphs for both the positive and negative ion species of a sample provides further enhanced ion species identification over three-dimensional plots of positive or negative mode measurements alone. 
     The three-dimensional color dispersion plots  906  and  908 , as illustrated above, may also show discontinuities in the branches, i.e., peak plots or traces, that are also useful for species identification. For example, the plot  906  includes a break in the trace or branch  901  that may be included as part of the stored signature for future comparisons. 
     As described above with respect to  FIGS. 15A ,  16 A,  17  and  25 , the dispersion plots of  FIGS. 33 and 35 , may employ color saturation, gray scale variations, black and white variations and/or peak outlines in place of the color variations depicted. 
     According to another feature, the identification above described analysis approaches may be made device-independent.  FIGS. 36A and 36B  show experimental detection data for a homologous group of ketones, including: acetone, butanone, pentanone, hexanone, heptanone, octanone, nonanone, decanone.  FIGS. 37 and 38  are tables showing monomers and clusters, respectively, for the above listed keytone species. As shown in  FIGS. 36A and 36B , each species has a unique mobility curve, and thus a unique mobility signature, for the given set of field conditions. As described above, the mobility signatures may be obtained and enhanced in any of a plurality of ways. However, the identification process can be further enhanced by making it device-independent. With device independence, signature data can be created that can be used on any device. According to one illustrative embodiment, the invention accomplishes this by determining the parameters of a function derived from the fundamental mobility coefficient associated with each species. 
     Therefore, for example, the multiple data represented in  FIGS. 36A ,  36 B,  37  and  48  each can be used to provide positive identification of a detected species by the unique and inherent mobility characteristic that identifies that species. According to one feature, the comparison can be made to a lookup library specific to the device in question, but also can be made to a universal set of data that is device-independent. Thus, in general, one does not wish to only compare the plot of abundance curves versus compensation voltage individually, but rather generate a plot of observed peak locations for specific compensation voltages, so that curves, slopes, signs, and various details can be discerned for each of the detected ions for comparison to a library of lookup data. 
     More specifically, in computing mobility signatures, we have found that an expression of the field-dependence of ion mobility, the so-called α coefficient, expressed as a function of field, can be used to generate a unique α function that is inherent for that species and is device independent. Thus the α function can be used as the unique signature of a species; this function expresses both a characteristic signature for the ion species and is device independent. In short, according to one feature, the invention recognizes that peaks change position in signature ways because they have different alpha signatures. 
     In one illustrative embodiment, the invention employs the α function as a mobility signature for detected species. The signature can be determined for a detected unknown compound, based on the field conditions that are used, and then this can be used to make an identification according to a lookup table of stored known signature data associated with known compounds. More particularly, in practice of a preferred embodiment of the invention, ion species are identified based on the mobility dependence of the species under various field conditions. Data is collected for the sample under test for at least two field conditions, the data is processed, and a comparison of detection data computed as an α function for the sample under test versus the stored data enables identification of the compounds in the sample. 
     Referring again to the discussion of the α parameter,  FIG. 3  is a plot of mobility versus electric field strength for three examples of ions, with field dependent mobility (expressed as the coefficient of high field mobility, α) shown for species at α greater, equal to and less than zero. For any given set of field conditions, the field strength and compensation can be correlated with an α value. This is shown in the work of Buryakov et. al.,  A New Method Of Separation Of Multi - Atomic Ions By Mobility At Atmospheric Pressure Using A High - Frequency Amplitude Asymmetric Strong Electric Field, Intl J. Mass Spec and Ion Proc . (1993), at p. 145. 
     We have observed that knowing the α parameter alone at a particular field strength does not prevent false positives. This would occur at the intersection of the two plots in  FIG. 4 , at the point indicated by reference numeral  100 . Without more information, knowledge of the α parameter for the respective ion species at that location does not provide unique mobility signatures for both compounds. Thus, without doing more, any number of readings at this intersection is likely to result in a detection error. 
     However, we have also found that we can express an ion&#39;s differential field mobility characteristic such as the α mobility characteristic, as a function of field, i.e., as α(E), and can define a unique mobility signature for the ion species which is device-independent. This α(E) or “alpha function” relates the size, effective cross-section, shape, and mass of the ion to field conditions. It is understood that as the applied electric field increases, the increasing electric field tends to displace, stretch, and/or break the bonds of the ion such that the stronger the field, the greater the induced dipole, quadripole, or higher order moments of the ion. These, in turn, affect the relative mobility of the specific ion. The result of relating these aspects is to define a unique mobility signature for the ion species of interest. This also turns out to be device-independent. A differential field includes both high and low field strengths which may exist, for example, in a varying RF field. A differential field mobility characteristic relates to the mobility properties of ions that are exposed to varying RF fields. 
     The relationship of the α(E) function to field conditions is shown in the following: 
                       V   c     ⁡     (   E   )       =       〈     α   ⁢           ⁢     E   s     ⁢     f   ⁡     (   t   )         〉       1   +     〈   α   〉     +     〈         ⅆ   α       ⅆ   E       ⁢     E   s     ⁢     f   ⁡     (   t   )         〉                 (   1   )               
where: Vcomp (peak position); Es—electric field strength; f(t)—waveform parameters (wave shape and so forth).
 
     Thus, for each spectral detection, we can compute α as a function of field conditions, i.e., α(E). Specifically, the asymmetric waveform in a planar field asymmetric waveform mobility spectrometer, E max (t)=E max f(t), is designed to satisfy the following conditions: 
                       1   /   T     ⁢           ⁢       ∫   0   T     ⁢         E   s     ⁡     (   t   )       ⁢     ⅆ   t           =       〈       E   s     ⁢     f   ⁡     (   t   )         〉     =   0             (     3   ⁢   a     )                 〈       f       2   ⁢   n     +   1       ⁡     (   t   )       〉     ≠   0           (     3   ⁢   b     )               
where ƒ(t)—is a normalized function which describes the waveform, and E max  is the maximum amplitude of the waveform. The waveform is designed such that its average value is zero (equation 3a) while the polarity of the electric field during one period is both positive and negative. The addition of the compensation field, C, to the waveform E s (t) yields Equation 4:
   E ( t )= E   s ( t )+ C=E   s ƒ( t )+ C    (4) 
so the average ion velocity over a period of the asymmetric waveform can be written as:
   V=&lt;V (t)&gt;=&lt; K ( E ) E ( t )&gt;  (5) 
     Only ions with average velocity of zero, v=0, will pass through the gap without neutralization. An expression for the compensation field required to enable an ion to pass through the gap can be obtained by substituting Equations 2, 3, and 4 into Equation 5 as shown in Equation 6: 
     
       
         
           
             
               
                 
                   C 
                   = 
                   
                     
                       
                         〈 
                         
                           α 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             E 
                             s 
                           
                           ⁢ 
                           
                             f 
                             ⁡ 
                             
                               ( 
                               t 
                               ) 
                             
                           
                         
                         〉 
                       
                       
                         1 
                         + 
                         
                           〈 
                           
                             α 
                             . 
                           
                           〉 
                         
                         + 
                         
                           〈 
                           
                             
                               
                                 ⅆ 
                                 α 
                               
                               
                                 ⅆ 
                                 E 
                               
                             
                             ⁢ 
                             
                               E 
                               s 
                             
                             ⁢ 
                             
                               f 
                               ⁡ 
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                           
                           〉 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     The value of this compensation electric field can be predicted precisely when the alpha parameter for the ion species, the waveform ƒ(t), and the amplitude of the asymmetric waveform E max  are known. 
     A procedure for extraction of α(E) from experimental measurements of the electric field dependence of the mobility scans is thus known. In this section, some additional considerations regarding the alpha parameter and methods to determine this parameter are described. First, emphasis must be given that the alpha parameter is a function (not a number) and the physical and chemical information about an ion is contained in the shape of the α(E) curve. The method of representing this curve is incidental to the topic. The only criterion critical in these methods is that the calculated values for the differential field mobility (i.e. K(E)=K o {1+α(E)]) should be as close as possible to the experimental values. The function for α(E) can be represented as an even power series or in complex form. In either instance, the curves of experimental results and calculations should agree closely. Thus, the quality of the approximation is limited by the accuracy of the experimental results and has been illustrated. Discerning the quality of a model based upon two parameters, three parameters, or a nonlinear function with five parameters was difficult. All approximations were located within the error of ΔC 1 (at ±9%). 
     In this work, a simple uniform method is described to represent the function of α(E), which will be suitable for comparison of results obtained under different experimental conditions. These methods could be used for differing asymmetric waveforms or different designs of IMS drift tubes: linear, cylindrical, or planar DMS. In general then, the criteria for choosing the level of approximation of alpha is first to ensure that the method of extracting the alpha parameter uses the least number of individual parameters of the experimental device. Second, the result should contain the fewest number of adjustable parameters, and the approximation curves should be within the experimental error bars. In the next section, the general method to extract the alpha parameter is described and then applied in the subsequent section. 
     The function of α(E) can be given as a polynomial expansion into a series of electric field strength E degrees as shown in Equation 7: 
     
       
         
           
             
               
                 
                   
                     α 
                     ⁡ 
                     
                       ( 
                       E 
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         n 
                         = 
                         1 
                       
                       ∞ 
                     
                     ⁢ 
                     
                       
                         α 
                         
                           2 
                           ⁢ 
                           n 
                         
                       
                       · 
                       
                         E 
                         
                           2 
                           ⁢ 
                           n 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Substituting Equation 7 into Equation 6 provides a value of the compensation voltage as shown in Equation 8 where an uneven polynomial function is divided by an even polynomial function. Therefore an odd degree polynomial is placed after the identity sign to approximate experimental results: 
     
       
         
           
             
               
                 
                   C 
                   = 
                   
                     
                       
                         
                           ∑ 
                           
                             n 
                             = 
                             1 
                           
                           ∞ 
                         
                         ⁢ 
                         
                           
                             α 
                             
                               2 
                               ⁢ 
                               n 
                             
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             S 
                             
                               
                                 2 
                                 ⁢ 
                                 n 
                               
                               + 
                               1 
                             
                           
                           ⁢ 
                           
                             〈 
                             
                               
                                 f 
                                 
                                   
                                     2 
                                     ⁢ 
                                     n 
                                   
                                   + 
                                   l 
                                 
                               
                               ⁡ 
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                             〉 
                           
                         
                       
                       
                         1 
                         + 
                         
                           
                             ∑ 
                             
                               n 
                               = 
                               1 
                             
                             ∞ 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 
                                   2 
                                   ⁢ 
                                   n 
                                 
                                 + 
                                 1 
                               
                               ) 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               α 
                               
                                 2 
                                 ⁢ 
                                 n 
                               
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               S 
                               
                                 2 
                                 ⁢ 
                                 n 
                               
                             
                             ⁢ 
                             
                               〈 
                               
                                 
                                   f 
                                   
                                     2 
                                     ⁢ 
                                     n 
                                   
                                 
                                 ⁡ 
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                               〉 
                             
                           
                         
                       
                     
                     ≡ 
                     
                       
                         ∑ 
                         
                           n 
                           = 
                           1 
                         
                         ∞ 
                       
                       ⁢ 
                       
                         
                           c 
                           
                             
                               2 
                               ⁢ 
                               n 
                             
                             + 
                             1 
                           
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           S 
                           
                             
                               2 
                               ⁢ 
                               n 
                             
                             + 
                             1 
                           
                         
                         ⁢ 
                         
                           〈 
                           
                             f 
                             
                               
                                 2 
                                 ⁢ 
                                 n 
                               
                               + 
                               1 
                             
                           
                           〉 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     This allows a comparison of the expected coefficient (approximated) to be compared to the values of alpha parameter as shown in Equation 9: 
     
       
         
           
             
               
                 
                   
                     c 
                     
                       
                         2 
                         ⁢ 
                         n 
                       
                       + 
                       1 
                     
                   
                   = 
                   
                     
                       
                         α 
                         
                           2 
                           ⁢ 
                           n 
                         
                       
                       ⁢ 
                       
                         〈 
                         
                           f 
                           
                             
                               2 
                               ⁢ 
                               n 
                             
                             + 
                             1 
                           
                         
                         〉 
                       
                     
                     - 
                     
                       
                         ∑ 
                         
                           k 
                           = 
                           1 
                         
                         
                           n 
                           - 
                           1 
                         
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               
                                 ( 
                                 
                                   n 
                                   - 
                                   k 
                                 
                                 ) 
                               
                             
                             + 
                             1 
                           
                           ) 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           c 
                           
                             
                               2 
                               ⁢ 
                               k 
                             
                             + 
                             1 
                           
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           α 
                           
                             2 
                             ⁢ 
                             
                               ( 
                               
                                 n 
                                 - 
                                 k 
                               
                               ) 
                             
                           
                         
                         ⁢ 
                         
                           〈 
                           
                             f 
                             
                               2 
                               ⁢ 
                               
                                 ( 
                                 
                                   n 
                                   - 
                                   k 
                                 
                                 ) 
                               
                             
                           
                           〉 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Alternatively, alpha parameters can be calculated by inverting the formula by using an approximation of the experimental results per Equation 10: 
     
       
         
           
             
               
                 
                   
                     α 
                     
                       2 
                       ⁢ 
                       n 
                     
                   
                   = 
                   
                     
                       1 
                       
                         〈 
                         
                           f 
                           
                             
                               2 
                               ⁢ 
                               n 
                             
                             + 
                             1 
                           
                         
                         〉 
                       
                     
                     ⁢ 
                     
                       { 
                       
                         
                           c 
                           
                             
                               2 
                               ⁢ 
                               n 
                             
                             + 
                             1 
                           
                         
                         + 
                         
                           
                             ∑ 
                             
                               k 
                               = 
                               1 
                             
                             
                               n 
                               - 
                               1 
                             
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 
                                   2 
                                   ⁢ 
                                   
                                     ( 
                                     
                                       n 
                                       - 
                                       k 
                                     
                                     ) 
                                   
                                 
                                 + 
                                 1 
                               
                               ) 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               c 
                               
                                 
                                   2 
                                   ⁢ 
                                   k 
                                 
                                 + 
                                 1 
                               
                             
                             ⁢ 
                             
                               α 
                               
                                 2 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     n 
                                     - 
                                     k 
                                   
                                   ) 
                                 
                               
                             
                             ⁢ 
                             
                               〈 
                               
                                 f 
                                 
                                   2 
                                   ⁢ 
                                   
                                     ( 
                                     
                                       n 
                                       - 
                                       k 
                                     
                                     ) 
                                   
                                 
                               
                               〉 
                             
                           
                         
                       
                       } 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     Any number of polynomial terms (say 2n), in principle, can be determined from Equation 10 though a practical limit exists as the number of polynomial terms in the experimental result of the approximation c 2n+1  should be higher than the expected number of alpha coefficients α 2n . Since the size of n depends on the experimental error, the power of the approximation of the experimental curves C(E s ) cannot be increased without limit. Usually N experimental points of C i (E si ) exist for the same ion species and experimental data can be approximated by the polynomial using a conventional least-square method. Finally, the number series terms cannot exceed the number of experimental points so increasing the number of series terms above the point where the fitted curves are located within the experimental error bars is unreasonable. In practice, two or three terms are sufficient to provide a good approximation shown in prior findings. The error in measurements must be determined in order to gauge the order of a polynomial for alpha. The sources of error in these experiments (with known or estimated error) were:
         1. Error associated with measurement and modeling of the RF-field amplitude (˜5%);   2. Error in C(E s ) from a first-order approximation of Equation 4 (˜3%), and   3. Error in measuring the compensation voltage (˜5-8%).
 
An approximate error may be ˜10% and there is no gain with approximations beyond two polynomial terms; thus, alpha can be expressed as α(E/N)=1+α 1 (E/N) 2 +α 2 (E/N) 4  with a level of accuracy as good as permitted by the measurements.
       

     A standard least-square method (regression analysis) was used to approximate or model the experimental findings. For N experimental points with C i (E si ) and for C=c 3 S 3 +c 5 S 5  a function y=c 3 +c 5 x can be defined where y=C/S 3 ; x=S 2  so c 5  and c 3  are given by Equations 11 and 12, respectively: 
     
       
         
           
             
               
                 
                   
                     c 
                     5 
                   
                   = 
                   
                     
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           N 
                         
                         ⁢ 
                         
                           
                             x 
                             i 
                           
                           ⁢ 
                           
                             
                               ∑ 
                               
                                 i 
                                 = 
                                 1 
                               
                               N 
                             
                             ⁢ 
                             
                               y 
                               i 
                             
                           
                         
                       
                       - 
                       
                         N 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               1 
                             
                             N 
                           
                           ⁢ 
                           
                             
                               x 
                               i 
                             
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     Through substituting experimental value c 3 , c 5 , values for α 2  and α 4  can be found per Equations 13 and 14: 
     
       
         
           
             
               
                 
                   
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     In order to calculate α 2n , knowledge is needed for the approximations of experimental curves for C(E 5 ) and for the function ƒ(t)—which is a normalized function describing the asymmetric waveform. 
     For example, nine data points were identified for each of the eight ketones of  FIGS. 36A ,  36 B,  37 , and  38 , based on the data collected in the tables of  FIGS. 37 and 38 . These can be used to compute the α curve for that species, such as with a piecewise linear approximation to the α curve. For example, two data points for butanone are a(Vcomp-a, Vrf-a) and b(Vcomp-b, Vrf-b). Between these two points, the slope and sign of the butanone curve can be computed. More complete characterization of the curve, such as with polynomial curve fitting, is also possible. 
     Now this data set becomes part of a data store for use in identification of the species of an unknown detected ion species for which two data points are collected and the corresponding curve data is computed. In short, in an illustrative practice of the invention, we collect data on at least two closely associated points (peaks) for a given ion sample and generate the curve data accordingly. Once we have the detected and computed data, we assume this approximates the alpha curve and therefore do a lookup to our stored data. Upon finding a match, we can then positively identify the sample. 
     In  FIGS. 39A and 39B  (monomers and clusters, respectively) we computed unique α curves for keytone ions (acetone, butanone, pentanone, hexanone, heptanone, octanone, nonanone, decanone) based on data collected in the tables of  FIGS. 37 and 38 , plotting the percent change in α against the change of field strength for the various data collected. These plots of percent change in α against field strength express a unique signature for each of these ion species. This is loaded in our data store for later comparison: the signature data includes the RF field strength and the compensation voltage at which the peak is detected. We also associate with it the identifying data for the known α function associated with that detected peak location and field conditions for each species. 
       FIGS. 39A and 39B  thus express the a function for individual ketones spanning electric fields of 0 to 80 Td (˜23 kV/cm), expressed as a percentage change in alpha as a function of field conditions. These plots are fundamental signature features of these ion species that are independent of the drift tube parameters and can be used in other mobility spectrometers. Thus, the α function can be favorably used in practice of the invention to provide a mobility identification data set that is device-independent. 
     These results are surprising and demonstrate that for chemicals with the same functional group, protonated monomers of a single type exhibit a broad range of behavior vis-à-vis the dependence of coefficients of mobility on electric fields. This difference in behavior for a common moiety suggests that the effect from the electric field must be associated with other aspects of molecular structure. One possible interpretation is that ions are heated during the high field and the effect on the protonated monomer should be striking. These ions with structures of (H 3 O) + M (H 2 O) n  or perhaps (H 3 O) + M (H 2 O) n (N 2 ) 2 , should be prone to dissociations with slight increases in ion temperature caused by the high field conditions. Thus, ion cross-sections and mobilities would accompany declustered small ions at high fields. 
     Referring again to  FIG. 39A , it should be noted that there is approximately a 20% increase in α(E) for the protonated monomer of acetone with high fields. As the molecular weight of the keytone is increased, ion heating is less pronounced and reflected in the α(E) function. The α(E) function for proton bound dimers (clusters) is consistent with decreases in mobility under high field conditions. Consequently, the basis for the α(E) function differs from that of protonated monomers. Indeed, the proton bound dimer for decanone undergoes about a 5% decrease at high fields. The cause for a decrease in mobility at high fields has no existing model but should be due to increased collisional size or increased strength of interaction between the ion and the supporting gas. 
     Furthermore, if we were to do the same for the cyclohexane and DMMP in  FIG. 4 , the computed alpha curves would differ accordingly. In this manner, the invention can distinguish ion species even when their mobility curves overlap, as long as we have at least a second detection data set to associate with each detected species in question. Therefore, the invention achieves a high level of assurance for the accuracy of identifications. 
     Thus we have shown that the fundamental dependence of mobility for ions in high electric field can be obtained from field asymmetric ion mobility spectrometry. Functions of dependence can be extracted from experiments using known methods to treat imperfect waveforms. These findings show an internal consistency with a homologous series of ketones, and also indicates a mass dependence not previously reported. 
     Focusing attention now on  FIGS. 40A-40F  a specific sequence of steps is described that may be carried out to perform species identification in several of the embodiments of the invention. These steps are provided by way of illustration and not limitation. In this illustration, the sequence of steps may be performed by the microprocessor  46  of the ion mobility spectrometer device  10  of  FIG. 5 . The microprocessor  46  provides digital control signals to the RF dispersion voltage (Vrf) generator  42  and compensation voltage (Vcomp) generator  44  to control the drive voltages for the filter  24 . The voltage generators  42  and  44  may also include, for example, digital-to-analog converters, not shown in detail in  FIG. 5 . 
     The microprocessor  46  coordinates the application of specific RF dispersion voltages Vrf and compensation voltages Vcomp, also taking into account the function of observing responses from the detector  26  as read through the analog to digital converter  48 . By detecting attributes (such as the peaks) of observed abundances of a particular ion species across a range of Vrf voltages, the microprocessor  46  can thus take steps to identify particular compounds. These may include, for example, comparing or correlating particular “response curve” data against a library of response curve data as stored in the memory  47 . They can also include computation of α curve parameters. The results of the comparison operation can be provided in the form of an appropriate output device such as a display or personal computer or the like, or maybe provided by electrical signals through an interface to other data processing equipment. 
     As shown more particularly in  FIG. 40A , a state  1000  is entered into the microprocessor  46  in which a compound is to be analyzed. Here, the compound is known and identified, such as by a user supplying an identifying text string to the computer. A sequence of steps is then performed by which data is to be acquired concerning the known chemical compound. From this state  1000 , a next state  1002  is entered in which a range of dispersion voltages Vrf and compensation voltages Vcomp are determined by the processor  46 . These ranges include a beginning voltage (b) and an end voltage (s) and step voltage(s) to be applied to each of the ranges Vrf is thus varied from an initial value Vrf(b) to a final value Vrf(e) by a step amount Vrf(s). Similarly, Vcomp is to be varied from Vcomp(b) to a final value Vcomp(e) by a step amount Vcomp(s). 
     The voltage ranges are then applied in the following steps. Specifically, step  1004  is entered in which the Vrf is allowed to step through a range of values. A state  1008  is entered next in which the compensation voltage Vcomp is also swept or stepped through a series of values or ranges. In state  1010 , the response to each applied voltage is stored as a value, (a). 
     If the last compensation voltage has not yet been tested, then processing returns to state  1008  in which the next compensation voltage is applied. However, in state  1012 , if all of the compensation voltages have been applied, then processing proceeds to a state  1014  wherein a test is made to see if all of the dispersion has been applied. 
     The loop continues until all of the compensation and dispersion voltages have been applied. The resulting set of data is then analyzed in a state  1018  to identify features of interest. In the specific example being described, it is the peak locations that are of interest. For each such peak in an observed response for a given applied dispersion voltage Vrf, a response value for a specific Vcomp is determined and its corresponding amplitude (a) is detected and stored. 
     The response curve data, or certain attributes thereof such as the peak locations are then stored as a data object P (or table) as shown in  FIG. 40B . Such an object illustratively contains an identification of the tested compound such as a text string. Also stored are a set of the applied dispersion voltages Vrf. For each such dispersion voltage Vrf, a corresponding peak compensation voltage is stored. Specifically, at least the compensation voltage Vcomp at which a peak was observed, and preferably, the corresponding amplitude of the response (abundance) observed at that peak is stored. 
     As previously described in detail, for a given Vrf, there may be a set of compensation voltages at which a number of “peaks” are observed. For example, as was described in connection with  FIG. 14A , the sample analyzed can be made up of a compound of specific ions, including monomers, cluster ions, and reactant ion peaks. Thus, illustratively, there is an accommodation in the structure of object P to anticipate that there will be more than one peak observed in any particular mobility scan, and that the number of peaks per response curve may not always be the same number. 
     An example, the illustrative object P of  FIG. 40B , includes a data element, where for a single RF dispersion voltage Vrf- 1 , peaks may be observed at compensation voltages Vc 11 , . . . , Vcmn having corresponding amplitudes a 11 , . . . , amn. This may correspond to the case of the lowest applied dispersion voltage in  FIG. 14A , where numerous peaks  601 -,  605 - 1 ,  608 - 1  are detected. However, at another dispersion voltage Vrf-m, only a single peak at Vcomp-m, am was detected. This might correspond to a case such as in the uppermost curve of  FIG. 6A , where only a single peak  601 -m was detected. 
     In an illustrative application, a library of data objects P (reference vectors) is developed by performing the steps of  FIG. 40A  for a plurality of known compounds of interest. This then permits an instrument to eventually enter a chemical recognition state  1200  as shown in  FIG. 40C . Next, a series of measurements are taken in states  1202 - 1214 . This series is similar to the measurements taken in  FIG. 40A . Specifically, a series of measurements are taken for a specified compensation and RF voltages. It should be understood that an entire set of all of the same measurements need not be taken in this mode as were taken in the chemical data acquisition mode. Specifically, not all points on a relatively dense response curve need to be taken, only enough to identify each compound. 
     Once the measurements are taken, a state  1220  is entered in which features, such as peaks of the response are identified for each peak, a corresponding compensation voltage and amplitude may be identified, and these stored to a candidate measurement vector P′. The candidate vector P′ thus represents a series of data that needs to be tested against a number of candidate compounds. The candidate vector P′ is then analyzed in states  1230  and/or  1240  by looking up corresponding counterparts in the library of reference vector objects P, and scoring a match between P and P′. These steps may be iterated until such time as a match or a best match is determined in a state  1250 . 
     It should be understood that any number of techniques may be used to determine a degree of match between P and P′. For example, if the elements (Vcomp, a) of P and P′ are considered to be data points in Euclidian geometry space, a distance can be computed. The comparison with the smallest Euclidian distance can then be selected as the best match. However, other recognition techniques may be used to determine an identity of an unknown compound, for example, more sophisticated signal processing techniques such as correlation may be used to resolve peaks; or other known pattern recognition algorithms, neural networks or artificial intelligence techniques may be used to find a best match for P′. This best match is then identified to a user, such as by looking up the compound identifier field and displaying it in state  1260 . 
       FIG. 40D  shows a series of steps, which may be added to the data acquisition phase and the chemical recognition phase to take advantage of second order data processing characteristics. For example, in the data acquisition state, a series of states  1020 ,  1022 ,  1024  and  1026  may be added to curve-fit specific attributes of the measured response. Specifically, a state  1020  may be entered in which for each data element of the object P a vector, z, is formed consisting of the peak compensation voltages vc 11 , vc 12 , . . . vc 1 m. 
     This vector is a vector of point locations for the peaks observed for a range of compensation voltages. Returning attention to  FIG. 14A , briefly, this may correspond, for example, to locating the points  601 - 1 , . . .  601 -m, . . .  601 -n corresponding to peak height and locations for the monomer ions of interest. A curve may then be fit through these peaks such as by applying a curve fitting algorithm, in state  1024 . In the illustrated example it is assumed that a quadratic equation is fitting the peaks of the form y 2 =βx 2 +γ. The β and γ coefficients can then be stored in the state  1026  associated with the vector. The chemical is thus identified by a curve fit to its peak locations approximating its mobility (α coefficient) behavior. 
     If this is done, a corresponding set of steps  1270 ,  1272  and  1274  can be added to the recognition process to identify peaks by performing a curve fit to observe data, and then, determining γ and β coefficients, rather than comparing raw data values in states  1270  and  1272 . In state  1274 , the β and γ coefficients are tested to determine closest matches in the P object library. 
       FIG. 40F  shows a series of steps that may be used to identify or distinguish peaks in the acquisition phase. Here initial data may be added to the objects P by identifying peaks as a cluster peak or monomer peak. Specifically, if a peak shift over a range of field condition voltages (e.g.,  FIG. 14A ) increases (i.e., shifts to the right), then this may be identified as a cluster peak. If the peak does not meet specific shifting criteria, it may be identified as a monomer peak. States  1310 ,  1331 , and  1332  may thus be added to the identification process. The results of these steps adds an additional parameter L associated with each data point in the object P to further identify each peak as a monomer cluster or other peak type, as shown in  FIG. 40E . 
     Other approaches to this may be used to label peaks. For example, reactant ion peaks (RIP) may also be identified by performing an analysis on a response of the instrument, with no sample S applied. In this mode, only the RIPs occur, and in their behavior across a range of compensation voltages can be stored. Information concerning the particular type of peak may be stored in pointer data in a state  1320 , at which such a peak is detected. This information can then be added to the objects P, specifically as shown in  FIG. 40E . 
       FIG. 40G  shows additional processing steps, which may be performed in the compound recognition state to take advantage of the situation of  FIGS. 36A-38  in which monomer and cluster ion behavior is observed. Specifically, the steps of  FIG. 40G  may be added as further steps  1280  in the recognition phase. Here, for every candidate peak P′, a corresponding monomer peak in the reference array P is compared. A score is then associated with the closest of the match in state  1284 . Similarly, in state  1286 , a cluster peak may be compared with its corresponding peak in the peak library P. A score sc is then determined in step  1288 , depending on the closest of this match. In a state  1290 , a final score sf can be associated with weighting the monomer peak score and the cluster peak score by weighting factors wm and wc. For example, in an instance where cluster peaks are expected to provide more information than monomer peaks, cluster peaks may be weighted highly and monomer peaks relatively low or zero factor. Using this weighting, both monomer and cluster peak identification can be combined to further refine compound analysis. 
     In various applications, the above described approaches to ion-based sample analysis may be employed in relatively compact, such as handheld, analyzer systems.  FIG. 41  is a conceptual diagram of such a compact DMS analyzer system  1400 . The DMS system may be used, for example, to analyze compounds, such as chemical warfare agents (CWAs), and Toxic Industrial Compounds (TICs), and Toxic Industrial Materials (TIMs) according to an illustrative embodiment of the invention. By operating the compact DMS analyzer system  1400  at less than atmospheric pressure, e.g., 0.5 atm, as described above, the system  1400  approximately doubles its resolution over existing state-of-the-art systems, while reducing its power consumption and size. By performing sample fragmentation, as described above, sample analysis may be further enhanced. By utilizing three-dimensional color dispersion plots, as also described above, analysis of CWAs, TICs, and TIMs is further enhanced. 
     The DMS analyzer system  1400  may employ an electromechanical pump, compressed gas or air, or the solid-state flow generator  1402 , which includes an ion source  1404 , an ion attractor  1406 , and a constrained flow channel  1408  for controlling sample flow and/or pressure within the system  1400 . The ion source  1404  provides a source of ions and the ion attractor  1406  attracts either positive or negative ions, depending on an applied bias voltage. The ion flow created in the constrained channel  1408  due to the ion flow generated by the interaction of the ion source  1404  with the ion attractor  1406  creates a fluid, e.g., a sample effluent, flow. In some illustrative embodiments, the DMS analyzer system  1400  may be miniaturized, such that the analyzer unit  1410  is included in application-specific integrated circuits (ASICs) embedded on a substrate  1412 . A solid state flow generator of the type employed by the invention is described in further detail in co-pending and co-owned U.S. patent application Ser. No. 10/943,523, filed on 17 Sep. 2004, the entire contents of which are incorporated above by reference. 
     The constrained channel  1408  includes an inlet end  1414  and an outlet end  1416 . The constrained channel  1408  also includes a sample introduction inlet  1418  to enable the analyzer  1410  to collect the sample gas for analysis. A pre-concentrator  1420  may be employed at the sample introduction inlet  1418  to concentrate the sample and improve analysis accuracy. An ionizer  1422  provides ionization of the sample using, for example, a radioactive Ni 63  foil, or non-radioactive plasma ionizer, or other suitable ionization source within ionization region  1424 . A plasma ionizer has the advantage of enabling precise control of the energy imparted to the sample gas for ionization. Ideally, the ionizer  1422  imparts only enough energy to ionize the sample gas, without producing nitric oxides (NOx&#39;s) and ozone. A fragmentation region may also be included in the system  1400 . NOx&#39;s and ozone are undesirable because they can form ion species that interfere with the ionization of CWA agents. Because diffusion and mobility constants generally depend on pressure and temperature, the DMS analyzer system  1400  may include a temperature sensor  1426  and/or a pressure sensor  1428  for regulating the temperature and/or pressure of the sample gas within the analyzer unit  1410  for more accurate analysis. The analyzer  1410  may also include a humidity sensor. The analyzer  1410  also includes an analytical region  1440  with filter plates  1442  and detector plates  1444 . A molecular sieve  1446  may be employed to trap spent analytes. 
     The controller  1446  provides control of filtering and detection while also providing an output of the detection results. The power supply  1448  provides power to the filter plates  1442 , solid-state flow generator  1402 , and any other component requiring electrical power. The controller electronics  1446  for Vcomp, Vrf, the ion heater pumping, the DMS ion motion, and the pre-concentrator  1420  heater may be located with the analyzer unit  1410 . Also, the detector  1444  electronics, pressure  1426  and temperature  1428  sensors, and the processing algorithm for a digital processor may reside within analyzer  1410 . 
     At atmospheric pressure, to realize the benefits of mobility nonlinearity, the DMS analyzer system  1400  illustratively employs RF electric fields of about 106 V/m, and a Vrf of about 200 Vpeak at about a 200×10 −6  μm gap. However, any suitable RF electric field parameters may be employed. The power supply  1448  may be remotely located relative to the analyzer unit  1410  to generate RF voltage for the filter plates  1442 . At less than atmospheric pressure, the RF electric field may be reduced as described above to further reduce the power consumption and size of the DMS analyzer system  1400 . 
     The DMS analyzer system  1400  may also interface with a personal computer (PC) or controller  1446  to utilized signal-processing algorithms that convert analyzer  1410  outputs into detection, identification, and/or measurement of analytes and concentration levels. The controller  1446  or an interfacing PC may also facilitate control and power management for the DMS analyzer system  1400 . The supporting electronics for the DMS analyzer system  1400  may be implemented, for example, on an ASIC, a discrete printed circuit board (PCB), or System on a Chip (SOC). 
     In operation, the solid-state flow generator or electromechanical transport pump  1402  draws samples into the DMS analyzer system  1400  at the inlet  1414  and past a CWA-selective chemical membrane concentrator  1420  having an integrated heater. The CWA-selective chemical membrane pre-concentrator  1420  may also serve as a hydrophobic barrier between the analytical region  1440  of the analyzer system  1400  and the sample introduction region  1450 . The membrane of the pre-concentrator  1420 , illustratively, allows CWA agents to pass, but reduces the transmission of other interferants and acts as a barrier for moisture. 
     The pre-concentrator  1420  may use selective membrane polymers to suppress or block common interferences (e.g., burning cardboard) while allowing CWA agents or CWA simulants to pass through its membrane. Although many selective membrane materials are available, poly-dimethyl siloxane (PDMS) may be a preferred membrane/concentrator/filter to reject water vapor and collect CWA analytes. At high concentration levels, water vapor molecules may cluster to the analytes, altering the analytes&#39; mobilities. Membrane materials such as hydrophobic PDMS tend to reduce the vapor to acceptable levels while absorbing and releasing analyte atoms. The thin membrane of the pre-concentrator  1420  may also be heated periodically to deliver concentrated analytes to the ionization region  1424  and analytical region  1440 . 
     Except for diffusion of analytes through the membrane/filter/pre-concentrator  1420 , the analytical region  1440  is generally sealed to the outside atmosphere. Thus, the analyzer system  1400  may employ elements for equalizing the pressure inside analytical region  1440  with the atmospheric pressure outside the analyzer system  1400  or maintain pressure in the analytical region  1440  at less than atmospheric pressure for improved ion intensity peak resolution. Once the sample gas molecules are ionized, the ions are driven longitudinally in the direction indicated by the arrow  1452  through the ion filter plates  1442  by static or traveling electrostatic fields, as opposed to being driven by the carrier gas. The filter plates  1442  apply transverse radio frequency (RF) field voltages and dc excitation electric compensation fields to the ions moving through analytical region  1440  to separate the species within a sample. 
     With water vapor removed, interferants (e.g., hydrocarbons and others) typically comprise roughly 0.10% of the incoming air volume by weight. Depending on the collection efficiency of the pre-concentrator  1420 , the molecular sieve  1446  may be sized to support about 6, 9, 12 or more months of substantially continuous or continuous operation before saturating. The molecular sieve  1446  may also be configured to allow movement of air in a circulatory fashion through the ion filter electrodes  1442  and back to the ionization region  1424 . 
     The DMS analyzer system  1400  may be used for detecting low concentrations (e.g., parts per trillion (ppt)) of CWAs, such as, without limitation, nerve and blister agents. In one illustrative embodiment, the DMS analyzer system  1400  includes a high-sensitivity, low-power, sample gas analyzer  1404  that builds on MEMS technology, but further miniaturizes the DMS analyzer system  1400  to achieve parts-per-trillion sensitivity, about 0.25 W overall power consumption (i.e., 1 Joule measurement every 4 seconds), and a size of about 2-cm 3  or less. 
     Because of the smaller analytical region  1440  and the resulting lower flow rate requirements, a low-power (e.g., mW) solid-state gas transport pump  1402 , using ionic displacement, may be employed to draw an air sample into the DMS analyzer system  1400  and onto the CWA-selective chemical membrane pre-concentrator  1420 . Compact DMS analyzer systems according to the invention have shown very high sensitivities to CWA simulants. By way of example, a compact DMS analyzer system according to the invention has been shown to detect methyl salycilate at parts-per-trillion (ppt) levels. The DMS analyzer system  1400  has the ability to resolve CWA simulants from interferants that cannot be resolved by current field-deployed detection technologies. 
       FIG. 42  is a graph depicting a DMS spectra showing resolution of dimethylmethylphosphonate (DMMP) from aqueous firefighting foam (AFFF) as measured in a DMS analyzer system  1400 . AFFF is one interferant that has proved extremely challenging for conventional IMS systems to resolve CWAs or other simulants. The AFFF ion intensity peak tends to overlap with the agent peak during sample detection in DMS or IMS systems. 
       FIG. 42  is a graph of multiple plots showing experimental results for a series of CWA simulants selectively mixed with 1% headspace of AFFF. The top plot  1460  of  FIG. 42  shows RIP for a DMS analyzer system  1400  with background air but no sample present with the sensor at atmospheric pressure. In the next plot  1462 , the AFFF interferant is added. This results only in a slight shift to the left (more negative compensation voltage) of the RIP ion intensity peak. Then, in plot  1464 , the CWA simulant DMMP is introduced into the spectrometer and the typical monomer and dimmer peaks appear together with a corresponding reduction in the RIP peak ion intensity. When 1% AFFF is added according to plot  1468 , the DMMP peaks are not effected and only a slight leftward shift of the RIP is observed. The same experiment was repeated with DIMP in plots  1468  and  1470 , and the effect of AFFF was negligible. In plot  1472 , MS is introduced, and according to monitored negative ion peaks, gives similar data illustrating the lack of interference with AFFF. The conclusion is that 1% AFFF has virtually no effect. Thus,  FIG. 42  illustrates the ability of the DMS analysis system  1400  to resolve CWA simulants from interferants. 
     In one illustrative embodiment, the compact hand-held DMS analyzer system  1400  is achieved by combining the following design characteristics: (a) using the analyzer/filter/detector  1410  with improved sensitivity and size reduction; (b) using the solid-state flow generator or electromechanical pump as a gas transport pump  1402  to sample and move analytes; (c) using the CWA-selective chemical membrane pre-concentrator  1420  with integrated heater (in some configurations provided by using a solid-state generator or electromechanical pump to transfer heat from other analyzer system components to the pre-concentrator  1420 ) to remove water vapor and to concentrate; and/or (d) using electric field propulsion of the ions  1454  through the analytical region  1440  of analyzer  1410 . 
     According to various illustrative embodiments, the invention improves the resolution of species identification over conventional systems, while decreasing size and power to achieve parts-per-trillion sensitivity, a less than about 0.25 mW overall power dissipation, and a size of about a 2-cm 3  or less in an entire system not including a power source or display, but including an RF field generator. According to some embodiments, an analyzer system of the invention has a total power dissipation of less than about 15 W, about 10 W, about 5 W, about 2.5W, about 1 W, about 500 mW, about 100 mW, about 50 mW, about 10 mW, about 5 mW, about 2.5 mW, about 1 mW, and/or about 0.5 mW. According to further embodiments, an analyzer system according to the invention, optionally including a display (e.g., indicator lights and/or an alphanumeric display) and a power source (e.g., a rechargeable battery) compartment, along with an RF field generator, may have a total package outer dimension of less than about 0.016 m 3 , 0.0125 m 3 , 0.01 m 3 , 0.0056 m 3 , 0.005 m 3 , 0.002 m 3 , 0.00175 m 3 , 0.0015 m 3 , 0.00125 m 3 , 0.001 m 3 , 750 cm 3 , 625 cm 3 , 500 cm 3 , 250 cm 3 , 100 cm 3 , 50 cm 3 , 25 cm 3 , 10 cm 3 , 5 cm3, 2.5 cm 3 , with the package being made, for example, from a high impact plastic, a carbon fiber, or a metal. According to further embodiments, an analyzer system, for example, according to the invention, including an RF generator, and optionally including a display, keypad, and power source compartment, may have a total package weight of about 5 lbs, 3 lbs, 1.75 lbs, 1 lbs, or 0.5 lbs. 
     Table 1 provides a comparison of drift tube (e.g., the constrained channel) dimensions, fundamental carrier gas velocities, and ion velocities for a various illustrative embodiments of a DMS analyzer system  1400  depending on the flow rate (Q) available to the analysis unit. Designs 1-4 provide flow rates of varying orders of magnitude ranging from about 0.03 l/m to about 3.0 l/m. Table 1 illustrates that as the flow rate is decreased through the DMS analyzer system  1400 , the filter plate dimensions and power requirements are reduced. Table 1 is applicable to a DMS analyzer system  1400  using either a sample gas or longitudinal field-induced ion motion. The time to remove an unwanted analyte is preferably less than about the time for the carrier to flow through the filter region (tratio). Also, for a particular target agent, the lateral diffusion as the ion flows through the analyzer  1410  is preferably less than about half the plate spacing (difratio). Based on this criteria, the plate dimensions may be reduced to about 3×1 mm 2  or smaller, while the ideal flow power may be reduced to less than about 0.1 mW. Thus, even for design 4, the number of analyte ions striking the detectors is sufficient to satisfy a parts-per-trillion detection requirement. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Illustrative DMS Analyzer System Design Specifications and Characteristics 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Design 1 
                 Design 2 
                 Design 3 
                   
               
               
                   
                   
                   
                 Q = 3 l/m 
                 Q = 0.3 l/m 
                 Q = 0.3 l/m 
                 Design 4 
               
               
                 Description 
                 Units 
                 Symbol 
                 Baseline 
                 Base dimen 
                 scaled 
                 Q = 0.03 l/m 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 plate dimensions 
                   
                   
                   
                   
                   
                   
               
               
                 *length 
                 m 
                 L 
                 0.025 
                 0.025 
                 0.005 
                 0.001 
               
               
                 *width 
                 m 
                 b 
                 0.002 
                 0.002 
                 0.001 
                 0.0004 
               
               
                 *air gap 
                 m 
                 h 
                 0.0005 
                 0.0005 
                 0.0005 
                 0.0002 
               
               
                 *volume flow rate 
                 l/min 
                 Qf 
                 3 
                 0.3 
                 0.3 
                 0.03 
               
               
                 Flow velocity 
                 m/s 
                 Vf 
                 50 
                 5 
                 10 
                 6.25 
               
               
                 pressure drop 
                 Pa 
                 dPf 
                 1080 
                 108 
                 43.2 
                 33.75 
               
               
                 flow power 
                 W 
                 Powf 
                 0.054 
                 0.00054 
                 2.16E−04 
                 1.69E.05 
               
               
                 RF excitation 
                 V 
                 Vrf 
                 650 
                 650 
                 650 
                 260 
               
               
                 design ratios 
               
               
                 Time to remove 
                 s 
                 tratio 
                 0.0128 
                 0.0013 
                 0.0128 
                 0.0160 
               
               
                 unwanted analyte 
               
               
                 divided by carrier time 
               
               
                 wanted ions-lateral 
                 s 
                 difratio 
                 0.200 
                 0.632 
                 0.200 
                 0.283 
               
               
                 diffusion divided 
               
               
                 by half gap 
               
               
                 ions to count per cycle 
                 — 
                 Nout 
                 1.22E+07 
                 1.22E+06 
                 1.22E+06 
                 1.22E+05 
               
               
                   
               
            
           
         
       
     
     For sample/carrier gases, there does not appear to be an electromechanical pump that operates at the preferred flow characteristics with an efficiency better than about 0.5%. With a 0.5% efficiency, an ideal flow loss of about 0.05 mW results in an actual power consumption of about 10 mW, about a factor of 100 greater than in the above discussed illustrative embodiment of the invention. 
     The DMS system  1400  may simultaneously detect both positive and negative ion intensity peaks which further improves detection selectivity. The combination of the positive and negative ion channel information, the shift in spectral peak as a function of applied field strength or voltage, and the display is this information in a three-dimensional manner provide a novel mechanism for chemical identification. 
       FIG. 43  is a three-dimensional dispersion plot  1750  of the detection of positive ions of agent GA over a range of field voltages and field compensation voltages with varying intensity represented in varying color according previously described illustrative embodiments of the invention. The plot  1750  illustrates the enhanced identification (selectivity) of a compound using a three-dimensional dispersion plot by, for example, a DMS system  1400 . In comparison,  FIG. 25  is a three-dimensional dispersion plot of negative ions of GA over a range of RF voltage versus compensation voltage with varying intensity represented in varying color that illustrates the enhanced identification (selectivity) of a compound using a three-dimensional dispersion plot by, for example, DMS system  1400 . Both measurements were performed with a concentration of GA at 0.14 ng/l, a Ni 63  source, 50% RH, 3 scan averaging, and 350 cc/min carrier gas flow. The differences between the three-dimensional plots  814  of  FIG. 25 and 1750  of  FIG. 50  illustrate that performing both positive and negative ion mode detection provides enhanced signature identification of ion species. 
     In certain illustrative embodiments, the compact DMS system  1400  of  FIG. 41  and various other figures may employ features and/or be incorporated into systems described in further detail in U.S. Pat. Nos. 6,495,823 and 6,512,224, the entire contents of both of which are incorporated herein by reference. 
       FIGS. 44-53  are conceptual block diagrams of chemical and/or biological agent detection systems using various configurations of a mobility detection analyzer system such as those depicted and described herein, a recirculation system, and other components according to illustrative embodiments of the invention. More particularly,  FIG. 44  is a conceptual block diagram of a CWA and/or biological agent detection system  1476  according to an illustrative embodiment of the invention. The system  1476  employs a mobility detection system  1478 , molecular sieve  1480 , pump  1482  with optional vent  1484 , optional second molecular sieve  1486 , circulating channel  1488 , sample inlet  1490 , exhaust  1492 , membrane  1494 , and orifice  1496 . The system  1476  may also employ filtered air or gas  1498  to circulate or transport a sample through the system  1476 . The mobility analyzer system  1478  may be a compact DMS analyzer system  1400  of  FIG. 48 , DMS system  10  of  FIG. 5 , an IMS, a TOF-IMS, a GC-IMS, an MS or the like. The system  1476 , like all of the previously described illustrative systems, may employ one or more dopants such as, methylene bromide (CH 2 Br 2 ), methylene chloride (CH 2 Cl 2 ), chloroform (CHCl 3 ), water (H 2 O), methanol (CH 3 OH), and/or isopropanol, introduced, mixed and/or flowed with the sample to enhance analysis. 
     In operation, the system  1476  receives a sample S at inlet  1490  and passes it through the membrane  1494  into the circulation channel  1498 . The membrane  1494  may filter out unwanted interferants, if desired, in the same or similar manner as the pre-concentrator  1420  of  FIG. 48 . The orifice  1496  may, in a fixed, controlled, or adjustable manner, regulate the gas and/or sample flow into the analyzer system  1478  and thereby regulate or control the pressure within the analyzer system  1478 . Thus, the analyzer system  1478  may operate at atmospheric pressure, below atmospheric pressure, or above atmospheric pressure. The pump  1482  maintains gas flow in the analyzer system  1478  and pressure control either independently or in coordination with the orifice  1496 . Thus, in one example, the pump  1482  draws sample flow through the orifice  1496  into the analyzer system  1478  to enable detection and identification of selected ion species. The analyzer system  1478  may be a DMS system  1400  that tunably detects certain ion species by adjusting its field/flow channel conditions, such as, its Vrf and Vcomp, parameters and in some configurations, controlling the pump  1484  and/or the orifice  1496  to control pressure within the system  1400 . 
     Once detection and identification are performed, the molecular sieve  1480  may trap spent analytes from the analyzer system  1478 . Again, the pump  1484 , whether electromechanical or solid-state, propels the gas, optionally through a second molecular sieve  1486 , through the circulating channel  1488 . The sample gas is then expelled through the membrane  1494  and the outlet  1492  or mixed and re-circulated with more sample S back into the orifice  1496 . 
       FIG. 45  is a conceptual block diagram of a CWA and/or biological agent detection system  1500 , configured for reduced pressure analysis, according to an illustrative embodiment of the invention. The system  1500  is similar to the system  1476  except that an additional sample flow channel  1502  is employed instead of a membrane. The system  1500  includes sample S inlet  1504 , orifice  1506 , ionization region  1508 , deflector plate  1510 , attractor plate  1512 , channel  1502  pump  1514 , second channel  1516 , analyzer system  1518 , molecular sieve  1520 , pump  1522 , and optional second molecular sieve  1524 . 
     In operation, the system  1500  draws sample S through the sample inlet  1504  and through the orifice  1506 . The orifice  1506  may be controlled, fixed, or adjustable to regulate sample gas flow and/or pressure in the channel  1502 . The pump  1514  may also be used in coordination with the orifice  1506  to regulate gas flow and/or pressure within the channel  1502 . The deflector plate  1510  may force, push, or selectively separate ions into the channel  1516  through the opening  1526  while the attractor  1512  may attract ions from the channel  1502  into the channel  1516 . A pressure drop across the opening  1526  may be adjusted so that only sample ions enter the channel  1516  while sample neutrals are prevented from entering. The sample ions may be directly introduced into the analyzer system  1518  or the ions may be neutralized and then re-ionized in the analyzer system  1518 . The analyzer system  1518  may be a DMS system, IMS system, or the like. The analyzer system  1518  may include multiple DMS, IMS, or other like systems or a combination of such systems to perform sample detection and identification. For example, system  748  of  FIG. 21  or system  754  of  FIG. 22  may be employed to apply conventional DMS detection in combination with fragmentation to enhance sample analysis. 
     The channel  1516  pump  1524  may then draw the sample S from the analyzer system  1518  through the molecular sieve  1520  and then propel the sample S, optionally through the second molecular sieve  1524 . The molecular sieves  1520  and  1524  will capture most of the spent sample S analytes. Any remaining sample S is mixed with new sample S gas and returned to the analyzer system  1518  via the channel  1516 . The outlet  1528  expels sample S gas from the channel  1502 . 
       FIG. 46  is a conceptual block diagram of a cylindrical or coaxial CWA and/or biological agent detection system  1530  according to an illustrative embodiment of the invention. The system  1530  includes a sample S inlet  1532 , constrictor  1534 , inner channel  1536 , opening  1538 , clean transport gas inlet  1540 , outer channel  1542 , analyzer system  1544 , channel  1542  outlet  1546 , and channel  1536  outlet  1548 . 
     In operation, the system  1530  draws the sample S into the channel  1536  through the constrictor or orifice  1534 . The constrictor  1534  may be adjustable, controllable or fixed to enable a pressure reduction below I atm, for example to 0.5, 0.65, or 0.85 atm, in the channel  1536 . The clean transport gas inlet  1540  receives clean transport gas into the channel  1542 . The channel  1542  may operate at pressures below 1 atm. The sample S may be drawn or attracted into the channel  1542  through the opening  1538  by a pressure differential with the channel  1536 , an ion attractor in channel  1542 , gas flow into channel  1542 , or other like technique. The analyzer system  1544  then detects and identifies the ion species of the sample S and expels the sample S through the outlet  1546 . The sample neutrals in the channel  1536  may be expelled through the outlet  1548 . 
       FIG. 47  is a DMS system  1550  including an orifice  1552  at the system  1550  inlet to control pressure within the system  1550  in coordination with a pump  1554 . The system also includes the molecular sieve  1556 , ion source  1558 , filter  1560 , and detector  1562 . In operation, the pump  1554  has sufficient power to draw a sample S through the orifice  1552  to then enable detection of the sample at a reduced pressure. 
       FIG. 48  is a DMS system  1564  including an orifice  1566 , ionization source  1568 , filter  1570 , detector  1572 , molecular sieve  1574 , pump  1576 , a second molecular sieve  1578 , a membrane  1580 , an inlet  1582 , and outlets  1584  and  1586 . Because the membrane  1580  is positioned upstream of the orifice  1566  and the sample flow is in direction  1588 , the membrane  1580  operates at atmospheric pressure while the ionization source  1568 , filter  1570 , and detector  1572  operate below atmospheric pressure due to a pressure drop across the orifice  1566 . It may be advantageous to operate the membrane  1580  at atmospheric pressure to prolong its useful life. 
       FIG. 49  is a DMS system  1590  including an orifice  1592 , ionization source  1594 , filter  1596 , detector  1598 , molecular sieve  1600 , pump  1602 , a second molecular sieve  1604 , a membrane  1606 , an inlet  1608 , and outlets  1610  and  1612 . Because the membrane  1606  is positioned downstream of the orifice  1592  and the sample flow is in the direction  1614 , the membrane  1606  operates below atmospheric pressure along with the ionization source  1594 , filter  1596 , and detector  1598  due to a pressure drop across the orifice  1592 . It may be advantageous to operate the membrane  1606  below atmospheric pressure. 
       FIG. 50  is a DMS system  1616  including an orifice  1618 , ionization source  1620 , filter  1622 , detector  1624 , molecular sieve  1626 , pump  1628 , a second molecular sieve  1630 , a membrane  1632 , an inlet  1634 , and outlets  1636  and  1638 . Because the membrane  1632  and the ionization source  1620  are positioned upstream of the orifice  1618  and the sample flow is in direction  1640 , the membrane  1632  and the ionization source  1620  operate at atmospheric pressure while the filter  1622  and detector  1624  operate below atmospheric pressure due to a pressure drop across the orifice  1618 . It may be advantageous to operate the membrane  1632  and ionization source  1620  at atmospheric pressure. 
       FIG. 51  is a DMS system  1642  including a first channel  1644  and a second channel  1646  operating at atmospheric pressure. The first channel  1644  includes an ionization source  1648 , deflector electrode  1650 , pump  1652 , inlet  1666 , and outlet  1668 . The second channel  1646  includes a filter  1654 , detector  1656 , molecular sieve  1658 , pump  1660 , and molecular sieve  1662 . An opening  1664  provides fluid communication between the channels  1644  and  1646 . 
     In operation, the system  1642  receives a sample S at the inlet  1666  into the channel  1644 . The ionization source  1648  ionizes the sample S. The ionized portions of the sample S, e.g., the positive ions, are deflected through the opening  1664  into the channel  1646  by the deflector  1650  having a positive charge. When the deflector  1650  is negatively charged, the deflector  1650  may deflect negative ions of sample S through the opening  1664  into the channel  1646 . The neutrals and non-deflected ions of sample S are then drawn by the pump  1652  to the outlet  1668  and expelled from the system  1642  while the ions in the channel  1646  are filtered by the filter  1654  and detected by the detector  1656 . The pump  1660  creates circulation flow in the direction  1670  within the channel  1646  to draw the sample S through the molecular sieve  1658  which collects spent analytes and then through a second molecular sieve  1662 . 
       FIG. 52  is a DMS system  1672  including a first channel  1674  and a second channel  1676  operating below atmospheric pressure without a membrane. The first channel  1674  includes an ionization source  1678 , deflector electrode  1680 , pump  1682 , inlet  1684 , outlet  1686 , and orifice  1700 . The second channel  1676  includes a filter  1688 , detector  1690 , molecular sieve  1692 , pump  1694 , molecular sieve  1696 , and orifice  1702 . An opening  1698  provides fluid communication between the channels  1674  and  1676 . 
     In operation, the system  1672  receives a sample S at the inlet  1684  into the channel  1674  and through the orifice  1700 . The orifice  1700  provides a pressure drop within the channel  1674  caused by the gas and/or air flow generated by the pump  1682 . The ionization source  1678  ionizes the sample S. The ionized portions of the sample S, e.g., the positive ions, are deflected through the opening  1698  into the channel  1676  by the deflector  1680  having a positive charge. When the deflector  1680  is negatively charged, the deflector  1680  may deflect negative ions of sample S through the opening  1698  into the channel  1676 . The neutrals and non-deflected ions of sample S are then drawn by the pump  1682  to the outlet  1686  and expelled from the system  1672  while the ions in the channel  1676  are filtered by the filter  1688  and detected by the detector  1690 . The pump  1694  creates circulation flow in the direction  1704  within the channel  1676  to draw the sample S through the molecular sieve  1692  which collects spent analytes and then through a second molecular sieve  1696 . 
       FIG. 53  is a DMS system  1706  including a first channel  1708 , a second channel  1710 , and a third channel  1712  with the second channel  1710  and third channel  1712  capable of operating at or below atmospheric pressure using a membrane  1714 . The first channel  1708  includes an inlet  1716  and an outlet  1718 . The second channel  1710  includes an ionization source  1718 , optional ionization source  1720 , deflector electrode  1722 , filter  1724 , and detector  1726 . The third channel  1712  includes an attractor electrode  1728 , filter  1730 , and detector  1732 . The combined circulation channel  1734  includes the chemical filter  1736 , pump  1738 , and optional chemical filter  1740 . An opening  1742  provides fluid communication between the channels  1710  and  1712 . 
     In operation, the system  1706  receives a sample S at the inlet  1716  into the channel  1708 . The sample S may be introduced from a GS column. The membrane  1714  may filter a portion of the sample S and provide a pressure barrier to enable a pressure below atmospheric pressure in the channels  1710  and  1712 . The channels  1710  and  1712 , along with the combined circulation channel  1734 , circulate filtered and clean carrier gas. The ionization source  1718  ionizes the sample S within this clean carrier gas. Optionally, a second ionization source  1720  may be employed in the channel  1710  to enhance the ability of the deflector  1722  and attractor  1728  to transfer selected ions from the channel  1710  to the channel  1712 . For example, the ionized portions of the sample S, e.g., the positive ions, are deflected through the opening  1742  into the channel  1712  by the deflector  1722  when the deflector  1722  is positively charged. When the deflector  1722  is negatively charged, the deflector  1722  may deflect negative ions of sample S through the opening  1728  into the channel  1712 . 
     The neutrals and non-deflected ions of sample S are then drawn by the pump  1738  through the channel  1710 , filter  1724  and detector  1726  while the selected ions are drawn through the channel  1712 , filter  1730 , and detector  1732 . The pump  1738  creates circulation flow in the direction  1744  within the channels  1710 ,  1712 , and  1734  to draw the carrier gas from the channels  1710  and  1712  into the channel  1734  and through the chemical filter  1736  and, optionally, the second chemical filter  1740 . The chemical filters  1736  and  1740  remove unwanted contaminants from the carrier gas. A make up gas may also optionally be introduced into the channel  1734  from an outside system. 
     The deflector  1722  and the attractor  1728  may be activated in a controlled manner to transport ions from the channel  1710  to the channel  1712 . In the channel  1710 , the non-deflected ions are filtered by filter  1724  and detected by detector  1726  while, in the channel  1712 , the deflected and attracted ions are filtered by the filter  1730  and detector  1732 . The resulting detected measurements from the channels  1710  and  1712  can then be compared, added, or subtracted from each other to enhance the identification of ion species. The controlled ionization of the sample S which is performed in a clean carrier gas, the detection in the channel  1712  of monomer or de-clustered ions, and the detection of clustered ions in the channel  1710  provide enhanced compound and ion species identification. 
     Other illustrative embodiments include systems, methods and devices for improving sample analysis, generally, and detection sensitivity, specifically, by performing sample ion species pre-separation and/or sample amplification. Such illustrative embodiments are discussed below. 
     Pre-separation of certain ion species of a sample reduces, and in some cases, eliminates the problem of competitive ionization within ion based mobility detection analyzers. At any atmospheric pressure or conditions where ion and/or neutral interactions have an effect on ion formation, atmospheric pressure chemical ionization (APCI) may occur. In such instances, compounds with the highest proton affinity (PA) and/or highest electron affinity (EA) preferentially capture or take up the charge from an ionization source. If there is a limited amount of charge available, for example, in a compact DMS system with limited power resources, the amount of available charge may not be sufficient to charge or ionize all of the molecules in a sample matrix. Thus, if only some of the molecules in a sample matrix are ionized, only that limited amount of molecules may be detected, resulting in erroneous analysis of a chemical matrix. Furthermore, certain compounds may not be ionized due to competitive ionization, resulting in no detection of these compounds. The invention includes embodiments that eliminate or mitigate the effects of competitive ionization by separating ion species before sample analysis or detection to prevent one ion species from consuming the charge intended to be used to ionize another ion species. 
     One technique for reducing the effect of competitive ionization is to use a gas chromatograph (GC) to pre-separate a sample matrix. A GC column may be used to separate multiple compounds, which may then be detected individually by a mobility-based analyzer, such as a DMS. Even a compound with a relatively low proton and/or electron affinity may be subsequently ionized and detected. A GC, however, is generally more complex, expensive, and often adds significant analysis time to provide sufficient compound separation. Typical analysis times are longer than one minute for sufficient compound separation. Thus, the invention includes systems, methods and devices for pre-separating a sample in a fast, efficient, and robust manner. According to other aspects, the invention provides such sample pre-separation in a compact package. Thus, the invention includes systems, methods and devices for pre-separating a sample in a fast, efficient, and robust manner. According to other aspects, the invention provides such sample pre-separation in a compact package. 
     Where further sample characterization is desired, neutrals, i.e., molecules of a sample that are not ionized, may be mixed with a new supply of charge, e.g., reactant ions or a plasma field, to enable further APCI reactions to occur. The newly created ions may then be removed for analysis or simply discarded. This process may be repeated until a desired compound type is ionized and detected using an analyzer. 
     In one embodiment of the invention, sample pre-fractionation is achieved by direct ionization of a sample matrix, competitive ionization by compounds of a certain type in the sample matrix, and then removal of the ionized compounds. The ionization source may be, for example, an UV source, laser, corona discharge, plasma source, soft X-ray source, or a source of reactant ions. Repeated interrogation of chemical compounds in a sample based on relative proton and electron affinities, using competitive ionization and the reaction of residual and/or un-reacted neutrals provides a comprehensive measure of the chemical composition of a sample without the need for traditional GC techniques. 
     The process of competitive ionization and the removal of product ions may be repeated, enabling incremental and selective isolation of product ions and neutrals. While chemical ionization involves the injection of fresh charge using reactant ions, non-chemical energy sources such as a laser or plasma or corona generator may ionize molecules of a sample. 
     In addition to being used for analysis, the invention may be used for selectively cleaning and/or conditioning samples, e.g., for removing selected molecules from a sample stream. For example, certain semiconductor industry or other process control applications require ultra pure or clean gasses. In these processes, water molecules are considered a contaminant in a gas stream of Nitrogen or Argon. In certain embodiments of the invention, water within a gas sample may be preferentially ionized and then removed from the gas stream while purified Argon or Nitrogen are then used in a low pressure chemical vapor deposition or for another semiconductor application. 
       FIG. 54A  is a conceptual diagram showing an example of a pre-separation process  1750  of a sample matrix  1752  including two types of compound molecules  1756  and  1758  according to an illustrative embodiment of the invention. The process  1750  begins by mixing reactant ions  1754  with a sample matrix of the two types of compound molecules  1756  and  1758  with a source of reactant ions  1754  to form a reactant ion and sample matrix mixture  1760 . 
     This mixing may involve injecting (e.g., via an injection pulse) the sample matrix  1752  into a re-circulating or circular flow of gas where the mixing of reactant ions  1754  with neutral molecules  1756  and  1758  can be controlled. Also, the injection of reactant ions  1754  and subsequent extraction of product ions, e.g., product ions  1762 , can be enabled using orifices in an ionization region, chamber, or gas flow path. Alternatively, a linear scheme may be employed where reactant ions  1754  are continuously introduced. In this scheme product ions, e.g., product ions  1762 , are removed at discrete or variable distances from the injection point of sample matrix  1752  or the product ion formation point. In either case, the effluent flow (e.g., the flow of gas) may be used to control or adjust the residence or contact times between reactant ions  1754  and neutral molecules  1756  and  1758  to control the formation of product ions such as product ions  1762  or  1764 . 
     Because the compound molecules  1756  are preferentially ionized by the reactant ions  1754 , the mixture  1760  includes un-ionized compound molecules  1756  and product ions  1762 . The product ions  1762  may be separated from the compound  1758  using chemical, electrical, magnetic, and/or a mechanical separation technique to remove the product ions  1762 . The ionized molecules or product ions  1762  may then be analyzed and characterized, for example, using a DMS, IMS, MS, or any suitable analyzer system or may be discarded. 
     Because the first type of compound molecules  1756  are preferentially ionized to form ionized molecules or product ions  1762 , the second type of compound molecules  1758  predominately are not ionized and retain a neutral charge. However, the source of reactant ions  1754  may be re-introduced to and mixed with the remaining neutral molecules  1758  to form ionized molecules or product ions  1764 . These product ions  1764  may then be separated and analyzed. The process  1750  may be repeated for any sample matrix with any number of compounds by repeatedly ionizing the sample matrix and removing the resulting product ions. Due to competitive ionization, the process incrementally removes different compounds with different ionization energies, enabling a comprehensive analysis of all compounds with a chemical sample. 
       FIG. 54B  is a conceptual diagram showing the pre-separation process  1768  of a sample matrix  1770  including two types of compound molecules  1772  and  1774  using an ionization source  1778  and electric field  1776  according to an illustrative embodiment of the invention. In this case, an ionization source  1778 , such as a plasma corona, laser, UV source, or the like, is used to ionize the sample matrix  1770 . Due to competitive ionization, the molecules  1774  predominantly are ionized into product ions  1780 . These product ions  1780  are then exposed to the electric field  1776  which substantially removes the product ions  1780  from the ionized sample matrix  1782 . The removed product ions  1780  may be analyzed or discarded. 
     The remaining non-ionized neutral molecules  1772  may then be ionized using the same ionization source  1778  or another ionization source to form product ions  1784 . The product ions  1784  may then be analyzed using a DMS or discarded. The electric field  1776  may be generated by any one of or combination of a deflector plate deflector array, attractor plate, attractor grid, and attractor array or various other electrodes. Alternatively, a magnetic field may be employed to remove selected product ions. 
       FIG. 55  is a conceptual block diagram of a sample pre-separation system  1786 . The pre-separation system  1786  uses first and second ionization regions  1788  and  1790  and first and second deflector regions  1792  and  1794  to separate a sample matrix S. Sample matrix S includes at least two compounds according to an illustrative embodiment of the invention. The sample pre-separation system  1786  includes an inlet  1796 , gas flow channel  1798 , first ionization region  1788 , first deflector region  1792 , first deflector plate  1800 , first attractor plates  1802 , first exhaust  1804 , first optional analyzer  1806 , pump  1808 , second ionization region  1790 , second deflector region  1794 , second deflector plate  1810 , second attractor plates  1812 , second exhaust  1814 , second optional analyzer  1816 , and the exhaust channel  1818 . 
     In operation, the sample matrix S is drawn into gas the flow channel  1798  through the inlet  1796  and then ionized in the first ionization region  1788 . The sample S may be ionized using reactant ions or any of the non-reactant ion sources described previously. Due to the chemical properties of the molecules, the limited supply of charge leads to competitive ionization where predominantly certain types of compound molecules are ionized into product ions while other types of compound molecules predominantly remain neutral. The first deflector plate or electrode  1800  and the first attractor plates or electrodes  1802  generate an electric field that propels the product ions out of the gas flow channel  1798  and through first exhaust  1804 . The first exhaust  1804  may deliver the product ions to an analyzer for detection and identification of the product ion species. Otherwise, the first exhaust  1804  may simply discard the product ions into the surrounding environment or neutralize them. 
     The remaining neutral molecules continue to travel in the gas flow channel  1798  and may pass through the first optional analyzer  1806 . The analyzer  1806  may be a DMS system that ionizes the remaining neutral molecules, performs a non-destructive detection and identification, and then neutralizes the molecules before returning the neutrals to the gas flow channel  1798 . The neutral molecules then continue to travel in the gas flow channel  1798  in the direction  1819  toward pump  1808  which propels the neutrals to the second ionization region  1790 . In the second ionization region  1790 , another type of compound molecule becomes predominantly ionized into product ions due to competitive ionization while one or more other compound molecules remain neutral in charge. 
     In the second deflector region  1794 , second deflector plate  1810  and second attractor plates  1812  generate an electric field that propels the product ions out of the gas flow channel  1798  and through the second exhaust  1814 . The second exhaust  1814  may deliver the product ions to an analyzer for detection and identification of the product ion species. Otherwise, the second exhaust  1814  may simply discard the product ions into the surrounding environment. 
     Again, the remaining neutral molecules continue to travel in the gas flow channel  1798  and may pass through an optional analyzer  1816 . The analyzer  1816  may be a DMS system that ionizes the remaining neutral molecules, performs a detection and identification, and then neutralizes the molecules before returning the neutrals to the gas flow channel  1798 . The neutral molecules may then continue to travel through the exhaust channel  1818  to yet further ionization regions and analyzers for further analysis or be discarded. 
       FIG. 56A  is a conceptual diagram of a sample pre-separation process  1820  according to a second illustrative embodiment of the invention. In the sample pre-separation process  1820 , a sample matrix  1822  may be re-circulated multiple times to interact with an ionization source such as reactant ions  1824 . The sample pre-separation process  1820  may remove different compound product ions  1826  from the sample matrix  1822  in each circulation. The process  1820  begins with the mixing of the sample matrix  1822  in an interaction region with a source of reactant ions  1824  to form an interaction region mixture  1828 . Due to competitive ionization, the types of compounds within the sample matrix  1822  with relatively higher proton and electron affinities tend to become ionized. Other types of compounds in the sample matrix  1822  with lower proton and electron affinities tend to remain neutral. 
     The ionized molecules, or product ions  1826 , are then removed from the mixture  1828  using previously described techniques such as an electric or magnetic field. The remaining neutral molecules  1830  are re-circulated for further ionization. Prior to further ionization, the remaining neutral molecules  1830  may optionally be subjected to mobility-based analysis using, for example, a DMS system  1832 . After the analysis, the neutral molecules  1830  are then delivered to an interaction region for further mixing with reactant ions  1824 . 
     The resulting neutral molecules  1830  may be re-circulated multiple times. Each time, the sample  1822  or remaining sample matrix  1830  interacts with reactant ions  1824  enabling the incremental removal of different compound molecules from the sample  1822 . The compound removed in each re-circulation tends to have a lower proton or electron affinity than the compounds removed in a previous re-circulation. The remaining neutral molecules  1830  may also be delivered to an analyzer  1834  after a sequence of iterations for detection and identification of a desired ion species. 
       FIG. 56B  is a conceptual diagram of a sample pre-separation process  1836  according to another illustrative embodiment of the invention. In the sample pre-separation process  1836 , a sample matrix  1838  including at least two types of compound molecules  1840  and  1842 , are re-circulated multiple times to interact with an ionization source  1844  and an electric field  1846 . As a result, the sample pre-separation process sequentially removes different compound product ions. In this case, the ionization source  1844 , such as a plasma field, laser, UV source, or like, is used to ionize the sample matrix  1838  to create an ionized sample matrix  1850 . Due to competitive ionization, molecules  1842  tend to be ionized into product ions  1848  in favor of molecules  1840 . These product ions  1848  are then exposed to the electric field  1846  which removes the product ions  1848  from the ionized sample matrix  1850 . The product ions  1848  may be analyzed or discarded. 
     The remaining non-ionized neutral molecules  1840  may then be ionized by re-circulating the molecules  1840  to the same ionization source  1844  to form product ions  1852 . The product ions  1852  may then be analyzed using a DMS or discarded. The electric field  1846  may be generated by any one of or combination of a deflector plate or electrode, deflector array, attractor plate or electrode, attractor grid, or attractor array. Alternatively, a magnetic field may be employed to remove selected product ions. This process may be applied to a matrix with an undetermined number of compounds by repeatedly re-circulating the sample through the same ion source to thoroughly characterize and/or identify all of the sample&#39;s constituents. The sample matrix may be introduced into the pre-separator as a plug or as a continuous stream. 
       FIG. 57  is a conceptual block diagram of a sample pre-separation system  1854  according to another illustrative embodiment of the invention. The sample pre-separation system  1854  re-circulates a sample matrix S through an ionization region  1856  multiple times. During each iteration, the sample pre-separation system  1854  removes a different compound from the sample matrix S. The compound removed on each iteration has a lower proton affinity or electron affinity than the compound removed in the prior iterations. The sample pre-separation system  1854  includes inlet  1858 , inlet valve  1876 , gas flow channel  1860 , ionization region  1856 , ion deflector/reflector region  1864 , exhaust  1862 , pump  1866 , bleed valve  1868 , analyzer valve  1870 , flow channel valve  1872 , and an optional analyzer  1874 . 
     In operation, a sample matrix S is drawn into the gas flow channel  1860  through the inlet  1858  and the inlet valve  1876 . The sample matrix S is then ionized in the ionization region  1856 . The ionization region  1856  may utilize an ionization source such as a reactant ion source, UV source, laser, and the like to ionize the sample S. Due to competitive ionization, certain types of compound molecules predominantly are ionized into product ions while other types of compound molecules remain neutral. The deflector/reflector region  1864  includes a deflector and/or attractor electrodes that generate an electric field to propel the product ions out of the gas flow channel  1860  and through exhaust  1862 . The first exhaust  1862  may deliver the product ions to an analyzer for detection and identification of the product ion species. Otherwise, the first exhaust  1862  may simply discard the product ions into the surrounding environment. 
     The remaining neutral molecules continue to travel in the gas flow channel  1860  through pump  1866 . Pump  1866  propels the neutral molecules toward the analyzer valve  1870 . The analyzer valve  1870  may be opened at certain times or in predetermined cycles to allow a portion of sample S through the valve  1870  to an analyzer. This controlled valve opening enables an analyzer such as a DMS, IMS, or MS to analyze a desired ion species without interference from undesired ion species. The gas flow channel  1860  may also include a bleed valve  1868  to enable makeup gas to be added or excessive gas to be removed from the gas flow channel  1860 . Until the analyzer valve  1870  is operated, the neutral sample S molecules will continue to flow through flow channel valve  1872  and may pass through an optional analyzer  1874 . The analyzer  1874  may be a DMS or like system that ionizes the remaining neutral molecules, performs a non-destructive detection and identification, and then neutralizes the molecules before returning the neutrals to the gas flow channel  1860 . The neutral molecules then continue to travel in the direction  1875  through the gas flow channel  1860  and eventually return to the ionization region  1856 . Another type of compound molecule becomes ionized into product ions due to competitive ionization while one or more other types of compounds molecules remain neutral in charge. This re-circulation process may be continued until an ion species is selected for analysis by operating the analyzer valve  1870 . 
       FIG. 58A  is a conceptual diagram of a sample pre-separation system  1878  where pre-selected reactant ions are intermixed with a sample stream to enable the pre-separation of ions having a particular proton or electronic affinity according to an illustrative embodiment of the invention. The pre-separation system  1878  includes a selected reactant ion type  1880 , a sample stream  1882 , a mixing unit  1884 , a controller unit  1886 , product ions  1888 , and neutral molecules  1890 . 
     In operation, a selected reactant ion species type  1880  is introduced to the mixer  1884  along with a sample stream  1882 . The reactant ion species can be, for example, oxygen ions or acetone ions. The mixer  1884  includes an interaction or mixer region that enables sample stream  1882  molecules of a relatively higher proton or electron affinity to be ionized into product ions  1888 . Most molecules of a relatively lower proton or electron affinity remain neutral molecules  1890 . 
     The controller  1886  is capable of regulating whether a single type or multiple types of reactant ions  1880  may be introduced into the mixer  1884  and mixed with the sample stream  1882 . The type of reactant ion species may be selected based on a particular property such as proton affinity, mobility, electron affinity, and/or chemical activity. By introducing a certain type of reactant ion or ions  1880 , the controller  1886  can determine which compound molecules or cluster of molecules of the sample stream  1882  are ionized. Thus, the controller  1886  may more precisely target particular compounds of the sample stream  1882  for further analysis or removal from the sample stream  1882 . The controller  1886  may also regulate effluent and/or gas flow through the mixing and/or ionization region to control the contact time between reactant ions and sample molecules and, thereby, control the amount of ionization that occurs. This technique also applies to other types of ionization sources such a lasers, UV source, and plasma generators. 
       FIG. 58B  is a conceptual diagram of a sample pre-separation system  1892  where pre-selected reactant ions  1894 , having been filtered and pre-selected, are then intermixed with a sample matrix  1896  to enable the pre-separation of ions having a particular proton or electronic affinity according to an illustrative embodiment of the invention. The pre-separation system  1892  includes pre selected reactant ions  1894 , a sample matrix  1896 , an ion mixing region  1898 , a product ion separator  1900 , optional analyzer  1902 , and an analyzer  1904 . 
     In operation, pre-selected reactant ions  1894  of a particular species are introduced to the ion mixing region  1898  along with a sample matrix  1896 . The reactant ions  1894  may be filtered and pre-selected using a DMS, IMS, MS, or like system. The ion mixer region  1898  enables the sample matrix  1896  molecules having proton or electron affinities above the proton and electron affinities of the pre-selected ions  1894  to be ionized into product ions  1906  while molecules of a relatively lower proton or electron affinity remain neutral molecules  1908 . As described previously, various techniques may be utilized to remove the product ions  1906 . An optional analyzer  1902  may be employed to analyze the product ions  1906 . The optional analyzer  1902  may also include an array of mobility-based analyzers. The analyzer  1904 , which may be a DMS, IMS, MS, or like, may be employed to analyze the remaining neutral molecules  1908 . 
     For example, ionized molecules such as Acetone may be selectively introduced as reactant ions and mixed with a sample matrix to remove substantially all molecules with proton affinities higher than Acetone&#39;s proton affinity (812 KJ/mol). By mixing a sample matrix with sufficient Acetone ions, charge will preferentially be transferred to molecules in the sample matrix with higher proton affinities than Acetone. The ionized molecules or product ions may then be separated form the sample matrix leaving neutral molecules having proton affinities less than Acetone&#39;s proton affinity. Thus, by using a particular reactant ion species to ionize a sample matrix, selected species of a sample matrix may be removed or isolated in a more precise and controlled manner. 
     In certain embodiment of the invention, multiple types of ionization sources or alternating ionization sources may be employed together or in a sequential flow arrangement. Different ionization sources may be employed to selectively remove ion species having incrementally lower proton or electron affinities. Thus, molecules with higher proton or electron affinities will be removed first. For example, a sample may first be exposed to a low ionization source such as a UV source, laser, or other photo-ionization source to remove ion species with high affinities. Then, the sample may be exposed to a higher ionization source such as a radioactive Ni 63  ionization source to remove ions species with relatively lower affinities. Additional ionization sources with pre-determined ionization energies may be employed to remove additional ion species until a desired ion species remains for analysis using a DMS, IMS, MS, and like mobility-based analyzer. 
       FIG. 59A  is a conceptual diagram of a sample pre-separation system  1910  including two flow channels wherein multiple ion separations are enabled by multiple ionization sources according to an illustrative embodiment of the invention. The sample pre-separation system  1910  includes an inlet  1916 , gas flow channel  1912 , inlet  1918 , gas flow channel  1914 , UV ionization source  1920 , first Ni 63  ionization source  1922 , second Ni 63  ionization source  1924 , first opening  1926 , second opening  1928 , third opening  1930 , outlet  1932 , and outlet  1934 . 
     In operation, a sample matrix S is introduced into gas flow channel  1912  through inlet  1916 . The UV ionization source  1920  then ionizes the sample S matrix at a relatively low energy. Due to competitive ionization, the compound molecules of sample S having the lowest ionization energies and highest affinities, e.g., ionization energies at about or below the UV ionization energy level, are ionized to form product ions. These product ions are then deflected from gas flow channel  1912  and/or attracted into gas flow channel  1914  through the first opening  1926  by the electrode  1919 . These low energy product ions may then be delivered by gas flow channel  1914  through outlet  1934  to an analyzer or discarded. The inlet  1918  accepts gas flow into gas flow channel  1914  to enable the flow of product ions delivered from gas flow channel  1912 . 
     For example, assume the sample matrix S includes nitric oxide species (NOx). In such a case, NO and NO 2  are formed by direct ionization because these are the only NOx species with ionization energies below the ionization energy of the UV source. Tables 2 and 3 provide lists of positive and negative NOx ion species equations respectively. Also, Table 4 shows the ionization energy, proton affinity, and electron affinity for selected NOx ion species. 
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Positive NOx Ion Species Equations 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 NO + hν → NO +  + e −   
               
               
                   
                 NO +  + H 2 O + M → (H 2 O)NO +  + M 
               
               
                   
                 (H 2 O)NO +  + H 2 O + M → (H 2 O) 2 NO +  + M 
               
               
                   
                 (H 2 O) n-1 NO +  + H 2 O + M → (H 2 O) n NO +  + M 
               
               
                   
                 (H 2 O) 3 NO +  + H 2 O → HNO 2  + (H 2 O) 3 H +   
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Negative NOx Ion Species Equations 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 NO 2  + hν → NO + O 
               
               
                   
                 NO 2  + O + M → NO 3  + M 
               
               
                   
                 NO 2  + e −  → NO 2   −   
               
               
                   
                 NO 2   −  + NO 3  → NO 2  + NO 3   −   
               
               
                   
                 NO 2   −  + NO 2  → NO + NO 3   −   
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 NOx Ionization Energy, Proton Affinity, and Electron Affinity 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Proton 
               
               
                   
                 Ionization 
                 Electron 
                 Affinity 
               
               
                   
                 Energy 
                 Affinity 
                 PA 
               
               
                   
                 EI (eV) 
                 EA (eV) 
                 (KJ/mol) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 NO 
                 9.26 
                 0.026 
                 531.8 
               
               
                   
                 NO 2   
                 9.58 
                 2.3 
                 591 
               
               
                   
                 NO 3   
                 12.57 
                 3.9 
                 n/a 
               
               
                   
                   
               
            
           
         
       
     
     After ionization occurs with respect to the UV ionization source  1920 , the non-ionized or neutral molecules remaining in the gas flow channel  1912  proceed to the first Ni 63  ionization source  1922 . Due to the competitive ionization process, the remaining molecules of the sample matrix S with the highest affinities are ionized to form product ions. These product ions are then deflected from the gas flow channel  1912  and/or attracted into the gas flow channel  1914  through the second opening  1928  by the electrode  1921 . These product ions may then be delivered by the gas flow channel  1914  through the outlet  1934  to an analyzer or discarded. 
     Assuming the sample matrix S includes NOx species, the negative polarity NOx compounds with the highest electron affinity such as NO 2  are NO 3  are removed as product ions.  FIG. 60  shows the electron affinities for NO 2  are NO 3  respectively. 
     After ionization occurs with respect to the first Ni 63  ionization source  1922 , the non-ionized or neutral molecules remaining in the gas flow channel  1912  proceed to the second Ni 63  ionization source  1924 . Due to the competitive ionization process, the remaining molecules of the sample matrix S with the highest affinities are ionized to form product ions. These product ions are then deflected from the gas flow channel  1912  and/or attracted into the gas flow channel  1914  through the third opening  1930  by the electrode  1923 . These product ions may then be delivered by the gas flow channel  1914  through the outlet  1934  to an analyzer or discarded. Any remaining neutral molecules may be delivered through the outlet  1932  to an analyzer for analysis. 
     Because the ionization process is dynamic and time dependent, the residence time for a sample matrix S within the proximity of an ionization source may be adjusted to form particular types of product ions. Thus, the interaction time between an ionization source and a sample matrix S may also be controlled to selectively remove certain product ion species. For example, with regard to the NOx ion species, the NO 3   −  ion species may be formed by direct photo-ionization using a UV ionization source according to the equations listed in Table 3. 
       FIG. 59B  is a conceptual diagram of a sample pre-separation system  1936  having two flow channels wherein multiple ion separations are enabled by multiple ionization sources including a plasma ionization source  1950  according to an illustrative embodiment of the invention. The sample pre-separation system  1936  includes an inlet  1942 , gas flow channel  1938 , inlet  1944 , gas flow channel  1940 , UV ionization source  1946 , Ni 63  ionization source  1948 , plasma ionization source  1950 , first opening  1952 , second opening  1954 , third opening  1956 , outlet  1958 , and outlet  1960 . 
     In operation, a sample matrix S is introduced into gas flow channel  1938  through inlet  1942 . The UV ionization source  1946  then ionizes the sample S matrix at a relatively low energy. Due to competitive ionization, the compound molecules of sample S having the lowest ionization energies and highest affinities, e.g., ionization energies at about or below the UV ionization energy level, are ionized to form product ions. These product ions are then deflected from gas flow channel  1938  and/or attracted into gas flow channel  1940  through the first opening  1952  by the electrode  1951 . These low energy product ions may then be delivered by the gas flow channel  1940  through the outlet  1960  to an analyzer or discarded. The inlet  1944  accepts gas flow into gas flow channel  1940  to enable the flow of product ions delivered from the gas flow channel  1938 . 
     After ionization occurs with respect to the UV ionization source  1946 , the non-ionized or neutral molecules remaining in the gas flow channel  1938  proceed to the Ni 63  ionization source  1948 . Due to the competitive ionization process, the remaining molecules of the sample matrix S with the highest affinities are ionized to form product ions. These product ions are then deflected from the gas flow channel  1938  and/or attracted into gas flow channel  1940  through the second opening  1954  by the electrode  1953 . These product ions may then be delivered by the gas flow channel  1940  through the outlet  1960  to an analyzer or discarded. 
     After ionization occurs with respect to the Ni 63  ionization source  1948 , the non-ionized or neutral molecules remaining in the gas flow channel  1938  proceed to the plasma ionization source  1950 . Due to the competitive ionization process, the remaining molecules of the sample matrix S with the highest affinities are ionized to form product ions. These product ions are then deflected from gas flow channel  1938  and/or attracted into gas flow channel  1940  through the third opening  1956  by the electrode  1955 . These product ions may then be delivered by gas flow channel  1940  through outlet  1960  to an analyzer or discarded. Any remaining neutral molecules may be delivered through the outlet  1958  to an analyzer for analysis. 
       FIG. 60  is a graph  1962  of ionization energies required for various NOx ion species to form either positive or negative ions by direct photo-ionization in air. The NOx species NO and NO 2  have relatively high affinities as reflected by their DMS and mass spectra which are shown in  FIGS. 61A ,  61 B, and  61 C.  FIG. 61A  is a graph  1964  of relative intensity versus mass units showing the mass-spectra to positive NOx ion NO.  FIG. 61B  is a graph  1966  of relative ion intensity versus mass units showing the mass-spectra for positive NOx ion NO 2 .  FIG. 61C  is a graph  1968  of ion intensity versus field compensation voltage showing the ion intensity peaks  1970  and  1972  for NO and NO 2  respectively. 
       FIG. 62  is a conceptual diagram of a cylindrical sample pre-separation system  1974  including an integrated cylindrical DMS  1976  or other analyzer according to an illustrative embodiment of the invention. The sample pre-separation system  1974  includes an inlet  1978 , gas flow channel  1980 , Ni 63  ionization source  1982 , ionization region  1984 , ionization region  1986 , outlet  1988 , and deflector outlets  1990 ,  1992 ,  1994 ,  1996 ,  1998 , and  2000 . The system  1974  also includes a deflector  2002 , deflector  2004 , voltage source  2006 , voltage source  2008 , insulator  2010 , insulator  2012 , and surrounding space  2014 . 
     In operation, a sample matrix S is drawn into the gas flow channel and/or path  1980  and ionized by the Ni 63  ionization source. Due to competitive ionization, certain compounds within the sample matrix S within the highest affinities are ionized into product ions. These product ions are then deflected from the gas flow channel  1980  into the surrounding space  2014  through deflector outlets  1990  and  1992 . The deflector  2002  resides within the center of the coaxial gas flow channel  1980  and holds an electric potential or voltage generated by the voltage source  2006 . The deflector  2002  potential is high enough to create an electric field of sufficient strength to propel the product ions from the gas flow channel  1980  into the surrounding space  2014 . The surrounding space  2014  may be an enclosed, substantially enclosed, or unenclosed path and/or channel. The surrounding space  2014  may be a second gas flow channel surrounding the system  1974  that directs product ions propelled from the gas flow channel  1980  to an analyzer or other system for further analysis. 
     The remaining neutral molecules of the sample matrix S then travel to the ionization region  1984 . Due to competitive ionization, certain compound molecules within the remaining sample matrix S with the highest affinities are ionized into a new group of product ions. These product ions are then deflected from the gas flow channel  1980  into the surrounding space  2014  through deflector outlets  1994  and  1996 . The deflector  2002  potential is high enough to create an electric field of sufficient strength to propel the product ions from the gas flow channel  1980  into the surrounding space  2014 . 
     The remaining neutral molecules of the sample matrix S then travel to the ionization region  1986 . Due to competitive ionization, certain compound molecules within the remaining sample matrix S with the highest affinities are ionized into a third group of product ions. These product ions are then deflected from the gas flow channel  1980  into the surrounding space  2014  through deflector outlets  1998  and  2000 . The deflector  2004  resides within the center of the coaxial gas flow channel  1980  and holds an electric potential or voltage generated by the voltage source  2008 . The deflector  2004  potential is high enough to create an electric field of sufficient strength to propel the product ions from the gas flow channel  1980  into the surrounding space  2014 . An insulator  2010  provides electrical separation and enables an electrical potential difference between deflector  2002  and deflector  2004 . 
     The remaining molecules of the sample matrix S, which preferably include the compound of interest for detection, are then delivered to the coaxial DMS system  1976  for analysis. The insulator  2012  provides electrical separation between the deflector  2004  and the DMS ion filter electrode  2016 . The ionization regions  1984  and  1986  may use any one of the previously described ionization sources to ionize the sample matrix S. 
     In certain illustrative embodiments, a variable and/or adjustable ionization energy source may be employed by the forgoing pre-separation systems. For example, a tunable laser may be used as an adjustable ionization source. A sample may then be repeatedly exposed to the laser ionization source while the energy level of the laser is changed for each ionization. By adjusting the laser energy level and the resulting ionization energy, different molecules of a sample are ionized and separated from the sample matrix. The wavelength or frequency of a laser may be adjusted to enable the selective removal of molecules from a sample. 
     In certain illustrative embodiments, the sample matrix environmental conditions may be altered at various stages during the ionization and separation process. For example, the level of moisture may be set at one level during the ionization process and then adjusted to another level during the extraction or removal process. By altering the environmental conditions such as the moisture level at different stages of the ionization and separation process the extraction of particular ions from a sample may be improved. 
     In other illustrative embodiments, dopants may be intermixed with a sample in the mixing region of a pre-separation system to improve and/or control the charge transfer to sample molecules from reactant ions. Different dopants may be added to a sample at different times and/or at various stages of the ionization and separation process. The dopants may be added before, during, or after selected analytes or product ions are removed from a sample matrix. For example, an inkjet like printer head may be loaded with various types of dopants. The head may deploy one or more dopants using injection pulses at various times and/or stages of an ionization and separation process. 
       FIG. 63  is a conceptual block diagram of a sample pre-separation system  2018  capable of mixing dopants with a sample matrix S in a controlled manner before or after reactant ions are added according to an illustrative embodiment of the invention. The sample pre-separation system  2018  includes the inlet  2020 , gas flow channel  2022 , dopant sources  2024 , ionization region  2026 , ion deflector/reflector  2028 , and pumps  2030 . 
     In operation, a sample matrix S is drawn through inlet  2020  into the gas flow channel  2022 . The dopant sources  2024  may include various types of dopants. Any one of the dopants or a combination of dopants may be added to the sample matrix S in the gas flow channel  2002 . Each dopant source  2024  may include an injection mechanism to inject or pulse controlled amounts of dopant into the gas flow channel  2002 . When the sample matrix S and dopant mixture enter the ionization region, certain types of compound molecules of the sample matrix S are ionized due to competitive ionization. 
     Upon leaving the ionization region  2026 , the ionized molecules or product ion are deflected by the deflector/reflector  2028  through outlet  2032  to either an analyzer or an exhaust. The pumps  2030  re-circulate the remaining neutral molecules through the gas flow channel  2022 . The dopant sources  2024  may again inject dopants into the gas flow channel  2022  to enable mixing of selected dopants with the remaining neutral molecules of the sample matrix S. This process may be repeated while the sample matrix S is re-circulated through the pre-separation system  2018  until a selected compound or group of compounds are extracted from the sample matrix S. 
     In one illustrative embodiment of the invention, a device and/or system may be employed that uses multiple flow paths to combine various combinations of ions to control the formation and delivery of a particular type of reactant ion species to a pre-separation system. By controlling the type of reactant ion species introduced into an ionization and mixing region, the type of compound of a sample that is ionized may be controlled. For example, logic circuits may arranged from an array of DMS, IMS, MS, and the like filters to control the flow and combination of various ion species into a pre-separation system. 
       FIG. 64  is a conceptual diagram of an array of logic circuits  2034  including an “or” flow circuit  2036  and an “and” flow circuit  2038  used to form a desired reactant ion species according to an illustrative embodiment of the invention. The “or” flow circuit includes the sample inlet  2040 , sample inlet  2042 , carrier gas inlet  2044 , flow channel  2050 , flow channel  2052 , flow channel  2054 , opening  2046 , opening  2048 , and outlet  2056 . The “and” flow circuit includes the sample inlet  2058 , sample inlet  2060 , carrier gas inlet  2062 , flow channel  2068 , flow channel  2070 , flow channel  2072 , opening  2064 , opening  2066 , and outlet  2074 . 
     In operation with regard to the “or” circuit  2036 , a sample A is drawn into the flow channel  2050  through inlet  2040  while a sample B is drawn into the flow channel  2052  through inlet  2042 . Either the sample A or the sample B, but not both sample A and B is deflected from the flow channels  2050  or  2052  into the center channel  2054 . Other means such as a microvalve or electromechanical switch may be employed to control the flow of ions from either channel  2050  or  2052  into the center flow channel  2054 . The deflected sample A or B is then delivered through the outlet  2056  to a target such as the ionization or mixing region of a pre-separation system. 
     In operation with regard to the “and” circuit  2038 , a sample C is drawn into the flow channel  2068  through inlet  2058  while a sample D is drawn into the flow channel  2070  through inlet  2060 . In this case both the sample C and D are deflected from the flow channels  2068  or  2070  into the center channel  2072 . The deflected samples C and D are then delivered through the outlet  2074  to a target such as the ionization or mixing region of a pre-separation system. 
     The logic circuit  2034  may deliver multiple combinations of sample reactant ions such as the sample combinations A only, B only, ACD, and BCD. Other combinations of reactant ions may be enabled dependent on the configuration of the logic circuit  2034  array. For instance, logic circuit may be arranged in parallel, in series, or in a combination of series and parallel in order to achieve a desired mixture. Although only two circuits are shown in  FIG. 64 , the number and types of circuits may be increased to facilitate the delivery of numerous combinations of reactant ions to a target. 
     In certain illustrative embodiments, arrays of analyzers may be employed for detecting and characterizing various compounds within a sample matrix. 
       FIG. 65  is a conceptual diagram of a sample pre-separation and analysis system  2076  using multiple ionization zones and multiple analyzers to analyze various ions of a sample matrix according to an illustrative embodiment of the invention. The pre-separation and analysis system  2076  includes sample inlet  2078 , gas flow channel  2080 , ionization region  2082 , ionization region  2084 , ionization region  2086 , analyzer  2088 , analyzer  2090 , analyzer  2092 , and outlet  2094 . 
     In operation, a sample matrix S is drawn into the gas flow channel  2080  through inlet  2078  and then ionized in ionization region  2082 . Due to competitive ionization, certain compound molecules are ionized into product ions that are then extracted from gas flow channel  2080  using any of the various techniques described previously. The extracted ions are then analyzed by an analyzer  2088  such as a DMS, IMS, MS and like system. 
     The remaining neutral molecules of sample matrix S are then ionized in ionization region  2084 . Again, the product ions are extracted and analyzed by an analyzer  2090 . The remaining neutral molecules of sample matrix S are then ionized in ionization region  2086 . The product ions are extracted and analyzed by an analyzer  2092 . Any remaining neutral molecules exit the gas flow channel  2080  through outlet  2094 . An additional analyzer may be employed at the outlet  2094  for further analysis of the sample matrix S. The number of analyzers and ionization regions may be varied depending on the number of product ions to be analyzed. 
       FIG. 66  is a conceptual diagram of a sample pre-separation and analysis system  2096  using multiple ionization zones and DMS analyzers, including a DMS analyzer  2116  with an arbitrarily curved drift tube and ion filter region  2122 , according to an illustrative embodiment of the invention. The pre-separation and analysis system  2096  includes sample inlet  2098 , gas flow channel  2100 , ionization region  2102 , ionization region  2104 , ionization region  2106 , DMS analyzer  2116  with a curved drift tube and ion filter region  2122 , analyzer  2118 , analyzer  2120 , deflector  2110 , deflector  2112 , deflector  2114 , and outlet  2124 . 
     In operation, a sample matrix S is drawn into the gas flow channel  2100  through inlet  2098  and then ionized in ionization region  2102 . Due to competitive ionization, certain compound molecules are ionized into product ions that are then deflected from gas flow channel  2100  by deflector  2110  into DMS analyzer  2116 . The extracted ions are then analyzed by the DMS analyzer  2116 . The DMS analyzer  2116  may include a arbitrarily curved drift tube and ion filter  2122  so that the electric field in the DMS is non-uniform. 
     A portion of the remaining neutral molecules of sample matrix S are then ionized in ionization region  2104 . The product ions are deflected by deflector  2112  from the gas flow channel  2100  into the analyzer  2118  and analyzed. The remaining neutral molecules of sample matrix S are then ionized in ionization region  2108 . The product ions are deflected by deflector  2114  into the analyzer  2120  and analyzed. Any remaining neutral molecules exit the gas flow channel  2100  through the outlet  2124 . An additional analyzer may be employed at the outlet  2124  for further analysis of the sample matrix S. The number of analyzers and ionization regions may be varied depending on the number of product ions to be analyzed. Also, the spacing between the ionization regions and analyzers may not be uniform. Furthermore, while not shown in  FIGS. 65 and 66 , multiple flow channels, each with one or more analyzers, may be arranged in parallel. In yet a further configuration, multiple analyzers may be employed in series or parallel after each ionization region to enhance sample analysis. 
       FIG. 67  is a conceptual diagram of a sample pre-separation and analysis system  2126  employing multiple ionization zones and analyzers along with a filtered gas source to control pressure within the analyzers according to an illustrative embodiment of the invention. The pre-separation and analysis system  2126  includes a sample inlet  2128 , gas flow channel  2130 , ionization region  2132 , ionization region  2134 , ionization region  2136 , analyzer  2138 , analyzer  2140 , analyzer  2142 , analyzer flow channel  2139 , analyzer flow channel  2141 , analyzer flow channel  2143 , pressure channel  2144 , pressure channel  2146 , pressure channel  2148 , deflector  2150 , deflector  2152 , deflector  2154 , ion attractors  2156 , exit port  2155 , exit port  2157 , exit port  2159 , ion attractors  2158 , ion attractor  2160 , outlet  2162 , and gas flow channel  2164 . 
     In operation, a sample matrix S is drawn into the gas flow channel  2130  through inlet  2128  and then ionized in ionization region  2132 . Due to competitive ionization, certain compound molecules are ionized into a group of product ions that are then deflected from gas flow channel  2130  by deflector  2150  and attracted by ion attractors  2156  through exit port and/or opening  2155  into the analyzer  2138 . The gas flow channel  2164  provides filtered and/or treated gas to the analyzer  2138  through pressure channel  2144  to establish a relatively higher pressure within the analyzer  2138 . The relatively higher and/or positive pressure within the analyzer  2138  and analyzer flow channel  2139  limits the entry of neutral molecules into the analyzer  2138 . The extracted product ions are then analyzed by the analyzer  2138 . 
     The remaining neutral molecules of sample matrix S are then ionized in ionization region  2134 . The product ions are deflected by deflector  2152  and attracted by ion attractors  2158  from the gas flow channel  2130  through exit port  2157  into the analyzer  2140  and analyzed. The gas flow channel  2164  provides filtered and/or treated gas to the analyzer  2140  through pressure channel  2146  to establish a relatively higher pressure within the analyzer  2140 . The relatively higher and/or positive pressure within the analyzer  2140  and analyzer flow channel  2141  inhibits and/or limits the entry of neutral molecules from gas flow channel  2130  into the analyzer  2140 . 
     The remaining neutral molecules of sample matrix S are then ionized in ionization region  2136 . The product ions are deflected by deflector  2154  and attracted by ion attractors  2160  through exit port  2159  into the analyzer  2142  and analyzed. The gas flow channel  2164  provides filtered and/or treated gas to the analyzer  2142  through pressure channel  2148  to establish a relatively higher pressure within the analyzer  2142 . The relatively higher and/or relatively positive pressure within the analyzer  2142  and analyzer flow channel  2143  inhibits and/or limits the entry of neutral molecules from the gas flow channel  2130  into the analyzer  2142 . 
     Any remaining group of neutral molecules exit the gas flow channel  2130  through the outlet  2162 . An additional analyzer may be employed at the outlet  2164  for further analysis of the sample matrix S. The number of analyzers and ionization regions may be varied depending on the number of product ions to be analyzed. Furthermore, the spacing between the ionization regions and analyzers may be non-uniform. 
       FIG. 68  is a flow diagram of a process showing the analysis of a sample matrix including re-circulation of the sample according to an illustrative embodiment of the invention. First, a sample matrix is mixed with reactant ions (Step  2166 ). Then, the reaction conditions are controlled to optimize the transfer of charge and ion species formation (Step  2168 ). Once the sample matrix is ionized to form product ions, the product ions are separated from the sample matrix (Step  2170 ). If desired, the separated product ions may be analyzed (Step  2172 ). Next, it is determined whether all desired ion species of the sample matrix have been removed (Step  2174 ). If all of the desired or selected ion species have been removed, the remaining neutral molecules of the sample matrix may be analyzed (Step  2178 ). If all of the desired ion species have not been removed, the remaining neutral molecules of the sample matrix are mixed with the reactant ions and the process is repeated (Step  2176 ). 
       FIG. 69  is a flow diagram of a process showing the analysis of a sample matrix composed of multiple molecule species according to an illustrative embodiment of the invention. First, the reactant ions are produced (Step  2180 ). Then, a sample matrix is mixed with the reactant ions (Step  2182 ). The reaction conditions are controlled to optimize the transfer of charge and permit target ion species formation (Step  2184 ). Once ionized, the product or target ions are separated from the sample matrix (Step  2186 ). If desired, the separated product ions may be analyzed (Step  2190 ). Then, the remaining neutral molecules of the sample matrix are mixed with the reactant ions ( 2192 ). Next, it is determined whether all desired ion species of the sample matrix have been removed (Step  2194 ). If all of the desired or selected ion species have been removed, the remaining neutral molecules of the sample matrix may be analyzed (Step  2196 ). If not all of the desired ion species have been removed, the process is repeated. 
       FIG. 70  is a conceptual diagram of a sample pre-separation (neutrals removal) system  2167  where the neutral molecules are removed from the ionized molecules rather than removing the ions from the neutral gas stream as described previously. The sample pre-separation system  2167  includes sample inlet  2169 , clean gas inlet  2171 , clean makeup gas inlet  2173 , clean makeup gas inlet  2175 , optional ionization source  2177 , gas flow channel  2179 , electrodes  2181 , flow permitting medium  2183 , neutral removal region  2185 , analyzer  2187 , and neutrals flow outlet  2189 . 
     In operation, a sample matrix S is drawn into the pre-separation device through sample inlet  2169  and ionized by reactant ions. The ions are transported by an electric field, generated by electrodes  2181 , towards an analyzer  2187  while sample S neutrals are drawn away from the sample ions through a “flow permitting” medium  2183 . The flow permitting medium  2183  may include a porous material or a region with small openings and/or holes to allow the neutrals to pass from the gas flow channel  2179  through the neutrals flow outlet  2189 . The sample S neutrals may then be removed from the sample S ions in the neutral removal region  2185  using a vacuum pump that creates a vacuum in the neutral removal region  2185 . The vacuum draws the neutrals out of the gas flow channel  2179 , e.g., a transport tube, while the ions are moved towards the analyzer  2187  by the electric fields of the electrodes  2181 . 
     The ions may move in direction  2191  counter to a clean gas flow-makeup stream which is free of sample neutrals. A gas flow-makeup stream may originate from a clean makeup gas inlet  2173  and/or clean makeup gas inlet  2175 . The sample pre-separation system  2167  may use discrete electrodes  2181 , or resistive ink or semi-conducting coatings with suitable voltages and currents applied to induce the desired electric fields. The gas flow channel  2179  may be enclosed substantially by a substantially circular and/or rectangular housing. With a substantially circular housing, the electrodes  2181  may be circular rings along the gas flow channel  2179 . With a substantially rectangular housing, the electrodes  2181  may reside on opposing facing planar surfaces with the gas flow channel  2179  in between. The sample pre-separation system  2167  may be planar in form. 
     The sample pre-separation (neutrals removal) system  2167  may interface with a DMS or IMS or MS or the like. In the illustrative embodiment of  FIG. 70 , the sample S may be mixed with reactant ions that are optionally introduced at inlet  2171  and ionized by ionization source  2177 . The mixture is transported into the separation region  2185  where product ions and some reactant ions are separated from the sample neutrals and transported to an analyzer  2187 . Alternatively, pre-ionized sample S molecules may be introduced into the gas flow channel  2179  at inlet  2169 . The separation, or neutral removal, region  2185  may then use a clean gas flow in the direction  2193 , which is transverse to the ion flow, to draw the neutral sample S molecules away from the ions in the gas flow channel  2179 . The remaining sample S molecules are then delivered to the analyzer  2187  for analysis. 
       FIG. 71  is a conceptual diagram of a sample pre-separation system  2198  employing an ionization region  2200 , ionization source inlet  2202 , analyzer flow channel  2221 , DMS ion filter  2204 , deflector  2206 , ion attractors  2208 , exhaust opening  2210 , neutral molecules  2212 , pump  2214 , and valve  2216  to selectively filter ion species for analysis according to an illustrative embodiment of the invention. In operation, a sample matrix S is drawn through valve  2216  when the valve  2216  is positioned to accept the sample matrix S. The valve  2216  may alternatively be positioned to only allow neutral molecules  2212  to re-circulate to the DMS inlet  2218  and ionization region  2200 . An ionization inlet  2202  may be employed to enable the introduction of reactant ions. The reactant ions may then mix with the sample matrix S and ionize selected compound molecules. 
     The DMS ion filter  2204  and an optional detector electrode  2220  may remain inactive until a sufficient amount of pre-separation iterations are performed to remove unwanted ion species from the sample matrix S. Once the sample matrix S is ionized, the deflector  2206  and ion attractors  2208  propel the product ions from the analyzer flow channel  2221  through the opening  2210 . These product ions may be further analyzed or discarded. 
     The remaining neutral molecules  2212  of the sample matrix are then propelled by the pump  2214  through the valve  2216  back to the DMS inlet  2218 . The remaining neutral molecules  2212  may be re-circulated until a desired type of compound remains. Then, the DMS filter  2204  and detector  2220  may be activated to analyze the remaining compound of the sample matrix S. Alternatively, the deflector  2206  and/or ion attractors  2208  may function as detector during the DMS analysis. 
       FIG. 72  is a conceptual diagram of a sample pre-separation system  2222  employing an ionization region  2224 , ion guiding region  2226 , DMS ion filter  2228 , positive electrodes  2230 , negative electrodes  2232 , optional analyzers  2234 , flow generator  2242 , selective concentrator  2244  and valve  2248  for ion species analysis according to an illustrative embodiment of the invention. The pre-separation system  2222  also includes DMS inlet  2250 , DMS flow channel  2240 , DMS outlet  2252 , flow generator  2242 , opening  2236 , and opening  2238 . 
     In operation, a sample matrix S is drawn through the valve  2248  and the DMS inlet  2250  into the ionization region  2224 . The sample matrix S is then ionized using one of the various ionization techniques previously described. Due to competitive ionization, certain compound molecules are ionized into product ions. The ion guiding region  2226  then concentrates the ions to the center of the DMS flow channel  2240 . The DMS ion filter  2228  may be activated at certain times to perform ion filtering. Then, the product ions are deflected from the DMS flow channel  2240  by either positive electrodes  2230  or negative electrodes  2232 . The positive electrodes  2230  act simultaneous as an attractor for negative product ions and as a deflector for positive product ions. Also, the negative electrodes  2232  act simultaneous as an attractor for positive product ions and as a deflector for negative product ions. Thus, both positive and negative product ions may be removed from the DMS flow channel  2240  simultaneously or at about the same time. 
     The remaining neutral molecules of the sample matrix S pass through the DMS outlet  2252  to the flow generator  2242 . The flow generator  2242  establishes gas flow in the DMS flow channel  2240  from the DMS inlet  2250  toward the DMS outlet  2252 . The flow generator  2242  may be a solid-state or electromechanical pump or the like. The flow generator  2242  then propels the neutral molecules through the selective concentrator  2244  which further concentrate the sample matrix S by removing unwanted compounds. The concentrator controller  2246  may regulate the conditions within the concentrator to enable sample matrix S concentration. 
     The remaining concentrated and neutral molecules of the sample matrix S then pass through the valve  2248  and return to the DMS inlet  2250  for further pre-separation if necessary. The valve  2248  may be positioned to allow an external sample matrix S to be collected, positioned to re-circulate only the neutral molecules, or positioned to allow both the external sample matrix S intake and re-circulation of the neutral molecules. 
     The previous pre-separation systems may be improved by use of molecular sieves, membranes, and the like, such as those described supra. For example, a membrane may be employed at various openings to maintain the pressure and re-circulated gas flow in a pre-separation system while allowing product ions to be removed. Also spectral changes may be monitored during the pre-separation process to provide an indication when adequate cleaning of a gas sample reached or when a particular compound may be sampled. 
     While current mobility based analyzers are sensitive, there is a need to detect concentrations in ranges lower than parts-per-trillion (ppt). For instance, a very small number of anthrax spores may cause significant health effects. However, existing analyzers may not be sensitive enough to detect the charge generated by such a small number of spores. One technique for overcoming this limitation is concentrating and/or amplifying the number of molecules of a sample, in time, to enable an analyzer to produce a larger signal for detection. 
     In certain embodiments of the invention, chemical amplification is employed to enable the detection of extremely low levels (e.g., concentrations of less than a few ppt) of analytes in a sample. The sample may be a fluid such as a vapor or liquid. By allowing selected molecules to circulate multiple times in an analyzer system, the concentration of an analyte may be increased to a detectable level. 
       FIG. 73A  is a conceptual diagram of a sample amplification system  2254  employing a DMS filter  2256 , detector and neutralizer  2258 , transport gas input  2260  and re-circulation loop  2262  for selected ion species analysis according to an illustrative embodiment of the invention. In operation, a sample S is drawn into the DMS filter  2256  which filters out and exhausts unwanted ion species. The selected ion species are delivered to the detector and neutralizer  2258 , which detects and neutralizes the selected ion species during the detection process. A transport effluent (e.g., a gas, liquid or vapor) input  2260  provides transport effluent (in the example a transport gas) to flow the neutralized ion species through the re-circulation loop  2262 . Upon return to the DMS filter  2256 , the neutralized ion species are mixed with more sample S molecules and then filtered by the DMS filter  2256 . The sample amplification process is repeated for a period of time until enough of the selected ion species are filtered by the DMS filter  2256  for the detector and neutralizer  2258  to detect the ion species of interest. 
       FIG. 73B  is a conceptual diagram of a sample amplification system  2264  employing a DMS filter  2266 , detector  2268 , ionization source  2270 , deflector  2272 , an attractor grid  2274 , DMS flow channel  2276 , re-circulation channel  2278 , inlet  2280 , exhaust  2282 , and an optional DMS  2284  for analysis of selected ion species according to an illustrative embodiment of the invention. 
     In operation, a sample S is drawn into the DMS flow channel  2276  through the inlet  2280 . The DMS filter  2266  filters out unwanted ion species while the detector electrodes  2268  detect the ion species of interest. Because the detected ions may be neutralized during detection, the ionization source  2270  then ionizes the sample S, including the neutralized ions. After ionization, the deflector  2272  propels the product ions through the attractor grid  2274  into the re-circulation channel  2278 . The unwanted and filtered compounds are exhausted from the DMS flow channel  2276  through exhaust  2282 . 
     The product ions may optionally be analyzed by analyzer  2284  before being circulated through re-circulation channel  2278  to inlet  2280  for mixing with more sample S molecules. Then, the mixture is circulated through the sample amplification system  2264  for another stage of filtering and detection. At the completion of each iteration of filtering and detection, the concentration of the target or desired ion species increases until the detector electrodes  2268  are able to detect the target species of interest. 
       FIG. 74  is a conceptual diagram of a sample amplification and analysis system  2286  employing a re-circulation channel according to an illustrative embodiment of the invention. The sample amplification and analysis system  2286  includes a inlet  2288 , ionization region  2290 , ionization source inlet  2292 , DMS filter region  2294 , deflection plate  2296 , guiding electrodes  2298 , detector and neutralizer electrode  2300 , exhaust  2302 , opening  2304 , transport gas inlet  2306 , re-circulation channel  2308 , and DMS flow channel  2310 . 
     In operation, a sample S is drawn into the DMS flow channel  2294  through the inlet  2288 . The sample S is ionized in the ionization region  2290 . The ionization source inlet  2292  enables the injection of reactant ions into the ionization region  2290 . Alternatively, an ionization source may reside within the ionization regions such as a plasma generator, UV source, or radioactive source. Once ionized, the sample is filtered in the DMS filter region  2294  to allow only a desired or selected ion species to reach the deflector  2296 . The unwanted, filtered, and neutralized ion species travel through the DMS flow channel  2310  and are discarded through the exhaust  2302 . 
     The selected ion species, however, are deflected by the deflector  2296  through the opening  2304  into the re-circulation channel  2308 . The guiding electrodes  2298  guide the selected ion species through the opening  2304  and toward the detector and neutralizer electrode  2300 . Once the selected ion species are detected and neutralized by electrode  2300 , transport gas from transport gas inlet  2306  propels the neutralized ions through the re-circulation channel  2308  toward the ionization region  2292 . In the ionization region  2292 , the neutralized ions are mixed with new sample molecules and ionized. The new mixture is then circulated through the amplification and analysis system  2286  and so on over a period of time until the concentration of the selected ion species reaches level that can be detected. 
       FIG. 75  is a flow diagram of a process of amplification of a selected ion species using an analyzer such as a DMS. First, a sample is collected and introduced (Step  2312 ). The sample is then passed through a DMS filter (Step  2314 ). The DMS filter may be controlled for designated time period to allow only a desired ion species to pass through the filter region without being neutralized (Step  2316 ). The compounds that are neutralized and/or not ionized are ejected from the DMS filter and analyzer (Step  2318 ). 
     Next, the remaining filtered ions are collected and neutralized (Step  2320 ). The neutralized ions are then mixed with additional sample molecules and/or a transport gas (Step  2322 ). The mixture is passed through the DMS filter for second stage of analysis (Step  2324 ). The process is repeated until a sufficient concentration of the desired ion species or compound is present for detection and analysis (Step  2326 ). 
     Sample analysis may also be enhanced by combining DMS techniques with sample detection using another type of device such as IMS, TOF IMS, FT IMS, MS, electrochemical detector, or the like. In one illustrative embodiment of the invention, DMS detection is combined with IMS detection to enhance sample identification. 
     IMS technology uses the coefficient of mobility (K) to identify chemical constituents of a sample by measuring the different values of mobility associated with different sample constituent ion species. The coefficient of mobility depends on the mass (μ) and cross section of an ion (Ω) as described in Equation 15: 
                   K   =         3   ⁢           ⁢   ⅇ       16   ⁢   N       ⁢       (       2   ⁢           ⁢   π       μ   ⁢           ⁢   k   ⁢           ⁢     T   eff         )       1   2       ⁢     1       Ω   1.1     ⁡     (     T   eff     )                   (   15   )               
The coefficient of mobility also depends on the electric field strength, the coefficient of mobility at low field conditions (K( 0 )), and the alpha parameter (α). The dependence is expressed in Equation 16:
   K=K (0)[1+α 2 ( E/N ) 2 +α 4 ( E/N ) 4   + . . . ]=K (0)[1+α( E/N )]  (16) 
The coefficient of mobility K may alternatively be expressed as:
   K ( E )= K (0)[1−α( E )]. 
     Because a conventional TOF IMS operates at low field conditions, a TOF IMS may be employed to plot and determine the K( 0 ) of a particular ion species. As described in further detail previously, because a DMS alternately operates at high and low field conditions, a DMS may be employed to plot and determine the alpha parameter α(E) of a particular ion species. Thus, by using a DMS in combination with a TOF IMS, the coefficient of mobility K(E) for a particular ion species may be plotted over a range of electric field strengths and, thereby, provide enhanced ion species identification based on the derived coefficient of mobility over a range of field strengths. 
     Also, by detecting a select ion species using multiple detection techniques, improved analysis may be achieved where one detection technique, e.g., DMS, provides better ion species differentiation and identification than another detection technique, e.g., TOF IMS, and visa versa. 
       FIG. 76  is a graph  2328  of ion intensity versus drift time in a conventional IMS for ions of benzene, acetone, and toluene respectively. In this instance, the ion intensity peaks for benzene, acetone, and toluene substantially overlap, inhibiting the IMS detector from distinguishing between the three possible compounds. Thus, an alternative detection technique, such as DMS detection, may be employed to provide improved ion species differentiation. 
       FIG. 77  is a graph  2330  of ion intensity versus field compensation voltage in a DMS for acetone, acetone and othoxylene, acetone and metaxylene, acetone and toluene, and acetone and benzene respectively. The graph  2330  provides different spectra plots of ion intensity for acetone, acetone and benzene, and acetone and toluene that, unlike the graph  2328 , enable the distinction between acetone, benzene, and toluene ion species. Thus, in this instance, the DMS detection graph  2330  enables the desired distinction between various ion species that was otherwise not possible based on the IMS graph  2328 . There may be instances, however, where IMS detection in combination with DMS detection enhances the distinction between ion species as opposed to relying on DMS detection alone. 
       FIG. 78  is a graph  2332  of ion intensity versus field compensation voltage in a DMS for ions of organo-phospates such as DEMP and DEEP respectively. The three ion peaks for DEMP occur at approximately the same field compensation voltages as the ion peaks for DEEP. While the DMS detection graph  2332  may adequately distinguish between the ion intensity spectra for DEMP and DEEP, additional information provided by another analytical detection technique in combination with the DMS analytical detection technique may, in certain circumstances, enhance the identification of one ion species over the other species. 
       FIG. 79  is a graph  2334  of ion intensity versus drift time in a conventional IMS for DEMP and DEEP, respectively. The ion intensity peaks  2336  and  2338  for DEMP are shifted left with respect to the ion intensity peaks  2340  and  2342  for DEEP, which provides further distinction information between these organo-phosphate ion species. Thus, in this instance, the IMS detection graph  2334  enhances the distinction between the DEMP and DEEP ion species that was not as clearly distinguishable based on the DMS graph  2332  alone. 
       FIG. 80  is a graph  2344  of compensation voltage versus mass in a DMS, along with drift time versus mass in an IMS. The graph  2344  illustrates the effect of ion mass on the type of detection method performed. As can be seen from the graph  2344 , DMS detection provides better ion species differentiation for lighter ions while IMS detection provides better ion species differentiation for heavier ions. By performing DMS and IMS detection in combination, the detection of both lighter and heavier ions may be enhanced. Again, by performing both DMS and IMS detection, the coefficient of mobility of a particular ion species may be plotted to further enhance chemical identification within a sample. 
       FIG. 81A  is a graph  2346  of the alpha parameter α(E) versus electric field strength for two ion species with similar alpha parameters. Because the alpha parameters of the two ion species are approximately the same, DMS detection alone likely cannot distinguish between them. However, even if the alpha parameters are approximately the same, K( 0 ) may be different, resulting in a different K(E) for the two ion species. 
       FIG. 81B  is a graph  2348  of the coefficient of mobility K(E) versus electric field strength for two ion species having similar alpha parameters α(E) but different low field mobility parameters K( 0 ). Interestingly, K( 0 ) acts as an offset for the alpha parameter, shown in  FIG. 81B  by the upward shift of the K(E) plot  2350  for the first ion species. This shifting or offset is analogous to a direct current (DC) voltage offset of an Alternating Current (AC) in an electronic circuit. The graph  2348  shows that, even when alpha parameters are nearly identical, ion species may be distinguished by the respective K(E) due to differences in K( 0 ). Again, by using a DMS to determine the alpha parameter and an IMS to determine K( 0 ), the K(E) may be plotted for enhanced ion species identification. 
       FIG. 82A  is a graph  2352  of the alpha parameter α(E) versus electric field strength for two ion species with different alpha parameters. In this instance, DMS detection alone may be sufficient to identify the ion species.  FIG. 82B  is a graph  2354  of the coefficient of mobility versus electric field strength for two ion species with similar low field mobility parameters K( 0 ) but different alpha parameters α(E). Because K( 0 ) is approximately the same for both ion species, the offset of K(E) for both ion species is approximately the same. However, because the alpha parameters α(E) for both ion species are different, as shown in the graph  2354 , the K(E) for both ion species are different and distinguishable. 
     If both parts of the coefficient of mobility, e.g., K( 0 ) and α(E), are different, then any portion of the K(E) plot may be enough to distinguish one ion species from another. In certain embodiments of the invention, a detection system may perform both DMS and IMS detection to determine K(E) or selectively perform DMS or IMS detection based on the target ion species weight according to  FIG. 80 . It may further be possible to combine IMS detection with the previously described enhanced DMS detection techniques such as fragmentation, pre-separation, amplification, and dispersion plotting to even further enhance the detection of ion species within a sample. 
     The determination of the alpha parameter α(E) has been described previously with regard to Equation 1. The low field coefficient of mobility K( 0 ) may be determined directly by using a conventional TOF IMS. The K( 0 ) is calculated by determining the drift time and peak position in the IMS ion intensity plot for certain DC electric fields levels applied to the drift region of the TOF IMS. The drift time enables the determination of ion velocity which, in turn, reveals the low field coefficient of mobility K( 0 ) based on the formula ν=K*E. 
     Alternatively, K( 0 ) may be determined by analyzing the frequency dependence of detector current, for example, within a cylindrical detector. This is shown in the work of Puton, et al.,  Measurement of Difference Ion Mobility Spectrum with Simple Cylindrical Detector, ISIMS  2003. By measuring the ion current vs. the RF frequency of the modulated AC voltage applied to two cylindrical electrodes in the ionization region of a radioionization detector, the K( 0 ) for positive and negative ions can be determined. The K( 0 ) can be determined by computing the second derivative of the frequency characteristic plot. 
     One deficiency with the Puton approach is that the ion current measurement is an average of all ions in a sample. Thus, it provides an average K( 0 ) as opposed to the K( 0 ) for a particular ion species. 
     According to one illustrative embodiment of the invention, this problem is resolved by employing a DMS to filter and isolate a particular ion species of interest prior to plotting the ion current vs. frequency. The K( 0 ) for the particular ion species is then determined by computing the second derivative of the frequency plot. This approach supports the determination of K( 0 ) for both positive and negative ions of a particular ion species, which may be concurrently or substantially simultaneously filtered by a DMS. 
       FIG. 83  is a conceptual diagram of a DMS-IMS detection system  2356  according to an illustrative embodiment of the invention. The DMS-IMS detection system  2356  includes the DMS  2358  and IMS  2360 . The DMS  2358  includes a sample S inlet  2362 , ionization region  2364 , ionization source  2396 , DMS filter region  2366 , filter electrodes  2368  and  2370 , field compensation voltage source  2372 , field voltage source  2374 , DMS flow channel  2398 , detector electrodes  2376  and  2378 , variable detector voltage sources  2380  and  2382 , and vents  2384  and  2386 . The IMS  2360  includes a shutter  2388 , drift region  2400 , gradient electrodes  2390 , optional shutter  2392 , and collector  2394 . 
     In operation, a sample S is drawn through the inlet  2362  into the ionization region  2364  and then ionized by the ionization source  2396 . The sample S is then filtered in the DMS filter region  2366  by applying a compensated high asymmetric RF field at the filter electrode  2370  while the filter electrode  2368  remains at a common or ground potential. The Vcomp is provided by the field compensation voltage source  2372  while Vrf is provided by the field voltage source  2374 . 
     Depending on the selected field voltage and field compensation voltage applied at the electrode  2370 , a selected portion of the ions of the sample S pass through the DMS filter region  2366  and are detected at the detector electrodes  2376  and  2378 . The sample S ions may be transported through the DMS flow channel  2398  by a carrier gas, electric field gradient, and the like. 
     Once the filtered ions are detected at either or both detector electrodes  2376  and  2378 , the neutrals may be re-ionized and delivered to the IMS  2360  for further analysis. As stated previously, the alpha parameter α(E) of the filtered ion species may be determined based on the detected ion intensity in the DMS  2358 . Alternatively, the detector electrodes  2376  and  2378  may be turned off or driven with voltages by the variable detector voltage sources  2380  and  2382  to prevent DMS detection while keeping the filtered ions within the DMS flow channel  2398  for delivery to the IMS  2360 . 
     Regardless of whether the filtered sample S ions are detected by the DMS  2358 , the filtered sample S ions are delivered from the DMS  2358  to the IMS  2360 . The vents  2384  and  2386  may be used to remove excess gas. Alternatively, the vents  2384  and  2386  may introduce reactant ions for re-ionization of the filtered and detected ions that were neutralized by the detector electrodes  2376  and  2378 . 
     In the IMS  2360 , the shutter  2388 , depending on its polarity, forms packets of the filtered ions, either positive or negative, from the DMS  2358 . The shutter  2388  may include a shutter grid, one or more electrodes, and a like type of ion trap. The shutter  2388  then injects or gates the filtered ion into the drift region  2400 . The filtered ions are then propelled through the drift region  2400  by a voltage gradient established by the gradient electrodes  2390 . For positive ions, the voltage gradient created by the gradient electrodes  2390  becomes relatively more negative as the filtered ions move toward the collector  2394 . For negative ions, the voltage gradient created by the gradient electrodes  2390  becomes relatively more positive as the filtered ions move toward the collector  2394 . The time between the gating of the ions by the shutter  2388  and the detection of the ions at the collector  2394 , e.g., the time of flight (TOF), may be used to determine the ion velocity and, subsequently, the low field coefficient K( 0 ) of the filtered ion species. The gradient voltage within the IMS  2360  may be approximately 500 volts (V), 400 volts, 250 volts, 100 volts, 50 volts, or as required to flow the ions across the drift region  2400  to the collector  2394 . Thus, the gradient field strength may be approximately 10,000 V/cm, 8,000 V/cm, 5,000 V/cm, 2,000 V/cm, 1,000 V/cm, or as required to flow the ions across the drift region  2400  to the collector  2394 . 
     The IMS  2360  may include an optional shutter grid  2392  for further filtering ions in the IMS  2360  by being gated at select times to allow certain ion species to reach the collector  2394 . The optional shutter grid  2392  may act as the second gate when and/or if the IMS  2360  functions as a Fourier Transform IMS (FTIMS). 
     A FTIMS is an improved form of IMS detection resulting in improved sensitivity, resolution, and processing time for sample detection and analysis. In a conventional IMS, ions are introduced into to drift region by pulsing open a gating grid such as the shutter grid  2388  of the IMS  2360 . The shutter grid  2388  may be pulsed open for approximately less than 1% of the analysis time of the IMS. Thus, in a conventional IMS, more than 99% of the ions formed may be discarded and never reach the collector, e.g., collector  2394 . 
     A FTIMS uses a two-gate design and performs a Fourier transform of the frequency domain ion mobility information, referred to as an interferogram, to reconstruct the detected ion species spectra. The interferogram is generated by the ions that are pulsed into the IMS which then interact with the second synchronously pulsed exit gate. The ions that reach the second gate are delayed by the time-of-flight across the IMS′ drift region, e.g., drift region  2400 . Thus, the stream of ions may be in or out of phase with the second gate, e.g., shutter  2392 . An interference signal is created that depends on the degree to which the second gate is open or closed. Ions with velocities that enable them to reach the second gate when the gate is open, e.g., at the appropriate frequency, provide maximal signal input. The gates are typically driven by a square wave. To identify a sample with multiple constituents having multiple ion velocities, the gates, e.g., input shutter gate  2388  and exit shutter gate  2392 , may be pulsed open using a square wave having a continually increasing frequency from a few hertz up to thousands of hertz. 
     A Fourier transformation of the interferogram enables the reconstruction of ion species spectra based on the relationship ion species velocity and the gating frequency applied the shutter grids  2388  and  2392 . Unlike a conventional IMS that may use an entrance gate with a 1% duty cycle, the entrance gate, e.g., shutter  2388 , for an FTIMS may operate with a 50% duty cycle which significantly increases the amount of ions introduced into the FTIMS and, thereby, significantly increases the sensitivity of the FTIMS analytical technique. 
     Instead of using an exit gate, e.g., shutter  2392 , an external second gate may be implemented within the electronics and/or electronic processing of a processor, e.g., MPU  46  of  FIG. 5 , to enable the Fourier transformation of the detected ion signals in an IMS with no second gate. Further details regarding the use of an external second gate for an FTIMS are described in the work of Edward E. Tarver,  External Second Gate, Fourier Transform Ion Mobility Spectrometry: Parametric Optimization for Detection of Weapons of Mass Destruction, Sensor  2004, 4, 1-13. 
       FIG. 84  is a conceptual diagram of a DMS-IMS detection system  2402  using a shutterless IMS according to an illustrative embodiment of the invention. The DMS-IMS system  2402  includes a DMS  2404  and IMS  2406 . The DMS  2404  includes a sample S inlet  2408 , ionization region  2410 , ionization source  2412 , DMS filter region  2414 , filter electrodes  2416  and  2418 , field compensation voltage source  2422 , field voltage source  2424 , DMS flow channel  2420 , detector electrodes  2426  and  2428 , detector voltage sources  2430  and  2432 , and vents  2434  and  2436 . The IMS  2406  includes a drift region  2444 , gradient electrodes  2438 , optional shutter  2440 , and collector  2442 . 
     In operation, a sample S is drawn through the inlet  2408  into the ionization region  2410  and then ionized by the ionization source  2412 . The sample S is then filtered in the DMS filter region  2414  by applying a compensated high asymmetric RF field at the filter electrode  2418  while the filter electrode  2416  remains at a common or ground potential. The field compensation voltage is provided by the field compensation voltage source  2422  while the field voltage is provided by the field voltage source  2424 . 
     Depending on the selected field voltage and field compensation voltage applied at the electrode  2418 , a desired portion of the ions of the sample S pass through the DMS filter region  2414  and are detected at the detector electrodes  2426  and  2428 . The sample S ions may be transported through the DMS flow channel  2420  by a carrier gas, electric field gradient, and the like. 
     Once the filtered ions are detected at either or both detector electrodes  2426  and  2428 , the resulting neutral ions may be re-ionized and delivered to the IMS  2406  for further analysis. As stated previously, the alpha parameter α(E) of the filtered ion species may be determined based on the detected ion intensity in the DMS  2404 . Alternatively, the detector electrodes  2426  and  2428  may be turned off or driven with voltages by the detector voltage sources  2430  and  2432  to prevent DMS detection while keeping the filtered ions within the DMS flow channel  2420  for delivery to the IMS  2406 . 
     Instead of using a shutter within the IMS  2406  to control the introduction of filtered ions into the drift region  2444 , the detector electrodes  2426  and  2428  may act as detectors for the DMS  2404  during one cycle and then be set to the same potential during an another cycle. During the cycle when the detector electrodes  2426  and  2428  are set to the same potential, the detector electrodes  2426  and  2428  act as an ion trap or shutter. The detectors electrodes  2426  and  2428  may then be used to control the injection of filtered ions from the DMS  2404  into the drift region  2444  of the IMS  2406 . The vents  2434  and  2436  may be used to remove excess gas and/or introduce reactant ions for re-ionization of the filtered and detected ions that were neutralized by the detector electrodes  2426  and  2428 . 
     In the IMS  2406 , the filtered ions are propelled through the drift region  2444  by a voltage gradient established by the gradient electrodes  2438 .  FIG. 84  shows a voltage gradient created by the gradient electrodes  2438  that is relatively more positive as the filtered ions move toward the collector  2442 . Thus, negative ions are propelled across the drift region  2444  to the collector  2442  for IMS  2406  detection. For positive ions, the voltage gradient created by the gradient electrodes  2438  may be configured to establish a relatively more negative potential as the filtered ions move toward the collector  2442 . The time between the gating of the ions by the detector electrodes  2426  and  2428  and the detection of the ions at the collector  2442  may be used to determine the ion velocity and, subsequently, the low field coefficient K( 0 ) of the filtered ion species. 
     The IMS  2406  may include an optional shutter grid  2440  for further filtering ions in the IMS  2406  by being gated at select times to allow certain ion species to reach the collector  2442 . The optional shutter grid  2440  may act as a second gate for the IMS  2406  if operating as an FTIMS. Otherwise, IMS  2406  may use an external second gate when acting as an FTIMS. 
       FIG. 85  is a conceptual diagram of a DMS-IMS detection system  2446  system where the IMS is connected to the DMS in manner that reduces the introduction of neutral molecules into the IMS according to another illustrative embodiment of the invention. The DMS-IMS detection system  2446  includes a DMS  2472  and IMS  2474 . The DMS  2472  includes a sample S inlet  2448 , ionization region  2450 , ionization source  2452 , DMS filter region  2454 , filter electrodes  2456  and  2458 , field compensation voltage source  2460 , field voltage source  2462 , detector electrodes  2464  and  2466 , detector power source  2484 , DMS flow channel  2468 , and outlet  2470 . The IMS  2474  includes a shutter  2476 , gradient electrodes  2478 , optional shutter  2480 , and collector  2482 . 
     In operation, a sample S is drawn through the inlet  2448  into the ionization region  2450  and then ionized by the ionization source  2452 . The sample S is then filtered in the DMS filter region  2454  by applying a compensated high asymmetric RF field at the filter electrode  2458  while the filter electrode  2456  remains at a common or ground potential. The field compensation voltage is provided by the field compensation voltage source  2460  while the field voltage is provided by the field voltage source  2462 . 
     Depending on the selected field voltage and field compensation voltage applied at the electrode  2458 , a desired portion of the ions of the sample S pass through the DMS filter region  2454  and are detected at the detector electrodes  2464  and  2466 . The detector electrode  2464  includes an orifice  2486  that allows ions to pass into the IMS  2474 . The sample S ions may be transported through the DMS flow channel  2468  by a carrier gas, electric field gradient, and the like. 
     Once the filtered ions are detected at either or both detector electrodes  2464  and  2466 , the neutrals may be re-ionized and delivered to the IMS  2474  via the orifice  2486  for further analysis. Otherwise, the neutral ion may be expelled through the outlet  2470 . As shown in  FIG. 85 , the IMS  2474  is oriented in manner, e.g., perpendicular to the DMS flow channel  2468 , that reduces the introduction of neutral molecules into the IMS  2474  by allowing neutral molecules to be expelled through the outlet  2470  while ions are directed through the orifice  2486  into the IMS  2474 . As stated previously, the alpha parameter α(E) of the filtered ion species may be determined based on the detected ion intensity in the DMS  2472 . To propel the ions into the IMS  2474  through the orifice  2486 , the detector electrode  2466  may be biased with a like potential as the filtered ions while the detector electrode  2464  is biased with an opposite potential to attract the filtered ions to the orifice  2484 . 
     The detector electrode  2464  may also have its potential configured to enable detection of filtered ions while concurrently or substantially simultaneously allowing a portion of the filtered ions through the orifice  2486  into the IMS  2474  for further IMS detection. The potential at the detector electrodes  2464  and  2466  may be selectively adjusted to control the fields and biases at the orifice  2486  and, thereby, determine the amount of detection at the DMS  2472  and/or injection rate into the IMS  2474 . 
     In the IMS  2474 , the shutter  2476 , depending on its polarity, forms packets of the filtered ions, either positive or negative, from the DMS  2472 . The shutter  2476  may include a shutter grid, one or more electrodes, and a like type of ion trap. The shutter  2476  injects or gates the filtered ion into the drift region  2488 . The filtered ions are then propelled through the drift region  2488  by a voltage gradient established by the gradient electrodes  2478 . For positive ions, the voltage gradient created by the gradient electrodes  2478  becomes relatively more negative as the filtered ions move toward the collector  2482 . For negative ions, the voltage gradient created by the gradient electrodes  2478  becomes relatively more positive as the filtered ions move toward the collector  2482 . The time between the gating of the ions by the shutter  2476  and the detection of the ions at the collector  2482 , e.g., the time of flight (TOF), may be used to determine the ion velocity and, subsequently, the low field coefficient K( 0 ) of the filtered ion species. 
     The IMS  2474  may include an optional shutter grid  2480  for further filtering ions in the IMS  2474  by being gated at select times to allow certain ion species to reach the collector  2394 . The optional shutter grid  2480  may act as a second gate for the IMS  2474  if operating as an FTIMS. Otherwise, the IMS  2474  may use an external second gate when acting as an FTIMS. 
       FIG. 86  is a conceptual diagram of a DMS-IMS detection system  2490  using a shutterless IMS which is connected to the DMS in a manner that reduces the introduction of neutral molecules into the IMS according to an illustrative embodiment of the invention. The DMS-IMS detection system  2490  includes a DMS  2492  and IMS  2494 . The DMS  2492  includes a sample S inlet  2496 , ionization region  2498 , ionization source  2500 , DMS filter region  2502 , filter electrodes  2504  and  2506 , field compensation voltage source  2508 , field voltage source  2510 , detector electrodes  2514  and  2516 , detector power source  2518 , DMS flow channel  2512 , orifice  2520 , and outlet  2522 . The IMS  2494  includes gradient electrodes  2524 , optional shutter  2526 , drift region  2530 , and a collector  2528 . 
     In operation, a sample S is drawn through the inlet  2496  into the ionization region  2498  and then ionized by the ionization source  2500 . The sample S is then filtered in the DMS filter region  2502  by applying a compensated high asymmetric RF field at the filter electrode  2506  while the filter electrode  2504  remains at a common or ground potential. The field compensation voltage is provided by the field compensation voltage source  2508  while the field voltage is provided by the field voltage source  2510 . 
     Depending on the selected field voltage and field compensation voltage applied at the electrode  2506 , a selected portion of the ions of the sample S pass through the DMS filter region  2502  and are detected at the detector electrodes  2514  and  2516 . The detector electrode  2514  includes an orifice  2520  that allows ions to pass into the IMS  2494 . The sample S ions may be transported through the DMS flow channel  2512  by a carrier gas, electric field gradient, and the like. 
     Once the filtered ions are detected at either or both detector electrodes  2514  and  2516 , the neutrals may be re-ionized and delivered to the IMS  2494  via the orifice  2520  for further analysis. Otherwise, the neutral ions may be expelled through the outlet  2522 . As shown in  FIG. 86 , the IMS  2494  is oriented in manner, e.g., perpendicular to the DMS flow channel  2512 , that reduces the introduction of neutral molecules into the IMS  2494  by allowing neutral molecules to be expelled through the outlet  2522  while ions are directed through the orifice  2520  into the IMS  2494 . As stated previously, the alpha parameter α(E) of the filtered ion species may be determined based on the detected ion intensity in the DMS  2492 . 
     Instead of using a shutter within the IMS  2494  to control the introduction of filtered ions into the drift region  2530 , the detector electrodes  2514  and  2516  may act as detectors for the DMS  2492  during one cycle and then act as guiding electrodes during another cycle. During the cycle when the detector electrodes  2514  and  2516  are acting as guiding electrodes, the detectors electrodes  2514  and  2516  may then be used to control the injection of filtered ions from the DMS  2492  into the drift region  2530  of the IMS  2494 . To propel the ions into the IMS  2494  through the orifice  2520 , the detector electrode  2516  is biased with a like potential as the filtered ions to repel the ions while the detector electrode  2516  is biased with an opposite potential to attract the filtered ions to the orifice  2520 . This cycling of the functionality of the detector electrodes  2514  and  2516  enables the detector electrodes  2514  and  2516  to alternately act like a shutter for the TOF measurement in the IMS  2494 . 
     In the IMS  2494 , the filtered ions are propelled through the drift region  2524  by a voltage gradient established by gradient electrodes  2524 . For positive ions, the voltage gradient created by the gradient electrodes  2524  becomes relatively more negative as the filtered ions move toward the collector  2528 . For negative ions, the voltage gradient created by the gradient electrodes  2524  becomes relatively more positive as the filtered ions move toward the collector  2528 . The time between the gating of the ions by the detector electrodes  2514  and  2516  and the detection of the ions at collector  2528  may be used to determine the ion velocity and, subsequently, the low field coefficient K( 0 ) of the filtered ion species. 
     The IMS  2494  may include an optional shutter grid  2526  for further filtering ions in the IMS  2494  by being gated at select times to allow certain ion species to reach the collector  2528 . The optional shutter grid  2526  may act as a second gate for the IMS  2494  if operating as an FTIMS. Otherwise, the IMS  2494  may use an external second gate when acting as an FTIMS. 
       FIG. 87  is a conceptual diagram of a DMS-IMS detection system  2532  using two IMS detectors according to an illustrative embodiment of the invention. The DMS-IMS detection system  2532  includes a DMS  2534 , IMS  2536 , and IMS  2538 . The DMS  2534  includes a sample S inlet  2540 , ionization region  2542 , ionization source  2544 , DMS filter region  2546 , filter electrodes  2548  and  2550 , field compensation voltage source  2554 , field voltage source  2556 , DMS flow channel  2552 , detector electrodes  2558  and  2560 , detector power sources  2572  and  2574 , orifices  2562  and  2564 , and outlet  2566 . The IMS  2536  includes a shutter  2568 , gradient electrodes  2576 , drift region  2582 , optional shutter  2578 , and collector  2580 . The IMS  2538  includes a shutter  2570 , gradient electrodes  2584 , drift region  2590 , optional shutter  2586 , and collector  2588 . 
     In operation, a sample S is drawn through the inlet  2540  into the ionization region  2542  and then ionized by the ionization source  2544 . The sample S is then filtered in the DMS filter region  2546  by applying a compensated high asymmetric RF field at the filter electrode  2550  while the filter electrode  2548  remains at a common or ground potential. The field compensation voltage is provided by the field compensation voltage source  2554  while the field voltage is provided by the field voltage source  2556 . 
     Depending on the selected field voltage and field compensation voltage applied at the electrode  2550 , a desired portion of the ions of the sample S pass through the DMS filter region  2546  and are detected at the detector electrodes  2558  and  2560 . The detector electrodes  2558  and  2560  include the orifices  2562  and  2564  that allow ions to pass into the IMS  2536  and IMS  2538  respectively. The sample S ions may be transported through the DMS flow channel  2552  by a carrier gas, electric field gradient, and the like. 
     The detector electrode  2558  may be negatively biased by the detector power source  2572  to attract positive ions into the IMS  2536  via the orifice  2562  and to repel negative ions toward the orifice  2564 . The detector electrode  2560  may be positively biased by the detector power source  2574  to attract negative ions into the IMS  2538  via the orifice  2564  and to repel positive ions toward the orifice  2562 . Thus, both positive and negative ions may be detected concurrently or substantially simultaneously by the DMS-IMS detection system  2532 . 
     Once the filtered ions are detected at either or both detector electrodes  2558  and  2560 , the neutrals may be re-ionized and delivered to either or both the IMS  2536  via the orifice  2562  or the IMS  2538  via the orifice  2564  for further analysis. Otherwise, the neutral ion may be expelled through the outlet  2566 . As shown in  FIG. 87 , the IMS  2536  and IMS  2538  are oriented in manner, e.g., perpendicular to the DMS flow channel  2552 , that reduces the introduction of neutral molecules into both the IMS  2536  and IMS  2538  by allowing neutral molecules to be expelled through the outlet  2566  while ions are directed through the orifices  2562  and  2564  into the IMS  2536  and IMS  2538  respectively. 
     A portion of the filtered ions may be detected and neutralized by the detectors  2558  and  2560 , allowing the remaining ions to enter the IMS  2536  and IMS  2538  for further analysis. The potential at the detector electrodes  2558  and  2560  may be selectively adjusted to control the fields and biases at the orifices  2562  and  2564  to determine the amount of detection at the DMS  2534  and/or the ion injection rate into the IMS  2536  and IMS  2538  respectively. As stated previously, the alpha parameter α(E) of the filtered ion species may be determined based on the detected ion intensity in the DMS  2534 . 
     In the IMS  2536 , the shutter  2568 , depending on its polarity, forms packets of the filtered ions, either positive or negative, from the DMS  2534 . The shutter  2568  may include a shutter grid, one or more electrodes, and a like type of ion trap. The shutter  2568  injects or gates the filtered ion into the drift region  2582 . The filtered ions are then propelled through the drift region  2582  by a voltage gradient established by the gradient electrodes  2576 . For positive ions, the voltage gradient created by the gradient electrodes  2576  becomes relatively more negative as the filtered ions move toward the collector  2580 . For negative ions, the voltage gradient created by the gradient electrodes  2576  becomes relatively more positive as the filtered ions move toward the collector  2580 . The time between the gating of the ions by the shutter  2568  and the detection of the ions at the collector  2580 , e.g., the time of flight (TOF), may be used to determine the ion velocity and, subsequently, the low field coefficient K( 0 ) of the filtered ion species. The TOF may also be used to identify the ion species directly. 
     The IMS  2536  may include an optional shutter grid  2578  for further filtering ions in the IMS  2536  by being gated at select times to allow certain ion species to reach the collector  2580 . The optional shutter grid  2578  may act as a second gate for the IMS  2536  if operating as an FTIMS. Otherwise, the IMS  2536  may use an external second gate when acting as an FTIMS. 
     In the IMS  2538 , the shutter  2570 , depending on its polarity, forms packets of the filtered ions, either positive or negative, from the DMS  2534 . The shutter  2570  may include a shutter grid, one or more electrodes, and a like type of ion trap. The shutter  2570  injects or gates the filtered ion into the drift region  2590 . The filtered ions are then propelled through the drift region  2590  by a voltage gradient established by the gradient electrodes  2584 . For positive ions, the voltage gradient created by the gradient electrodes  2584  becomes relatively more negative as the filtered ions move toward the collector  2588 . For negative ions, the voltage gradient created by the gradient electrodes  2584  becomes relatively more positive as the filtered ions move toward the collector  2588 . The time between the gating of the ions by the shutter  2570  and the detection of the ions at the collector  2588 , e.g., the time of flight (TOF), may be used to determine the ion velocity and, subsequently, the low field coefficient K( 0 ) of the filtered ion species. The TOF may also be used to identify the ion species directly. 
     The IMS  2538  may include an optional shutter grid  2586  for further filtering ions in the IMS  2538  by being gated at select times to allow certain ion species to reach the collector  2588 . The optional shutter grid  2586  may act as a second gate for the IMS  2538  if operating as an FTIMS. Otherwise, the IMS  2538  may use an external second gate when acting as an FTIMS. The IMS  2536  and IMS  2538  are connected to the DMS  2534  in an adjacent manner respectively to substantially reduce and/or eliminate the introduction of neutral ions into either IMS. 
       FIG. 88  is a conceptual diagram of a DMS-IMS detection system  2592  using two shutterless IMS detectors according an illustrative embodiment of the invention. The DMS-IMS detection system  2592  includes a DMS  2594 , shutterless IMS  2596 , and shutterless IMS  2598 . The DMS  2594  includes a sample S inlet  2600 , ionization region  2602 , ionization source  2604 , DMS filter region  2606 , filter electrodes  2608  and  2610 , field compensation voltage source  2614 , field voltage source  2616 , DMS flow channel  2612 , detector electrodes  2618  and  2620 , detector power sources  2628  and  2630 , detector orifices  2622  and  2624 , and outlet  2626 . The IMS  2596  includes gradient electrodes  2632 , drift region  2638 , optional shutter  2634 , and a collector  2636 . The IMS  2598  includes gradient electrodes  2640 , drift region  2646 , optional shutter  2642 , and a collector  2644 . 
     In operation, a sample S is drawn through the inlet  2600  into the ionization region  2602  and then ionized by the ionization source  2604 . The sample S is then filtered in the DMS filter region  2606  by applying a compensated high asymmetric RF field at the filter electrode  2610  while the filter electrode  2608  remains at a common or ground potential. The field compensation voltage is provided by the field compensation voltage source  2614  while the field voltage is provided by the field voltage source  2616 . 
     Depending on the selected field voltage and field compensation voltage applied at the electrode  2610 , a desired portion of the ions of the sample S pass through the DMS filter region  2606  and are detected at the detector electrodes  2618  and  2620 . The detector electrodes  2618  and  2620  include the orifices  2622  and  2624  that allow ions to pass into the IMS  2596  and IMS  2598  respectively. The sample S ions may be transported through the DMS flow channel  2612  by a carrier gas, electric field gradient, and the like. 
     The detector electrode  2618  may be negatively biased by the detector power source  2628  to attract positive ions into the IMS  2596  via the orifice  2622  and to repel negative ions toward the orifice  2624 . The detector electrode  2620  may be positively biased by the detector power source  2630  to attract negative ions into the IMS  2598  via the orifice  2624  and to repel positive ions toward the orifice  2622 . The detector electrodes  2618  and  2620  may alternately act as shutters for IMS  2596  and IMS  2598  respectively. 
     During one cycle, e.g., the DMS cycle, the electrodes  2618  and  2620  may be biased to act as DMS detectors. During another cycle, e.g., the shutter cycle, the detector electrodes  2618  and  2620  may be set to equal potentials or potentials that encourage the introduction of ions into the IMS  2596  and IMS  2598  respectively. At one cycle, strong negative and positive potentials may be applied to detector electrodes  2618  and  2620  respectively to facilitate DMS detection of positive and negative ions. At the next cycle, a neutral or common bias may be placed on both detector electrodes  2618  and  2620  to allow ions to pass through the orifices  2622  and  2624  into IMS  2596  and IMS  2598  respectively for further analysis. Again, both positive and negative ions may be detected concurrently or substantially simultaneously by the DMS-IMS detection system  2532 . 
     Once the filtered ions are detected at either or both detector electrodes  2618  and  2620  during a detection cycle, the neutrals may be re-ionized and delivered to either or both the IMS  2596  via the orifice  2622  or the IMS  2598  via the orifice  2624  during the shutter cycle for further analysis. Otherwise, the neutral ion may be expelled through outlet  2626 . As shown in  FIG. 88 , the IMS  2596  and IMS  2598  are oriented in manner, e.g., perpendicular to the DMS flow channel  2612 , that reduces the introduction of neutral molecules into both the IMS  2596  and IMS  2598  by allowing neutral molecules to be expelled through the outlet  2626  while ions are directed through the orifices  2622  and  2624  into the IMS  2596  and IMS  2598  respectively. 
     A portion of the filtered ions may be detected and neutralized by the detectors  2618  and  2620 , allowing the remaining ions to enter the IMS  2596  and IMS  2598  during the shutter cycle for further analysis. The potential at the detector electrodes  2596  and  2598  may be selectively adjusted to control the fields and biases at the orifices  2622  and  2624 . As stated previously, the alpha parameter α(E) of the filtered ion species may be determined based on the detected ion intensity in the DMS  2594 . 
     In the IMS  2596 , the filtered ions are received from the orifice  2622  during the shutter cycle of the detector electrode  2618 . The filtered ions are then propelled through the drift region  2638  by a voltage gradient established by the gradient electrodes  2632 . For positive ions, the voltage gradient created by the gradient electrodes  2632  becomes relatively more negative as the filtered ions move toward the collector  2636 . For negative ions, the voltage gradient created by the gradient electrodes  2632  becomes relatively more positive as the filtered ions move toward the collector  2636 . The time between the gating of the ions by the detector electrode  2618  and the detection of the ions at the collector  2636 , e.g., the time of flight (TOF), may be used to determine the ion velocity and, subsequently, the low field coefficient K( 0 ) of the filtered ion species. The TOF may also be used to identify the ion species directly. 
     The IMS  2596  may include an optional shutter grid  2634  for further filtering ions in the IMS  2596  by being gated at select times to allow certain ion species to reach the collector  2580 . The optional shutter grid  2634  may act as a second gate for the IMS  2596  if operating as an FTIMS. Otherwise, the IMS  2596  may use an external second gate when acting as an FTIMS. 
     In the IMS  2598 , the filtered ions are received from the orifice  2624  during the shutter cycle of the detector electrode  2620 . The filtered ions are then propelled through the drift region  2646  by a voltage gradient established by the gradient electrodes  2640 . For positive ions, the voltage gradient created by the gradient electrodes  2640  becomes relatively more negative as the filtered ions move toward the collector  2644 . For negative ions, the voltage gradient created by the gradient electrodes  2640  becomes relatively more positive as the filtered ions move toward the collector  2644 . The time between the gating of the ions by the detector electrode  2620  and the detection of the ions at the collector  2644 , e.g., the time of flight (TOF), may be used to determine the ion velocity and, subsequently, the low field coefficient K( 0 ) of the filtered ion species. The TOF may also be used to identify the ion species directly. 
     The IMS  2598  may include an optional shutter grid  2642  for further filtering ions in the IMS  2598  by being gated at select times to allow certain ion species to reach the collector  2644 . The optional shutter grid  2642  may act as a second gate for the IMS  2598  if operating as an FTIMS. Otherwise, the IMS  2598  may use an external second gate when acting as an FTIMS. 
       FIG. 89  is a conceptual diagram of a DMS-IMS detection system  2648  that supports a DMS mode and an IMS mode according to an illustrative embodiment of the invention. The DMS-IMS detection system  2648  includes a sample S inlet  2650 , ionization region  2652 , ionization source  2654 , DMS filter region  2656 , filter detectors  2658  and  2660 , field compensation voltage source  2662 , field voltage source  2664 , DMS flow channel  2668 , detector electrodes  2670  and  2672 , detector power sources  2674  and  2676 , and outlet  2678 . 
     In the DMS operating mode, a sample S is drawn through the inlet  2650  into the ionization region  2652  and then ionized by the ionization source  2654 . The sample S is then filtered in the DMS filter region  2656  by applying a compensated high asymmetric RF field at the filter electrode  2660  while the filter electrode  2658  remains at a common or ground potential. The field compensation voltage is provided by the field compensation voltage source  2662  while the field voltage is provided by the field voltage source  2664 . 
     Depending on the selected field voltage and field compensation voltage applied at electrode  2660 , a desired portion of the ions of the sample S pass through the DMS filter region  2656  and are detected at the detector electrodes  2670  and  2672 . The sample S ions may be transported through the DMS flow channel  2668  by a carrier gas, electric field gradient, and the like. 
     Once the filtered ions are detected at either or both detector electrodes  2670  and  2672 , the neutral ion may be expelled through the outlet  2678 . As stated previously, the alpha parameter α(E) of the filtered ion species may be determined based on the detected ion intensity at the detector electrodes  2670  and  2672 . 
     The IMS mode of operation is used to determine the low field mobility K( 0 ) based on analyzing the frequency dependence of detector current within a simple cylindrical detector as described in the work of Puton, et al.,  Measurement of Difference Ion Mobility Spectrum with Simple Cylindrical Detector, ISIMS  2003. 
     In the IMS mode of operation, a sample S is filtered in the DMS filter region  2656  such that a select ion species is delivered to the detector electrodes  2670  and  2672 . A modulated AC voltage is then applied by detector power source  2676  to detector electrode  2672  to expose the filtered ions to a modulated AC field. The ion current of detector electrodes  2670  and/or  2672  is then plotted versus the RF frequency of the modulated AC voltage applied to the detector electrode  2672 . Based on the plot, the low field mobility K( 0 ) may then be determined for the filtered ion species. 
     Thus, the alpha parameter may be determined during the DMS mode and the low field mobility K( 0 ) during the IMS mode which may be combined to determine the coefficient of mobility K(E) for the selected ion species. With the K(E), the detected ion species may be identified with a high degree of confidence. 
       FIG. 90  is a conceptual diagram of a DMS-IMS detection system  2680  where IMS and DMS detection occur concurrently and/or near simultaneously according to an illustrative embodiment of the invention. The DMS-IMS detection system  2680  includes a DMS  2722  and IMS  2720 . The DMS  2722  includes a sample S inlet  2682 , ionization source inlet  2684 , ionization region  2686 , DMS filter region  2686 , detector electrodes  2688  and  2690 , field compensation voltage source  2692 , field voltage source  2694 , DMS flow channel  2696 , detector electrodes  2698  and  2700 , detector power sources  2702  and  2704 , and DMS outlet  2706 . The IMS  2720  includes a shutter  2708 , gradient electrodes  2710 , optional shutter  2712 , collector  2714 , drift region  2718 , and IMS outlet  2716 . 
     In operation, a sample S is drawn through the inlet  2682  into the ionization region  2686  and ionized. The ionization inlet  2684  may introduce reactant ions into the ionization region  2686  to facilitate the sample S ionization. Alternative ionization sources may be employed as described previously to enable sample S ionization. The ionized sample S may then be drawn concurrently or near-simultaneously into both the DMS  2722  and the IMS  2720  for DMS and IMS analysis. 
     In the DMS  2722 , the sample S is filtered in the DMS filter region  2686  by applying a compensated high asymmetric RF field at the filter electrode  2690  while the filter electrode  2688  remains at a common or ground potential. The field compensation voltage is provided by the field compensation voltage source  2692  while the field voltage is provided by the field voltage source  2694 . 
     Depending on the selected field voltage and field compensation voltage applied at the electrode  2690 , a desired portion of the ions of the sample S pass through the DMS filter region  2686  and are detected at the detector electrodes  2698  and  2700 . The sample S ions may be transported through the DMS flow channel  2696  by a carrier gas, electric field gradient, and the like. 
     Once the filtered ions are detected at either or both detector electrodes  2698  and  2700 , the neutrals may are expelled through the DMS outlet  2706 . As stated previously, the alpha parameter α(E) of the filtered ion species may be determined based on the detected ion intensity in the DMS. 
     In the IMS  2720 , the shutter  2708 , depending on its polarity, forms packets of the sample S ions, either positive or negative, from the ionization region  2686 . The shutter  2708  may include a shutter grid, one or more electrodes, and a like type of ion trap. The shutter  2708  then injects or gates the ions into the drift region  2718 . The filtered ions are then propelled through the drift region  2718  by a voltage gradient established by the gradient electrodes  2710 . For positive ions, the voltage gradient created by the gradient electrodes  2710  becomes relatively more negative as the filtered ions move toward the collector  2714 . For negative ions, the voltage gradient created by the gradient electrodes  2710  becomes relatively more positive as the filtered ions move toward the collector  2714 . The time between the gating of the ions by the shutter  2708  and the detection of the ions at the collector  2714 , e.g., the time of flight (TOF), may be used to determine the ion velocity and, subsequently, the low field coefficient K( 0 ) of the filtered ion species. The TOF may also be used to identify the ion species directly. 
     The IMS  2720  may include an optional shutter grid  2712  that may further filter ions in the IMS  2720  by being gated at select times to allow certain ion species to reach the collector  2714 . The optional shutter grid  2712  may act as a second gate for the IMS  2720  if operating as an FTIMS. Otherwise, the IMS  2720  may use an external second gate when acting as an FTIMS. 
     It should be understood that  FIGS. 83-90  provide various exemplary combinations of DMS and IMS detection which are not exhaustive of the possible combinations of ion mobility based analyzers and detection techniques. Ion mobility based analyzers of one type may be combined in parallel, in series, in a combination of series and parallel. One or more analyzers of one type, e.g., DMS, may be employed in series and/or parallel with one or more analyzers of another type, e.g., IMS, to identify an ion species and/or sample constituent. It may not be necessary to use one type of analyzer before using another type of analyzer or to use multiple analyzers and/or analyzer types in a particular order. While the only two types of analyzers in combination have been featured, more than two types of ion mobility based analyzers may be employed in combination to identify sample constituent if necessary. 
     Although the invention has been described with regard to particular illustrative embodiments, it should be appreciated that the invention is broader in scope. For example, although the above described illustrative embodiments are directed to DMS-IMS and DMS-FTIMS combinations, in other illustrative embodiments, a DMS may be combined in a similar fashion with one or more GCs, FTIRs, MSs, and/or LCMS. 
     Additionally, the invention may be employed with any system for identification of unknown species of ions traveling through a varying controlled excitation field, the identification being based on the known characteristic travel behavior of the species under the varying field conditions. The ion or ions to be identified may be traveling alone or in a group of ions of same or differing characteristic travel behavior. Additionally, the ion or ions to be identified may be transported through the systems and devices of the invention by any suitable effluent, including transport gasses, liquids and/or vapors. The filter field may be compensated in any of various manners as long as a species of interest is returned to the center of the flow and permitted to pass through the filter while all other species are retarded or neutralized. Identification is made based on known field-dependent differential mobility behavior of at least one species of ions traveling in the field at known field conditions. 
     It should also be appreciated that, in various practices, the invention provides improved systems, methods and devices for ion species identification. According to some features, the invention varies one or more filter field/flow channel conditions to improve species discrimination. For example, according to some illustrative embodiments, the invention determines changes in ion mobility, based, for example, on changes in: Vrf; Vcomp; field strength; Vrf duty cycle; Vrf wavelength; Vrf frequency; and/or flow channel temperature, pressure, humidity, flow rate, doping and/or carrier gas CG composition. According to other features, the invention takes multiple scans of the sample S, for example, by recirculating the sample S and/or processing the sample S in parallel and/or in series with one or more additional DMS, IMS, TOFIMS, FTIMS, GC, FTIR, MS, or LCMS, at differing flow channel/filter field conditions. 
     According to further features, the invention employs approaches, such as, fragmenting, lowering pressure, three-dimensional dispersion plotting, ion pre-separation, and/or ion amplification to enhance detection resolution. According to other features, the invention stores a library of signatures for known compounds and pattern matches data from unknown compounds with the stored library to identify the unknown compounds. It should be understood that the invention is applicable not only to planar DMS systems, but may be applied in general to ion mobility spectrometry devices of various types, including various geometries, ionization arrangements, detector arrangements, and the like, and brings new uses and improved results even as to structures that are all well known in the art. 
     Thus, the invention is not limited to configurations of the illustrative embodiments and may be practiced in any other suitable configurations, including radial and cylindrical DMS devices. Additionally, various modifications and variations may be made to the invention without departing from the spirit and scope herein.