Abstract:
A system and methodology for the detection of threat substances is described. The detector system consists of a method to evaporate the sample into a primary separator and thermal release of trapped target materials into a secondary separator like conventional GC. The GC column is thermally ramped to elute all substances and the end of the column terminates into an atmospheric pressure chemical ionization source of an axial ion mobility spectrometer (AIMS). Both polarity ions are pulsed into a single construction separator tube at different timing. Their arrival time is detected on a collector plate, which allows registering their ion mobility spectra of both polarities for a single GC peak.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the field of substance detection (for example, explosives, narcotics, chemical warfare agents, environmental pollutants), which are typically but not necessarily threat substances. 
       BACKGROUND OF THE INVENTION 
       [0002]    The increasing terrorist threat internationally has made it crucial to detect all kinds of explosives and other threat substances in order to provide security for important locations such as airports, border crossings, embassies, seaports, governmental buildings, power stations or transportation systems. A number of techniques for detecting threat substances are known, such as X-ray screening, fluorescence quenching, neutron and gamma-ray spectroscopy, LC-MS, UV gated Raman spectroscopy, laser induced breakdown spectroscopy, electrochemical and immunosensors, chemiluminescence, SPME-HPLC, GC-ECD, GC-SAW devices, GC-differential mobility spectrometer. More recently, metal oxide semiconductor (MOS) nanoparticle sensors have been used for the detection and discrimination of low concentrations of explosives. 
         [0003]    Ion mobility spectrometry (IMS) has been shown over the past 20 years to be a reliable method for trace detection of explosives, drugs, chemical warfare agents, toxic industrial chemicals and various organic environmental pollutants, due to its low detection limit, relatively fast response, hardware simplicity, and portability. IMS-based equipment is presently used in vulnerable places, such as airports, for screening of both people and carry-on luggage. 
         [0004]    Although IMS technology has been successful in many areas, it is undesirably limited in cases where the sample material is presented in complex matrices. Under these conditions, when other materials are liberated with the analytes of interest, those other materials can selectively compete in the ionization process. Their ionization levels may be less than those for the analytes of interest, so they competitively react in the ionization process and greatly reduce the sensitivity and selectivity of the IMS. 
         [0005]    IMS is a gas-phase ion separation technique that operates under atmospheric pressure. A drift tube consisting of a reaction region and drift region is the main element of the IMS instrument. In conventional IMS instruments, the electric field is created by a series of conducting guard rings, and in more simplified drift tube designs the ion drift tube is formed of single-piece, conductive glass tube. This more recent drift tube design is disclosed in U.S. Pat. No. 7,081,618, which describes a reaction-ionization/drift tube chamber constructed with one or more single-piece conductive ceramic or glass tubes having specified conductivity. The glass tube or ceramic is used in place of the stack assemblies of metal and ceramic annular components that were typically used in previous drift tubes. This approach provides a simpler design, fewer parts and improved performance for fast switching of ion polarity during a scanning mode. 
         [0006]    Ion mobility spectrometers for detection of explosives, narcotics and other contraband are disclosed in U.S. Pat. Nos. 3,699,333, 5,027,643, and 5,200,614. U.S. Pat. No. 5,491,337 shows still further improvements to ion trap mobility spectrometers and U.S. Pat. No. 6,690,005 describes a pulsing mechanism for ions entering the rings-stack drift tube with front trapping capability and switching of ion polarity entering the drift chamber. U.S Patent application 2002/0134933 provides a method for detecting both positive and negative mobility spectra wherein the first and second selected switching times are less than 20 msec and 15 msec, respectively, with a transition time of less than 5 msec. 
         [0007]    U.S. Pat. No. 7,528,367 describes an ion mobility spectrometer with an inlet that communicates to an ionization chamber and a drift chamber. Stacked grid electrodes with applied potential hold ions between them until they are pulsed into the drift chamber. This patent claims sharper peak shape and improved resolution. U.S. Pat. Nos. 6,124,592 and 6,407,382 describe methods to separate and store ions by exploiting mobility characteristics of the ions by applying an electric field to trap the volume of ions prior to pulsing them into the ring stacked drift chamber. US application 2009/0113982 discloses a multi-dimensional detection system based on the ultraviolet detection of molecules produced in the thermal decomposition of explosive compounds separated by gas chromatography. 
         [0008]    Meanwhile, the combination of GC and IMS has been established for the use of the IMS as a detector to the effluent from a GC, wherein the IMS has been interfaced to the GC effluent column and operates continuously and is used no differently than other conventional GC detectors such as flame photometric, flame ionization and electron capture detectors. 
       SUMMARY OF THE INVENTION 
       [0009]    What is desired is a system and/or method having better performance, simpler assembly, reduced cost and/or greater reliability for field deployment, than the prior art. Also, the present invention will preferably provide sufficient selectivity and specificity for analyzing the complex chemical matrix that is normally encountered in sampling maritime containers, air cargo, luggage and the like. 
         [0010]    In an aspect of the invention, a GC is used as a tool to separate out predetermined analytes of interest to be selectively fed to the IMS on an intermittent and temporally separated basis. Thus, samples are cleaned up prior to analysis by IMS, as opposed to using an IMS as an alternative detector to a constant effluent flow from a GC separation column. The GC acts as a pre-analysis separator such that effluent is introduced to the IMS only when certain predetermined elution conditions have been met. Furthermore, detection criteria may be selectively set to alarm when both positive and negative IMS peaks are detected at a certain ratio for a specific elution time. 
         [0011]    The advantages of IMS detection, such as high sensitivity, good specificity, and fast detection rates can be more effectively utilized when the sample is preconditioned. A major difficulty with the use of IMS under field conditions is the heavily contaminated samples and complex chemical matrices often found in the field. These can cause detector overload and system contamination. Such conditions occur, for example, in forensic investigations following bombing incidents, and in the search of shipping containers for drugs and explosives. The problem arises within the ionization process where the background contaminants which are present in much greater abundance than the analytes of interest dominate the ionization process and preferentially take the available electrons or charge reservoir for ionization to the detriment of the analytes, such that limits of detection and selectivity are significantly degraded. 
         [0012]    This effect is shown in equation (1) where the available charge reservoir in the ionization source of the IMS is affected by numerous compounds entering the ionization process with different concentrations and affinity for electron charge. 
         [0000]      [Reagent Ions] EAR=[A]EAA+[B]EAB+[C]EAC+   (1)
 
         [0013]    [R]=reagent ion reservoir concentration in the ionization source 
         [0014]    EA=Electron affinity of compound A, B, C entering the source 
         [0015]    [A]=Concentration of analyte of interest 
         [0016]    [B], [C]=Concentration of contaminants in the sample 
         [0017]    By pretreating the sample in the sample vaporization process, and separating it in a sample loop and in the GC column such that only analytes that are very close to those earmarked for detection are fed into the IMS, significant improvements in signal to noise ratio can result, and false alarm rates can be considerably reduced. 
         [0018]    Chromatography is a very mature well established technique. The capillary columns contain liquid phases chemically bonded to the column walls. The passage of the vapour through the column is retarded to varying degrees depending upon carrier gas flow rates, column temperature and chemical properties of the compounds injected onto the column. Different chemicals will travel through the GC at different rates and emerge at different times. In the preferred form of the invention, the eluent portion of interest is taken to the detector, by time separation, while other fractions are allowed to bypass the detector (preferably IMS) as needed. This prevents unwelcome competition in the ionization process and/or overloading of the detector. 
         [0019]    In an aspect of the invention, a pre-separator is used after the desorber step. This allows venting of volatile contaminants in the sample and trapping of the analytes of interest. The temperature of the pre-separator is pulsed to expel the trapped analytes into the head of the GC column. The temperature of the column is ramped by applying power to the conducting tubing of the column to provide separation of various fractions of the pre-separated sample. The analytes of interest are then introduced into the ionization source of the AIMS at different intervals. 
         [0020]    In an aspect of the invention, the substances detection system includes, optionally, a desorption apparatus which receives the sample, and operates to clean the sample by removing volatile compounds not of interest while vaporizing remaining potential analytes of interest from the sample, which travel to a pre-separator. The pre-separator functions to release potential analytes of interest from the cleaned sample sequentially, separated in time. The potential analytes of interest are delivered sequentially to an IMS detector for short cycle processing, are processed there, and analytes of interest, if any, provisionally identified. 
         [0021]    Simultaneously, potential analytes of interest are also delivered to a GC for conditioning and/or further separating (long cycle). Once separated, the potential analytes of interest are delivered to the IMS detector having been separated and cleaned more thoroughly than the short cycle potential substances of interest. Thus, if the short cycle identifies a substance of interest, that identification can either be confirmed as correct, or as a false alarm, by the long cycle. The long cycle result is more reliable because the long cycle provides a more thoroughly conditioned and separated sample to the detector. 
         [0022]    Also, in an aspect of the invention, introductions of various chemical ionization reagents (CIR) used in the IMS into the ionization source with the effluents from the GC are timed to the particular GC peak of interest. Thus, when there is no GC peak, no CIR is being introduced at all. Also, each particular predetermined CIR used for a corresponding particular substance of interest is introduced only concurrently with the GC peak associated with that substance of interest. 
         [0023]    In an aspect of the invention, there is provided system for detecting the presence of one or more predetermined analytes in a sample, wherein the predetermined analytes number two or more, the system comprising: 
         [0024]    a temporal separation means for temporally separating the predetermined analytes within the sample; 
         [0025]    an ion mobility spectrometer detector programmed to detect the predetermined analytes of interest in the sample; 
         [0026]    wherein the temporal separation means is configured to deliver the analytes of interest one by one, separated in time, to the ion mobility spectrometer detector. 
         [0027]    In an aspect of the invention, there is provided a method of detecting the presence of one or more predetermined analytes in a sample, wherein the predetermined analytes number two or more, the method comprising the steps of: 
         [0028]    pre-separating the sample by temporally separating the predetermined analytes in the sample; then 
         [0029]    splitting the pre-separated sample into a bypass sample and a main sample; then 
         [0030]    delivering the bypass sample to the detector for preliminary detection of one or more predetermined analytes; 
         [0031]    delivering the main sample to the gas chromatograph to further temporally separate predetermined analytes; 
         [0032]    then delivering the further separated main sample to the detector to confirm or disconfirm detection of one or more predetermined analytes. 
         [0033]    In an aspect of the invention, there is provided a method of detecting the presence of one or more predetermined analytes in a sample, wherein the predetermined analytes number two or more, the method comprising the steps of: 
         [0034]    temporally separating potential predetermined analytes in the sample; 
         [0035]    delivering the temporally separated potential predetermined analytes one-by-one to the ionization chamber of and IMS detector; 
         [0036]    deploying chemical ionization reagent to the ionization chamber concurrently with the delivery of each potential predetermined analyte; 
         [0037]    withholding chemical ionization reagent when there is no delivery of predetermined analyte to the ionization chamber occurring; 
         [0038]    if one or more predetermined analytes is present, detecting their presence. 
         [0039]    In an aspect of the invention, there is provided an apparatus for detecting the presence of one or more predetermined analytes in a sample, wherein the predetermined analytes number two or more, the apparatus comprising: 
         [0040]    a detector configured to receive and detect the presence of predetermined analytes carried in a carrier gas; 
         [0041]    a carrier gas generator, the generator comprising a single reservoir and configured to selectively operate in a gas delivery mode in which clean carrier gas is delivered to the detector and a cleaning mode in which the generator generates clean carrier gas for subsequent use in the detector; 
         [0042]    wherein the detector and the generator and positioned in a common housing. 
         [0043]    In an aspect of the invention, there is provided an apparatus for detecting the presence of one or more predetermined analytes in a sample, wherein the predetermined analytes number two or more, the apparatus comprising: 
         [0044]    a detector configured to receive and detect the presence of predetermined analytes carried in a carrier gas; 
         [0045]    a carrier gas generator, the generator comprising first and second reservoirs and configured such that the first reservoir operates in a gas delivery mode in which clean carrier gas is delivered to the detector while the second operates in a cleaning mode in which clean carrier gas is generated for subsequent use in the detector; 
         [0046]    the generator being configured to switch the first reservoir to the cleaning mode and the second reservoir to the gas delivery mode 
         [0047]    wherein the detector and the generator and positioned in a common housing. 
         [0048]    In an aspect of the invention, there is provided a detector for detecting the presence of one or more predetermined analytes in a sample, wherein the predetermined analytes number two or more, the detector comprising an IMS detector configured receive potential predetermined analytes from a GC during GC peaks, the detector being configured to simultaneously ionize the potential predetermined analytes positively and negatively, and to scan across the GC peaks to obtain both positive and negative scans across each GC peak. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0049]    Reference will now be made, by way of example only, to preferred embodiments of the invention and in which: 
           [0050]      FIG. 1  is a schematic representation of the preferred analyser system; 
           [0051]      FIG. 2  is a schematic representation of a carrier gas generating means; 
           [0052]      FIG. 3  is a schematic representation of an alternate carrier gas generating means; 
           [0053]      FIG. 4  is a schematic representation of another alternate carrier gas generating means; 
           [0054]      FIG. 5  is a schematic representation of the detector circuitry; 
           [0055]      FIG. 6  is a schematic representation of alternative detector circuitry; and 
           [0000]      FIG. 7  shows a sample detector output display. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0056]    Referring now to  FIG. 1 , a schematic representation of the preferred analyser system  10 , according to an aspect of the invention, is shown. The sample  12  is acquired through interfacing with desorber  14 . Desorber  14  communicates with pre-separator  16 , which communicates both with GC  18 , and AIMS  20 . Processing means  22  and  24  are in communication with AIMS  20 , and the outputs of means  22 ,  24  are used to identify substances of interest, after which identification information is disseminated. In the preferred embodiment, a carrier gas (discussed below) carries the sample from the desorber  14 , to the pre-separator  16 , the GC  18  and the AIMS  20 . 
         [0057]    The sample may, for example, be positioned on a sample collection slide, card or filter disk sized and configured to interface with the desorber  14 . Preferably, the desorber  14  includes means for ramping up temperature upon receipt of a sample to evaporate volatile compounds not of interest, thus cleaning the sample. These volatile contaminants are preferably vented. As the temperature continues to rise, the cleaned sample is then evaporated and travels to the pre-separator  16 . Preferably, the desorber  14  communicates with the pre-separator  16  via a six-port heated valve, which functions to keep the sample evaporated until it condenses in the pre-separator  16 . The pre-separator  16  is kept cool while the sample is transferred from the desorber  14 , so that the sample will condense and thus be trapped. 
         [0058]    The pre-separator  16  preferably operates as follows. It is heated in a ramping fashion with power pulses ranging from 100-500 msec to assist in the thermal separation of different compounds based on their physical and chemical properties. Each compound will be released at a different temperature, and thus at a different time, creating a temporal separation between the individual predetermined analytes present. The pre-separator  16  also functions to release other volatile compounds not of interest that were not removed by the desorber  14 , while separating in time the release of potential analytes of interest as the pulsed increase in temperature proceeds. 
         [0059]    Thus, the desorber  14  and pre-separator  16  function to eliminate unwanted compounds and/or contaminants (such as volatile compounds), and thus to preselect for analysis compounds likely to be of interest. 
         [0060]    Preferably, the pre-separated sample emerging from the pre-separator  16  is split into main and bypass samples. The bypass sample is carried directly to AIMS  20 , permitting a faster analysis as a result of the GC step being skipped for the bypass sample. This faster analysis can, in the preferred embodiment, take about 20-30 seconds, providing a quick detection of threat substances followed by confirmation after GC analysis of the main sample is completed is completed. This offers flagging of the sample for further investigation and circumvents the need to call on dog screeners and other measures which will slow down air cargo movement, luggage or other items. 
         [0061]    On the other hand, if the short cycle shows no detection, there is a strong likelihood that the sample is clean. Preparations can begin to test the next sample. In the unlikely event that the long cycle shows detection when the short cycle did not, the relevant object (e.g. shipping containers, luggage, etc.) can be extracted and dealt with accordingly. 
         [0062]    Preferably, the main sample is carried to the GC, and the preferred GC operates to evaporate the main sample by upward ramping of temperature. The main sample molecules are preferably trapped by adsorption, condensation, surface interaction on a cooled trapping material consisting of an inert coated metal surfacelike GC liquid phase and other means of trapping molecules. The trap is resistively heated by applying power across its terminals to release trapped materials into the carrier gas and transfer the evaporated main sample into the analytical GC column. The preferred GC column can contains polar, semi-polar or non-polar bonded liquid phase for effective separation of explosives molecules like NG, DNT, TNT, PETN, RDX, TATP, HMTD, HMX, and narcotics like cocaine, heroin, amphetamines, methamphetamines and other illicit drugs. The GC may also be configured to work for other compounds, including but not limited to alkaloids from tobacco, and human odors like lactic and pyruvic acids. An example of GC based explosive detector is described by R.Batlle, et al., Anal.Chem.75, 3137 (2003), the disclosure of which is incorporated herein by reference. 
         [0063]    Temperature ramping of the preferred GC column is accomplished by resistive heating of the column from 40 to 220 degrees Celsius, which allows separation of volatile and non-volatile (higher boiling point) compounds, typically in a span of 1-3 minutes. The initial temperature of the GC before heating is preferably maintained by an electrically driven cooling fan. 
         [0064]    Referring now to  FIG. 2 , the carrier gas supply is preferably generated internally to the analyser system  10 . Ambient air is delivered to a gas module  22  by a diaphragm pump  24 , preferably internal to the module. The preferred gas module includes a reservoir  26  containing an adsorber in the form of moisture- and hydrocarbon-absorbing materials to clean the incoming ambient air, and a second reservoir  28  containing the same materials. Preferably, heating means associated with the second reservoir  28  function to heat it to 200 degrees Celsius. The two reservoirs are connected such that the second reservoir  28  is purged by a small stream of gas from the first reservoir  26 . Subsequently, when the second reservoir is clean and the first dirty, the first is heated by heating means and purged by clean air from the second. 
         [0065]    The preferred module further includes a timing circuit  30  and microprocessor  32  to control the use of each reservoir to supply clean gases. Preferably, the reservoirs are configured to clean the gas to a moisture content of less than 2 ppm and organic compounds content of less than 1 ppm. Also, preferably, the two reservoirs are contained in a common housing with the IMS. 
         [0066]    It will be appreciated that in this configuration, either reservoir can be used to supply clean carrier gas to the system  10 , including the desorber, pre-separator, GC and IMS. 
         [0067]    In the preferred system, the gas module supplies clean carrier gas independently to the desorber  14 , the the pre-separator  16 , the GC  18  and the IMS  20 . In each case, the carrier gas in used to advance the sample through each component, allowing for separation and/or analysis. 
         [0068]    In another embodiment of the invention ( FIG. 3 ) there is an external carrier gas supplied from an external gas cylinder, reservoir assembly, or commercial zero air generator operated externally to the analyzer system. In such an embodiment, typically, an AC to DC converter  34  would provide DC to an external gas supply module  36  which would then deliver carrier gas to the system  10 , preferably independently to each component as described above. In another embodiment a compressed gas supply  38  or other pure air gas generator  40  could be used instead of module  36 . 
         [0069]    In another embodiment, there is a gas supply module  42  ( FIG. 4 ) comprising a single scrubbing tower that is capable of operating for 8-10 hours continuously and is heated to purge contaminants at the end of the cycle while the system is purged with a clean gas generated from use of membrane separator, hollow fiber air dryer modules offering high selectivity for water over air. Drying capability of 50-100 ppm of water and low hydrocarbon content can be achieved and sufficient to purge the reservoir for a full day operation. The module may be contained in a common housing with the IMS. 
         [0070]    This module comprises of inlet filter  44 , pump  46 , coalescence filter  48 , and fiber tubes dryer  50 . Heater  52  heats reservoir  54  during the purge cycle, and dryer  56  cleans the gas, which is returned to reservoir  54  for use during normal operation. The module of  FIG. 4  can supply clean carrier gas during normal operation, and taken offline for a purge cycle, typically after 8-10 hours of normal operation. 
         [0071]    Preferably, the gas cleaning process will be microprocessor controlled, to provide precision control of the heating mechanism and purging cycle associated with cleaning the gas. Thus, preferably, the cleaned carrier gas has moisture content of less than 5 ppmv concentration, and hydrocarbon concentration of less than 1 ppmv. It is also preferred that the temperature control, gas flow and switching mechanisms of the adsorber enclosures are microprocessor controlled, which also allows for tracking the status of adsorber interaction time and use. This also allows precise conditions to be restored after a power failure. 
         [0072]    Those skilled in the art will appreciate that the analysis using the IMS  20  involves ionization, typically both positive and negative, of the sample entering the IMS. IMS devices, in general terms, identify analytes of interest by measuring mobility of associated ions using a drift tube and detector. CIRs are deployed in the IMS&#39; ionization chamber to facilitate ionization of the substances in the sample for detection. 
         [0073]    The preferred embodiment of the system is configured to time the deployment of CIRs to be concurrent with the GC peaks of analytes of interest. This is in contrast to the prior art, in which CIRs are typically fed into the IMS constantly. In the preferred embodiment, then, CIRs are conserved, and wastage reduced, since CIRs are deployed only when needed for ionization. In the preferred embodiment, the microprocessor controlling the system  10  is programmed to as to release CIRs to the IMS only concurrently with GC peaks, that is, when potential analytes of interest are arriving for analysis. CIRs are preferably withheld during the absence of GCpeaks. 
         [0074]    Referring now to  FIG. 5 , the IMS assembly preferably comprises a microprocessor or CPU  57  which is configured to switch on and off high voltage power supply  58  (HVPS). HVPS  58  and CPU  56  are operatively connected to switching and monitoring circuit  60 , which is used by CPU  56  to monitor the voltage from the HVPS and to actually switch the voltage. 
         [0075]    The AIMS  20  receives the switching voltage and provides the raw output used to calculate ion mobility and identify, if appropriate, analytes of interest. The output is amplified by a pre-amplifier  62  prior to delivery to a data grabber circuit  64 . It will be appreciated that the pre-amplifier is vulnerable to damage from sudden large changes in electric field resulting from changes in polarity and ionization of the sample. Specifically, damage may result from sudden change of voltages and voltage surge on the guard electrode located in front of the IMS&#39; Faraday collector plate. The system  10  is thus configured to provide a protective blanking pulse signal to the pre-amplifier timed to coincide with the changes in the electric field, thus preventing the aforementioned damage. 
         [0076]    Circuit  60  preferably provides the high voltage polarity needed to operate the axial ion mobility spectrometer (AIMS) in one polarity and the appropriate gating pulse to introduce single polarity ions into the single glass or ceramic tube drift tube. The process is under CPU control. The signal generated at the preamplifier  62  is fed to the data grabber board  64  which controls the blanking pulse and feedback to the switching and monitoring circuit and to the CPU  56 . 
         [0077]    In the preferred embodiment, the circuit  60  comprises a half H instead of four H bridge, which offers a simpler and faster switching circuit capability over prior art. 
         [0078]    Alternation between ion polarities is preferably governed by a timing circuit of duration varying from 100-500 msec, depending on the eluting GC peak from the chromatography column. In this mode, several positive ion scans are collected in one polarity and several negative ion scans are collected in the opposite polarity mode. This is possible because the GC peak is wide enough, and the switching frequency high enough, to provide sufficient numbers of data points associated with a single GC peak, for both positive and negative polarities. Preferably, a time gap is afforded between each polarity to allow stabilization of reagent ions and baseline. 
         [0079]    In an alternate embodiment shown in  FIG. 6 , there are instead two HVPSs,  58   a  and  58   b , one set to output positive voltage, and the other negative. In this embodiment, supplies  58   a  and  58   b  may both draw power from a 24 VDC power supply  66 . The power supplies  58   a  and  58   b  themselves do not switch polarity. Rather, the circuit  60  switches between one HVPS and the other. Preferably, the data grabber rate is 100 k samples/sec or down to 10 microseconds/sample for improved peak resolution. The advantage of two separate high voltage power supply is ability to adjust the polarity independently for each HVPS. Also switch time is reduced, because polarity does not switch—preferably, switch time is reduced as low as 500 microseconds. 
         [0080]      FIG. 7  shows, by way of example, the output and display associated with selective detection of the explosive Tetryl. In the preferred embodiment, the generated positive and negative ions for specific GC peak are averaged and displayed in a plot of ions intensity versus drift time in milliseconds and separation time in seconds. Tetryl is an example of a substance that forms both negative and positive ions for a single GC peak. Tetryl is separated at retention time of 105.6 seconds and produced a positive ion peak at drift time 5.82 milliseconds and reduced mobility constant of 1.412 cm 2 /V.sec. The negative ion detected at the same retention time at drift time of 5.53 msec and reduced mobility constant 1.502 cm2/V.sec. More generally, the detection algorithm used by the system  10  (and executed by the microprocessor) identifies the substance or analyte based on retention time, specific reduced mobility constants and the ratio of the positive and negative ion signals for specific analyte. 
         [0081]    It will be appreciated by those skilled in the art that system  10  is preferably programmed to detect specific, pre-determined substances, or analytes of interest. It is thus known in advance, which potential analytes of interest are sought to be detected. For each analyte of interest, basic properties such as boiling point, retention time, reduced mobility, drift time and ion intensity are known in advance. This allows the pre-separator  16 , GC  18 , IMS  20  and microprocessor to detect and identify the pre-determined analytes of interest.