Abstract:
A portable system for sampling the ambient air of a selected environment for the presence of unwanted chemical warfare vapors, such as nerve or blister gases, is disclosed. The excitation for the electrical elements of the system can be originated from a battery or from an ac excitation. The system comprises a detector unit having first and second ion mobility spectrometers which simultaneously detect and monitor for the presence of the chemical agent vapors so as to provide an accurate and quick determination of the unwanted chemical vapor within the selected environment, without false alarming to non-chemical warfare agent vapors, which act as interferents. The system design also allows monitoring in the presence of electromagnetic interference (EMI).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to the invention described in U.S. patent application Ser. No. 90/853,926, and which is filed concurrently herewith. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a system used to analyze compositions to determine what chemical elements are present therein and, more particularly, to a system that analyzes an air sample and, if undesired chemical warfare vapors are present therein, provides signals to activate alarms. 
     The ambient by which one is surrounded is of utmost importance. However, the ambient may suffer from pollution that allows the surrounding atmosphere to be contaminated, especially by man-made waste and vapor pollutants. 
     The environment by which one is surrounded may also be invaded by more serious pollutants, especially during terrorist situations or during warfare, in particular, chemical warfare. Chemical warfare involves the use of chemicals, such as blister and nerve gases, that attack humans and animals, with the human suffering severe bodily pain and/or death within minutes of exposure. 
     Current state of the art portable chemical warfare agent detectors have not demonstrated the capability to function properly on board ship due, in part, to the presence of electromagnetic interference (EMI) and non-chemical warfare (CW) agent vapors which act as interferents. These interferent vapors tend to cause false positive alarms or impede the detection and identification of the chemical warfare vapor. 
     Systems for measuring samples to determine the contents thereof are known and some of which may employ ion mobility spectrometers (IMSs), such as described in U.S. Pat. No. 4,445,038; 5,083,019; 5,300,773; 5,491,337; and 5,587,581, and all of which are herein incorporated by reference. The IMS provides a quantitative measurement of the contents of the molecules being sampled by measuring a time of “flight” of the ions of the molecules through a drift region of the IMS which is determined by the ion mobility characteristic of the ions being sampled and which, in turn, provides the identity and the concentration of the composition being measured. Accordingly, it is desired that means be provided employing ion mobility spectroscopy technology that analyzes the environment to detect the presence of unwanted chemical warfare agent vapors and provide alarm thereto, but without alarming to common interferents and EMI found in a shipboard environment. More particularly, it is desired that an Ion Mobility Spectrometer (IMS) be provided that yields a quick and improved accurate determination of these unwanted chemical warfare agent vapors so that the environment may be quickly purged thereof. 
     OBJECTS OF THE INVENTION 
     It is a primary object of the present invention to provide a system utilizing an IMS that accurately detects and monitors for the presence of undesired chemical warfare agent vapors in an environment. It is also equally important for the system not to alarm in an environment when specific chemical warfare agent vapors are not present. 
     It is another object of the present invention to provide a system employing an IMS that quickly, yet accurately, detects and monitors for the presence of undesired chemical warfare vapors in an environment and, upon detection thereof, provides an alarm indication. 
     Another object of the present invention is to provide a system having at least a first and second configuration so that an alarm condition is only generated if there is an agreement between the detection derived separately from the first and second configurations. 
     In addition, it is an object of the present invention to provide a system employing first and second IMSs to advantageously detect ions having both predominately positive and negative polarities, respectively, so as to simultaneously detect separate gaseous samples having respective positive and negative charge characteristics. 
     It is another object of the present invention to provide for an instrument that uses ion mobility spectroscopy technology that analyzes molecules of chemical agent vapors by determining the cluster arrangement of the ions making up the chemical vapor agents and conditions the molecules of selected vapors so that these molecules are more easily and accurately detected by an IMS operated to more advantageously detect ions manifesting a positive or negative charge. 
     Furthermore, it is an object of the present invention to provide for an IMS that generates an electrical signal which is routed to means for comparing the electrical signal against predetermined signals indicative of unwanted and/or dangerous compositions of gaseous vapors, and if a match exists therebetween, an alarm is generated. 
     In addition, it is an object of the present invention to provide a system that has the ability to operate on standard ship&#39;s power or provide rechargeable means so as to operate on battery power making the system portable. 
     Another object of the present invention is to reduce the weight and size of the associated elements making up the system so as to further contribute to the portability of the system. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a system for sampling the ambient of a selected environment for the presence of unwanted, predetermined chemical vapors therein. 
     The system comprises means for obtaining a sample of the selected environment and means for conditioning the sample into a vapor containing known molecules. The system further comprises means for receiving the vapor comprising ion clusters that define ions of the molecules. The means for receiving comprises first and second ion mobility spectrometers with one of the ion mobility spectrometers having arranged therewith a reagent source. The means for receiving is capable of being powered by a battery. Each of the first and second ion mobility spectrometers provides an electrical signal representative of the respectively received defined ions of the molecules. The system further comprises means for comparing each of the representative electrical signals of the first and second ion mobility spectrometers against predetermined signals representative of predetermined chemical vapors and generating an alarm signal if a match exists therebetween. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention may be realized when considered in view of the following detailed description, taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a block diagram of the system of the present invention; 
     FIG. 2A is a schematic of the detector unit of FIG. 1 while FIG. 2B illustrates an alternative arrangement of selected elements of FIG. 2A; and 
     FIG. 3 is a schematic of the IMS cells of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein the same reference number indicates the same element throughout, there is shown in FIG. 1 a block diagram of the system  10  of the present invention. The system  10  samples the ambient of a selected environment  12  by means of an intake fitting  14  having an internal filter to remove dust/lint particles, a detector unit  16  comprising an ion mobility spectrometer (IMS) cell (POSITIVE)  18  and an ion mobility spectrometer (IMS) cell (NEGATIVE)  20 , a central processing unit (CPU)  22  having pre-stored quantities, and a display  24 , as well as an audible alarm  25 . In actuality, the intake fitting  14 , the central processor unit (CPU)  22  and the display  24  are integral with the detector unit  16 . 
     The detector unit  16  has the ability to operate on standard ship&#39;s power or is provided with rechargeable means so as to operate on battery power further contributing to, along with other features of the present invention, making the system  10  portable. The detector unit  16  serves as means for receiving and ionizing the treated vapors into cluster arrangements that define ions of the molecules and as means for receiving the clusters of the defined ions and providing corresponding electrical signals thereof. The CPU  22  serves as the means for comparing the electrical signals generated by the detector unit  16 . The CPU  22  has reprogramming capabilities so that the routines running therein may be easily updated to accommodate new/future sampled vapors. The CPU  22  incorporates parallel processors. One processor&#39;s sole function is a detection routine, while the other processor runs the system functions, thereby decreasing response time. 
     The system  10  employs at least one detector unit  16  having at least two ion mobility spectrometers  18  and  20 , with the ion mobility spectrometer  18  operating in a mode to detect ions predominately having a positive polarity and operatively cooperating with a reagent source that treat an associated portion of sample being measured so as to be more easily detected by the ion mobility spectrometer  18  operating in the positive mode. 
     In general, the intake  14  draws air, along flow path  26 , which serves as a sample from the selected environment  12 . After the intake (with integral filter), the sampled air passes over a semipermeable membrane. The membrane minimizes the introduction of water vapor into the cells  18  and  20  (which are arranged in a closed-loop system). The intake  14  preconditions the sample and delivers the sample on to flow path  28  which, in turn, is delivered to IMS cell  18  via flow path  30  and to IMS cell  20  via flow path  32 . The IMS cells  18  and  20  provide electrical signals on signal paths  34  and  36 , respectively, that are routed to a central processing unit  22 . The central processing unit  22  compares the received signals on signal path  34  and  36  against pre-stored quantities and, if a comparison exists therebetween, provides an electrical signal on signal path  38  that is delivered to display  24 . If the comparison fails, the central processing unit  22  delivers via signal path  38  an electrical signal to the audible alarm  25 . The pre-stored quantities correspond to electrical signals representative of gaseous vapors of unwanted or dangerous compositions, such as nerve or blister gases used in chemical warfare, or pollutants that can contaminate the environment  12  being monitored. 
     FIG. 1 illustrates a system  10  referred to herein as Shipboard Chemical Agent Monitor-Portable (SCAMP) comprised of an arrangement of a single detector unit  16  and allows the CPU  22  to generate an error signal upon the detection of an alarm condition therein. The system  10 , in particular, the detector unit  16 , as will be described, is designed by appropriate means, such as the confinement of associated circuitry onto single circuit boards and into single confined compartments, as well as providing all operating circuit with appropriate protection against electromagnetic interference (EMI) discussed in the “Background” section. This EMI protection provides the system  10  with the capability (lacking in prior art devices) of detecting chemical-warfare (CW) agent vapor in the presence of shipboard EMI. Furthermore, as will be described, the CPU is provided with operating routines, that are detection algorithms, that are designed not to alarm to common shipboard interferents, also discussed in the “Background” section. 
     In general, each of the ion mobility spectrometer (IMS) cells  18  and  20  accepts ions in a vapor sample, and then separates those ions in an electric field. The acceleration of the ion in an electric field is a function of its charge and mass and, at atmospheric pressure, of its shape and size as well. The characteristics that tell how fast a particular ion can move through an electric field at a given temperature and pressure is called the mobility of the ion, and such is an indication for determining the make up of the molecules of the vapor sample being analyzed and measured by the IMS cells  18  and  20 . 
     At atmospheric pressure, ions and molecules can cluster together in a way unique to the molecule producing the ions. This clustering does not need to be with similar molecules. These non-similar molecules are called reagents. As used herein, G-agent vapor molecules cluster with acetone molecules, forming positively charged cluster ions. As further used herein, H-agent vapor molecules cluster with hydroxyl ions to form negatively charged cluster ions. Further, as used herein, a single-agent ion clustered with reagent molecules is called a monomer. Further still, as used herein, a two- and a three-agent molecule clustered with reagent molecules is called a dimmer and a trimmer, respectively. 
     In the separation method of the IMS cells  18  and  20  to be further discussed with reference to FIG. 3, the ions start from rest at the same time and travel a known distance along a drift region having a high-voltage gradient which is applied thereto. A cathode electrode is located at the end of the drift region in each IMS cells  18  and  20  to detect the traveling ions. The smaller ion clusters have greater mobility and reach the end of the drift region first, as compared to other clusters. Heavier clusters arrive later at the cathode electrode, and their arrival time is on the order of their mass. The ion mobility is sometimes referred to as determining the time of “flight” as more fully disclosed in the previously incorporated by reference U.S. Pat. No. 5,587,581 (hereinafter the&#39;581 patent). As used herein, the arrival time at the cathode electrode is primarily a measure of the size and shape of the cluster ions. 
     Each substance or composition operated on by each IMS cells  18  and  20  that can be ionized produces a unique electrical IMS signal. As will be further described with reference to the CPU  22 , an unknown substance can be identified by comparing its unique IMS signal, also called its IMS signature, with a set of previously recorded signatures of known substances making up a reference library. The waveforms of the IMS may have peaks that represent information regarding the identity and concentration oft he samples being measured in a manner more fully described in the &#39;581 patent. The reference library may be made up to identify any substance at any concentration thereof with such substances being, for example, nerve or blister gases. If the unknown substance IMS signature matches one of the known signatures in the reference library, that unknown substance is identified. 
     The overall fluid flow of system  10  may be further described with reference to FIG.  2 A. 
     As seen in FIG. 2A, the fluid flow on path  28  having a first flow rate of 2.0 LPM, first encounters a manifold  40  that serves as a dividing means that receives the sample at the first flow rate and separates the sample flowing at the first flow rate into first and second paths  30  and  32 , respectively, (also shown in FIG. 1) having second and third flow rates which are equal to one-half of the first flow rate. More particularly, the dividing means  40  divides the fluid flow on path  28  into two paths  30  and  32 , each having a preferred positive and negative flow rate of 1.00 LPM which are respectively directed into membrane retainers  42  and  44 , respectively, containing, semipermeable membranes  42   a  and  44   a . The flow rate of 1.00 LPM is shown in FIG. 3, for the sake of clarity, as being directed into two membrane retainers  42  and  44 , but in actuality, the retainers  42  and  44 , along with their membranes, advantageously can be a one piece retainer, again for the sake of reducing weight and increasing the portability of the system  10 , as illustrated in FIG.  2 B. In FIG. 2B, a single retainer  43  supporting semipermeable membranes  42   b  and  44   b  divide the retainer  43  into three chambers, the center chamber passing the sample gas at the first flow rate, which is drawn through the retainer  43  by pump  74 ′. It will be appreciated that the ports  50 ′,  52 ′,  60 ′,  62 ′in FIG. 2B correspond to ports  50 ,  52 ,  60 , and  62  in FIG.  2 A. 
     Referring again to FIG. 2A, the retainer  42  has first, second, third and fourth ports  46 ,  48 ,  50 , and  52 , respectively, with the first port  46  having fluid communication with the second flow rate  30  and the second port  48  accepting the fluid flow that does not migrate through the membrane in retainer  42 . The third port  50  is fluidly coupled to a port  54  of the IMS cell  18 . The second retainer  44  has first, second, third and fourth ports  56 ,  58 ,  60 , and  62 , respectively, which are fluidly coupled in a manner similar to those of retainer  42 . The port  60  of retainer  44  is fluidly connected to port  64  of the IMS cell  20 . 
     Each of the retainers  42  and  44 , as previously discussed, is merged into a single retainer, which is preferably comprised of stainless steel and has a mount that holds a 1.0 mi-thick semipermeable membrane of a dimethyl silicone/polycarbonate hybrid material. The semipermeable membranes within the retainers  42  and  44  serve as means to selectively allow sample molecules of interest, such as those contained in nerve or blister gases or other pollutants contained in the sample being measured, into the IMS cells  18  and  20 , while excluding excess water therefrom. As the air sample passes over each of the semipermeable membranes, a few sample air molecules migrate through the semipermeable membranes and get entrained in the recirculating air flows (to be described) of the detector unit  16 . The few sample air sample molecules that pass through the semi-permeable membranes are the only part of the original ambient air sample that actually get analyzed by the IMS cells  18  and  20 . 
     The port  48  of retainer  42  and the port  58  of retainer  44  are fluidly coupled to a manifold  66  which, in turn, provides an output of fluid path  68  that is fluidly coupled to a port  70  of a metering manifold  72 . The output of port  70  is fluidly coupled to a sample pump  74  by way of fluid path  76 . The sample pump provides an output on fluid path  78  which is exhausted from the detector unit  16 . 
     The detector unit  16  further comprises recirculation means comprising a recirculating pump  80  having an input  82  and an output  84 . The input  82  is fluidly coupled, via a manifold  86 , to an air recirculation port  88  of the IMS cell  18  and also to an air recirculation port  90  of the IMS cell  20 . 
     The output  84  of the recirculation pump  80  is fluidly connected to a cartridge  92  having an input and an output  94  and containing a desiccant. 
     The desiccant cartridge  92  is interposed in the recirculating air of the detector unit  16  so as to clean and dry the recirculating air. Care should be exercised in the selection of the size of the desiccant cartridge  92  so as to keep it as small as feasible, thereby further contributing to the portability of the system  10 . The recirculating air of the detector unit  16  includes desiccant cartridge  92  that filters out all of the contaminates from the reduced air, that is, the sample air which permeates through the membrane of the second and third flow rates on paths  30  and  32 , respectively. The desiccant cartridge  92  may be filled with a molecular sieve material (size  4 A) and a charcoal (untreated 6×16 mesh wire) which may be a BPL type, known in the art. The molecular sieve material removes residual water vapor and the BPL charcoal removes any organic contaminants. 
     During operation, the desiccant cartridge  92  may typically become slowly loaded with contaminants and become unable, over a period of time, to maintain a clean and dry environment inside the recirculating air circuit of the detector unit  16  and, thus, desiring replacement thereof. The average life of the desiccant cartridge  92  is approximately 500 operating hours. 
     The output  94 , having a flow rate of approximately 2.4 LPM, of the desiccant cartridge  92  is routed to second dividing means comprising the metering manifold  72 . The second dividing means  72  is a series of flow valves  96 ,  98 ,  100  and  102  that separates the output  94  of desiccant cartridge  92  into first  104 , second  106 , third  108 , and fourth  110  flow paths, respectively carrying forth, fifth, sixth and seventh flow rates. The first and second flow paths  104  and  106 , which are routed to the positive IMS cell  18 , separate the output  94  of the desiccant cartridge  92  into fourth and fifth flow rates, wherein the fourth flow rate is greater than that of the fifth flow rate. More particularly, it is preferred that the fourth flow rate be approximately 0.7 LPM, whereas the fifth flow rate is preferred to be approximately 0.25 LPM. The sixth flow rate is preferred to be approximately 1.2 LPM, whereas the seventh flow rate is preferred to be approximately 0.25 LPM. The sample flowing at the fourth flow rate is routed, via flow path  104 , to port  112  of the IMS  18 , whereas the fifth flow rate is routed, via flow path  106 , to a reagent source  114  having an input port and an output port  116 . The sample flowing at the sixth flow rate is routed, via flow path  108 , to port  118  of the IMS  20 , whereas the sample flowing at the seventh flow rate is routed, via flow path  110 , to port  62  of the second retainer  44 . 
     The reagent source  114  may be an acetone vapor source consisting of a Teflon diffusion tube immersed in liquid acetone contained in a stainless steel vessel that is mounted next to the positive IMS cell  18 . The output  116  of the reagent source  114  is routed to the port  52  of the retainer  42 . In operation, just prior to entering the positive IMS cell  18 , the recirculating air within the detector unit  16  passes through the immersed tube of the reagent source  114  and the acetone molecules therein diffuse into the tube and mix with the recirculating air at a constant rate of approximately 5000 ng/min at 60° C. The acetone molecules increase the positive polarity of the ions of the molecules being measured by the IMS cell  18  and, thus, increase the sensitivity of the positive IMS cell  18  operated in a manner to be further described with reference to FIG.  3 . 
     A separate reagent vapor source similar to the reagent vapor source  114  is not required for the negative IMS cell  20 . A small amount of residual atmospheric water vapor migrates through the semipermeable membrane of retainer  44  with the sample vapor and enters, by way of port  60  of the retainer  44  and fluid path  64 , the ionization chamber of the IMS cell  20 , to be described. These water molecules act as the reagent for the negative polarity ion reactions within the IMS cell  20  and, thus, negate the need for a separate reagent vapor source  114  for IMS cell  20 . 
     The components comprising the IMS cell  18  are preferably placed into a compartment  16 B and, similarly, the components comprising IMS cell  20  are also preferably placed into a compartment  16 C with both compartments being located in a single housing  16 A. The single housing  16 A is provided with appropriate electromagnetic interference (EMI) protection so that the system  10 , in particular, the IMS cells  18  and  20  successfully detect chemical warfare (CW) agent vapor in the presence of shipboard electromagnetic interference (EMI). The single housing  16 A has a heater  16 D operatively disposed in the single housing. Preferably, the heater  16 D is operated so as to maintain the temperature of the IMS cells  18  and  20  at a constant temperature of about 180° F. The heater advantageously can be either a 163 watt strip heater (AC) and/or 21 watt strip heater (DC) mounted under the cells  18  and  20  so as to maintain their temperature and heat their surrounding components in order to prevent sample vapor from condensing as it travels through the detector unit  16 . 
     The IMS cells  18  and  20  may be further described with reference to FIG. 3, which is a schematic that is generically applicable to both the IMS cells  18  and  20 , even though the IMS cell  18  predominately operates with positive potentials and the IMS cell  20  predominantly operates with negative potentials. The descriptions of the IMS cells  18  and  20  with reference to FIG. 3 are generic, but point out, as needed, the differences in the operation of the IMS cells  18  and  20 . 
     FIG. 3 illustrates an ionization chamber outlined in phantom, which can represent either of IMS cell  18  or  20 . IMS cell  18  is referred to as the first ionization chamber; IMS cell  20  is referred to as the second ionization chamber. The use of this first and second terminology to refer to the IMS cells  18  and  20  respectively is maintained throughout. The first IMS cell contains ports  54 ,  88 , and  112 , whereas the second IMS cell includes ports  64 ,  90  and  118 , all of which were previously  10  described with reference to FIG.  2 . 
     Each IMS cell is composed of  11  conducting rings  144 - 1  . . .  144 - 11  carrying gradually increasing or decreasing voltages, a radioactive source  120 , which lines the inside of the first ring  144 - 1 , a shutter grid or gate  122  which resides in the middle of the fourth ring  144 - 4 , an aperture grid  164  (also known as pole guard) and a collector  150 . The conducting rings  144 - 1  . . .  144 - 11  are insulated from one another as well as from the gate  122 , aperture grid  164  and collector  150 . The region on the side of the gate  122  including the radioactive source  120  may be thought as the ionization region  141 , and the region on the collector  150  side of the gate  122  may be thought of as the  142 . 
     The radioactive source  120  is preferably a foil containing 100 microcuries of Americium— 241  which emits beta particles that collide with the mixture of the sample and reagent molecules. The reagent molecules of IMS cell  18  are acetone molecules from reagent source  114 , whereas previously discussed, the reagent molecules of IMS cell  20  are from residual water vapor in the system. The beta particles ionize the reagent molecules, and the reagent ions react with the sample molecules, causing the formation of sample molecule ion clusters. The ion clusters formed in IMS cell  18  are positive ion clusters, whereas the ion clusters formed in IMS cell  20  are negative ion clusters. 
     In the positive cell  18 , the  11  conducting rings  144 - 1  . . .  144 - 11  are connected to a high voltage source  130  through a voltage divider  186  so that voltage sequentially and evenly decreases from 2000 Volts (V) at the ring  144 - 1  holding the radioactive source  120  to ground at the ring  144 - 11  holding the collector  150  in approximately 200 V decrements to provide a negative voltage gradient in the positive IMS cell  18 . The grounded ring  144 - 11 , which holds the collector  150 , is electrically insulated from the collector  150 . When the gate  122  is opened as described hereinafter, the positive ion clusters formed in the ionization region are swept down the gradient through the drift region  142 , through the pole guard  164  to the collector  150 , which causes a current pulse in the collector  150 . The current into the collector  150  is converted to a voltage and amplified by a circuit  158  attached to the IMS cell  18 . The time between the gate opening and the arrival of current pulses is proportional to the time required for the ion clusters to move through the drift region. The time to move through the drift region in the presence of an electric field is proportional to the reduced ion mobility which is a characteristic of the particular ion cluster involved. The pattern of pulses which occurs from the time the gate  122  is opened can be used as a signature to identify a substance. The voltage on signal path  34  from the amplifier  158  is digitized by one channel of the analog-to-digital converter  160  on the processor board  162  so the digital signal processor  168  can use the pattern for substance identification. 
     In the negative cell  20 , the  11  conducting rings  144 - 1  . . .  144 - 11  are connected to a high voltage source  130  through a voltage divider  186  so that voltage sequentially and evenly increases from −2000 V at the ring  144 - 1  holding the radioactive source  120  to ground at the ring  144 - 11  holding the collector  150  in approximately 200 V increments to provide a positive voltage gradient in the negative IMS cell  20 . The grounded ring  144 - 11 , which holds the collector  150 , is electrically insulated from the collector  150 . When the gate  122  is opened as described hereinafter, the negative ion clusters formed in the ionization region  141  are swept up the gradient through the drift region  142 , through the pole guard  164  to the collector  150 , which causes a current pulse in the collector  150 . The current into the collector  150  is converted to a voltage and amplified by a circuit  158  attached to the cell assembly  20 . The time between the gate  122  opening and the arrival of current pulses is proportional to the time required for the ion clusters to move through the drift region  142 . The time to move through the drift region in the presence of an electric field is proportional to the reduced ion mobility which is a characteristic of the particular ion cluster involved. The pattern of pulses that occurs from the time the gate  122  is opened can be used as a signature to identify a substance. The voltage on signal path  36  from the amplifier  158  is digitized by one channel of the analog-to-digital converter  160  on the processor board  162  so the digital signal processor  168  can use the pattern for substance identification. Information from both the positive cell signature on signal path  34  and the negative cell signature on signal path  36  can be combined as well to identify a substance. 
     The positive gate assembly  122  is a two wire grid  124 ,  126  in the same plane placed in the middle of the fourth ring  144 - 4 , which is actuality a two-piece ring with the same high voltage on each of the pieces. The wires alternate so that adjacent wires in the plane belong to different grids. Voltages derived front the same high voltage source  130  that is used by the rings  144 - 1  . . .  144 - 11  are applied to the wire grids  124  and  126 . These high voltages are somewhat higher than the high voltage on the fourth ring. When the gate  122  is closed, the voltage on one grid  124  is 24v higher than the high voltage on the other grid  126  so that an electric field is produced between adjacent wires in the plane. This transverse field sweeps ions to the more negative wires on the grid  126  where they are neutralized and, thus, resist the movement of the ion clusters into the drift region  142 . When the gate  122  is opened, the wire grids  124  and  126  are shorted together, removing the transverse field and allowing the ion clusters to move into the drift region  142  where they are swept down the voltage gradient to the collector  150 . 
     The negative gate assembly  122  is identically constructed as the positive gate assembly and similarly placed in the middle of the fourth ring  144 - 4  in the negative IMS cell  20 . The fourth ring  144 - 4  in the negative cell  20  is similarly a split ring with same highly negative voltage applied to both sides of the split ring  144 - 4 . Voltages derived from the same high voltage source  130  as the rings  144 - 1  . . .  144 - 11  are, applied to the wire grids  124  and  126 . These highly negative voltages are somewhat more negative than the highly negative voltage on the fourth ring  144 - 4  when the gate  122  is closed, the voltage on one grid  124  is 24v more negative than the highly negative voltage on the other grid  126  so that an electric field is produced between adjacent wires in the plane. This transverse field sweeps ions to the less negative wires in the grid  126  where they are neutralized and thus resist the movement of the ion clusters into the drift region  142 . When the gate  122  is opened, the wire grids  124  and  126  are shorted together, removing the transverse field and allowing the negative ion clusters to move into the drift region where they are swept up the voltage gradient to the collector  150 . 
     The aperture grid, i.e., pole guard,  164  is another wire grid in which all the wires are at the same voltage and that voltage is approximately 30 V above the ground potential on the ring  144 - 11  holding the collector  150  in the positive IMS cell  18 . The pole guard  164  has a focusing effect which causes pulses to be more narrow and improves the resolution of the IMS cell  18 . The pole guard  164  in the negative IMS cell  20  performs the same function but is at a voltage that is approximately 30 V below the ground potential on the ring  144 - 11  holding the collector  150 . 
     The processor board  162  contains a microcontroller  170  and a digital signal processor (DSP)  168  running in parallel. The microcontroller  162  and associated input output circuitry  172  handle all detector input and output including that of signal controlling the alarm visual display  24 , whereas the digital signal processor  168  and associated analog-to-digital converter circuitry  160  handle alarm detection. The digital signal processor  168  also provides an output that controls the audible alarm  25 . The use of parallelism between the processors  168  and  170  as well as within the digital signal processor  168  itself reduces the time to alarm. The high degree of circuit integration within the two (2) processors  168  and  170  allows the incorporation of all electronics except the high voltage source  130 , amplifier circuitry  158 , and DC to DC converter  166  onto a single board  162 , reducing the size of the system and contributing to its portability. 
     The processor board  162 , high voltage sources  130 , amplifiers  158 , pumps, heaters and transducers are powered by DC to DC converter  166  preferably located in detector unit  16 . The power for these converters comes from a power management system  174  that can select between a rechargeable battery  134  or ship&#39;s power  136  using automatic electronic switching  138  controlled by a microcontroller  178 . The power management system  174  also contains a battery charger  180  and can recharge the battery  134  while powering the detector unit  16  from ship&#39;s power  136  through the primary converter  182 . The power management system  174  contains a display  176  to inform the operator of pertinent information concerning the battery state and power system state in general. The use of the rechargeable battery  134  allows the system  10  to be portable. 
     OPERATION OF THE SYSTEM OF THE PREFERRED EMBODIMENT 
     In operation and with first reference to FIG. 2, the detector unit  16  receives the sample air flow at a first rate of 2.0 LPM which is then split between the positive and the negative IMS cells  18  and  20  via the flow paths  30  and  32 , respectively. The sample first flow rate is directed into the retainers  42  and  44  and directed across semipermeable membranes in the retainers  42  and  44 . A few of the molecules of the sample migrate through the semipermeable membranes and are entrained in the circulating air flow of the detector unit  16 . The remaining sample air is directed to the metering manifold  72  via ports  48  and  58  of retainers  42  and  44 , respectively, then is immediately exhausted out of the system by way of port  70  of the metering manifold  72  and the sample pump  74 . 
     The detector unit  16  has a recirculating air path formed essentially by recirculating pump  80  and the metering manifold  72 . The recirculating pump  80  provides a recirculating air flow of 2.4 LPM which is provided so as to maintain a clean and dry condition inside each IMS cell  18  and  20 . The recirculating air is routed through the desiccant cartridge  92  containing a 50% molecular sieve material and a 50% activated BPL charcoal to remove any contaminants from the recirculating air flow. 
     The acetone vapor reagent source  114  is included in the recirculating air flow circuit of the positive IMS cell  18 . This vapor reagent source  114  provides a trace amount the reagent molecules required for the reaction with the G-agent vapor molecules to form the positive ions predominant in the operation of the positive IMS cell  18 . The negative IMS cell  20  does not need a separate vapor reagent source similar to the reagent vapor source  114  because there is enough residual water molecules in the air to form hydroxyl ions (negative ions) needed to react with the H-agent molecules predominant in the operation of the negative IMS cell  20 . 
     The two IMS cells  18  and  20  are provided, which is of importance to the present invention, so that IMS cells  18  and  20  operate simultaneously, one in the positive mode and the other in the negative mode. This allows the system  10  to continuously detect both nerve (IMS cell  18 ) and blister (IMS cell  20 ) agent vapors. 
     The sample molecules that migrate through the semipermeable membranes, located in the retainers  42  and  44 , become entrained in the recirculating air that contains the reagent vapor molecules. This sample-reagent vapor mixture enters the ionization region  141  (shown in FIG. 3 for both IMS cells  18  and  20 ). The ionization region  141  for each IMS cell  18  and  20  is surrounded by the radioactive source  120 , which gives off beta particles that collide with the associated mixture of the sample-reagent molecules. The reagent molecules ionize and react with the sample molecules to create ion clusters for the molecules thereof. 
     The gate  122  of each of the IMS cells  18  and  20  is arranged so as to set up an electric field that prevents the ion clusters within the ionization region  141  from passing through into the drift region  142 . However, every 30 millisecond interval, the grids  124  and  126 , which make up the gate  122 , are shorted together. This momentarily removes the potential difference between grids  124  and  126 , and eliminates the electric field between them. This “opens” the gate  122  and a small, discrete group of ion clusters enter the drift region  142 . 
     As the ion clusters travel through the length of the drift region  142 , the ion clusters separate due to their different ion mobilities in the electric field and arrive at the collector  150  at different times, i.e., the smaller ion clusters have greater mobility and reach the collector  150  ahead of the heavier clusters. As the ion clusters impact on the collector  150 , they discharge and create a small ion current. This ion current is made available at the output of the collector  150  in the form of a signal which is amplified by amplifier  158  located in each IMS cells  18  and  20  and converted to a digital voltage by the A/D converter  160 . This digital voltage at the output of A/D converter  160  serves as an IMS signature of the sample vapor being measured by each of the IMS cells  18  and  20 . These IMS signatures are then analyzed by the application routines running in the CPU  22 , e.g., DSP  168  and microprocessor  170 . 
     The CPU  22  advantageously may contain a DSP Chip having routines that analyze digital signals. More particularly, the application programs running in the CPU  22  operate in conjunction with pre-stored quantities, each indicative of a signature of a vapor of interest, such as a vapor that may be created by either of the nerve or blister gas. The CPU  22  compares the IMS signatures present on signal paths  34  and  36  (IMS cells  18  and  20 , respectively) with the pre-stored quantities, and if a match exists therebetween, generates an alarm signal via signal path  38  which notifies the operator of the undesired condition. As previously mentioned, the CPU  22  may be easily reprogrammed so as to upgrade the routines to detect new/future vapor agents. 
     The system  10  may be calibrated by placing a confidence sample in the path of the intake fitting  14  so that its content is analyzed by the system  10 . More particularly, the confidence sample may be used to create one or more IMS signatures on signal paths  34  and  36  of FIG. 2, which may be or may not be recognized by the application routines running in the CPU  22  so that a calibration check is generated, which is indicative that the associated elements of the system  10  are operating correctly. This confidence sample, along with the detection algorithms embodied in the operating routines running in the CPU  22 , ensures that the system  10  does not alarm to common shipboard interferents. 
     It should now be appreciated that the present invention provides for an improved detection system that samples the ambient of an environment and detects and monitors for the presence of unwanted chemical agent vapors. The detector unit uses two different ion mobility spectrometers (IMSs) to analyze the air sample and, if unwanted chemical vapors are detected, provides appropriate signals to activate visual displays. 
     Although the invention has been described relative to the specific embodiments thereof, there are numerous variations and modifications that will become readily apparent to those skilled in the art in the light of the above teaching. It is, therefore, to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.