Abstract:
An apparatus is provided for identifying trace amounts of substances of interest in an air flow. The apparatus includes an inlet for receiving the air flow and an outlet for exhausting major portions of the air flow. The outlet is offset from the inlet. A porous impactor is disposed in alignment with the inlet and is impacted by particles entrained in the air flow. The porous impactor is heated sufficiently to vaporize particles impinging thereon. A detector communicates with the porous impactor and is operative for identifying substances of interest in the vaporized particles.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to detectors for detecting trace amounts of particles of interest.  
         [0003]     2. Description of the Related Art  
         [0004]     Terrorism risks continue to exist at transportation facilities, government buildings and other high profile locations where there is a significant flow of pedestrian or vehicular traffic. As a result, most airports and many government buildings now include apparatus for detecting trace amounts of explosives. These devices typically operate on the principle that small amounts of the explosive materials will be transferred to the body, clothing and luggage of people who had handled the explosive.  
         [0005]     Some detectors employ small flexible fabric-like traps that can be wiped across a package or piece of luggage. The trap removes residue from the surface of the package or luggage. The trap then is placed in an apparatus, such as an ion trap mobility spectrometer, that tests the residue on the trap for trace amounts of explosive materials. A device of this type is disclosed in U.S. Pat. No. 5,491,337 and is marketed by the GE Ion Track. These devices typically are employed in proximity to metal detectors at airports, and security personnel will perform screening on some of the passengers based on a random sampling or based on a determination that the passenger has met certain criteria for enhanced screening.  
         [0006]     The ion trap mobility spectrometer disclosed in U.S. Pat. No. 5,491,337 also can operate in a mode for detecting trace amounts of narcotics. Narcotics are illegal and insidious. Furthermore, it is known that many terrorists organizations fund their terrorism through the lucrative sale of narcotics.  
         [0007]     Only a fraction of airline passengers have their carry-on baggage checked for trace amounts of explosives or narcotics using fabric-like traps and the available ion trap mobility spectrometers or similar devices. Efforts to use such devices to check all carry-on bags for trace amounts of explosives or narcotics would impose greater time and cost penalties on the airline industry. Additionally, the above-described explosive detectors typically are used only on luggage and other parcels. An apparatus of this type would not identify plastic explosives worn by a passenger who had no carry-on luggage.  
         [0008]     U.S. Pat. No. 6,073,499 discloses a walk-through detector. The detector shown in U.S. Pat. No. 6,073,499 operates under the principle that a boundary layer of air adjacent to a person is heated by the person. This heated air adjacent a person is less dense than air farther from the person. Less dense air rises. Accordingly, a thermal plume of air referred to as a human convection plume flows up adjacent to the person at a rate of about 50 liters per second. Minute particles, including particles of explosives or narcotics that may have been handled by the person, will be entrained in this human convection plume of air and will flow up from the person. U.S. Pat. No. 6,708,572 shows a walk-through detector with a plurality of high-pressure air jets that direct small puffs of air towards the torso of the person in the portal. These jets of air help to stimulate a release of particles from the clothing and hands of the person so that a greater concentration of particles can become entrained in the human convection plume.  
         [0009]     The above-described walk-through detector has a metallic screen incorporated into the ceiling of the portal. A vacuum pump or fan above the screen generates an airflow that is intended to match the volumetric flow of air generated by the human convection plume (e.g., about 50 liters/second). An airflow generated by the vacuum pump or fan that is too low will permit particles entrained in the human convection plume to dissipate into the ambient air on currents of ambient air near the detector. A flow rate generated by the vacuum pump or fan that is too high will draw additional air through the screen and hence will dilute the concentration of particles of interest. Some of the particles entrained in the thermal plume will attach to the screen. The screen in the ceiling of the portal is moved into a desorber after a sufficient sampling time (e.g., 5 seconds) and is heated to temperatures in the range of 220° C.-250° C. so that particles thereon are vaporized. The vaporized particles then are drawn into the inlet of the ion mobility spectrometer, the ion trap mobility spectrometer or other such detecting device to determine whether any particles of interest were entrained in the human convection plume. An alarm or other signal will be triggered if a particle of interest is detected. The above-described walk-through detector typically operates at an overall average sampling efficiency of about 1% at mid-torso level. This level of efficiency typically meets government standards for detection and offers low false alarm rates. Thus, the walk-through detector disclosed in U.S. Pat. No. 6,073,499 and in U.S. Pat. No. 6,708,572 is very effective for detecting whether a person is carrying explosives or narcotics and whether the person has recently handled explosives or narcotics. However, improved efficiencies would be received well and could be even more effective for detecting even smaller amounts of particles of interest without increasing the false alarm rate.  
         [0010]     The walk-through detector disclosed in U.S. Pat. No. 6,073,499 and U.S. Pat. No. 6,708,572 is marketed by GE Ion Track as the EntryScan3® and currently operates at about a fifteen second cycle to sample, desorb and analyze a passenger. The person being screened must pause in the portal of the walk-through detector for at least the sampling phase of that cycle, and typically at least about 5 seconds. A system that achieves a shorter cycle time would be well received in the industry.  
         [0011]     Inertial impactors use principles enunciated in Stokes Law and function to collect particles entrained in a gas. Such impactors have been used, for example, in the analysis of air quality and are shown, for example, in U.S. Pat. No. 6,435,043 and U.S. Pat. No. 6,732,569. Inertial impactors have not been used to identify extremely small particles (e.g., 1 micron) of explosives or narcotics that may exist at low concentration in an air stream of 50-100 liters per second.  
         [0012]     In view of the above, it is an object of the subject invention to provide a detector with improved sample collection efficiency.  
         [0013]     Another object of the invention is to provide a detector with a shorter cycle time from sample collection to analysis, and hence a detector with a higher passenger throughput.  
         [0014]     A further object of the invention is to provide a detector with fewer or no moving parts.  
       SUMMARY OF THE INVENTION  
       [0015]     The subject invention relates to a preconcentrator for use with a detecting apparatus that analyzes samples of air to determine whether any particles of interest exist in the sample of air. The preconcentrator improves collection efficiency at any selected flow rate. The invention also relates to a detecting apparatus that incorporates such a preconcentrator.  
         [0016]     The preconcentrator of the subject invention preferably comprises a flow impactor in the path of the air that is flowing from the person or object being sampled. The impactor is configured to alter the flow path of at least part of the gas in which the particles of interest are entrained. However, particles, including particles of interest, that are entrained in the air stream have greater mass and hence greater inertia than the gas molecules. Thus, the particles, especially those of 1 μm or larger, are less likely to be diverted by the impactor, and accordingly will continue along a more linear path towards the impactor. As a result, the concentration of particles approaching the impactor can be increased significantly. An impactor differs significantly from a screen or filter in that a screen or filter permits the air to pass therethough. The impactor, however, does not pass most of the air, but rather diverts most of the air. The geometry of the impactor can be selected in view of the flow rates and known characteristics of the substances of interest. The impactor may be in the form of a substantially honeycomb with substantially hexagonal spaces.  
         [0017]     The detector of the subject invention preferably is configured to sample and analyze continuously rather than employing the sample/desorb/analyze cycle of known devices. The continuous operation can be achieved by providing an impactor that can be heated sufficiently to desorb the particles of interest or an impactor that can be used in combination with such a continuous heater. In this regard, the impactor may be formed from a metallic sintered material that can be heated to sufficiently high temperatures to vaporize or desorb particles impinging thereon. Alternatively, the impactor can be formed from a non-metallic or ceramic sintered material that is plated with metal. The metal material of the impactor can have an electric current applied thereto. The current is controlled to heat the impactor sufficiently to vaporize particles impinging thereon. Air flowing through the impactor will transport the vaporized particles of interest to the inlet to the detector. Thus, the detector has the potential of operating substantially continuously at a passenger throughput rate of 5 seconds or less. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a schematic view of a walk-through portal that incorporates the preconcentrator and detector of the subject invention.  
         [0019]      FIG. 2  is a schematic diagram of a known ion trap mobility spectrometer that can be used with the preconcentrator of the subject invention.  
         [0020]      FIG. 3  is a cross-sectional view taken along a vertical line and illustrating a first embodiment of a preconcentrator according to the subject invention.  
         [0021]      FIG. 4  is a cross-sectional view similar to  FIG. 3 , but showing a preconcentrator in accordance with a second embodiment of the invention.  
         [0022]      FIG. 5  is a plan view of the impactor incorporated into the preconcentrator of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     A portal detection system that incorporates the preconcentrator and detector of the subject invention is identified generally by the numeral  10  in  FIG. 1 . The portal detection system  10  includes a portal  12  that is similar to the portal disclosed in U.S. Pat. No. 6,073,499 and U.S. Pat. No. 6,708,572. More particularly, the portal  12  has a passage  14  extending therethrough. The portal  12  typically will be installed at a security checkpoint and the passage  14  thereof will be dimensioned to conveniently accommodate a human pedestrian who desires clearance at the security checkpoint.  
         [0024]     The portal detection system  10  operates partly upon the theories described in U.S. Pat. No. 6,073,499. In particular, a boundary layer of air adjacent to a person is heated by the person and generally is hotter than ambient air at farther distances from the person. Hot air is less dense than cooler air and rises relative to the more dense cooler air. As a result, a significant human convection plume of hot air rises in the boundary area adjacent to the person. The human convection plume generally achieves flow rates of 50-100 liters/second. This significant flow of warm air tends to entrain particles that had been on the skin or clothing of the person passing through the portal  10 . Thus, these microscopic particles travel upwardly with the plume of heated air.  
         [0025]     The portal  10  includes air jets  16  that direct short puffs of air towards the person in the passage  14 . The jets  16  are directed at an area of the person extending roughly from the knees to the mid torso and help to dislodge particles from the skin and clothing to stimulate particle separation and hence to increase the concentration of particles entrained in the human convection plume. The portal detector apparatus  10  further includes a compressed air supply  17  that is controlled to fire the jets sequentially from bottom to top as explained in U.S. Pat. No. 6,708,572. However, other jet firing patterns can be used.  
         [0026]     The portal detection apparatus  10  further includes a controller  18 , a preconcentrator  19  and a detector  20 . The detector  20  preferably is an ion trap mobility spectrometer as disclosed in U.S. Pat. No. 5,491,337 and as illustrated schematically in  FIG. 2 . More particularly, the detector  20  includes a heated membrane  21  formed from a microporous refractory material or from dimethyl silicone and disposed to communicate with a portion of the airflow generated by the thermal plume in the passage  14  of the portal detector apparatus  10 . The heated membrane  21  blocks passage of at least selected constituents of the air but enables passage of other constituents of the air, including the constituents of interest.  
         [0027]     The sample air, carrier gas, and dopant molecules pass through the inlet  22  and are spread by a diffuser  24  into an ionization chamber  26 . The ionization chamber  26  is in the form of a shallow cylinder with a diameter D, length L, and cylindrical wall  28  of a radioactive material, e.g., nickel 63  or tritium, which emits beta particles. Inlet  22  communicates with one end of the ionization chamber  26 . A grid electrode E 1  is provided at the end opposite the inlet  22 , and is normally maintained at the same potential as the inlet end and the walls of the ionization chamber  26 . Thus a largely field-free space is provided in which electrons and ion charges build up and interact with the sample molecules under bombardment by the beta-particles from the radioactive walls. Beyond the ionization chamber  26 , the ionized sample gases pass through open electrode E 1  and into an ion drift region  30  having several field-defining electrodes E 2 -E n . A collector electrode or plate  32  is disposed at the end of the drift region  30  for receiving the ion samples reaching that end. Periodically a field is established across the ionization region  26 , by creating a potential difference between the grid electrode E 1  and the inlet diffuser  24  and radioactive source  28 , for about 0.1-0.2 mS, to sweep the ions through the open grid E 1  into the drift region  30  with the assistance of the switching of the field between electrodes E 1  and E 2 . The ions in the drift region  30  experience a constant electric field, maintained by the annular electrodes E 2 -E n , impelling them along the region and down toward the collector electrode  32 . The electrode  32  detects the arriving charge, and produces signals that are amplified and analyzed through their spectra in the spectrometer. The gases exit through an outlet in the wall next to the electrode  32 . After about 0.2 mS the field across the ionization region  26  is again reduced to zero and the ion population is again allowed to build up in the chamber  26  preparatory to the imposition of the next field. The polarity of the fields is chosen on the basis of whether the detector is operated in a negative or positive ion mode. When detecting explosives, a negative ion mode is usually appropriate, but when detecting narcotic samples positive ion mode is preferred.  
         [0028]     The preconcentrator  19  of the subject invention is disposed between the passage  14  and the detector  20  as shown in  FIG. 3 . The preconcentrator  19  includes a channel  36  with an inlet  38  and an outlet  40 . A porous impactor  42  is mounted in proximity to the outlet  40  and is sufficiently small to permit a main sample exhaust flow between the periphery of the porous impactor  42  and the outlet  40  of the channel  36 . A chamber  44  is mounted to the porous impactor  42  and defines a plenum  45  that surrounds a side of the porous impactor  42  opposite the inlet  38  to the channel  36 . The plenum  45  includes an outlet  46  that communicates with the inlet  22  of the detector  20 . A main sample air pump  50  communicates with the outlet  40  of the channel  36  and generates an air flow rate that approximately matches the flow rate of the human convection plume (e.g. 50-100 liters/second). A detector sample air pump  52  communicates with the outlet  46  from the plenum  45  and with the detector  20  and produces a much lower flow rate that matches the flow rate preferred for the detector (e.g. 300 cc/mim).  
         [0029]     The porous impactor  42  creates an impediment to the substantially linear flow of air A 1  in the inlet  38  to the channel  36 . Gaseous molecules in the flow of air A 1  in the inlet  38  have very low inertia due to their low mass and density. Hence, a significant portion of the gaseous molecules in the flow A 1  will be diverted through curved paths indicated generally by A 2  in  FIG. 3  to bypass the porous impactor  42  and to follow a path A 3  between the outer periphery of chamber  44  and the outlet  40  of the channel  36 . The flows A 1  and A 3  are offset and do not align. Particles P entrained in the inlet airflow A 1  have more mass and higher density than the gaseous molecules and hence have more inertia. Accordingly, particles P entrained in the inlet air flow A 1  will follow more linear paths towards the porous impactor  42 . As a result, the concentration of particles P impinging on the porous impactor  42  will exceed the concentration of particles in the inlet  38 .  
         [0030]     The main sample air pump  50  is operative to generate airflow rates approximately equal to the airflow rates in the human convection plume. As noted above, airflow rates in the human convection plume surrounding the person in the portal  10  typically are in the range of 50-100 liters/second. The size and shape of the channel  36  and the size, composition and porosity of the impactor  42  are selected to optimize the concentration of particles P impinging upon and/or passing through the porous impactor  42  and to minimize the concentration of particles by-passing the chamber  44  and exiting the outlet  40 . These flow characteristics of the particles P are dependent upon a Stokes number analysis. In this regard, Stokes law relates to the velocity characteristics of an object settling in a fluid, wherein:  
         V   term     :=       (     1   18     )     ·   ρ   ·       Size   particle     2     ·     g   µ           
 
         [0031]     In this equation, ρ relates to, density between the particle and the fluid, μ is the viscosity of the fluid and g is the acceleration of gravity. The Stokes number is an index of particle impactability. Molecules with a Stokes number near zero have little inertia and are not impactible. Particles with high Stokes numbers (e.g., approaching 1) are impactible. The Stokes number is determined by the equation  
       St   :=         ρ   particle     ·       Size   particle     2         18   ·   µ   ·   τ           
 
         [0032]     In this equation, τ equals characteristic time which is calculated as Lchr/Vchr, where Lchr is the dimension of the body upon which particle impaction takes place, and Vchr is the relative velocity between the air stream and the body. As noted above, the flow rate of air A 1  is about 50 liters per second, and the flow velocity is about 5 meters per second. The porous impactor has an area of about 16 in 2 .  
         [0033]     The angular acceleration produced by the change in directional flow in the preconcentrator  19  is calculated as 
 
Acceln=(Velocity Airflow) 2 /radius 
 
         [0034]     This result can be used in a Stokes law equation for calculating particle lag due to inertia by replacing the above-calculated acceleration for the acceleration of gravity considered in the original Stokes law. Thus, Stokes law as applied to particle lag due to inertia is  
         V     particle   ⁢               :=       (     1   18     )     ·   ρ   ·       Size   particle     2     ·     acceln   µ           
 
         [0035]     The Stokes law relationship can be utilized with changes in geometry and air velocity to maximize impaction of particles on the porous impactor  42 . As a result, greater concentrations of particles can be placed in communication with the detector  20 , thereby enhancing reliability. The porous impactor  42  is designed using these relationships to collect particles with sizes of 1 micron and larger.  
         [0036]     As noted above, existing portal detector devices have a cycle that includes sampling, desorbing and analyzing. Additionally, the known devices typically require a mechanical translation of a sampling screen from a sampling location to the desorber. The cycle time can be reduced significantly in the apparatus  10  by substantially continuously heating the porous impactor  42  in situ sufficiently to desorb particles impinging thereon. The vaporized particles then are transported substantially continuously to the inlet  22  of the detector  20 , thereby automatically cleaning the impactor  42 . Heating can be achieved by forming the porous impactor from a sintered metal and then passing an electric current through the porous impactor  42  for heating the porous impactor  42  sufficiently to vaporize particles impinging thereon. The chamber  44  also can be heated for maintaining desired temperature of the particles for transfer to the detector  20 . The airflow mechanics of the preconcentrator  19  are such that a low pressure exists in the plenum  45  and on the rear side of the porous impactor  42 . As a result, the vaporized particles are transported from the porous impactor  42  into the low pressure plenum  45 . A detector sample flow exhaust  52  is employed to generate a flow of the vaporized particles into the detector inlet  22 . The volume of the plenum  45  partly determines the flow rate across the porous impactor  42 , and hence also affects the concentration of particles. Thus, the volume of the plenum and the flow rate of the detector sample flow exhaust  52  preferably are small and are selected to achieve an optimum flow rate into the detector inlet  22 . An alternative to the above-described porous impactor  42  can be formed with a sintered ceramic material that has the front surface plated with metal. An electric current then can be applied across the metallic material to heat the porous impactor  42  sufficiently to vaporize the particles impinging thereon.  
         [0037]     The preconcentrator  19  illustrated in  FIG. 3  utilized a continuous porous impactor  42  and diverts air around the periphery of the chamber  44  that contains the porous impactor  42 . The porous impactors  42  of  FIG. 3  is most effective for capturing particles that are entrained near the centerline of the flows stream A 1 . However, the momentum transfer is less for particles that are entrained in parts of the flow stream A 1  further from the centerline. Thus, particles trajectories at more outward positions in the flow stream are less likely to divert from the flow stream. In this regard, the efficiency of particle impact will drop explanationally at further distances from the centerline of the flow stream A 1 .  FIGS. 4 and 5  show an alternate preconcentrator  19 A that effectively divides the preconcentrator  19 A into several smaller areas. The alternate preconcentrator  19 A includes a channel  36  similar or identical to the channel  36  illustrated in  FIG. 3  and described above. However, several porous impactors  62  are mounted across the front of a chamber  64  to create a system of porous impactors  62  with multiple centerline flow patterns so that particles can impact more uniformally over the entire area of the preconcentrator  19 A. The preconcentrator  19 A illustrated in  FIG. 5  may have an overall width of approximately 10 cm. However, each of the porous impactors  62  may have a width of about 1 cm and are separated from one another by channels  66  that communicate with the periphery of the chamber  64  to accommodate the main sample flow exhaust. The preconcentrator  19 A of  FIGS. 4 and 5  provides a different geometry and hence a different flow path for both air and particles. The geometry illustrated in  FIGS. 4 and 5  can achieve greater sampling efficiency.  
         [0038]     In both embodiments, the control  18  controls the operation of the detector  20  and generates an alarm signal if a particle of interest is detected.  
         [0039]     While the invention has been described with respect to certain preferred embodiments, various changes can be made without deporting from the scope of the invention as defined by the claims. For example, the preconcentrator is shown as being used with a walk through detector portal. However, the preconcentrator can be used with other detection devices that rely upon transporting particles of the interest on a flow of air. The porous impactor also can take other geometries depending upon the sizes of the particles that are to be collected, flow rates of air towards the impactor and the dwell time targets. Additionally, an ion trap mobility spectrometer is shown as the detector. However, other detectors, including detectors that rely upon Raman spectroscopy can be employed.