Abstract:
A device for concentrating particles from a high volume gas stream and delivering the particles for detection in a low volume gas stream includes first and second preconcentrators. The first preconcentrator has a first structure for retaining particles in a first gas flow path through which a first gas flows at a relatively high volume, valves for selectively stopping the first gas flow; and a second gas flow path through which gas flows at an intermediate flow volume for moving particles from the first structure. The second preconcentrator includes a second structure for retaining particles in the second gas flow path; a valve for selectively stopping the second gas flow; and a third gas flow path through which gas flows at a low volume for moving particles from the second structure to a detector. Each of the particle retaining structures is preferably a metal screen that may be resistively heated by application of an electric potential to release the particles.

Description:
The United States Government has rights in this invention pursuant to Department of Energy Contract No. DE-AC04-94AL85000 with Sandia Corporation. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     (Not Applicable) 
     BACKGROUND OF THE INVENTION 
     Trace chemical detection of explosives, i.e., the art of detecting explosive materials from minute quantities of vapor and/or microscopic particles (hereinafter referred to as ‘particles’), can be an important aspect of many physical security systems. Among the challenges currently confronting researchers in this area is the problem of how to collect the explosive sample and transport it to the detector without major losses. In many applications, especially applications involving the general public such as airport passenger screening, swipe collection of particles via direct physical contact with the person or object to be screened for explosives is either too physically invasive or time consuming, so it is necessary to base the collection process on air flows. But the vapor and/or airborne particle material that is collected in such air flows is usually far more dilute than the detector is capable of measuring, and the air flow is often too large to be directly accommodated by the detector. These disparities give rise to preconcentrators, devices which take a trace sample of a material from a large incoming air flow and concentrate the material into a smaller volume before it is introduced into a trace detector. 
     U.S. Pat. No. 5,854,431 of Linker et al discloses a single stage preconcentrator  10  for use in collecting particles from an air stream that passes over a person or object under observation. Preconcentrator  10  includes a screen  14  disposed between input and output air stream valves  22  and  22 ′, respectively, which valves are secured together to form a layered arrangement whereby gas to be tested passes through open valve  22 , through screen  14  (where particles are deposited), and exits through open valve  22 ′. A fan or other gas moving device may be situated downstream of output valve  22 ′, and the source upstream of input valve  22  may be the output of a booth through which people or objects being tested may pass. 
     As set forth in the &#39;431 patent, screen  14  is preferably formed of a metallic felt made from very thin metal filaments, with diameters ranging from 1 to 80 micron. As a comparison, human hair has a diameter between 70 and 100 micron. The felt is a pleatable and weldable stainless steel matrix, produced by the sintering of a composite metal fiber. The preferred material is Bekipor® ST, produced in North Carolina by Bekaert Fibre Technologies of Belgium. The preferred configuration of screen  14  was pleated, with the folds being parallel to the flow of gas during desorption. A particular advantage of this screen material is that it may be resistively heated to release the particles from the surface by applying an electric potential across its surface. 
     Single stage preconcentrator  10  of the &#39;431 patent operates as follows: first—both valves  22 ,  22 ′ are open and air to be tested flows through screen  14 , which is not being heated and which absorbs particles from the air to be tested; second—both valves  22 ,  22 ′ are closed and screen  14  is heated to desorb collected particles; and third—a carrier gas  34  (air or inert gas) is provided to move desorbed particles in a direction parallel to the pleated surface of screen  14  to a detector at output  40 . 
     A problem with this device is that the output  40  flows at about 4 liters/minute in order to move the particles from screen  14 , while the detector prefers an input on the order of 0.1 to 0.5 liters/minute. 
     Simple flow restrictors would not work in this application, because the particles may adsorb to the restrictor instead of proceeding to the detector. In addition, any solution to this problem must be capable of being reset after every test so that the results of the previous test do not effect the next test. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a 2-stage preconcentrator to enable particles to be moved to a detector under conditions acceptable to the detector, and to provide for cleaning of one stage while the other stage is operating. 
     To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention may comprise a device for concentrating particles in a high volume gas flow for detection in a low volume gas comprising a first preconcentrator comprising a first structure for retaining particles in a first gas flow path through which a first gas flows at a relatively high flow; means for selectively stopping the first gas flow; and a second gas flow path through which gas flows at an intermediate flow for moving the particles from the first structure to a second preconcentrator coupled to said first preconcentrator. The second preconcentrator comprises a second structure for retaining particles in the second gas flow path; means for selectively stopping the second gas flow; and a third gas flow path through which gas flows at a relatively low rate for moving the particles from the second structure to a detector. 
     Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form part of the specification, illustrate an embodiment of the present invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 shows a schematic representation of the invention. 
     FIGS. 2A and 2B show an end view of the previous and present configurations of first preconcentrator screen. 
     FIG. 3 shows a perspective view of the second stage of the invention. 
     FIG. 4 shows an exploded view of the output stage of the second stage. 
     FIG. 5 shows an end view of a portion of the output stage. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, a two-stage preconcentrator for detection of particles in a gas stream may include a first stage preconcentrator  10  and a connected second stage preconcentrator  50 . A preferred embodiment of first stage preconcentrator  10  is disclosed in U.S. Pat. No. 5,854,431, the disclosure of which patent is incorporated herein by reference. As discussed in the aforementioned &#39;431 patent, a first gas flow is through flow control means such as air stream valves  22 ,  22 ′ and screen  14  may be multi-bladed irises similar to a camera shutter. However, since  6 ″ diameter shutters of this type are not readily available, valves  22 ,  22 ′ may also be single leaf shutters that cover a hole and pivot about a point adjacent the outer perimeter of the hole. Tests have shown that valves  22 ,  22 ′ do not have to be well sealed against leaks to permit the invention to function as designed; in fact, for reasons that are not totally understood, shutters that were very well sealed against leaks did not perform as well as less well-sealed shutters. 
     The &#39;431 patent used a pleated screen  14  with a zig-zag cross-section as shown in cross-section in FIG. 2A, with pleat height being about ½ inch and the distance between peaks being about ¼ inch in a 6″×6″ screen; the invention uses a scalloped screen  14  with a cross-section as shown in FIG. 2B wherein every other pleat is rounded, with a pleat height of about ⅜ inch and distance between peaks of about ¾ inch. The configuration of FIG. 2B has been found to give improved performance over the configuration of FIG. 2A during desorption of particles after collection. It is believed this improved performance results from fewer particles being caught in the valley between adjacent pleats. 
     As seen in the &#39;431 patent, the output  40  of first stage preconcentrator is a slot that is approximately the same dimension as the edge dimensions of screen  14  (approximately 6″×0.5″). The output  40  is fed into a funnel  42  that has a similarly shaped input  44  and a round smaller output  46  connected to the input of a second stage preconcentrator  50 . 
     Second stage preconcentrator  50  captures the particles from screen  14  of first stage preconcentrator  10  and pass them to a detector at a lower flow rate with minimal loss of particles. Second stage preconcentrator  50  preferably comprises an input stage  60  connected in series with an output stage  80 . As shown in FIG. 3, input stage  60  is adjacent and attached to output stage  80 , and both stages are held by a bracket  59 . A motor  69  is attached to one side of bracket  59  and drives, through appropriate gears, a rotating valve  70  in input stage  60  as discussed hereinafter. A second motor  89  is also attached to bracket  59  and drives, through other gears, a rotating valve  100  in output stage  80  as discussed hereinafter. 
     One embodiment of input stage  60  includes a solid rectangular housing  62  having a cylindrical bore extending between two opposed faces. Rotary valve  70  is formed of a solid cylinder of a material such as stainless steel having a diameter sized to fit tightly within the bore of housing  62 . (As illustrated, the axis  74  of valve  70  extends into the page in FIG.  1  and extends parallel to the axis of motor  69  in FIG. 3) Valve  70  includes a first hole  76  extending in a straight line perpendicular to axis  74  and through valve  70 , and a second hole  78  extending perpendicular to and from hole  76  at axis  74  to the surface of valve  70 . A hole  66  through a side of housing  62  provides an input for input stage  60  enabling gas from first stage preconcentrator  10  to communicate with valve  70 . Another hole  64  through an opposing side of housing  62  provides an output from input stage  60  enabling gas passing through valve  70  to communicate with output stage  80 . 
     Holes  66  and  64  are preferably aligned so they and first hole  76  of valve  70  provide a clear, straight gas path from first stage preconcentrator  10  to output stage  80  when valve  70  is in the position shown in FIG.  1 . This position is referred to as the 90° C. position of input stage  60 . 
     A third hole  68  through housing  62  enables valve  70  to communicate with a thrust jet  52 . (According to literature of the manufacturer, Artx Ltd of Cincinnati, Ohio, thrust jet  52  is a tubular device that releases a tiny amount of compressed air at near-sonic velocity through an internal ring-shaped nozzle. The high-speed ‘tube’ of air ejected through the front of the device creates a strong vacuum which pulls additional surrounding air through the rear of the device.) 
     If valve  70  is rotated 90° C. counter-clockwise from the position shown in FIG. 1, housing hole  66  is aligned with cylinder hole  78  and housing hole  68  is aligned with cylinder hole  76 , enabling gas to flow between housing holes  66  and  68 . This position is referred to as the 180° C. position of input stage  60 . 
     If valve  70  is rotated 90° C. clockwise from the position shown in FIG. 1, housing hole  64  is aligned with cylinder hole  78  and housing hole  68  is aligned with cylinder hole  76 , enabling gas to flow between housing holes  64  and  68 . This position is referred to as the 0° C. position of input stage  60 . 
     Output stage  80  of second stage preconcentrator  60  is shown in FIGS. 1 and 4 to include a second particle collecting screen  110 , preferably flat and made of the same metal fiber felt material as screen  14  in the first stage preconcentrator  10 . Screen  110  is mounted on, and electrically insulated from, valve  100  that rotates within a housing  82 . Means are provided to heat screen  110  to desorb particles, as discussed hereinafter. 
     Each of housings  62  and  82  are preferably made of a material to which particles being detected do not readily adhere. Teflon® is a good particle-resistant material for systems designed to detect explosive particles. However, because it is difficult to make mechanical attachments to Teflon®, each housing is preferably surrounded by aluminum plates through which suitable holes are bored to facilitate attachment to neighboring elements of the invention. 
     As shown in FIG. 4, housing  82  is preferably a regular parallelogram (such as a cube) with a central bore  83  that receives valve  100 . The four surfaces that surround bore  83  are covered by aluminum plates  91 - 94 . Each plate has holes which align with the holes in the surface of housing  82 , such as  86  and  86 ′. For most of the discussion of this embodiment of the invention, hole  86  should be understood to mean the hole through both plate  93  and housing  82 , and the surface  93  of housing  82  should be understood to mean the surface of plate  93 . 
     Alternatively, housings  62  and  82  and parts of valve  100  could be manufactured from aluminum or other material, and the surfaces which are exposed to particles could be coated with Teflon® or other particle-resistant material such as ceramic. 
     Housing  82  has a plurality of gas passage holes arranged perpendicular to the axis  90  of valve  100 . An input port  84  and an aligned third port  86  extend through one pair of opposite surfaces  91 ,  93  of housing  82 , and a fourth port  85  and an aligned output port  87  extend through the other pair of opposite surfaces  92 ,  94  of housing  82 . A thrust jet  96  (FIG. 1) is attached to third port  86 , and input port  84  is coupled with output port  64  of input stage  60 . The detection device  36  is connected to output port  87 . 
     Housing  82  also includes another pair of holes  81   a ,  81   b  for directing cooling air to the screen  110  through valve body  120  via holes  111   a  and  111   b . Vortex coolers  99   a ,  99   b  are connected to each of holes  81   a ,  81   b  (FIGS. 1,  4 ), which holes are formed so that their ends inside valve body  120  are adjacent to the input side of screen  110  when valve  100  is in the position shown in FIG.  1 . (Vortex coolers have been known since the 1930s and are available from ARTX Ltd, among other sources. A vortex cooler consists of a hollow tube which has a side opening for compressed air. Due to the construction of the tube, heated air comes out of one end and cooled air comes out of the other end of the hollow tube.) 
     Valve  100  is shown in FIGS. 4 and 5 to include a plurality of parts. An electrically conducting assembly includes rectangular screen  110 , which is affixed at each end to a different one of a pair of spaced electrically conducting rods  112   a ,  112   b . Each rod  112   a ,  112   b  is split in half lengthwise from an end a distance corresponding to the width of screen  110 . The half-cylindrical split pieces  114   a ,  114   b  are then fastened back into their original place with the screen between that piece and the other half of the rod, as shown in FIGS. 4 and 5. This assembly is held by an electrically insulating, particle-resistant, holder  102  (preferably Teflon® or high-temperature ceramic) that includes an end cap  103  and a generally rectangular slab  104  extending from end cap  103 . As shown in FIG. 5, a pair of spaced holes  105   a ,  105   b  extend into slab  104  from the end opposite end cap  103 , and a slot  106  extends between holes  105   a ,  105   b . These pieces are sized such that rod  112   a  slides into hole  105   a , rod  112   b  slides into hole  105   b , and screen  110  slides into slot  106 . A hole  108  extending through slab  104  permits gas to flow through screen  110 . An electrical power supply  130  (1.5 v AC) is connected through holes in end cap  103  to the ends of rods  112   a ,  112   b  to apply a voltage across screen  110 . 
     Valve  100  further comprises a generally cylindrical stainless steel body  120  that is sized to fit into bore  83  in housing  82 . A generally rectangular slot  122  is cut into body  120  from one end thereof for holding slab  104  and the electrically conducting assembly discussed above. A hole  128  extends through body  120  and communicates with hole  108  in slab  104 . A pair of holes  111   a ,  111   b  in body  120  complete the passages  81   a ,  81   b  between vortex coolers  99   a ,  99   b  and screen  110  when valve  100  is in the position shown in FIG.  1 . An insulating spacer  118  is placed between the exposed ends of rods  112   a ,  112   b  and the interior of valve body  120 . 
     Motor  89  (FIG. 3) rotates valve  100  to any of three positions. As shown in FIG. 1, valve  100  is in the 0° C. position with the input side of screen  110  facing input hole  84  and holes  111   a ,  111   b  aligned with passages  81   a ,  81   b  in housing  82 . If valve  100  is rotated 180° C. to face hole  86 , it is in the 180° C. position. If it is rotated 90° C. counterclockwise from the position shown in FIG. 1 to face hole  87 , it is in the 90° C. position. 
     The operation of the invention involves several process steps as outlined below: 
     1. First Stage Adsorb From Source 
     Valve  70  is placed in the 0° C. position to block gas flow through hole  66  and to permit flow between holes  64  and  68  when the second stage is cleaned. Valves  22 ,  22 ′ are open, and a large volume of air or other gas flows from the source to be tested through valves  22 , screen  14 , and out through valve  22 ′. The heater for the first stage is off, as particles in the gas stream adsorb to a cool screen more than a hot screen. Explosive particles from the gas flow impinge on and adsorb to cool felt screen  114 . 
     2. Clean Second Stage 
     This step may occur simultaneously with step 1. Valve  100  is placed in the 180° C. position with the screen facing hole  86 . Voltage from supply  130  is applied to heat screen  110  to desorb any remaining particles. Thrust jet  96  is activated to create a flow through screen  110 . Input gas passes from input stage  60  (through thrust jet  52  (which is off), hole  68 , valve  70 , and hole  64 ) to output stage  80  (through hole  84 , hole  128 ,hole  108 , screen  110 , hole  108 , hole  128 , and hole  86 ) to be exhausted by thrust jet  96 . 
     3. End First Stage Adsorb From Source 
     The source of air from the object under test is stopped and valves  22 ,  22 ′ are closed. 
     4. Desorb Particles From The First Stage  10  To The Second Stage  50   
     The second stage heater  130  is turned off and the first stage heater  38  is turned on. In input stage  60 , valve  70  is rotated to the 90° C. position and valve  100  is rotated to the 0° C. position (the position of the valves shown in FIG.  1 ). Thrust jet  96  is turned on to draw gas with particles from first stage  10 , through funnel  42 , input stage  60  and through screen  110  in output stage  80 . The gas and particles from screen  14  are quite warm (over 140° C.) when they reach output stage  80  as a result of heating of screen  14 . This heat, and the relatively high flow rate, keeps particles from adsorbing to the aluminum funnel  42 . Since warm particles will not adsorb to screen  110 , vortex coolers  99   a  and  99   b  are turned on to provide a flow of cold air that cools the particles and screen  110 . 
     5. Particle Detection 
     Thrust jet  96  and vortex coolers  99   a ,  99   b  are turned off. Valve  100  is rotated counterclockwise to the 90° C. position so that screen  110  faces the detector and gas flow through holes  84  and  86  is blocked. Heater  130  for screen  110  is energized to desorb the trapped particles. The detector generates a low volume gas flow towards the detector to cause particles to be carried from the screen  110  through hole  87  to the detector. Opposing hole  85  provides the input for this flow. 
     6. Clean First Stage 
     This step may occur simultaneously with step 5. Valve  70  is rotated to the 180° C. position and power supply  38  is energized to heat screen  14 . Thrust jet  52  is turned on to draw air from inlet  34  over screen  14 , through hole  66  to hole  68 . 
     With two stage preconcentration the demonstrated sensitivity of the system has increased by at least an order of magnitude over the single stage preconcentrator of the &#39;431 patent, from low nanograms (10 −9 ) to high picograms (10 −12 ). This increase in sensitivity provides a significant increase in the capability of a portal system using the preconcentrator to detect explosives on people passing through the portal. 
     The particular sizes and equipment discussed above are cited merely to illustrate a particular embodiment of this invention. It is contemplated that the use of the invention may involve components having different sizes and shapes as long as the principle of using two preconcentrator stages, is followed. For example, the sizes of openings, screens, and gas flow may be adjusted to meet requirements of an application. 
     Although aluminum has been utilized for many of the metal parts of this invention, it should be understood that these parts may also be formed of stainless steel or other materials that are less attractive to explosive or other particles being detected. Particles have not been observed to stick to aluminum funnel  42  because of the relatively high air flow. If particles did adsorb to funnel  42 , it could be made of a more particle-resistant material, or means such as an additional air source or holes could be provided in funnel  42  to provide a flow of air along the inside edges that keep particles towards the center of funnel  42 . 
     Detector  36  is preferably an ion mobility spectrometer, a time-of-flight mass spectrometer, and other known device for measuring mass and time of flight. 
     In addition, alternative materials or structures may be utilized for screens  14  and  110 . Any screen or surface that has the desired properties of retaining particles (for steps 1 and 4) until they are controllably released (for steps 2, 4, and 6) may be utilized in place of screens  14  and  110 . It is intended that the scope of the invention be defined by the claims appended hereto.