Abstract:
A device samples chemicals adsorbed to a surface by applying a pulse of fluid to desorb the particles adhered to the surface. After the pulse of fluid, the region above the surface is enriched with particles dislodged from the surface. Suction is applied in the region above the surface to collect these dislodged particles, which are then transferred to a chemical detector for detection, identification, and quantification. 
     A pulsed air sampler collects particles adhered to a surface and delivers the particles to a chemical sensor. An outlet ejects a fluid, preferably gas, pulse to dislodge particles from the surface and thereby enrich the density of particles above the surface. An inlet collects the dislodged particles for delivery to the chemical sensor.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 09/151,743 filed on Sep. 11, 1998, now U.S. Pat. No. 6,269,703. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for collecting chemical samples, and more specifically, to an apparatus that is capable of collecting particles or molecules adhered to a surface. 
     Many situations arise requiring the capability to detect the presence or absence of a chemical on a surface. One important example is the detection of toxic and hazardous substances in the environment, such as explosives and chemical agents. Searching for toxic or hazardous substances involves monitoring a variety of different surfaces and checking for the presence of particular chemicals. 
     In general, the process of detecting a chemical comprises three main steps: acquiring the sample, conditioning the sample, and employing a chemical detector to detect, identify and quantify the specific chemical of interest, herein referred to as the target analyte or target. Sample acquisition comprises the removal of the target analyte from a surface or host matrix to which it may be attached. Sample conditioning comprises the preparation, conditioning, or processing of the sample prior to its introduction to a chemical detector. Detecting the target analyte with the chemical detector involves determining the presence or absence of the target chemical on or in the chemical detector. 
     Although chemical detection is conventionally viewed as being defined by the lower limit of the detection ability of the chemical detector, the performance of the chemical detector alone does not accurately characterize the ability of the entire system to detect the presence of a specific chemical. In many cases, performance is significantly affected by sample acquisition and conditioning. 
     Often, it is difficult to acquire a chemical sample when the chemical is adhered to a surface, particularly if the vapor pressure of the chemical is low, or if the temperature is low. Under these circumstances, only a small amount of molecules may be in the gas phase and available for collection. Additionally, the surface of the chemical may become crusted-over with time, which further reduces the quantity of vapors in the gas phase available for detection. 
     Accordingly, a need exists for an improved system for collecting chemical samples, particularly when the chemical is adhered to a surface. 
     SUMMARY OF THE INVENTION 
     The present invention comprises an apparatus for collecting particles, such as molecules, that are adhered to a surface. The apparatus comprises an outlet that ejects fluid for dislodging the particles from the surface and an inlet for collecting the particles once dislodged. The particles may, for example, be in the form of an aerosol or vapor. 
     In the preferred embodiment, the fluid is warm air and a plurality of outlets are employed to eject pulses of the warm air. A blower draws air from the atmosphere and supplies that air to the outlets to dislodge or desorb the particles while a pump provides suction to draw the particles into a plurality of inlets. Intermittent flow of air through the outlets may be achieved by means of a valve, or alternatively, by blower control electronics that switch the blower on and off. Intermittent suction at the inlets may be provided by activating the pump intermittently or, alternatively, by using a valve. 
     According to another aspect of the invention, a method for collecting particles, such as molecules, that are adhered to a surface comprises ejecting fluid onto the surface to desorb the particles from the surface and drawing the desorbed particles into an inlet. The particles can then be detected using a chemical detector. In the preferred embodiment, the ejected fluid comprises pulses of air which are diverted against the surface and the drawing is accomplished using intermittent suction. Such ejection may be alternated with the drawing. In another embodiment, continuous suction is provided throughout a plurality of blowing periods. In the preferred embodiment, the fluid as well as the desorbed particles are heated. For example, the desorbed particles may be heated by heating the inlet. Preferably, the fluid directs the desorbed particles toward the inlet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a preferred embodiment of the present invention. 
     FIG. 2 is a plan view of the bottom of the embodiment of FIG. 1 showing the plurality of inlet and outlet orifices arranged in concentric arrays. 
     FIG. 3 is a schematic drawing of the embodiment of FIGS. 1 and 2. 
     FIG. 4 is a fragmented cross-sectional view taken along the lines  4 — 4  of FIG. 3 showing the air flow distribution network. 
     FIG. 5 is a plan view of the bottom of another embodiment of the present invention wherein a plurality of outlet orifices surrounds a single inlet orifice. 
     FIG. 6 a  is an elevational view of the embodiment of FIG. 5 that depicts the application of suction to draw particles into the inlet. 
     FIG. 6 b  is an elevation view of the embodiment of FIG. 5 showing fluid exiting the plurality of outlets to dislodge particles adhered to a surface, and showing suction being simultaneously applied to draw the desorbed particles into the inlet. 
     FIG. 6 c  is an elevation view of the embodiment of FIG. 5 that illustrates the continued application of suction to draw the desorbed particles into the inlet after the fluid ejection shown in FIG. 6 b  has ceased. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As illustrated in FIG. 1, a pulsed air sampler  10  comprises a display assembly  66  having a display and/or alarm  68 . The display assembly  66  is mounted on one end of a shaft  64  and contains a master CPU. The other end of the shaft  64  is connected to an interface assembly  62 . A collector assembly  60  is attached to the interface assembly  62 . The pulsed air sampler  10  is powered entirely by batteries located in the display assembly, and thus is completely portable. 
     As shown in FIG. 2, the collector assembly  60  includes an array of inlet orifices or inlets  38  and an array of outlet orifices or outlets  32 . The inlets  38  are interspersed with the outlets  32  to form a pattern in which each inlet  38  is located between an adjacent pair of outlets  32 , such that the outlets and inlets are in concentric arrays. 
     As shown in FIG. 3, the display assembly  66  comprises an air blower  12  with an air intake or inlet  14  and an air outlet attached to a pneumatic conduit  16  that extends through the shaft  64 . An outlet heater  20  has a heating element  18  in thermal contact with the conduit  16 . The heater  20  includes a heater power supply  22 . The pneumatic line  16  conducts air from the inlet  14  to a chamber  24  in the interface assembly  62 . The chamber  24  has an outlet valve  26  electrically connected to electronics  28  to control air flow to an outlet line  30 . The outlet line  30  conducts air from the valve  26  to an air flow distribution network  34  in the collector assembly  60 . The air travels through a manifold (FIG. 4) in the network  34  and is output from the plurality of outlets  32 . 
     The air distribution network  34  additionally comprises a second manifold (FIG. 4) that conducts air from the plurality of inlets  38  to an inlet line  36 . An inlet heater  42  comprises a heater power supply  44  and a heater element  40  that is in thermal contact with the inlet line  36 . The inlet line  36  is attached to an inlet valve  46  within the interface assembly  62 . The valve  46  is electrically connected to valve control electronics  48  to control flow from the inlet line  36  to a detector housing  50  in the interface  62 . The detector housing  50  contains a chemical detector, such as a sensor  52 . Electrical cable  54  connects the chemical sensor  52  to detection electronics  56 . A sample acquisition pump  58 , attached to the detector housing  50  containing the chemical sensor  52 , is provided to draw air from the inlets  38  to the sensor  52 . Electrical cable  54  connects the chemical sensor  52  to detection electronics  56  in the display assembly  66 . 
     As shown in FIG. 4, the air flow distribution network  34  is comprised of three plates, an upper plate  70 , a middle plate  72 , and a lower plate  74 . The upper plate  70  is spaced from the middle plate  72  to form a passageway or plenum  76  therebetween. This outlet line  30  is connected to the plenum  76  and is in fluid communication therewith. The inlet heater element  40  extends from the conduit  36  into the passageway  76  between the upper plate  70  and the middle plate  72 . This heater element  40  is disposed in thermal contact with the middle plate  72 . Preferably, the middle plate  72  is formed of a thermally conducting material, such as aluminum, so that application of heat by the heater element causes both the top and bottom surfaces of the middle plate to be warmed. Tubular members or inserts  80  extend from the middle plate  72  to the lower plate  74 . The ends of each tubular member  80  have reduced diameter portions which are press fit or glued into openings in the middle plate  72  and lower plate  74 , respectively. The bores of the members  80  form respective linear passageways  78  that extend from the plenum  76  to the outlet orifices  32 . The tubular members  80  also function as spacers to mount the lower plate  74  in spaced relationship to the middle plate  72 . The spacing between the middle plate  72  and the lower plate  74  creates a passageway or plenum  82  between the middle plate  72  and the lower plate  74 . This plenum  82  is connected to the inlet line  36 , and is in fluid communication therewith. The inlet openings  38  in the lower plate  74  are in fluid communication with the interior of the plenum  82 . Thus, the three plates  70 ,  72 ,  74  provide two intertwined manifolds, each having a sealed air flow path, one of which extends from the inlets  38  through the plenum  82  to the inlet line  36 , and the other of which extends from the outlet line  30 , through the plenum  76 , through the tubes  80  and to the outlets  32 . Air flow from the inlets  38  is illustrated by arrows  88  while air flow to the outlets is illustrated by arrows  86 . 
     In operation, the pulsed air sampler  10  collects particles such as molecules that are adhered to a surface by employing a timed sequence of short air pulses to disturb the layer of molecules at the interface between the surface and the surrounding region of air. The pulses of air have sufficient velocity to both disturb the molecules and to move them away from the surface. In order to facilitate the desorption of the molecules adsorbed to the surface, warm air is used to warm the molecules, and thereby increase their thermal activation energy and enhance the likelihood that they will be desorbed from the surface. After application of an air pulse, the vapor above the surface will be enriched by the desorbed molecules, and the enriched vapor is d&amp;awn into the inlets  38  by suction. The enriched vapor may contain aerosols which are comprised of high concentrations of the target molecule. 
     Air blower  12  generates the air required to dislodge the adsorbed molecules from the surface. The air blower  12  draws ambient air from the air intake  14  and forces the air into the pneumatic line  16  to the accumulation chamber  24 . Preferably, the air blower  12  is capable of pressurizing the chamber  24  to a pressure in range of about 0.5 to 5 pounds per square inch (psi) above atmospheric pressure, and provides a flow rate from about 10 to 2000 cubic centimeters of air per minute. The air blower  12  forces air under pressure through the pneumatic line  16  in the shaft  64 . The heating element  18  heats the conduit  16  so that the air is heated as it travels down the shaft. The shaft  64  may be surrounded with thermal insulating material to minimize heat loss and conserve power. Preferably, the outlet heating element  18  extends substantially the entire length of the pneumatic line  16  and the shaft  64  (about 3-4 feet in the preferred embodiment). 
     The pressure within the chamber  24  is released by opening the outlet valve  26 . The outlet valve  26  is a spring-loaded gate valve that is controlled by the flow control electronics  28 . (Alternatively, the blower  12  itself can be switched on and off with a blower controller  84 , instead of employing the outlet valve  26 .) Once the outlet valve  26  is opened, a pulse of warm air will rush through the outlet line  30  and into the flow distribution network  34  shown in FIG.  4 . Specifically, the jets of air flow through the outlet line  30  and into the passageway  76  between the upper plate  70  and the middle plate  72 , in the direction of the arrows  86 . The air proceeds down the tubular members  80  and through the outlet orifices  32  to dislodge the molecules from the surface being monitored. 
     The warm air exits the outlets  32  at a sufficient velocity and temperature and in sufficient volume to dislodge the molecules from the surface. The temperature of the air ejected from the outlets, for example, may be between about 25° C. and 60° C. 
     To collect the sample, the pump  58  is activated to provide suction at the inlets  38  to draw the dislodged molecules into the inlet line  36  and the detector housing  50 . The pump  58  may, for example, draw from 50 to 1000 cubic centimeters of air per minute. Providing suction on the inlet line  36  will cause air to be drawn from the passageway  82  between the middle plate  72  and the lower plate  74 . Consequently, air is drawn through the inlet orifices  38  formed in the lower plate  74 . Arrows  88  indicate the flow of the air into the inlet orifices  38 , through the passageway  82  between the middle plate  72  and the lower plate  74 , and into the inlet line  36 . The incoming air, enriched with dislodged molecules, travels through the inlet line  36  and into the sensor housing  50 . 
     In some cases, the target molecules may tend to adsorb onto the inlet line  36  or onto the middle  72  and lower plates  74  in the passageway  82  or onto the inlet orifices  38  as the sample is drawn towards the sensor. Such adsorbed molecules will not be detected by the chemical sensor  52  and, thus will result in an inaccurate measurement of the sample concentration. Conversely, random desorption of molecules previously adsorbed on the inlet line  36  or on the middle  72  or lower plates  74  will be detected by the chemical sensor  52  and will also create inaccurate measurements of the sample concentration. Such adsorption and/or desorption can cause measurement errors, particularly when the concentration of molecules is low. 
     To minimize the adsorption of the molecules onto the surfaces of the inlet orifices  38 , the air flow distribution network  34 , and the inlet line  36 , the incoming air may be heated using the heating element  40  to heat the inlet line  36  and the middle plate  72  (FIG.  4 ). The inlet heater  42  applies thermal energy to the middle plate  72 , which warms the incoming air to a temperature, for example, between about 25° C. and 60° C. 
     To further minimize adsorption of the dislodged molecules, the inlet line  36 , the inlet orifices  38 , as well as the sides of the middle  72  and lower  74  plates that form the plenum  82  can be chemically deactivated by applying a coating, such as polytetrafluoroethylene (e.g., Teflon®), which does not provide a reactive surface on which the molecules can adsorb. 
     In the preferred embodiment, the suction provided to draw the molecules into the inlets  38  is intermittent. The valve  46  situated between the air pump  58  and the inlets  38  is opened and closed to switch the suction on and off. Valve control electronics  48  are employed to open and close the valve  46  in an intermittent fashion. Alternatively, the pump  58  itself can be switched on and off with a pump controller  90 . 
     The suction provided by the pump  58  ultimately transports the dislodged target molecules from the surface being monitored to the sensor housing  50 . Once in the sensor housing  50 , the molecules can be detected by the chemical sensor  52 . It will be appreciated that the chemical sensor  52  may comprise any sensor capable of detecting the presence of the specific molecules or other particles sought to be detected. Well known examples of such chemical sensors  52  include surface acoustic wave, chemi-resistors, and solid-state sensors. A chemical sensor employing an array of SAW devices is disclosed in the co-pending application of William D. Bowers, et al. entitled “Chemical Sensor Array”, Ser. No. 09/151,747, filed on the same date as the present application which is hereby incorporated herein by reference. 
     Preferably, the chemical sensor  52  is capable of identifying as well as detecting the presence of the target molecules or particles. In either case, the chemical sensor  52  outputs an electrical signal that indicates that target molecules have been detected. This electrical signal is carried by electrical cable  54  to the sensor electronics  56  and ultimately to a display  68  in the display assembly  66  shown in FIG.  1 . 
     Although the preferred pulsed air sampler  10  employs air to dislodge molecules adhered to a surface, other fluids, gaseous and liquid, may be employed in separate embodiments of the invention. Additionally, the apparatus of the present invention may be used to detect the presence of particles other than molecules, such as sub-micron, neutrally charged particles. These particles may be adhered to either a liquid or solid surface. 
     As discussed above, in the embodiment shown in FIG. 2, the plurality of inlets  38  as well as the plurality of outlets  32  are arranged in concentric arrays as shown in FIG.  2 . The arrays are alternated, each inlet  38  or array of inlets being surrounded by an array of outlets  32 . Also, each outlet array, except for the outermost, is surrounded by an array of inlets  38 . An alternative configuration is depicted in FIG.  5 . In this configuration, a single inlet  38  is surrounded by a single concentric array of outlets  32 . FIG. 5 also shows the inlet  38  and outlet  32  orifices as circular openings. In yet another configuration (not shown), a single common inlet/outlet orifice is used in place of separate outlets and inlets. This configuration can be implemented by connecting both the outlet line  30  and the inlet line  36  to a single line terminated by an inlet/outlet orifice. Employing the same hole as both the outlet for ejecting fluid and the inlet for collecting the dislodged particles can minimize false positive readings caused by the desorption of particles stuck to the surface of the orifice or the inlet line  36 . Warm fluid exiting from outlet line  30  could remove particles adsorbed onto the walls of the inlet/outlet orifice and carry them away from the chemical sensor  52 . Thus, this embodiment may prevent the random desorption of particles adhered to the inlet and detected by the chemical sensor  52 , and thereby improve the accuracy of estimates of the concentration of particles on the surface being monitored. As described above, the suction provided to draw the particles into the inlets is preferably intermittent, and the drawing of the particles into the inlets is preferably alternated with the step of blowing air through the outlets. 
     To test a surface for the presence of the target particle, such as a molecule, the operator moves the pulsed air sampler  10  towards the surface to be monitored. In order to avoid contamination, the bottom of the collector assembly  60  (FIG. 1) should preferably be at least 10-15 mm from the surface to be sensed, so that the molecules do not adhere to it. As described above, a pulse of warm air is emitted from the plurality of outlet orifices  32  thereby dislodging or desorbing the particles on the surface to be sampled. The use of short intermittent output pulses of warm air is preferred, since a continuous flow of warm air onto the surface tends to dilute the sample. By way of example, the pulse duration may be from about a few milliseconds to about one second. The warm air pulse can be controlled either automatically or by the user. The multiple jets of air emerging from the plurality of outlets  32  disturb the molecules residing within the surface/air boundary, causing the molecules to be desorbed from the surface being monitored. The jets are turned off as the target molecules become airborne. These airborne molecules may take the form of chemical vapor or aerosols containing the target molecule. Suction is subsequently provided to draw the air located below the pulsed air sampler  10  into the inlets  38  and to the chemical sensor  52 . 
     In an alternative preferred embodiment, suction can continuously be applied while fluid, such as warm air, is periodically ejected from the outlets  32 . FIGS. 6 a  to  6   c  show an embodiment of the sampler  10  having a single inlet  38  and a concentric array of outlets  32  situated over a contaminated surface  92 . In FIG. 6 a , suction has been applied, but fluid has not yet been ejected from the outlets  32 . Particles  94  are shown on the contaminated surface  92 . A portion  96  of the particles  94  are airborne and are being drawn into the inlet  38  by suction. Lines  98  indicate the flow of the particles  94  toward the inlet  38  as a result of this suction. 
     In FIG. 6 b , suction is being applied while fluid is simultaneously being ejected from the array of outlets  32 . Arrows  100  indicate the flow of fluid ejected from the outlets  32 . FIG. 6 b  depicts the situation where the pulse of fluid dislodges or desorbs the particles  94  and yet does not dilute the sample. In particular, the particles  94  that are shown in FIG. 6 a  on the surface  92  are portrayed as airborne in FIG. 6 b  as a result of the pulse of fluid emanating from the plurality of outlets  32 . FIG. 6 b  additionally shows suction being applied and the particles  94  that are dislodged or desorbed from the surface  92  being drawn into the inlet  38 . As discussed above, the optimum duration of the pulse of fluid needs to be determined experimentally. 
     In FIG. 6 c , no fluid is being ejected from the outlets  32 , although suction is still being applied to draw the desorbed particles  92  into the inlet  38 . Thus, in this embodiment, suction is continuously applied while the flow of fluid through the outlets  32  is switched on and off. 
     In summary, the preferred embodiments entail utilization of a combination of fluid ejection, such as blowing, and suction to collect chemical samples. To sample a chemical adsorbed on the surface  92 , the apparatus relies on enhancing the number density of airborne molecules or particles  94  available for detection by the chemical sensor  52 . As described above, the sample is taken by providing a controlled velocity pulse of fluid to disturb the surface  94  and desorb the particles  92 , thereby suspending the particles in the region above the surface  94 . During this time, the sample acquisition pump  58  can apply suction to draw the chemically enriched sample to the chemical sensor  52 . This sample will be enriched with particles  94  desorbed from the surface  92 . The apparatus is also useful for breaking up and sampling chemicals that have become crusted over with time. 
     The present invention may be embodied in other specific forms without departing from the essential characteristics as described herein. The embodiments described above are to be considered in all respects as illustrative only and not restrictive in any manner. The scope of the invention is, therefore, indicated by the following claims rather than the foregoing description. Any and all changes which come within the meaning and range of equivalency of the claims are to be considered in their scope.