Abstract:
Embodiments of an apparatus for collecting biological aerosols from an air sample include a hollow tube adapted for pumping a liquid through an interior volume to an outer surface and a collection surface disposed on the outer surface and adapted for collecting the airborne particles from the surrounding air sample. Collection efficiency is enhanced by a charging mechanism that applies a charge to the airborne particles such that the airborne particles are deflected toward the collection surface. Embodiments of operation for the apparatus include the steps of providing the air sample, directing the air sample toward the hollow tube, and applying a charge to the airborne particles such that the airborne particles deposit on the collection surface/outer surface of said hollow tube.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 10/603,119, filed Jun. 24, 2003 (entitled “Method And Apparatus For Concentrated Airborne Particle Collection”), which is herein incorporated by reference in its entirety. This application also claims the priority of U.S. Provisional Patent Application No. 60/574,803, filed May 27, 2004 (entitled “Electrostatic Particle Collection System”), and to U.S. Provisional Patent Application No. 60/659,362, filed Mar. 7, 2005 (entitled “Spinning Disc Electrostatic Collection System”), both of which are herein incorporated by reference in their entireties. 
     
    
     REFERENCE TO GOVERNMENT FUNDING  
       [0002]     This invention was made with Government support under contract number DMD13-03-C-0041, awarded by Defense Advance Research Projects Agency and under contract number W911SR-04-C-0025 awarded by The U.S. Army Robert Morris Acquisition Center. The Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention generally relates to the sampling of air, and more particularly relates to the collection of pathogen and aerosol particles from air samples.  
       BACKGROUND OF THE INVENTION  
       [0004]     There is an increasing demand for air sampling systems for military, private or individual use that are capable of collecting aerosol and pathogen particles or spores. While current air sampling systems have been proven to function reliably, they are often quite large and thus not only consume a great deal of power, but also produce a lot of noise. These systems also tend to produce very large liquid samples, analyses of which can take several days or even weeks. Thus current air sampling systems are not practical for private or individual use, or for environments or circumstances in which analysis of an air sample must be performed quickly.  
         [0005]     Therefore, there is a need in the art for a compact, high-efficiency bio-aerosol collector that can produce a relatively small volume of liquid sample for expedited analysis.  
       SUMMARY OF THE INVENTION  
       [0006]     Embodiments of an apparatus for collecting biological aerosols from an air sample include a hollow tube adapted for pumping a liquid through an interior volume to an outer surface and a collection surface disposed on the outer surface and adapted for collecting the airborne particles from the surrounding air sample. Collection efficiency is enhanced by a charging mechanism that applies a charge to the airborne particles such that the airborne particles are deflected toward the collection surface. Embodiments of operation for the apparatus include the steps of providing the air sample, directing the air sample toward the hollow tube, and applying a charge to the airborne particles such that the airborne particles deposit on the collection surface/outer surface of said hollow tube. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0008]      FIG. 1  is a cut away view of one embodiment of an airborne particle collection apparatus according to the present invention;  
         [0009]      FIG. 2  is an exploded view of the airborne particle collection apparatus illustrated in  FIG. 1 ;  
         [0010]      FIG. 3  is a top view of the cyclone array illustrated in  FIG. 1 ;  
         [0011]      FIG. 4  is a top view of the vortex breaker section illustrated in  FIG. 1 ;  
         [0012]      FIG. 5  is an exploded view of the capture section illustrated in  FIG. 1 ;  
         [0013]      FIG. 6  is a schematic illustration of corona charging section adapted for use with the capture section illustrated in  FIGS. 1 and 5 ;  
         [0014]      FIG. 7  is a second embodiment of a capture section and corona charging section;  
         [0015]      FIG. 8  is a second embodiment of a collection apparatus according to the present invention;  
         [0016]      FIG. 9  is a schematic illustration of a third embodiment of a capture section according to the present invention;  
         [0017]      FIG. 10A  is a plan view of a third embodiment of a collection apparatus according to the present invention;  
         [0018]      FIG. 10B  is a cut away view of the collection apparatus illustrated in  FIG. 10A ;  
         [0019]      FIG. 11  is a cut away view of a fourth embodiment of a collection apparatus according to the present invention;  
         [0020]      FIG. 12  is a schematic diagram illustrating a fifth embodiment of a particle collection system for depositing aerosol particles into a liquid, according to the present invention;  
         [0021]      FIGS. 13A and 13B  are schematic diagrams illustrating a typical pore of the particle collection system of  FIG. 12 ;  
         [0022]      FIG. 14  is a schematic diagram illustrating a sixth embodiment of a particle collection system for depositing aerosol particles into a liquid, according to the present invention; and  
         [0023]      FIG. 15  is an isometric view illustrating a seventh embodiment of a particle collection system for depositing aerosol particles into a liquid, according to the present invention. 
     
    
       [0024]     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
       DETAILED DESCRIPTION  
       [0025]     Embodiments of the invention generally provide a compact, lightweight, low power and low noise device capable of collecting respirable airborne particles and focusing them into a small liquid volume. In one embodiment, the device is capable of achieving a particle concentration in the range of approximately 1 to 10 microns, and can achieve sampling rates of up to approximately 1000 liters per minute (lpm).  
         [0026]      FIG. 1  is a cut away view of one embodiment of a particle collection apparatus  100  according to the present invention. In the embodiment illustrated, the apparatus  100  is constructed in a substantially cylindrical shape; however, those skilled in the art will appreciate that embodiments of the invention may be configured in any number of alternate forms and shapes without departing from the scope of the invention. The apparatus  100  comprises a housing  102 , within which is contained an air intake assembly  104 , a sample separation section  106 , and a particle capture zone  108 .  
         [0027]     The air intake assembly  104  is adapted to draw air flow into the collection apparatus  100  and comprises a motor  110 , first and second fans  112 A,  112 B, and an air duct  114 . The first fan  112 A is disposed proximate a first end  101  of the collection apparatus  100  and is coupled to the fan motor  110 . The optional second fan  112 B is positioned inward of the first fan  112 A along a longitudinal axis of the apparatus  100 , and in one embodiment, the second fan  112 B is smaller than the first fan  112 A. The air duct  114  begins at an aperture  116  in the second end  103  of the apparatus  100  and extends at least partially therethrough to provide an inlet path for the air that is drawn in by the fans  112 A,  112 B when in operation. In one embodiment, the duct  114  is disposed through the center  105  of the housing  102 . Optionally, the air intake assembly  104  may further comprise an impactor  150  positioned between the duct  114  and the fans  112 A,  112 B and adapted to act as a pre-filter. That is, the impactor  150  includes a plurality of tubes or channels  152  for filtering large particles out of the primary flow as it is drawn into the apparatus  100 .  
         [0028]     The sample separation section  106  comprises a substantially circular array of cyclones  118  positioned radially outward of the center  105  of the apparatus  100  (i.e., in the embodiment illustrated in  FIG. 1 , radially outward of the air duct  114 ) and a vortex breaker  120  (shown in  FIGS. 2 and 4 ).  FIG. 3  is a top view of the cyclone array illustrated in  FIG. 1 . Although  FIG. 3  depicts an array of eight cyclones  118 , those skilled in the art will appreciate that a greater or lesser number of cyclones  118  may also be used to advantage. Referring simultaneously to  FIGS. 1 and 3 , each cyclone  118  in the array is connected to the air duct  114  by a tangential inlet  124 . The inlets  124  are adapted to carry incoming air from the duct  114  to the sample separation section  106 . Each cyclone  118  is adapted to separate airborne particles from the primary air flow. A vortex finder  154  positioned proximate to the first ends  107  of the cyclones  118  comprises a plurality of short channels that project into the cyclones  118  to establish first exits ports  122  for the primary flow. That is, a first exit port  122  at the first end  107  of each cyclone  118  is adapted to collimate and guide the primary flow out of the cyclones  118 , so that the primary flow may be discharged from the separation section  106 . A second exit port  126  located proximate a second end  109  of each cyclone  118  carries the separated particle flow to the vortex breaker  120 .  
         [0029]     Referring to  FIGS. 1, 2  and  4 , the vortex breaker  120  is located proximate the second ends  109  of the cyclones  118  and in one embodiment comprises a series of chambers  128 . One chamber  128  is positioned adjacent the second end  109  of each cyclone  118  and has an interior volume adapted to concentrate the particle flow carried from the cyclones  118  into a relatively denser, low velocity flow. Alternatively, one chamber (not shown) may be substantially annular in shape and be adapted to receive aerosol flow from all cyclones  118 . A tangential slot  136  in a wall  138  of the vortex chamber  128  allows the aerosol flow to be drawn out of the chamber  128  and toward the capture section  108 .  
         [0030]     The vortex breaker  120  is separated from the capture section  108  by a controllable air/fluid boundary  130 . The air/fluid boundary  130  is positioned adjacent the exterior of the vortex chambers  128 , and in one embodiment the mechanism comprises a liquid plate  132  having a high porosity hydrophobic membrane  134  disposed thereon. The hydrophobic membrane  134  is adapted to establish a liquid seal or boundary between the vortex chamber  128 , which is adapted to contain air or particle flow (i.e., a gaseous medium), and the capture section  108 , which is adapted to contain a liquid as described further herein. In one embodiment, the membrane  134  comprises a nylon mesh that is thermally imbedded over at least a portion of the capture section  108 . The nylon mesh is optionally treated with polytetrafluoroethylene (PTFE) or an equivalent substance to increase its hydrophobic properties.  
         [0031]     Referring to  FIGS. 1 and 5 , the capture section  108  comprises at least one microfluidic, or nanofluidic, channel  140  within which a small volume of liquid is contained for transporting the aerosol or other particles that have been focused therein. In one embodiment, a nylon mesh such as that described above is thermally embedded over the at least one channel  140 . The capture section  108  may additionally comprise a liquid collection chamber  142 , where the liquid flow (including the particles focused therein) is collected, or may alternatively be coupled to a means for transporting the flow to a separate analysis or collection device (not shown).  
         [0032]     The air/fluid boundary  130  described above optionally includes an electrostatic focusing mechanism such as a corona charging section  500  for electrostatically manipulating the particles to enhance the focusing of the particles into the liquid in the at least one channel  140  of the capture section  108 . One embodiment of a corona charging section  500  is illustrated in a schematic view in  FIG. 6 . The corona charging section  500  comprises a corona array  602  and a ground electrode  604 . The corona array  602  comprises a plurality of corona tips  606  positioned proximate to the at least one channel  140  of the capture zone  108 . The electrode  604  is positioned a distance away from the array, and in one embodiment is positioned across the channel  140  from the array  602 . An electrostatic field  608  is thereby generated between the array  602  and the electrode  604 . The electrostatic field  608  charges the particles in the liquid flow and drives them toward the middle of the channel  140 . The corona charging section  500  is thereby adapted to enhance the manipulation of the particles into the liquid by urging the particles into the center of the liquid flow for quicker and more efficient transport. The electrostatic field generated by the corona charging section  500  also ensures a substantially uniformly charged particle stream.  
         [0033]      FIG. 7  is a schematic view of a second embodiment of a collection section  700  including a corona charging section  702 . In this embodiment, the collection section  700  includes a corona array  704  and electrode  706 , a translating particle-collecting material such as a tape  708 , a reservoir  710  and a particle removal device  712 . In one embodiment, the collection tape  708  has a first surface  701  and a second surface  703 , and is adapted to translate around several bearings  714  (e.g., three or more) in a closed loop. In one embodiment, the closed loop resembles a triangle. The corona array  704  and electrode  706  generate an electrostatic field  716  that drives particles through an aperture  722  in the channel  740  and onto the adjacent first surface  701  of the collection tape  708 . The reservoir  710  is positioned adjacent the lower bearing or bearings  714  and is adapted to wick a thin layer  718  of fluid onto a first surface  701  of the tape  708  as it translates past or through the reservoir  710 . The liquid layer  718  enhances collection of aerosol particles on the tape surface  701 . The particle removal device  712  is positioned to remove particles from tape  708  after particles have been deposited, but before the tape  708  translates past the reservoir  710 . The collection device  712  may be a squeegee, a blade, a vacuum or any other device that is capable of removing the liquid layer  718  from the tape  708  so that the liquid and particles therein are transferred to a collection chamber  720 . Optionally, the first surface  701  of the tape  708  is treated to become hydrophilic, and the second surface  703  is treated to become hydrophobic. The area of the collection tape  708  may be very small to enable higher concentration of particles.  
         [0034]      FIG. 9  is a schematic illustration of a third embodiment of a capture section  900  according to the present invention. The capture section  900  comprises a channel  902 , a hydrophobic membrane  904 , an electrostatic focusing electrode  906  and an electrophoretic electrode  908 . The hydrophobic membrane  904  is substantially similar to that described previously herein, but is additionally made to be conductive and is embedded over a portion of the channel  902  adjacent the vortex breaker section (not shown). The electrophoretic electrode  908  is positioned across the channel  902  from the hydrophobic membrane  904 . The electrostatic focusing electrode  906  is positioned outside of the channel  902 , proximate the side on which the electrophoretic electrode  908  is positioned. A differential voltage V is applied across the channel  902  to create an electrophoretic pumping cell within the channel  902 , between the hydrophobic membrane  904  and the electrophoretic electrode  908 . An electrostatic effect created by the electrostatic focusing electrode  906  enhances the particle manipulation through the hydrophobic membrane  904  and into the liquid flow. The electrophoretic effect created by the pumping cell charges the particles in the liquid flow and drives them toward the center of the liquid flow for quicker and more efficient transport. In the event that there is interference between the electrostatic and electrophoretic effects, the two competing effects can be operated in a cyclic manner at an established optimum frequency that allows efficient electrostatic transport in the particle flow and also allows electrophoretic transport of the particles in the liquid flow.  
         [0035]      FIG. 8  illustrates another embodiment of the present invention, in which a collection apparatus  800  also includes an electrostatic precipitator section  802 . In one embodiment, the electrostatic precipitator section  802  comprises a plurality of precipitator plates  804  and at least one corona electrode  806 , both located proximate the entries  801  (i.e., the first ends) of the cyclones  810 . The electrostatic precipitator section  802  is adapted to attract small charged particles (i.e., charged within the cyclones  810  by the at least one corona electrode  806 ) that escape from the cyclones  810  along with the exiting primary flow rather than pass to the capture section  808 .  
         [0036]     Referring back to  FIG. 1 , in operation, the intake assembly  104  is activated to draw air into the apparatus  100  through the air duct  114 . The air passes through the duct  114  to the tangential inlets  124 , which carry the air flow to the the cyclones  118 .  
         [0037]     The cyclones  118  separate particles from the primary air flow. As the flow field is rapidly revolved within the cyclone  118 , centrifugal force drives the aerosol particles to the walls of the cyclone  118 , where the particles may be tribo-charged by rubbing against the wall surface. As the flow continues to spiral through the cyclone  118  to the second end  109 , additional particles are separated from the flow. The flow of aerosol particles exits the cyclones  118  through the second ends  109  and enters the chamber  128  of the vortex breaker  120 , where it is concentrated into a denser, low velocity flow.  
         [0038]     The primary flow reverses direction and flows back through the centers of the cyclones  118 , where it passes out of the first ends  107  of the cyclones  118  and is carried past the fans  112 A,  112 B and through exhaust ports  144  in the first end  101  of the housing  102 , to exit the collection apparatus  100 . If a precipitator section such as that illustrated in  FIG. 8  is incorporated, small charged particles that are not separated out of the primary flow by the cyclones  118  will be attracted to precipitator plates as the primary flow passes through the precipitator plates on the way to the exhaust ports  144 . The use of an array of small cyclones  118  (rather than, for example, a single large cyclone) to separate the aerosol and primary flows provides improved separation efficiency at a low pressure drop, thereby enabling the construction of a quieter and more compact apparatus  100  that consumes less power. For example, in one embodiment, the entire apparatus  100  is only six inches in diameter.  
         [0039]     The densified aerosol flow is drawn through the tangential slots  136  in the walls  138  of the vortex breaker chambers  128 . As the particles flow outward from the chambers  128 , the particles are electrostatically focused into an array of capillaries formed by the hydrophobic mesh membrane  134 . The particles are drawn through the capillaries in the mesh  134  and into the liquid of the capture section  108 , where a continuous liquid flow through the microfluidic channels  140  transports the captured particles into the collection chamber  142 . Alternatively, the capture section  108  may be coupled to a port or line (not shown) that is adapted to transport the fluid out of the collection apparatus  100  and into, for example, a separate collection container or an analysis device.  
         [0040]     As the flow of particles arrives at the air/liquid interface (i.e., the hydrophobic membrane  134 ), the particles reside in a boundary layer where the liquid flow velocity approaches zero. Particle transport in the liquid is enhanced by positioning the corona electrode ( 604  in  FIG. 6 ) adjacent the collection chamber  142 , but isolated from the collection liquid. In this manner, the electrostatic field  608  continues to act upon the particles after they have entered the collection liquid in the microfluidic channels  140  of the capture section  108 , which urges the particles into the higher velocity flow in the central portions of the channels  140  so that the particles can be rapidly carried away. This positioning of the electrode  604  also alleviates the need to bias the liquid in the channels  140  to a high voltage to attract the aerosol particles. Other means for enhancing particle transport in the liquid include, but are not limited to, electro-kinetic pumping, pulsed pumping, ultrasonic techniques and incremental pumping.  
         [0041]     Over the course of operation, the hydrophobic mesh membrane  134  may become clogged with large particles, dust or debris. In such an instance, the water in the channels  140  may be pressurized to a level exceeding the retention pressure of the mesh membrane  134 . Consequently, the boundary established by the membrane  134  will be broken and water will flow out through the mesh  134 , carrying dust and debris away with the flow. The water pressure is subsequently reduced, allowing the mesh membrane  134  to re-establish the liquid seal. Thus the hydrophobic membrane  134  may be easily cleaned without having to disassemble the collection apparatus  100 .  
         [0042]     Although a collection apparatus according to the present invention has been heretofore described as a device having a substantially cylindrical configuration, those skilled in the art will appreciate that a collection apparatus may be constructed in alternate shapes and configurations without departing from the scope of the invention. For example,  FIGS. 10A-10B  illustrate an embodiment of a collection apparatus  1000  having a substantially box-shaped housing  1002 .  
         [0043]     The collection apparatus  1000  is constructed as a box having an air inlet side  1004  for the intake of air samples and an air outlet side  1006  opposite the inlet side  1004  for the expulsion of separated primary flow air. The inlet and outlet sides  1004 ,  1006  have a plurality of apertures  1010  for the intake or expulsion of air. In addition, at least one capture liquid outlet  1008  may be coupled to the housing  1002  to transport liquid and particles captured therein to a collection or analysis device (not shown).  
         [0044]     As illustrated in  FIG. 10B , the collection apparatus  1000  comprises an air intake section  1018 , a separation section  1012 , a vortex breaker section  1014  and a capture section  1016 . The air intake section comprises a plurality of channels  1020  coupled to the apertures  1010  formed in the air inlet side  1004  of the housing  1002 . Each channel  1020  has a tangential inlet  1022  that is coupled to the separation section  1012  for transporting air samples to the separation section  1012 .  
         [0045]     As in the previous embodiments, the separation section  1012  comprises at least one cyclone  1024  coupled to the inlets  1022  for receiving air samples and separating airborne particles in the samples from the primary flow. The at least one cyclone expels clean primary flow through a first exit port  1040 , and expels separated particles through a second exit port  1026 .  
         [0046]     The second exit port  1026  transports the separated particles to a chamber  1028  of the vortex breaker section  1014 , where the particle flow is concentrated for passage to the capture section  1016 .  
         [0047]     The capture section  1016  is coupled to the vortex breaker section  1014 . Concentrated particle flow is passed through an exit port  1030  in the vortex chamber  1028  to a capture section channel  1032 . The channel  1032  contains a liquid for transporting the particles to a collection or analysis device (i.e., via the capture liquid outlet  1008  illustrated in  FIG. 10A ). Electrostatic focusing mechanisms such as the hydrophobic mesh and/or corona biasing assembly discussed herein may be used to enhance particle manipulation in the channel  1032 .  
         [0048]     A fourth embodiment of a collection apparatus according to the present invention is illustrated in  FIG. 11 . The collection apparatus  1100  is substantially similar to the apparatus  800  illustrated in  FIG. 8 , but instead of an electrostatic precipitator section, the apparatus  1100  includes a condensation section  1102 . In one embodiment, the condensation section  1102  comprises an evacuable volume  1104  that is adapted to cool and condense small airborne particles that escape from the cyclones  1106  along with the exiting primary flow. The condensation section  1102  may be adapted for coupling to an analysis or extraction device (not shown), for example by a port or connection that transports the condensed particles out of the apparatus  1100 . Optionally, the apparatus  1100 , or any of the alternate embodiments described herein, may include a detector section  1108  located adjacent to the capture section  1110  for retaining a device (not shown) to analyze the particles collected and condensed within the capture section  1110 . The analysis device may be formed integral with the apparatus  1100 , or the detector section  1108  may be manufactured for interface with a number of separate compatible analysis devices.  
         [0049]      FIG. 14  is a schematic diagram illustrating a fifth embodiment of a particle collection system  1400  for depositing aerosol particles into a liquid, according to the present invention. The particle collection system  1400  may be implemented, for example, in place of the previously disclosed mechanisms (e.g., the sample separation and particle capture zones) for collecting and concentrating airborne particles into a liquid medium. However, the particle collection system  1400  may also be implemented in other forms of collection apparatuses as well (e.g., such as those without an inertial separator front end).  
         [0050]     The particle collection system  1400  comprises a hollow tube  1402  coaxially disposed within the air duct  1404 , which contains a flow of aerosol particles. The hollow tube  1402  is open at both a first end  1406  and an opposite second end  1408 . In one embodiment, the hollow tube  1402  is comprised of at least one of: a sintered metal, a sintered glass and a sintered polymer.  
         [0051]     In one embodiment of operation, a liquid is received near the first end  1406  of the hollow tube  1402  (e.g., via at least one inlet  1412  that is coupled to a reservoir or other liquid source, not shown) and pumped through the interior volume of the hollow tube  1402  toward the open second end  1408  of the hollow tube  1402 . As the liquid approaches the open second end  1408  the hollow tube  1402 , the liquid exits the hollow tube  1402  and spills over the second end  1408  of the hollow tube  1402  and along the outer surface of the hollow tube  1402 . Thus, as evaporation occurs at the outer surface of the hollow tube  1402 , more liquid is automatically delivered to the surface of the hollow tube  1402 . Airborne particles from an incoming air sample within the air duct  1404  deposit in the liquid on the outer surface of the hollow tube  1402 . The liquid, including the deposited particles, flows along the outer surface of the hollow tube  1402  to a particle collection or analysis device (e.g., via an outlet  1414  positioned near the outer surface of the hollow tube  1402 ).  
         [0052]     In another embodiment of operation, airborne particles from an incoming air sample within the air duct  1404  deposit on a dry outer surface of the hollow tube  1402 . The deposited particles are then “rinsed” from the outer surface of the hollow tube  1402  by pumping liquid through the hollow tube  1402  as described above.  
         [0053]     Further embodiments of the particle collection system  1400  may be enhanced by providing a charging section comprising a first electrode  1418  at the surface of the hollow tube and at least one array  1420  of second electrodes (i.e., corona tips) proximate to the region in which the incoming air sample, including the particle flow, is received. In one embodiment, the first electrode  1418  comprises a thin (e.g., approximately 0.0005 to 0.002 inches thick) layer of conductive material (e.g., vapor deposited for sputtered metals such as tin, titanium or the like) disposed on the outer surface of the hollow tube  1402  (e.g., such that the outer surface of the hollow tube  1402  functions as a ground electrode). In another embodiment, the material that comprises the hollow tube  1402  may be a conductive or semiconductive material such as a sintered metal (e.g., stainless steel, titanium or the like) or a mixture of sintered polymer and sintered metal (e.g., a conductive plastic), such that the hollow tube  1402  itself functions as the first electrode  1418  (i.e., without a coating). In one embodiment, the array  1420  of corona tips is radially disposed, e.g., around an inner perimeter of the air duct  1404 .  
         [0054]     The array  1420  of corona tips, in cooperation with the first electrode  1418 , generates an electrostatic field therebetween. When the array  1420  of corona tips is biased to a voltage that is sufficient to create a corona discharge, particles passing through the electrostatic field acquire charges due to field charging (i.e., in accordance with the Pauthenier equation). The trajectories of the charged particles are then influenced such that each particle has a high probability of depositing within the liquid on the outer surface of the hollow tube  1402  (e.g., the particles are deflected toward the first electrode  1418 ). In one embodiment, charging incoming particles achieves a collection efficiency of approximately ninety-nine percent or greater for particles of approximately 2 μm in size, where collection efficiency is defined as the number of particles collected on the first electrode  1418  divided by the total number of incoming particles (e.g., as measured at the inlet of the air duct  1404 ). In further embodiments, additional arrays of corona tips may be implemented along the length of the air duct  1404 , near points further along the length of the hollow tube  1402  (e.g., closer to the first end  1406  of the hollow tube  1402 ), to enhance deflection of particles along substantially the entire length of the hollow tube  1402 .  
         [0055]     The particle collection system  1400  thus combines a charging mechanism (e.g., the array  1420  of corona tips operating in conjunction with the first electrode  1418 ) with a collection mechanism (e.g., the hollow tube  1402 ) in order to achieve more efficient collection of airborne particles. Particles are thereby charged and collected in a single stage process (e.g., as opposed standard methods of charging particles in a first stage and depositing the particles onto a collection surface in a second stage). The implementation of the single-stage charging and collection mechanism substantially increases the quantity of airborne particles that are captured on the outer surface of the hollow tube  1402 , thus providing better sample concentration for analysis than is currently achieved by existing collection devices.  
         [0056]      FIG. 12  is a schematic diagram illustrating a sixth embodiment of a particle collection system  1200  for depositing aerosol particles into a liquid, according to the present invention. Like the particle collection system  1400 , the particle collection system  1200  may be implemented, for example, in place of the previously disclosed mechanisms (e.g., the sample separation and particle capture zones) for collecting and concentrating airborne particles into a liquid medium.  
         [0057]     The particle collection system  1200  is substantially similar to the particle collection system  1400  and comprises a hollow tube  1202  coaxially disposed within the air duct  1204  of a particle collection apparatus. The hollow tube  1202  is open at a first end  1206  and closed at an opposite second end  1208 . The hollow tube  1202  is comprised of a porous material that is capable of wicking liquid onto its surface. To that end, the hollow tube  1202  comprises a plurality of pores  1210 . For example, in one embodiment, the hollow tube  1202  is comprised of at least one of a sintered glass and a sintered polymer.  
         [0058]     In one embodiment of operation, a liquid is received near the open first end  1206  of the hollow tube  1202  (e.g., via at least one inlet  1212  that is coupled to a reservoir or other liquid source, not shown) and pumped through the interior volume of the hollow tube  1202  toward the closed second end  1208  of the hollow tube  1202 . As the liquid is pumped through the hollow tube  1202 , the liquid is drawn through the pores  1210  of the hollow tube  1202  and onto the outer surface of the hollow tube  1202  by capillary action. Thus, as evaporation occurs at the outer surface of the hollow tube  1202 , more liquid is automatically delivered to the surface of the hollow tube  1202 . Airborne particles from an incoming air sample within the air duct  1204  deposit in the liquid on the outer surface of the hollow tube  1202 . The liquid, including the deposited particles, flows along the outer surface of the hollow tube  1202  to a particle collection or analysis device (e.g., via an outlet  1214  positioned near the outer surface of the hollow tube  1202 ).  
         [0059]     In another embodiment of operation, airborne particles from an incoming air sample within the air duct  1204  deposit on a dry outer surface of the hollow tube  1202 . The deposited particles are then “rinsed” from the outer surface of the hollow tube  1202  by pumping liquid through the hollow tube  1202  and out through the pores  1210  to the outlet  1214  as described above.  
         [0060]     Similarly to the particle collection system  1400 , further embodiments of the particle collection system  1200  may be enhanced by generating an electrostatic field that deflects incoming particles into the liquid on the outer surface of the hollow tube  1202 . In one embodiment, this electrostatic field is generated by providing at least one array  1220  of corona tips proximate to the region in which the incoming air sample, including the particle flow, is received. The array  1220  of corona tips works in conjunction with a first electrode  1218  deposited on the outer surface of the hollow tube  1202  to deflect incoming particles into the liquid on the outer surface of the hollow tube  1202 , as described above with reference to  FIG. 14 . In one embodiment, the array  1220  of corona tips is radially disposed, e.g., around an inner perimeter of the air duct  1204 . In further embodiments, additional arrays of corona tips may be implemented near points further along the length of the hollow tube  1202  (e.g., closer to the first end  1206  of the hollow tube  1202 ) to enhance deflection of particles along substantially the entire length of the hollow tube  1202 . In another embodiment, the porous material that comprises the hollow tube  1202  may be a conductive or semiconductive material such as a sintered metal (e.g., stainless steel, titanium or the like) or a mixture of sintered polymer and sintered metal (e.g., a conductive plastic), such that the hollow tube  1202  itself functions as the first electrode  1218  (i.e., without a coating).  
         [0061]     In further embodiments, the hollow tube  1202  further comprises an electrokinetic pump for enhancing the flow of the liquid through the pores  1210  of the hollow tube  1202 . The electrokinetic pump comprises a third electrode  1216  that is disposed coaxially within the hollow tube  1202 , such that the third electrode is spaced apart from the first electrode by a dielectric (e.g., the hollow tube  1202  itself, which in this embodiment may be formed, for example, of a sintered glass or sintered polymer upon which the first electrode  1218  is deposited as a coating). The third electrode  1216  and the first electrode  1218  are of different potentials such that when an electric field between the third electrode  1216  and the first electrode  1218  is biased, an electrokinetically induced pressure deflects the liquid meniscus outwardly at the pores  1210  of the hollow tube  1202 .  
         [0062]      FIGS. 13A and 13B  are schematic diagrams illustrating a typical pore  1210  of the hollow tube  1202 . Specifically,  FIG. 13A  illustrates a pore  1210  absent the effects of electrokinetic pumping, while  FIG. 13B  illustrates the effects of electrokinetic pumping, as described above, applied to the same pore  1210 . As illustrated, the effects of the electrokinetic pumping urge the meniscus  1300 B of the liquid outwardly through the pore  1210 , so that the outer surface of the hollow tube  1202  is substantially coated with at least a thin layer of liquid. In some embodiments, this may enhance the ability of the particle collection system  1200  to collect particles from an incoming air sample, as compared with an embodiment in which electrokinetic pumping is not applied (e.g., see the meniscus  1300 A).  
         [0063]     The location of the electrokinetic pump near the particle collection surface (e.g., the outer surface of the hollow tube  1202 ) provides several advantages. For example, such an arrangement facilitates liquid distribution in a multi-unit configuration. Additionally, the electrokinetic pump utilizes space that would normally remain unoccupied, and therefore requires no additional volume to achieve enhanced particle collection capabilities. Moreover, the configuration of the particle collection system  1200  including the electrokinetic pump is substantially orientation-independent and requires a minimal volume of liquid for collecting particles.  
         [0064]      FIG. 15  is an isometric view illustrating a seventh embodiment of a particle collection system  1500  for depositing aerosol particles into a liquid, according to the present invention. Like the particle collection systems  1200  and  1400 , the particle collection system  1500  may be implemented, for example, in place of the previously disclosed mechanisms (e.g., the sample separation and particle capture zones) for collecting and concentrating airborne particles into a liquid medium.  
         [0065]     The particle collection system  1500  is similar in some ways to the particle collection systems  1200  and  1400  and comprises a hollow tube  1502  adapted to be coaxially disposed within the air duct of a particle collection apparatus. The hollow tube  1502  is open at both a first end  1506  and an opposite second end  1508 .  
         [0066]     In addition, the particle collection system  1500  comprises a rotatable disk  1504  positioned at the second end  1508  of the hollow tube  1502 . The rotatable disk  1504  is positioned such that a rotational axis of the rotatable disk  1504  is orientated substantially coaxially with the longitudinal axis of the hollow tube  1502 ; thus, the rotatable disk  1504  is rotatable about the longitudinal axis of the hollow tube  1502 .  
         [0067]     The rotatable disk  1504  comprises a flat surface  1510  having a port  1514  disposed substantially in the center thereof and a first radius r 1 . The first radius r, is smaller than the radius r 2  of the entire rotatable disk  1504 , such that a trench  1512  is formed between the flat surface  1510  of the rotatable disk  1504  and the outer circumference of the rotatable disk  1504 .  
         [0068]     In one embodiment of operation, a liquid is received near the first end  1506  of the hollow tube  1502  (e.g., via at least one inlet that is coupled to a reservoir or other liquid source, not shown) and pumped through the interior volume of the hollow tube  1502  toward the second end  1508  of the hollow tube  1502 . As the liquid is approaches the second end  1508  the hollow tube  1502 , the liquid is exits the hollow tube  1502  through the port  1514  of the rotatable disk and spills over onto the flat surface  1510  of the rotatable disk  1504 . Thus, as evaporation occurs at the flat surface  1510  of the rotatable disk  1504 , more liquid is automatically delivered to the flat surface  1510  of the rotatable disk  1504 . Airborne particles from an incoming air sample within the air duct deposit in the liquid on the flat surface  1510  of the rotatable disk  1504 . As the rotatable disk  1504  rotates, the rotational motion causes the liquid, including the deposited particles, to be drawn away from the port  1514  and centrifugally pumped toward the trench  1512 , where the liquid collects. The collected liquid, including the deposited particles, may then be siphoned, pumped or otherwise transported to a particle collection or analysis device (e.g., via an outlet, not shown, positioned near the trench  1512  or the outer surface of the hollow tube  1502 ).  
         [0069]     In another embodiment of operation, airborne particles from an incoming air sample within the air duct deposit on a dry flat surface  1510  of the rotatable disk  1504 . The deposited particles are then “rinsed” from the flat surface  1510  of the rotatable disk  1504  by pumping liquid through the hollow tube  1502  and rotating the rotatable disk  1504  as described above.  
         [0070]     Similarly to the particle collection systems  1200  and  1400 , further embodiments of the particle collection system  1500  may be enhanced by providing at least one array of corona tips proximate to the region in which the incoming air sample, including the particle flow, is received. The array of corona tips works in conjunction with a first electrode deposited on the outer surface of the hollow tube  1502  to deflect incoming particles into the liquid on the flat surface  1510  of the rotatable disk  1504 , as described above with reference to  FIG. 14 . In one embodiment, the array of corona tips is radially disposed, e.g., around an inner perimeter of the air duct.  
         [0071]     In one embodiment, the rotatable disk  1504  is rotated at a high enough speed to render gravitational forces substantially insignificant. In such an embodiment, the particle collection system  1500  affords a greater degree of orientation capability for a particle collection device incorporating the particle collection system  1500 , since gravity is not depended on to transport the liquid in which the particles are deposited.  
         [0072]     In further embodiments, the rotatable disk  1504  may be substituted with a different mechanism such as a traveling tape or wire collections means that travels in and out of the air duct in a direction of motion that is substantially perpendicular to the airflow through the duct.  
         [0073]     Thus, the present invention represents a significant advancement in the field of bio-aerosol collection. An apparatus is provided that achieves highly efficient collection of airborne particles into a small volume of liquid, which may be easily analyzed for the detection of pathogen, aerosol or other undesirable particles. The efficiency of the apparatus is belied by the compact dimensions of the apparatus, which enable the apparatus to be easily incorporated in portable particle collection devices. Moreover, the orientation-independent configuration of the apparatus makes the apparatus suitable for use in a variety of environments and devices.  
         [0074]     While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.