Patent Publication Number: US-6337213-B1

Title: Apparatus and method for collection and concentration of respirable particles into a small fluid volume

Description:
RELATED APPLICATION 
     This application relates to U.S. Provisional Application No. 60/113,067 filed Dec. 21, 1998, and claims priority thereof. 
    
    
     The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to sample collection of particles, particularly to the collection and concentration of small (1-10 μm) respirable particles for analysis, and more particularly to a low power, man-portable apparatus to capture and concentrate small respirable particles into a sub-milliliter volume of fluid for analysis. 
     Substantial development work has been done in recent years on portable Polymerase Chain Reaction (PCR) and cytometric detectors with the ultimate goal of producing man-portable biological agent detection systems. At the front end of these systems is a sample collector, which collects and concentrates the agents of interest into a liquid sample volume. The agents sampled are generally in the form of respirable particles with sizes in the range of 1-10 μm. 
     The sample collectors now used with mini PCRs and mini-flow cytometers are large (typically several liters in volume), requiring relatively large fans which draw too much power for portable use. These prior collectors concentrate their sample into milliliter (mL) size volumes, and are inefficient for low-concentration samples. Detectors and their associated sample volumes are becoming smaller (&lt;100 μL for PCR), so currently available mL sample volumes are much too large. A recent approach to a small sample collector is described in “A Portable High-Throughout Liquid-Absorption Air Sampler for Respirable Aerosol Particles”, A. Birenzvige et al, Aerosol Science and Technology, 29: 133-140 (1998). In this approach, a liquid extractant was injected into a conical cavity and a fan was then activated so as to draw air into the cavity, whereby the highly turbulent and swirling motion of the air caused the liquid to swirl up and wet the wall of the sampling tube such that particles in the air collected in the liquid, and after a few minutes the fan was shut off and the liquid drained for analysis. While the above described sample collector reduced the sample volume size, the collection apparatus is complex, expensive, and has low efficiency. 
     An ideal sample collector, for aerosol liquids, airborne pathogens, or other small particles of interest, should be low power, man-portable, inexpensive, and can concentrate the sample into a volume less than 100 μL. The present invention provides a solution to the above reference need for a small volume sample collector, and involves an apparatus and method to capture and concentrate small (1-10 μm) respirable particles into a sub-milliliter volume of fluid, and thus provides a sample collector wherein the fluid volume with concentrated particles is small and compatible with current mini-PCRs and mini-flow cytometers. The method of the invention is of a two step, or phase operation, inspiration and concentration, where the inspiration step basically collects particles on a wetted wall having small slits therein that act as capillary channels, and the concentration step basically involves spinning the wetted wall to drive the particles and fluid through the small slits and such impinge on a non-wetted surface and slide off into a container. The basic difference in this apparatus and methods from previous approaches is that the collection phase uses a fixed volume defined by the capillary slits, and the amount of particles collected in the concentration phase is determined by the sampling time. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a small volume sample collector. 
     A further object of the invention is to provide a small volume, low power, man-portable sample collector for small particles of interest. 
     A further object of the invention is to provide for the capture and concentration of 1-10 μm size respirable particles into a sub-milliliter volume of fluid. 
     Another object of the invention is to provide a two phase method of collecting and concentrating small particles into a small volume fluid sample. 
     Another object of the invention is to provide a sample collector having wetted capillary channels, and a concentrator that forces the collected particle by centrifugal force through the capillary channels into a container. 
     Another object of the invention is to provide a man-portable, small fluid volume, sample collector and concentrator for 1-10 μm size particles of interest that is compatible with mini-PCRs or mini-flow cytometers. 
     Another object of the invention is to provide a man-portable, small fluid volume particle collector and concentrator wherein the collection phase uses a fixed volume, defined by capillary slits, and the concentration phase is determined by the sampling time. 
     Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically the invention involves a low power, man-portable, small fluid volume sample collector for 1-10 μm size aerosol liquids, airborne pathogens, or other small particles of interest. The invention involves an apparatus and method to capture and concentrate small (1-10 μm) respirable particles into a sub-milliliter volume of fluid. The method is a two step or phase operation involving first collection and second concentration. The collection (inspiration) phase involves a cylindrical liner with small vertical slits that act as capillary channels connected to a fluid reservoir and provide a wetted surface to trap particles present in air being blown through the cylindrical liner. The concentration phase involves a transfer operation wherein the cylindrical liner is spun and the collected sample volume is forced through the capillary channels and against a non-wetted surface that deflects the sample to the bottom of the device where it is collected. The collection phase uses a fixed volume, defined by the capillary slits, and the concentration is determined by the sampling time. Thus, to get a higher concentration of particles into the sample, the inspiration step is longer, but the sample size stays the same. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated into and form a part of the disclosure, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 schematically illustrates an embodiment of an apparatus made in accordance with the present invention operating in the collection (inspiration) phase of the method of the invention. 
     FIG. 2 illustrates the apparatus of FIG. 1 in the concentration phase of the method of the invention. 
     FIG. 3 illustrates another embodiment of the apparatus of the present invention. 
     FIG. 4 illustrates an embodiment of the central cylinder of the apparatus of FIG. 1 showing a portion of the vertical capillary slits which are located around the circumference of the cylinder. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to an apparatus and method for the collection of respirable particles and concentration of said particles into a small fluid volume. The apparatus is of a man-portable type and designed to capture and concentrate small (1-10 μm) aerosol liquids, airborne pathogens, or other particles of interest into a sub-milliliter volume of fluid. The fluid with concentrated particles can then be analyzed by a portable flow cytometer or PCR DNA analysis system. Thus, the invention provides a solution for the need of a low power, man-portable sample collector which can concentrate the sample into a volume less than 100 μL. 
     The method of the invention involves a two step or phase process. In the first phase or step, referred to herein as “inspiration” or collection, a fan draws turbulent air into a central cylinder that is lined along its circumference with small vertical slits that act as capillary channels. The capillary channels are connected to a central fluid reservoir and provide a wetted surface to trap particles in the air. The reservoir will also replenish fluid as it evaporates. Fouling of the system by larger particles may be minimized by a pre-collection fractionator. The inspiration step or phase lasts for several minutes, the length of this phase determines the concentration of the particles into the sample. 
     In the second step or phase referred to herein as “transfer” or concentration, at least the central cylinder is spun, and the sample volume is forced out of the capillary channels by centrifugal force. Alternately, the cylinder may be pressurized, or a containing vessel evacuated to force the liquid out of the slits. The liquid impinges on a non-wetting cylindrical surface, located about the central cylinder, that deflects the liquid to the bottom of the device where it is finally collected. 
     Sample collectors, as currently known use a continuous stream of fluid to capture particles. This means that for a low concentration sample, a large volume must be collected to get enough of the target particles to be sensed. For the sample to be used in very small volume sensors (such as a mini-PCR), this means subdividing the collected sample into smaller portions, greatly reducing the effect of concentration. The basic difference in the current collector/concentrator with the prior known approaches, is that in this current device the collection or inspiration phase uses a fixed volume, defined by the capillary slits, and the concentration factor of the particles is determined by the sampling time. This means that to get a higher concentration of particles into the sample, the inspiration (collection) phase or step is longer, but the sample size stays the same. 
     The apparatus of the invention basically comprises a pair of cylinders with the central cylinder having small vertical slits in the wall, the lower end being in contact with a liquid reservoir, and being mounted within an outer cylinder having an unwetted tapering surface. A fan directs air into the central cylinder. The central cylinder is spun causing the liquid sample to be driven from the slits therein onto the inner surface of the outer cylinder which deflects the sample into a collection container. The outer cylinder may be held stationary or spun with the central cylinder. Depending on the construction and cooperation of the central cylinder and the reservoir, it may be necessary to lower the reservoir prior to spinning of the central cylinder. 
     Referring now to the drawings, FIGS. 1 and 2 schematically illustrate the overall operation or method of the invention which involves an inspiration (collection) cycle and a spin (concentration) cycle, respectively. In the inspiration cycle, fractionated air is drawn through a central cylinder having capillary slits and which are connected to a fluid reservoir, particles stick to fluid in the capillary slits (100 μm×100 μm×50 mm slit=0.5 μl fluid), and evaporating fluid in the slits is replenished from the reservoir. In the spin cycle, the air passing through the central cylinder is shut off and the cylinder, if necessary, is separated from the fluid reservoir, the central cylinder is then spun forcing fluid and captured particles through the capillary slits, and the captured particles and fluid move outward and contact an outer capture frustum, cylinder or cone and pass down the inner surface thereof to a collection point at the bottom of the cone, whereafter the collected particles can be analyzed, such as by a portable flow cytometer or PCR. The capillary size affects the spin speed requirement and sample volume, and a small intake fan can be utilized to draw the air through the central cylinder. 
     As shown in the schematic embodiment of FIGS. 1 and 2, the collection/concentration apparatus generally indicated at  10  comprises a central cylinder  11 , and an outer cylinder  12  having an upper end of each indicated at  13  and  14 , mounted to a member  15  having openings  16  (tub shown) and to which a spin motor  17  is mounted. A lower end  18  of central cylinder  11 , as shown in FIG. 1, extends into a fluid or liquid reservoir  19 , while a lower end  20  of outer cylinder  12  is located above a collector or container  21 . An optional deflector  22  is positioned within central cylinder  11  and a suction impeller or fan  23  driven a motor  24  is located beneath the central cylinder  11 . Central cylinder  11  is provided with vertically extending slits about the circumference thereof which function as capillary channels. FIG. 4 illustrates in greater detail an embodiment of the central cylinder  11 . 
     The method of the present invention involves a two phase or step operation. In the first phase or step, the collection or inspiration operation, the fan  23  draws turbulent air through the central cylinder  11 , as indicated by arrows  25 , cylinder  11  being lined along its circumference with small vertical slits that act as capillary channels. The capillary channel in central cylinder  11  is connected to the reservoir  19  and provide a wetted surface to trap particles  26  present in the air. The reservoir  19  will also replenish fluid as it evaporates. Note that deflector  22  functions to direct particles  26  as indicated by arrow  27  onto the inner (or capillary) surface  27  of central cylinder  11  where they are trapped in liquid in the capillary slits. 
     After the inspiration step, shown in FIG. 1, which may last for several minutes, there is a transfer or collection step, shown in FIG.  2 . As shown, the air flow, through central cylinder  10  is shut off and the reservoir  19  is lowered from the end  18  of central cylinder  11 , after which the cylinders  11  and  12  are spun by motor  17  as indicated by arrow  28 . In some cases, only central cylinder  11  needs to be spun. As the central cylinder  11  is spun, the sample volume (particles  26  trapped in liquid) is forced outwardly through the capillary channels in cylinder  11 , as indicated by arrows  29 , by centrifugal force. The particle containing liquid droplets  26 ′ impinges on a non-wetting inner wall surface  30  of outer cylinder  12  that deflects the droplets  26 ′ to the bottom and  20  of cylinder  12  where they are collected by collector on container  21  for analysis as described above. 
     Alternately, the central cylinder  11  may be pressurized or the outer cylinder  12  may be evacuated to force the particle container liquid droplets  26 ′ through the capillary slits in the wall of central cylinder  11 . 
     As pointed out above, the apparatus of FIGS. 1 and 2 provides a collection phase using a fixed volume, defined by the capillary slits, and concentration of the particles is determined by the sampling time. Thus, to get a higher concentration of particles into the sample, the inspiration step is longer, but the sample size stays the same. 
     FIG. 3 illustrates an embodiment of the apparatus which incorporates the components of the schematic embodiment of FIGS. 1 and 2. The apparatus, generally indicated at  40  includes a housing  41  on which is mounted a spin motor  42  and a fan or impeller motor  43 . Housing  41  is provided with slots or openings  44  adjacent motor  43  and within the housing  41  is mounted an impeller  45  connected to be driven by motor  43 . Spin motor  42  is operatively connected by a mechanism generally indicated at  42 ′ to a capillary slit containing cylinder  46  and a non-wettable cylinder or capture cone  47  located within housing  41 . Mounted to housing  41 , as by bolts  48 , only one shown, is an annular member  49  containing annular grooves  50  and  51 , with groove  50  functioning as a fluid reservoir to supply fluid to the slits in cylinder  46 , and groove  51  functioning as a collector or container for particle containing fluid droplets. The components of the apparatus of FIG. 3 function as described above with respect to FIGS. 1 and 2. 
     FIG. 4 illustrates an embodiment of the central cylinder  11  of FIGS. 1-2 and  46  of FIG. 3, and is generally indicated at  60 . Cylinder  60  includes annular outer rim sections  61  and  62  connected by an annular wall section  63  having vertically extending slits  64  around the circumference of wall section  63 , only partially illustrated. Rim section  62  may be provided with an outwardly flange or lip, not shown, extending illustration. By way of example, the cylinder  50  may have an overall length of 1.250 inch, an outer diameter or width of 1.00 inch, with rim section  61  having a length of 0.150 inch, and rim section  62  having a length of 0.100 inch, with the slits  64  having a width of 10 μm to enable passage of particle containing fluid droplets of 1-10 μm. An embodiment of a tested cylinder had a height of ½″, diameter of 2″, with slits of 75 μm. The slits may be 50-75 μm for example. 
     It has thus been shown that the present invention provides an apparatus and method for collecting and concentrating particles in the range of 1-10 microns, and can concentrate the sample into a volume less than 100 μL. Thus, this invention enables collection and concentration of small (1-10 μm) particles that can be analyzed by a portable flow cytometer or PCR DNA analysis system. The apparatus of the invention provides a low power, manportable collector/concentrator, thus providing a solution to the need for such an apparatus. 
     While particular embodiments and parameters have been illustrated and/or described to exemplify and teach the principles of the invention, such are not intended to be limiting. Modifications and changes may become apparent to those skilled in the art, and it is intended that the invention be limited only by the scope of the appended claims.