Abstract:
A biological aerosol detector is provided. The biological aerosol detector uses a semiconductor optical source with an ultraviolet emission band to excite biological molecules in an aerosol sample. Filtering optics are configured to attenuate radiation from a secondary emission band of the optical source to prevent false signals due to scattering of secondary emission band radiation from non-biological molecules. An intake/exhaust manifold that includes an intake pipe that fits within a concentric exhaust pipe is also provided. The intake/exhaust manifold planarizes the flow of the sampled aerosol to maximize the time of irradiation. An electrostatic sampling grid is also provided to selectively draw biological molecules having a net charge into the optical chamber.

Description:
RELATED APPLICATIONS 
   Applicant claims the benefit under 35 U.S.C. § 119(e) of provisional application Ser. No. 60/659,119 filed Mar. 7, 2005. 

   GOVERNMENTAL INTEREST 
   The invention described herein may be manufactured, used and licensed by or for the U.S. Government. 
   TECHNICAL FIELD 
   This document relates to detection of aerosols by illumination with ultraviolet radiation. 
   BACKGROUND 
   The detection of aerosols within fluid samples can be accomplished by optical methods. Such methods are useful in detecting potentially harmful aerosols, such as biological aerosols that may be present after a biological agent attack or industrial accident. It is well known that biological molecules fluoresce when excited by ultraviolet (UV) radiation. As a result, biological molecules in an aerosol sample can be optically detected by irradiating the sample with ultraviolet radiation, and observing the fluorescence response. Since differing excitation wavelengths may be used to detect different classes of biological molecules, the excitation wavelength can be chosen to detect specific classes of biological molecules such as proteins, flavinoids, and metabolite products. 
   A biological aerosol detector of the type described above is described in more detail in U.S. patent application Ser. No. 10/720,877, now allowed, which is incorporated herein by reference. The detector described in patent application Ser. No. 10/720,877, includes a light source 4, which can be a UV laser light source or an LED light source. Recently, UV light sources in the form of semiconductor ultraviolet optical sources or SUVOS have become available. These light sources typically have both a primary emission band with a center wavelength in the ultraviolet region (i.e., a primary emission band that is capable of eliciting a fluorescence response from a biological aerosol), and a secondary emission tail at longer wavelengths that overlaps and interferes with the fluorescence response. When SUVOS type light sources are used in the biological aerosol detector described in patent application Ser. No. 10/720,877, light from the source&#39;s secondary emission band can be scattered by particles in the aerosol detector&#39;s optical cavity, thereby creating a positive response signal in the aerosol detector regardless of whether the scattering particle was a biological molecule or not. To reduce the occurrence of these types of false positive signals, the radiation from the SUVOS&#39;s secondary emission band must be attenuated to prevent it from entering the aerosol detector&#39;s optical cavity. 
   SUMMARY 
   A biological aerosol detector is provided. In one aspect, the biological aerosol detector includes an optical excitation source having a primary emission band and a secondary emission band. The excitation source can be a semiconductor ultraviolet optical source with a primary emission band in the ultraviolet spectrum, and a secondary emission band at longer wavelengths. The primary emission band is configured to excite biological molecules in an aerosol sample in the detector&#39;s optical cavity. Filtering optics are included to attenuate radiation that is emitted in the secondary emission band which, when scattered, can mimic the fluorescence response of a biological molecule. The filtering optics can include a dichroic mirror configured to reflect radiation in one of the primary or secondary emission bands, and to transmit radiation in the other of the primary or secondary emission bands. A focusing optic, such as a ball lens, can be positioned in the optical path between the excitation source and the dichroic mirror. The filtering optics can include an optical filter positioned in the optical path between the dichroic mirror and the optical cavity to attenuate the intensity of radiation in the secondary emission band. The filtering optics can also include a lens positioned in the optical path between the dichroic mirror and the optical filter. An orifice, located in a housing that encloses the optical cavity, can be configured as a control aperture to limit stray radiation from entering the optical cavity. The orifice can be made of any material with a UV absorbing coating, e.g., graphite. 
   In another aspect, the biological aerosol detector can include an optical excitation source, an optical cavity in which a sampled aerosol is irradiated by the optical excitation source, a housing that encloses the optical cavity, and a combined intake and exhaust gas manifold that passes through the housing and into the optical cavity. The combined intake and exhaust gas manifold can collect an aerosol sample and direct it into the optical cavity, and can exhaust the aerosol sample from the optical cavity after it has been irradiated by the optical excitation source. The sampling, transport, and exhaust of the aerosol can occur in the same plane to increase the time the aerosol is irradiated by the excitation source. The combined intake and exhaust gas manifold can include an intake pipe of a first length having one end external to and one end internal to the housing forming the optical cavity, an exhaust pipe of a second length that is less than the first length, and that completely surrounds the intake pipe over at least a portion of the length of the intake pipe, and an end cap. The end cap can cover the end of the exhaust pipe that is external to the housing, and can include a face plate with a hole in it that allows passage of the intake pipe through it. 
   In another aspect, the biological aerosol detector includes an optical excitation source, an optical cavity in which a sampled aerosol is irradiated by the optical excitation source, a housing that encloses the optical cavity and that has two holes that define a sampling axis, a pair of electrical grids located external to the housing and on either side of the holes forming the sampling axis, and a power source attached to the electrical grids to generate an electric field between them. The electric field across the electrical grids allows the preferential sampling of aerosol molecules having a net electrical charge. The electrical grids can be plates or meshes. The biological aerosol detector can include a Faraday based ion detector to electrically detect sampled aerosol molecules. The Faraday based ion detector can include a collection grid and an ion detection circuit. The biological aerosol detector can also include an optical detection circuit to detect a fluorescence signal emitted by a sampled aerosol molecule that has been excited by the optical excitation source, and a correlation circuit to correlate signals from the optical detection circuit and the Faraday based ion detector. The correlation circuit can be a temporal coincidence circuit. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic representation of a biological aerosol detector according to an embodiment of the present invention; 
       FIG. 2  is a detailed schematic representation of the filtering optics of the biological aerosol detector of  FIG. 1 ; 
       FIGS. 3A and 3B  shows the excitation assembly and dichroic mirror assembly of  FIG. 1 ; 
       FIGS. 4A and 4B  illustrate an intake/exhaust configuration of a biological aerosol detector according to an embodiment of the present invention; 
       FIGS. 5A-5C  illustrate component views of the intake/exhaust configuration of  FIGS. 4A and 4B ; 
       FIG. 6  illustrates the principle of electrostatic sampling of manmade biological aerosols; 
       FIG. 7  illustrates the biological aerosol detector of  FIG. 1  utilizing the electrostatic sampling principle of  FIG. 6 ; and 
       FIG. 8  illustrates the biological aerosol detector of  FIG. 7  including an ion detection component. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic representation of a biological aerosol detector  100  according to one embodiment of the present invention. In one aspect, the biological aerosol detector  100  shown in  FIG. 1  improves upon the biological aerosol detector described in patent application Ser. No. 10/720,877 by providing an alternative optical excitation configuration and accompanying filtering optics. The biological aerosol detector  100  has at least one excitation assembly  110  that includes an excitation source and collection/collimation optics that are discussed in more detail in connection with  FIG. 2 . Excitation assembly  110  emits radiation toward a dichroic mirror assembly  120  that transmits a portion of the radiation toward a beam dump  208  ( FIG. 2 ), and reflects a portion of the radiation toward a lens  130 . The excitation assembly  110  and the dichroic mirror assembly  120  are mounted on a plate  112  to allow for a fixed relative positioning of the two components. The radiation reflected from dichroic mirror assembly  120  passes through the lens  130  to a filter  132  that further reduces a portion of the radiation passing through lens  130 . 
   The biological aerosol detector  100  also includes an optical cavity  133  formed in part by the housing for an elliptical mirror  134 . An aerosol sample drawn into optical cavity  133  is excited by radiation that is emitted from excitation assembly  110  and that passes through lens  130  and filter  132 . The lens  130  focuses this radiation onto the inner focal point of elliptical mirror  134 , and the optical path from lens  130  to the inner focal point of elliptical mirror  134  defines a UV or excitation beam axis  136 . 
   As shown in  FIG. 1 , detector  100  includes first and second excitation assemblies with corresponding filtering optics placed at opposite ends of the UV beam axis  136 . This configuration allows particles to be illuminated from two optical sources, thereby providing twice the excitation power of devices that use a single excitation assembly. In addition, the wavelengths emitted by the multiple excitation assemblies  110  may be different, thereby allowing the detection of different biological aerosols or discrimination between detected aerosols. 
   The housing of elliptical mirror  134  includes an inlet  138  through which aerosol samples are drawn into the optical cavity formed by the housing. As described in patent application Ser. No. 10/720,877, the biological aerosol detector shown in  FIG. 1  utilizes an opposing intake flow design that defines an aerosol sampling axis  137 . The UV beam axis  136  is perpendicular to this aerosol sampling axis  137 , and both of these axes are perpendicular to a fluorescence detection axis  139  that runs orthogonal to the top plan view of the detector that is shown in  FIG. 1 . A detector placed along fluorescence detection axis  139  can detect the fluorescence radiation emitted by a biological molecule in the sampled aerosol that is excited by the ultraviolet light from excitation assembly  110 . 
     FIG. 2  is a more detailed schematic representation of the excitation assembly  110  and filtering optics of biological aerosol detector  100  of  FIG. 1 . Excitation assembly  110  includes an excitation source  202  and a collection/collimation optic  204 . The excitation source  202  is a semiconductor ultraviolet optical source or SUVOS in the form of a light emitting diode or LED. A SUVOS based LED typically emits radiation in both primary and secondary emission bands. The primary emission band, with a center wavelength in the ultraviolet region, is used to irradiate and optically excite biological molecules in an aerosol sample within the optical cavity. The secondary emission band, with a center wavelength in the visible spectrum, is generally an unintended and unwanted companion to the primary emission band since its spectrum sometimes and undesirably overlaps the expected fluorescence spectrum of biological molecules excited by radiation from the primary emission band. As a result of this overlap, radiation from the secondary emission that is scattered by non-biological molecules in the sampled aerosol can mimic or appear to be fluorescence radiation that is emitted by excited biological molecules in the aerosol. 
   To reduce the likelihood of such false positive signals, and to improve detector  100 &#39;s ability to distinguish between biological and non-biological molecules in a sampled aerosol, detector  100  includes several features to attenuate the radiation from the SUVOS excitation source&#39;s secondary emission band. More particularly, detector  100 &#39;s excitation assembly  110  includes a collection/collimation optic  204  in the form of a ball lens having a short focal length and large numerical aperture. The collection/collimation optic  204  focuses and directs radiation emitted from the SUVOS excitation source  202  onto a dichroic mirror  206 . The dichroic mirror  206  reflects light within the SUVOS excitation source  202 &#39;s primary emission band with high efficiency, but reflects only a small portion of light within the SUVOS source  202 &#39;s secondary emission band. Instead, the majority of light in the SUVOS source  202 &#39;s secondary emission band is transmitted through dichroic mirror  206  to a beam dump  208 , where it is absorbed. 
   The radiation that is reflected from dichroic mirror  206  is reflected toward a lens  130  that focuses the radiation to the inner focal point of the elliptical mirror  134  shown in  FIG. 1 . Lens  130  is a 30 mm effective focal length plano-convex lens, or any other lens having a sufficiently long focal length that allows it to be positioned outside of the optical cavity  133  ( FIG. 1 ) formed by elliptical mirror  134 . An optical filter  132 , positioned between lens  130  and optical cavity  133 , further attenuates the radiation from the SUVOS source  202 &#39;s secondary emission band while substantially transmitting the radiation from the SUVOS source  202 &#39;s primary emission band. Preferably, optical filter  132  has an optical density of at least 2 with regard to the light from the secondary emission band. Optical filter  132  may be a Schott UG-11 filter, multi-layer dielectric notch filter, a colored glass filter, or any other type of filter having suitable characteristics with regard to the transmission and attenuation of the primary and secondary emission bands. The combination of dichroic mirror  206  and optical filter  132  provides a suitable compromise between maintaining high throughput of desired radiation from the SUVOS excitation source  202 &#39;s primary emission band, and attenuating the unwanted radiation from the SUVOS excitation source  202 &#39;s secondary emission band. The filtering optics also include a graphite orifice  242 , located in the housing of the elliptical mirror  134 , that serves to attenuate stray radiation that enters the opening  140  in the housing of elliptical mirror  134 . 
     FIGS. 3A and 3B  represent this embodiment of excitation assembly  110  and dichroic mirror assembly  120 . Excitation assembly  110  includes a cover  301  that is mounted over a SUVOS based LED  202  (shown in  FIG. 3B ). Ball lens  204  is fixed to cover  301 , which is mounted over SUVOS based LED  202  via threads  305  that allow radiation transmitted through ball lens  204  to be focused. Dichroic mirror assembly  120  includes a dichroic mirror  206  mounted within a lower portion  303  and upper portion  307  of a holder. Both dichroic mirror assembly  120  and excitation assembly  110  are mounted on a plate  112  to allow for a fixed relative positioning of these components. 
   The inlets  138  of detector  100  ( FIG. 1 ) may be modified to serve as both the intake and exhaust ports of a sampled aerosol. Such an arrangement is shown in the combined intake/exhaust manifold  400  shown in  FIGS. 4A ,  4 B, and  5 A through  5 C. Manifold  400  includes an intake pipe  401 , exhaust pipe  405 , and exhaust cap  403 . The intake pipe  401  fits inside exhaust pipe  405 , and is held in place partially by exhaust cap  403 . The exhaust cap  403  includes a nozzle  409  that can be connected to a vacuum pump. The exhaust pipe  405  fits securely within an opening of the housing of elliptical mirror  134 , and is made of any suitably rigid material, such as aluminum coated with an enhanced aluminum coating. 
   As indicated by the reference arrows shown in  FIG. 4B , an aerosol sampled through intake pipe  401  flows toward the inner focal point of elliptical mirror  134 . To maximize the time the sampled aerosol is exposed to radiation from the excitation assembly  110  (see  FIG. 1 ), flows from opposing intake pipes  401  are typically collided at the focal point of elliptical mirror  134 , and thereby stalled. Eventually, the sampled aerosol is evacuated from the optical cavity formed by elliptical mirror  134  through holes  407  bored through either side of exhaust pipe  405 . The evacuated aerosol flows along the cylindrical gap between the outer wall of intake pipe  401  and the inner wall of exhaust pipe  405  and exits the exhaust pipe through the nozzle  409  of the exhaust cap  403 . This arrangement of the inflow and exhaust trajectories of the sampled aerosol, which is largely planar in nature, reduces the vertical motion of sampled aerosol molecules. The reduced vertical motion allows the sampled aerosol molecules to remain in the optical cavity formed by elliptical mirror  134  for a longer period of time, thus increasing the likelihood of exciting and detecting any biological molecules in the sampled aerosols. 
   As shown in  FIGS. 5A and 5B , intake pipe  401  fits within exhaust pipe  405  and exhaust cap  403 . Holes  407  are bored through exhaust pipe  405 . As illustrated in the cutaway view shown in  FIG. 5C , intake pipe  401  is secured within exhaust pipe  405  by exhaust cap  403  on one end, and by a plate  411  that is fastened to the end of exhaust pipe  405  on the other end. Plate  411  has a hole bored through it with a diameter that is approximately equivalent to the outer diameter of intake pipe  401 , so that intake pipe  401  fits securely within exhaust pipe  405 . 
   Detector  100  ( FIG. 1 ) can also be modified to sample aerosols using an electrostatic sampling method as shown in  FIGS. 6 and 7 . This sampling method tends to preferentially sample biological aerosols since aerosolized biological materials tend to have a net electrostatic charge due to the manner in which they are released. Referring to  FIG. 6 , the basic principle behind the electrostatic sampling method utilizes an electrostatic field, such as that created by two grids  602  and  604  of differing electrostatic potential, to attract aerosol molecules with a net electrostatic charge. For example, grid  602  can be held at a positive electrostatic potential relative to grid  604 , thereby creating an electrostatic field such that positively charged particles  608  (open circles) traveling between the grids will be attracted to grid  604 , while neutrally charged particles  606  (black circles) will proceed along their original trajectory. Of course, the grid potentials can be reversed to attract and sample negatively charged particles rather than positively charged particles. 
   Referring now to  FIG. 7 , the detector  100  of  FIG. 1  is shown modified to utilize the electrostatic sampling method discussed in connection with  FIG. 6 . Inlet  138  ( FIG. 1 ) is replaced by plates  702  and  704 . Plate  702  is held at a positive electrostatic potential relative to plate  704  by a direct current (DC) power source  706 , thereby creating an electrostatic field between plates  702  and  704  that is directed along a sampling axis  137 . The plates  702  and  704  may each include a hole to allow charged particles to pass through them. Alternatively, screens could be used in place of plates  702  and  704  for this purpose. The electrostatic field created by plates  702  and  704  draws positively charged particles into the optical cavity formed by elliptical mirror  134 , so that the particles can be irradiated by ultraviolet radiation along the UV beam axis  136 . The particles proceed along the sampling axis  137  to the plate  704 , where they are neutralized and collected. The field strengths required to implement the sampling method shown in  FIGS. 6 and 7  depend on the charge of the particles sought to be captured, the desired velocity of the particles along the sampling axis, and any pressure difference across the sampling orifice. Typically, field strengths on the order of 200 Volts/cm are sufficient to electrostatically sample biological aerosols. 
     FIG. 8  illustrates a further modification of the biological aerosol detector shown in  FIG. 7 . The configuration of  FIG. 8  includes a collection grid  810  and a detection circuit  812  that collectively operate as a Faraday based ion detector. Plate  704  is maintained at a positive electrostatic potential relative to collection grid  810  by DC power source  808 . Charged particles traveling along sampling axis  137  proceed past plate  704  to collection grid  810  where they are detected by detection circuit  812 . The electrical response from detection circuit  812  can be correlated with any fluorescence signal reflected by elliptical mirror  134  onto a photodetector to further enhance the detection of biological aerosol signal events relative to background event signals. For example, the electrical response from detection circuit  812  can be put in temporal coincidence with the fluorescence signal detected by the photodetector to improve the signal to noise response of detector  100 . 
   A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example,  FIG. 1  illustrates one possible configuration of filtering optics for use in attenuating radiation from an optical source in an undesired secondary emission band. Other configurations, including alternative ordering of the described components, may achieve similar results and are within the spirit and scope of the present invention. For example, a dichroic mirror that largely transmits radiation from the first emission band and reflects radiation from the second emission band can be used. With this mirror the excitation source and mirror would have to be repositioned so that the reflected radiation is largely directed to a beam dump, while the transmitted radiation is largely focused to the inner focal point of the elliptical mirror  134 . Also, while specific components and numbers have been given in reference to some of the filtering components, it will be understood that different components may be chosen depending on the exact wavelengths of the excitation source. Such modifications are within the spirit and scope of the present invention. 
   In addition, the configuration of  FIGS. 6-8  has been described in connection with electrostatic fields; however, electrodynamic fields can also be used. The electrostatic sampling configuration shown in  FIGS. 7 and 8  can be used in connection with the vacuum/pressure sampling flow design such as that shown in  FIG. 1 . In one alternate embodiment, a vacuum or pressure induced intake flow can be configured to oppose an electrostatically induced intake flow such that the two flows stall near the inner focal point of the elliptical mirror  134 . Such an embodiment increases the amount of time the aerosol sample is irradiated by the excitation source, thereby increasing the likelihood that a biological molecule in the sample will be excited to a fluorescent state. Accordingly, these and other embodiments are within the scope of the following claims.