Abstract:
Physical property determination of a particle or classification of the particle as a function of the physical property by evaluating scattered light profile from a single particle is disclosed. The particle may include chemical structures that vibrate as a function of a physical property of the particle. The physical property may include an absorptive property of the particle or a chemical composition. From a detected scattered light spectrum, at least two anomalous dispersive regions may be identified. The physical property of the particle may be determined as a function of the at least two regions. A system employing the physical property determination can achieve sensitivities useful for low particle density applications such as detection for biological and chemical agents.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/125,534, filed on Apr. 25, 2008, attorney&#39;s docket no. 0050,2118-000, entitled “Method and Instrumentation for Determining a Physical Property of a Particle.” The entire teachings of the above application is incorporated herein by reference. 
     
    
     GOVERNMENT SUPPORT 
       [0002]    The invention was supported, in whole or in part, by a grant FA8721-05-C-0002 from United States Air Force. The Government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    The detection and discrimination of biological aerosols from an ambient aerosol became a heightened concern during first Gulf War with Iraq in 1991. As a result of this perceived threat, the development of biological aerosol detectors for use as triggers in a system context began in earnest. The flurry of activity to develop this capability is evidenced by various works published in the United States, United Kingdom, and Canada. 
         [0004]    Aerosol sensors that were being developed and field-tested were a departure from the instruments that had been developed for much of aerosol science in previous decades. Present day needs typically require very small concentrations of biological aerosols to be detected. For example, single aerosol particles in a liter of air, in a background of a very large number of ambient aerosol particles, or 100&#39;s to 1000&#39;s of particles per liter of air. The first instruments of this type simply measured the particle size through optical scattering or aerodynamic means. Rapid advances included measuring the particle shape in addition to size and measuring a fluorescent light scatter in multiple bands, with or without measurement of particle size. 
       SUMMARY 
       [0005]    Due to the present day needs of detection systems, instruments designed to measure individual aerosols particles one at a time in a very rapid fashion rather than measuring mixtures, or ensembles, of aerosol particles, are needed. There is also a need to meet the challenge for on-the-fly aerosol sensor developers, which is to apply more discriminating techniques, such as vibrational spectroscopy, to individual aerosol particles in a manner consistent with rapid analysis. There is also a need to develop instruments that can detect and discriminate types of aerosols in addition to biological aerosols. 
         [0006]    In example embodiments, a system, and corresponding method, which replaces the complexity of an imaging system requiring a multi-element detector with one, or a small number, of detectors that collect all of the infrared light scattered in an appropriate direction is presented. Using example embodiments of the system presented herein, the scattered light may be produced by a single particle to utilize on-the-fly aerosol detection. 
         [0007]    The system may be configured to determine a physical property of a particle. The particle may be a simple particle or a complex particle, for example a DNA strand, and the vibrational excitation may include low energies, for example energies lower than 0.5 eV. The physical property of the particle may be a chemical composition and/or an absorption region. The system may include a light detector that may be configured to detect scattered light from a particle, where the particle may have chemical structures that vibrate as a function of the physical property of the particle. The system may also include an identifying unit that may be configured to identify at least two regions of a spectrum of the scattered light that may be based on vibrations of the chemical structures. The chemical bonds may vibrate with vibrational excitation energies less than 0.5 eV. The system may further include a determination unit that is configured to determine the physical property of the particle as a function of the at least two regions. 
         [0008]    The determination unit may be further configured to classify the particle. The determination unit may also be configured to identify the at least two regions by identifying a region in the spectrum of the scattered light having a non-decrease of intensity with increasingly longer wavelengths. The determination unit may also be configured to determine the physical property of a single particle by analyzing the scattered light produced solely by the single particle. 
         [0009]    The system may further include a light source that may be configured to produce light in a direction of propagation to illuminate the particle resulting in the scattered light. The light source may be an infrared light source configured to generate wavelengths greater than 2.5 μm. The light source may further be configured to generate the at least two wavelengths of light to include wavelengths as short as a fourth of the diameter of the particle. The light source may also be configured to emit light of multiple wavelengths. 
         [0010]    The light detector may be configured to detect scattered light of different wavelengths in different regions of the detector. The light detector also may be configured to detect the scattered light in an angular direction substantially equal to an angular direction of propagation of the light. Alternatively, the light detector may be configured to detect the scattered light in an angular direction between an angular direction of propagation of light from the light source and an angular direction perpendicular to the propagation of the light. 
         [0011]    It should be appreciated that the system described herein may be configured to detect any type of aerosol or chemical particle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The foregoing will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the example embodiments. 
           [0013]      FIGS. 1A and 1B  are diagrams with examples of particle detection systems; 
           [0014]      FIG. 2  is a schematic of an optical system of a conventional particle size analyzer; 
           [0015]      FIG. 3  is a schematic of a forward scattering particle detection system according to example embodiments; 
           [0016]      FIG. 4  is a graphical representation of the real and imaginary indices of refraction of propylene glycol; 
           [0017]      FIG. 5A  is a graphical representation of detected scattered light versus wavelength obtained using the system of  FIG. 3  according to example embodiments. 
           [0018]      FIG. 5B  is a flow diagram of example operational steps used in analyzing the data of  FIG. 5A  according to example embodiments; 
           [0019]      FIG. 6  is a schematic of a side scattering particle detection system according to example embodiments; 
           [0020]      FIG. 7  is a graphical representation of detected scattered light versus wavelength obtained using the system of  FIG. 6  according to example embodiments; 
           [0021]      FIG. 8  is a schematic of a near back scattering particle detection system; 
           [0022]      FIG. 9  is a graphical representation of detected scattered light versus wavelength obtained using the system of  FIG. 8 ; and 
           [0023]      FIGS. 10A-10D  are schematics of alternative designs of the forward scattering particle detection system of  FIG. 3  according to example embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    A description of example embodiments follows. 
         [0025]      FIG. 1A  provides an overview example  100  of a particle detection system  101 . The particle detection system  101  may be situated to detect particles  104  in an airvent system  105  of a building  103 . The particle detection system  101  includes an inlet (not shown) in which an airflow enters the particle detection system  101 . An outlet  106  of the particle detection system  101  may be used as a pathway to shunt the airflow if particles  102  detected are deemed unsafe for breathing. Otherwise, the airflow can continue into the airvent system  105 . 
         [0026]    As another example, a liquid stream may also need to be evaluated. For instance, a water reservoir may need to be continuously monitored to ensure harmful particles are not introduced into a water supply. 
         [0027]      FIG. 1B  provides an overview example  107  of a particle detection system  111  detecting particles  113  in a liquid stream  109 . The particle detection system  111  may include an inlet  115  used to supply a sample of the liquid flow  109  to the particle detection system  111 . Once the liquid flow  109  has been checked for a presence of foreign particles, an outlet  117  may be used to remove the sample from the particle detection system  111 . 
         [0028]      FIG. 2  provides an example of the components of a prior art system used for analyzing particles. The system of  FIG. 2  includes a laser source  222   a  configured to emit a laser beam onto a condenser lens  222   b , spatial filter  222   c , and a collimator lens  222   d , resulting in collimated light  228 . The collimated light  228  is configured to interfere with a number of particles  229 , held in suspension within a sample volume  231 , in order to produce a spatial light intensity distribution pattern of scattered light. Light scattering in an angle smaller than 35° in a forward angle is condensed by a condenser lens  223  to form a scattering image on a ring detector  224  positioned at a focal point of the lens. Light scattered forwardly in a larger angle range (e.g., &gt;35°) and light scattered laterally and rearwardly are detected by forward large angle scattered light sensors  225 , sideward scattered light sensors  226 , and rearward scattered light sensors  227 , formed of independent light sensors, respectively. 
         [0029]    The intensity distribution data of the light scattered by the particles  229  to be measured vary according to the sizes of the particles. Since the actual particles  229  to be measured include particles of different sizes, the intensity distribution data of the scattered light generated by the particles  229  to be measured become a superposition of the lights scattered from the respective particle. Analyzing the superpositioned light scattering provides data on the particle size distribution of the particles included in the sample volume. Note that the prior art point detection system illustrated in  FIG. 2  requires the use of a single wavelength laser source and multiple detectors measuring multiple particles at one given time. Further note that the system of  FIG. 2  is not capable of determining the absorption of the particles as it is difficult to directly measure the amount of light a particle absorbs due to the difficulty of separating the light scattered from the light adsorbed. 
         [0030]    In example embodiments of a particle detection system, a particle point detection system includes a single detector that may be configured to determine a particle absorption and/or chemical composition from data obtained from a single particle is disclosed. The particle absorption and/or chemical composition may be determined by analyzing an amount of light scattered by the particle. Furthermore, the example system presented herein may also be configured for “on-the-fly” particle detection. It should be appreciated that alternative embodiments may employ multiple detectors. 
         [0031]      FIG. 3  illustrates an example particle point detection system  300  according to example embodiments. The detection system  300  may measure size and absorption of a single particle or an ensemble of particles. The detection system  300  may include an infrared light source  301  that may be configured to emit a light beam  303 . The light source  301  may be configured to emit light of multiple wavelengths in succession or simultaneously. The light beam  303  may illuminate a particle  305  traveling in a fluid stream, where the fluid may include air, water, vapor, or any other known liquid or gas. Upon illuminating the particle, diverging scattered light  307  may be produced. The scattered light  307  may be used to obtain a scattering spectrum that may provide characteristics of the illuminated particle as a function of measured intensity and wavelength. 
         [0032]    A beam block  309  may be configured to block the light beam  311  upon particle illumination, such that a light detector  317  does not receive light directly from the light beam, therefore preventing saturation of the light detector  317 . A focusing element  313  may be used to focus the scattered light  315  onto the light detector  317 . Any form of light detection known in the art may be used as the light detector  317 . 
         [0033]    The configuration of the detection system  300  features forward scattering detection, where the scattered light  315  is detected in approximately the same direction of propagation as the light beam  303 . 
         [0034]    The light detector  317  may be coupled to a processing unit  339 . The light detector  317  may be configured send data measurements  343  to the processing unit  339  in the form of an analog electrical signal. The processing unit  339  may be configured to determine physical properties of the particle as a function of the measured data and wavelength used to produce the light scattering. 
         [0035]    The processing unit  339  may be configured to send measurement instructions  345  to the light detector  317  in the case that the light detector  337  includes an intelligent programmable configuration. The measurement instructions  345  may include, for example, on/off instructions or reading instructions. The light detector  317  and the processing unit  339  may be connected via a connection link  341 . It should be appreciated that the connection link  341  may be a wired, optical, or wireless connection, or any other data transfer connection known in the art. 
         [0036]    The processing unit  339  may also be connected to a database storage  347 . The processing unit  339  may send the database storage  347  a particle identification request, and/or a data storage request  351 . The data storage request  351  may include the data measurements  343 , or representation thereof, provided by the light detector  317 . The particle identification request may include a request to compare information stored in the database storage  347  with the obtained data measurements  343 , optionally for the purpose of classifying and identifying the particles in the sample volume  327 . The database storage  347  may send a particle identification result  353  to the processing unit  339 . The particle identification result  353  may comprise a listing of possible particle matches with respect to the data measurements  343 . 
         [0037]    The processing unit  339  may also be coupled to a network  355 . The processing unit  339  may send a particle identification request, a data storage request, and/or a data sharing request  359  to the network  355 . The particle identification request and data sharing request  359  may be similar to the request  351  sent to the database storage  347 . The data sharing request  359  may be a request to share data with a user  369  that may be connected to the network  355 , or another detection system  371  that may be connected to the network  355 . The network  355 , or more specifically, a server or other network element (not shown) connected to the network  355 , may also send a message  361  in the form of particle identification results, similar to the result  353  sent by the database storage  347 , or instructions to the processing unit  339 . The instructions  361  may comprise measurement instructions similar to the instructions  345  sent to the light detector  317 . 
         [0038]    The database storage  347  and the network  355  may also include a bidirectional data transfer connection  363 . The database storage  347  may send identification results and/or a data sharing request  365  to the network  355 . The network  355  may send an identification request  367  to the database storage  347 . It should be appreciated that the data transfer connections  349 ,  357 , and  363  between the processing unit and the data storage, the processing unit and the network, and the network and the data storage, respectively, may include or be supported by any data transmission link known in the art. 
         [0039]    Once a scattering spectrum, or measured data, has been obtained, the physical properties of the particle may be determined. In an example embodiment, a method for detecting and classifying individual micron-sized particles is based on their infrared (IR) absorption spectra. The IR absorption spectra may result from vibrations of chemical or molecular bonds of the particle. These vibrations may occur naturally or as a result of the illumination of the particle. 
         [0040]    Normally, scattered light intensity from an object (particle) decreases with increasing wavelength. In this context, anomalous scattering refers to a relatively sharp increase in scattering intensity as wavelength increases. This effect may be caused by a relatively rapid change in the real refractive index at wavelengths associated with an absorption peak, an inherent property of the particle material. Therefore, achieving discrimination between certain agent aerosols and natural background aerosols may be to interrogate individual aerosol particles “on-the-fly” with a discrete set of IR wavelengths that, in effect, may provide an IR absorption spectrum of the particle material via the anomalous scattering signals. Many biological warfare (BW) and chemical warfare (CW) agent materials have distinctive IR spectrums, and may thus be characterized via an IR spectrum. 
         [0041]    Currently, there are no comparable capabilities for determining aerosol absorption. It is generally not feasible to infer true absorption in either single particles or aerosol populations from direct extinction (transmission loss) measurements because: (a) significant wavelength, and particle size, shape, and composition dependencies of the elastic scattering are too complex to accurately model without a priori information, and (b) scattering losses from aerosols are typically the dominant contribution to extinction, so that small errors in determining the scattered energy translate into larger errors in estimating absorption. Consequently, absorption determination based attenuation measurements have been historically accomplished for aerosols only by first collecting them onto a filter substrate. 
         [0042]    In example embodiments, a rapid, on-the-fly, aerosol point detection capability, comparable with fluorescence triggers currently employed, may be achieved with the development of a new approach to aerosol particle absorption determination, as a function of an anomalous scattering approach. 
         [0043]    The anomalous scattering approach may be based on the connection between absorbed and scattered incident radiation results from a well-known fundamental relationship between the real and imaginary parts of any material&#39;s effective complex refractive index. Therefore, the proposed technique is quite general and leads to a new on-the-fly aerosol diagnostic technique for major constituent compositional analysis based on IR spectral absorption profiles, which are analogous to well-established conventional IR absorption spectroscopy for bulk materials. 
         [0044]      FIG. 4  illustrates the real  401  and imaginary  403  parts of the refractive index (y-axis  402 ) of a propylene glycol particle for a 8-11 μm wavelength region (x-axis  404 ). Within this wavelength range a propylene glycol may have two prominent absorption features  409  and  411  that may be seen as peaks at 8.75 μm and 9.55 μm, λ 1  and λ 2 , respectively, in the imaginary index of refraction, n i    403 , and highlighted with vertical lines  406  and  408 , respectively, running the height of the plot. Associated with peaked regions  409  and  411  in the imaginary part of the index of refraction  403  are regions of anomalous dispersion  405  and  407 , respectively, in the real part of the index of refraction, n r    401 . The anomalous dispersion region  405  may be defined by points  405   a  and  405   b , whose relation reflect an increase in the real part of the index of refraction within a region of increasing wavelength. Similarly, the anomalous dispersion region  407  may be defined by points  407   a  and  407   b.    
         [0045]    Since the anomalous dispersion in the real part of the index of refraction is associated with the absorption peaks in the imaginary part of the index of refraction, the scattering spectrum may be analyzed to determine the absorption peaks. 
         [0046]      FIG. 5A  illustrates the power detected  509  (y-axis) versus the wavelength  511  (x-axis) for propylene glycol spherical droplets for a scattering configuration similar to the system illustrated in  FIG. 3 .  FIG. 5B  shows a flow diagram of example operations that may be taken in the evaluation of data shown in  FIG. 5A . Each trace  501 ,  503 ,  505 , and  507  represents a single particle size 10, 5, 2, and 1 microns in diameter, respectively. As illustrated in  FIG. 5A , each trace  501 - 507  includes two prominent anomalous dispersion regions associated with wavelengths λ 1  and λ 2 . The peaks in the scattered light profile may be the result of vibrations of chemical bonds included in the particle due to absorption properties of the particle ( 515 ). 
         [0047]    As explained in relation to  FIG. 4 , the anomalous dispersion regions may be identified by regions of increasing detected power with increasing wavelengths ( 517 ). For example, within trace  501 , obtained with particles including a diameter of 10 microns, there is an increased power detection between points  501 A- 501 B and  502 A- 502 B defining adsorption peaks at wavelengths λ 1  and λ 2 , respectively. Within trace  503 , obtained with particles including a diameter of 5 microns, there is an increase in power detected between points  503 A- 503 B and  504 A- 504 B defining adsorption peaks at wavelengths λ 1  and λ 2 , respectively. Within trace  505 , obtained with particles including a diameter of 2 microns, there is an increased power detection between points  505 A- 505 B and  506 A- 506 B defining adsorption peaks at wavelengths λ 1  and λ 2 , respectively. Within trace  507 , obtained with particles including a diameter of 1 micron, there is an increased power detection between points  507 A- 507 B and  508 A- 508 B defining adsorption peaks at wavelengths λ 1  and λ 2 , respectively. 
         [0048]    As can be seen in  FIG. 5A , the scattering spectrum encodes the absorption peaks as regions of anomalous scattering and is nearly independent of particle size as the peaks for traces  501 - 507  are roughly within the same wavelength range λ 1  and λ 2 . 
         [0049]    It should be appreciated however that for smaller particle sizes, the scattering mirrors the anomalous dispersion of the index of refraction. As the particle size increases the minimum in the scattering signal versus wavelength slightly red shifts. For example, the regions of increased detection  502 A- 502 B and  504 A- 504 B of traces  501  and  503 , respectively, are slightly out of the wavelength range λ 2 . Eventually, when the particle size is much greater than the wavelength of light, the scattering minimum, or wavelength at which the second derivative with respect to wavelength of the scattering is greater than it is for neighboring wavelengths, may coincide with the peak in absorption. 
         [0050]    However, when the particle size is much greater than the wavelength of light, the value of the particle size resonances, also contributing to the scattering spectrum. For cases where the particle size is more than four times the wavelength, the scattered light spectrum has minima and maxima that result from destructive and constructive interference of different paths the light can take through the particle. In this regime the association between the spectrum of the index of refraction, regions of anomalous dispersion, and the associated regions of anomalous scattering are not a strict dependence. In this regime of particle size to scattering wavelength, the scattering spectrum may not a reliable indicator of the absorption features of the aerosol particle. 
         [0051]    Once the anomalous dispersion regions have been identified, various physical properties of the particle may be determined ( 519 ). These physical properties may include, but are not limited to, a chemical composition of the particle or an absorption region. Utilizing the physical properties of the particle, a classification, or discrimination, of the particle may be made ( 521 ). 
         [0052]    It should be appreciated that although propylene glycol spherical droplets were used in obtaining the data of  FIG. 5A  any other particle may be classified using embodiments presented herein. For example a complex particle, such as a DNA strand may be classified. 
         [0053]      FIG. 6  illustrates an alternative detection configuration according to example embodiments. The detection system of  FIG. 6  utilizes a side scattering detection method. The side scattering system may include a light source  301  that may be configured to emit a light beam  303 . The light beam  303  may interfere with a particle  305 , thereby creating a diverging light scattering. A lens  313  may be position in a location approximately traverse with respect to the direction of propagation of the laser beam  303 . The location of the lens  313  allows for a diverging side scattering  307  to be focused onto an infrared detector  317 . It should be appreciated that the processing unit, network, and database connections illustrated in  FIG. 3  may also be applied to the system of  FIG. 6 . 
         [0054]      FIG. 7  illustrates the power detected  709  (y-axis) versus the wavelength  711  (x-axis) for propylene glycol spherical droplets. The measured data illustrated in  FIG. 7  may be obtained using a detection configuration similar to the system illustrated in  FIG. 6 . Each trace  701 ,  703 ,  705 , and  707  represents a single particle size 10, 5, 2, and 1 microns in diameter, respectively. As illustrated in  FIG. 7 , each trace  701 - 707  includes two prominent anomalous dispersion regions associated with wavelengths λ 1  and λ 2 . The anomalous dispersion regions for trace  701  are labeled as  701 A- 701 B and  702 A- 702 B, corresponding to wavelengths λ 1  and λ 2 , respectively. The anomalous dispersion regions for trace  703  are labeled as  703 A- 703 B and  704 A- 704 B, corresponding to wavelengths λ 1  and λ 2 , respectively. The anomalous dispersion regions for trace  705  are labeled as  705 A- 705 B and  706 A- 706 B, corresponding to wavelengths λ 1  and λ 2 , respectively. The anomalous dispersion regions for trace  707  are labeled as  707 A- 707 B and  708 A- 708 B, corresponding to wavelengths λ 1  and λ 2 , respectively. 
         [0055]    As is illustrated in  FIG. 7 , a side scattering detection system provides a light spectrum that may be unreliable as the size of the particle under investigation increases. 
         [0056]    For example, in the traces  701  and  703  associated with particles featuring a diameter size of 10 and 5 microns, respectively, the anomalous dispersion region associated with the wavelength λ 2  is red shifted in comparison to the traces  705  and  707 . The red shifting may be caused by a lack of scattered light in the side scattering direction. The lack of increase in scattering from 10 micron-diameter particles from those of 5 micron-diameter may be due to the scattering pattern becoming more biased towards the forward direction with increasing particle size. 
         [0057]    However, it should be appreciated that the scattering spectrum of  FIG. 7  is still dominated by the absorptive features of the aerosol particle. For the smaller particle sizes the minimum in the scattering spectra still coincide with the minimum in the real part of the refractive index. 
         [0058]      FIG. 8  illustrates yet another alternative detection configuration. The detection system of  FIG. 8  utilizes a near back-scattering geometry. The near back-scattering system may include a light source  301  that may be configured to emit a light beam  303 A. The light beam  303 A may be reflected and redirected by a mirror  304 . The redirected beam  303 B may interfere with a particle  305  traveling in a same direction as the beam  303 B, thereby creating a backward diverging light scattering  307 . A lens  313  may be position in a location approximately along the same axis with respect to the direction of propagation of the laser beam  303 B. The location of the lens  313  allows for the backward diverging scattering  307  to be focused onto an infrared detector  317 . It should be appreciated that the processing unit, network, and database connections illustrated in  FIG. 3  may also be applied to the system of  FIG. 8 . 
         [0059]      FIG. 9  illustrates the power detected  909  (y-axis) versus the wavelength  911  (x-axis) for propylene glycol spherical droplets for a scattering configuration similar to the system illustrated in  FIG. 8 . Each trace  901 ,  903 ,  905 , and  907  represents a single particle size 10, 5, 2, and 1 microns in diameter, respectively. As shown in  FIG. 9 , the spectrum associated with a near back-scattering system may be particle size dependent. For example, the traces  905  and  907  corresponding to particle having a diameter size of 2 and 1 microns, respectively, have well defined regions of anomalous dispersion. The anomalous dispersion regions of trace  905  are labeled as  905 A- 905 B and  906 A- 906 B for wavelengths λ 1  and λ 2 , respectively. The anomalous dispersion regions of trace  907  are labeled as  907 A- 907 B and  908 A- 908 B for wavelengths λ 1  and λ 2 , respectively. 
         [0060]    Conversely, the traces  901  and  903  associated particles having diameter sizes of 10 and 5 microns, respectively, are greatly red shifted and include large fluctuations in detected power. Thus, in detection systems featuring a near back-scattering geometry, as the diameter size of a particle increases, the reliability of the measured data may decrease. 
         [0061]      FIGS. 10A-10D  illustrate alternative embodiments of the particle detection system according to example embodiments. The forward scattering detection system of  FIG. 10A  includes a light source  319  that may be configured to simultaneously emit parallel light beams of different wavelengths  321 . An aerosol particle  305  may be configured to travel in a fluid stream in a direction approximately traverse to the direction of propagation of the light beams  321 . The resulting diverging scattered light  307  may include a temporal dependence that corresponds to the scattering spectrum. A lens  313  may be used to collect and focus the scattered light  307  onto an infrared photodetector  317 . The focused light  327  may include multiple focus points where each point may correspond to a different wavelength. A beam block  309  may be used to prevent the laser beams  321  from reaching and saturating the photodetector  317 . 
         [0062]      FIG. 10B  illustrates an alternative design where the infrared detector includes an array of photodetectors  317 A- 317 C, where each detector may be configured to receive light of a corresponding wavelength. 
         [0063]      FIG. 10C  illustrates an alternative design where the light source  331  may be configured to emit overlapping beams of multiple wavelengths  303  through a technique known as wavelength combining. Upon intersecting with the particle  305  traveling in a direction traverse to the direction of propagation of the beam  303 , scattered light  307  may be produced. An optical system including a pair of collecting lenses  313 A and  313 B and an optical dispersing element  329 . The optical dispersing element, such as a prism or a grating  329  may be located between the two collecting lenses  313 A and  313 B and may be used to demultiplex the multi-wavelength scattered light. Similarly to the system illustrated in  FIG. 10B , the demultiplexed scattered light may be focused onto an array of detectors  317 A- 317 C, where each detector may be configured to detect light of a single wavelength. 
         [0064]      FIG. 10D  illustrates another alternative design that is similar to the design illustrated in  FIG. 10C  with the exception that the light source  331  is configured to produce an overlapping beam where the different wavelengths may be modulated in time so that the light scattering may be generated from one wavelength at any give time. Therefore, the all the scattered light may be focused in a same location on the infrared detector  317 , thus eliminating the need for multiple detectors and thereby allowing for a reduction the necessary detection size. 
         [0065]    It should be appreciated that the detection systems of  FIGS. 10A-10D  may also include the processing unit, network, and database connections illustrated in  FIG. 3 . 
         [0066]    It should also be appreciated that the embodiments described herein may be used regardless of a particle&#39;s shape. Thus, the shape of a particle does not impose limitations on the example embodiments presented. Furthermore, it should be appreciated that the embodiments described herein may be used to detect any known aerosol or chemical particle. 
         [0067]    It should be understood that the process, disclosed herein, may be implemented by a computer controlled machine using instructions implemented in hardware, firmware, or software. If implemented in software, the software may be stored on any form of computer readable medium, such as random access memory (RAM), read only memory (ROM), compact disk read only memory (CD-ROM), and so forth. In operation, a general purpose or application specific processor loads and executes the software in a manner well understood in the art. It should also be appreciated that the embodiments presented herein may be employed in general spectroscopy systems. For example, the system presented may be employed in terahertz spectroscopy. 
         [0068]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.