Abstract:
Particle detection using a system that enlarges particles, concentrates them, and then dries them to return them to their original sizes. Semiconductor components may be used to maintain better control over the process.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional Application Serial No. 60/648,879, filed on Jan. 31, 2005. The disclosure of the prior application is considered part of (and is incorporated by reference in) the disclosure of this application. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     The U.S. Government may have certain rights in this invention pursuant to South Coast AQMD Grant No. 04062. 
     
    
     BACKGROUND  
       [0003]     Detection of aerosol concentration may be used for various purposes.  
         [0004]     At least one study has linked the level of ambient particulate matter or PM with adverse health effects. Studies are ongoing to detect the specific physical and/or chemical properties of the particulate matter that are responsible for the adverse health effects. Fast and accurate measures of particulate matter characteristics may be helpful for these features.  
         [0005]     Different kinds of particle concentrators have been used to study the characteristics of particulate matter. Previous systems have used virtual impaction, slit virtual impactors, as well as other techniques.  
         [0006]     A versatile aerosol concentration enrichment system, or VACES, may be used to detect particles. The VACES system first grows the particles to larger sizes, via supersaturation in water vapor, and then concentrates them by inertial virtual impaction, and returns the particles to their original size by diffusion and drying.  
         [0007]     The current VACES configuration consumes, however, significant electrical power, and also has typically required attended operation by experts.  
       SUMMARY  
       [0008]     The present application describes a special, miniaturized VACES system, with certain structural differences from the prior art. These differences may enable miniaturization and unattended operation.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     These and other aspects will now be described in detail with reference to the accompanying drawings, wherein:  
         [0010]      FIG. 1  shows a block diagram of the overall system.  
     
    
     DETAILED DESCRIPTION  
       [0011]     The device described herein may form particle enriched air that can be used as an elevated exposure atmosphere for exposure studies, or can be used to collect the material. The particle enriched material can also be used in sampling instrumentation, to provide elevated levels of ambient particulate matter. The particle enriched air may also be used for collection of particles in aqueous solutions. The These systems do not significantly alter the physical or chemical properties of the particles.  
         [0012]     An embodiment is described of a miniaturized particle enricher. This device can be used for any of the purposes described above. Another embodiment may use the concentrated aerosol stream to form continuous particle stream for a mass spectrometer. The continuous aerosol stream may increase the spectrometer&#39;s hit rate or sensitivity.  
         [0013]     In this application, a low intake flow rate, for example, less than 1 L per minute, may be used and an unattended 24-hour a day sampling technique may also be used.  
         [0014]     The system described herein has a nominal intake flow rate of 30 L per minute, a nominal minor flow rate of 1-1.5 L per minute, and more automated operation.  
         [0015]     The system uses humidification of the air stream using a special saturator. The saturator uses a heated moist absorbent material surrounding the intake flow. Cooling is carried out to achieve supersaturation and particle growth. Improved control of temperature, and miniaturization is obtained by using a solid-state thermoelectric chiller. A draining system to a closed vessel removes extra water vapor. In addition, by maintaining better control of the temperature, freezing and ice are reduced.  
         [0016]     The particles in the aerosol are caused to increase in size by the supersaturation and freezing. The grown particles are then concentrated using a virtual impactor.  
         [0017]     The concentrated particles are then returned to their original size using a diffusion dryer that is filled with silica gel.  
         [0018]     As a result of laboratory evaluation, this new system has been found to include near ideal enrichment factors for particles of different compositions, and has also been found to not materially change the particle size distributions.  
         [0019]     Details of the embodiment are described herein with respect to  FIG. 1 . The air inlet  100  may be a 2.54 cm inner diameter inlet, and air may travel at 30 L per minute. The system may operate with a small pressure drop, e.g. that of the type associated with standard PM 2.5 , inlet impactor or cyclone.  
         [0020]     The following describes the specific sizes and dimensions of the structure used in this system. However, it should be understood that any of these dimensions may be varied while still be maintained within the teachings herein. In particular, any of these dimensions may be varied by 50%, or more than 50%.  
         [0021]     A saturator  110  is located in the path of the inlet air. The saturator is formed of a 2.54 cm inner diameter and 45 cm long circular channel, surrounded by a cellulose sponge  112  contained within an aluminum cylinder  114 . A heating tape  116  may be wrapped around the exterior of the saturator, and controlled by a heat controller  118 . For example, the heat controller may be a variable transform. A voltage is maintained, such as to heat the air, so that the air leaving the saturator is maintained between 28 and 29° C., and having a relative humidity greater than 90%.  
         [0022]     A temperature/humidity probe  120  may be used immediately downstream of the saturator  110  in order to measure the temperature and humidity. In an embodiment, the temperature and humidity probe may be a model 37960,available from Cole-Parmer Instruments of Vernon Hills, Ill..  
         [0023]     The saturator is maintained wet using water from water reservoir  105  which is pumped by peristaltic pump  106 . A closed system is used, to maintain the pressure differential.  
         [0024]     The saturated material flow passes through a section  124  of tubing, which is substantially in a U-shape. The U-shape is formed with a drain  126  at a bottommost portion thereof. The drain forms a gravity fed drain that removes excess condensed water from the saturator  110 , (and also from the condenser, which is to be described herein). The drain  126  forms a basin as a closed system to compensate for certain pressure drops. The bottom portion  125  of the U tube  124 , where the drain is located, is physically lower than other portions.  
         [0025]     After the U section  124 , flow passes through the condenser  130 . The condenser  130  is formed of a 2.54 cm inner tube  132  surrounded by a 7.62 cm outer tube  134 . Both tubes of the condenser are 27 cm long. The air flow passes through the inner tube  132 , while the space between the outer tube  134  and inner tube  132  is cooled. In the embodiment, a continuous flow of chilled 1:1 mixture of ethylene glycol to water forms a coolant which fills the space between the inner and outer tubes.  
         [0026]     A recirculating chiller  140 , circulates the coolant  138  through the space. The chiller  140  is formed of a thermoelectric cooler is run to maintain the temperature of the outer wall of the condenser at −1° C. The thermoelectric chiller may be duty cycle modulated to maintain the desired temperature.  
         [0027]     In the embodiment, the chiller may be a Thermocube 300-1D-1-LT, available from solid State cooling systems Pleasant Valley N.Y. Since the thermoelectric cooler may be more easily controlled than other cooling devices, the temperature may be kept at −1° C.: substantially higher than the temperatures used in the larger devices. This may thus may eliminate or minimize any problem of ice buildup on the inner walls.  
         [0028]     In operation, the condenser  130  supersaturates the air stream, and particles grow by condensation to a diameter that is above the cut point of the virtual impactor  150 . The virtual impactor is a minimized size virtual impactor with a 50% cut point of about 1.5 μ in aerodynamic diameter. Inertial forces are used to concentrate the particle containing droplets in the minor flow  155  of the impactor. The minor flow continues through the conduit  155 . The major flow  156  is substantially particle free, and is drawn away by vacuum pump  157 . In the embodiment, the vacuum pump may be a model 0523-101Q-G582 DX available from Gast of Benton Harbor Mich..  
         [0029]     In the embodiment, the minor flow can range between 0.6 L per minute and 2 L per minute depending on the application of the desired amount of enrichment  
         [0030]     The minor flow in conduit  155  is then sent to a dryer  160 . The dryer includes an inner tube  161  which is 1.1 cm in diameter and 15 cm in length. The tube is formed of a metal screen surrounded by baked silica gel  162 . The gel may be re-baked or changed periodically. The dryer removes the water from the droplets, and returns the particles to their original size. The output  170  is a particle enriched flow which is ready for sampling.  
         [0031]     In an embodiment, the entire system including the pumps and the chiller weighs less than 30 kg and occupies a space less than 40 cm wide by 60 cm deep by 150 cm high.  
         [0032]     The performance of this system and its components have been thoroughly tested. Test techniques have included collection of particles, fluorescence analysis of the collected particles, and others.  
         [0033]     One test technique has been to detect monodisperse fluorescent particles by collection and fluorescence analysis. Another has included detection of other materials. It was found that particle losses are less than 10% independent of particle diameter for minor flows up 1 ½ liters per minute. The standard deviation for the virtual impactor was 1.8, 1.25 and 2.32 for minor flows of 1.5 L per minute (minor flow ratio 0.05) 1 L per minute (ratio 0.033) and 0.6 L per minute (ratio 0.2) respectively. It was also found that particle volatility does not influence the amount of concentration of the aerosols.  
         [0034]     In the embodiment, the controller  118  may control the heating element, the war or control by the peristaltic pump  106 , the amount of cooling by the recirculating chiller  140 , and may also maintain information for example when the silica gel needs recharging.  
         [0035]     The general structure and techniques, and more specific embodiments which can be used to effect different ways of carrying out the more general goals are described herein.  
         [0036]     Although only a few embodiments have been disclosed in detail above, other embodiments are possible and the inventor(s) intend these to be encompassed within this specification. The specification describes specific examples to accomplish a more general goal that may be accomplished in another way. This disclosure is intended to be exemplary, and the claims are intended to cover any modification or alternative which might be predictable to a person having ordinary skill in the art. For example, the sizes given herein may differ, and additional or other parts may be used.  
         [0037]     Also, the inventor(s) intend that only those claims which use the words “means for” are intended to be interpreted under 35 USC 112, sixth paragraph. Moreover, no limitations from the specification are intended to be read into any claims, unless those limitations are expressly included in the claims.  
         [0038]     The controller described herein may be any kind of computer, either general purpose, or some specific purpose computer such as a workstation or formed of dedicated logic or a configurable logic block. The computer may be a Pentium class computer, running Windows XP or Linux, or may be a Macintosh computer. The programs may be written in C, or Java, or any other programming language. The programs may be resident on a storage medium, e.g., magnetic or optical, e.g. the computer hard drive, a removable disk or other removable medium. The programs may also be run over a network, for example, with a server or other machine sending signals to the local machine, which allows the local machine to carry out the operations described herein.