Sensor Systems for Biological Agent Attacks Protecting Buildings and Military Bases (2005) / Chapter Skim
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4 Bioaerosol Samplings Systems for Near-Real-Time Detection
4 Bioaerosol Samplings Systems for Near-Real-Time Detection
Pages 46-70
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From page 46... ...
,' the aerosol is collected in the dry state prior to flhe addition of B cells for analysis. In the future, there may be optical devices that can speciate airborne organisms, in which case neither a liquid-based nor a dry collector would be needed, because the particles would be identified in the aerosol state 2 Commercial bioaerosol sampling systems are available from several companies, and the Department of Defense has developed prototype bioaerosol detection systems that include sampler technologies.
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From page 47... ...
As a consequence, the use of novel materials and manufacturing processes may be required during development and production of DTW bioaerosol samplers. PARTICLE SIZE CONSIDERATIONS In general temms, a sampling system must be able to collect aerosol particles sized such ~ at they can most efficiently be deposited in the human respiratory system.
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From page 48... ...
Sampling from Occupied Environments For sampling of aerosols in building environments, either from ductwork or from the occupied environment, panicles with sizes outside the nominal range 1 to 10 pm AD should be considered. The upper size for occupied environment sampling applications should be selected after considering several factors such as the potential methods for aerosolization of threat bioagents, the loss of particles during transport from the site of aerosolization to the sampling location, the effectiveness of filters and air conditioning components in duct sampling applications, and a realistic assessment of being able to collect a sample of the aerosol particles and transport it to the detector.
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From page 49... ...
There are standardized methods for batch-sampling aerosol particles fnam stacks such as those codified by the EPA.8 The batch techniques compensate for irregularities in the concentration and velocity profiles by sequentially sampling at prespecified points on a geometrical grid across the stack cross section. At each traverse point on the grid, the velocity of gas at the inlet plane of the sampling nozzle is set to equal that of the undisturbed stack velocity at the particular point.
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From page 50... ...
were typically used to span a duct cross section. However, there are two problems with this approach: The nozzles of such rakes will not all be isokinetic because of natural spatial variations in a velocity profile, and, more importantly, substantial aerosol particle losses can occur on flhe inner walls of the nozzles." Tests with an ANSI-1969'2 nozzle operated isokinetically in an aerosol wind tunnel at 10 meters per second showed losses of 75 percent for 1 0-pm AD aerosol particles,'3 and tests with a rake of nozzles in a nuclear stack showed only 41 percent of the radionuclide activity was associated with material collected on a sampling filter, with the remainder lost in the sampling system.'4 Because of problems with flhe extractve sampling approach used in ANSI N13.1-1969, a more robust approach was developed for continuous emission monitoring of the stacks and ducts of the nuclear industry.'5 This methodology is single-point representative sampling, whereby a sample is extracted at a location in the duct where both fluid momentum and contaminant concentration are well-mixed, as manifested by the unifommity of flhe velocity and contaminant concentration profiles.
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From page 51... ...
transmission of aerosol parades with sizes less than or equal to 10 pm AD, as is illustrated in Figure 4.4; however, they have not been tested with aerosol particles as large as 30 pm AD. Aerosol transmission of a sampling nozzle or probe is the ratio of aerosol concentration at the exit plane of the nozzle to the undisturbed aerosol concentration of the flow stream at the location of the probe.
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From page 52... ...
26:111-126. The Deposition software, which is acceptable methodology for demonstrating compliance with flhe nules of the Nuclear Regulatory Commissions and ANSI N13.1-1999 for estimating aerosol particle losses in sampling systems, can be used to evaluate flhe effectiveness of systems for sampling variously sized bioaerosol particles in ductwork.
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From page 53... ...
The variation of collection efficiency of the cyclone with particle size is shown in Figure 4.7. Ideally a fractional efficiency curve such as that shown in Figure 4.7 would be a step function, where particles smaller than the cutpoint would pass through the cyclone and those larger than the cutpoint would be collected.
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From page 54... ...
classical impactor and (b) virtual impactor.
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From page 55... ...
where C = Cunningham's slip correction ,24 which has a value very nearly equal to 1 for particles greater than 10 pm AD; pa = density of water; Dr'= aerodynamic particle diameter, Ul = velocity at the exit plane of the acceleration jet; ~ = air viscosity; and, Al = diameter of the acceleration jet at its exit plane. The cutpoint Stokes number is approximately a constant, so for a fixed flow rate different cutpoint sizes can be achieved by varying the jet diameter.
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From page 56... ...
. Were it not for wall losses in a virtual impactor, the concentration of aerosol particles with sizes smaller than the cutpoint size in the fine particle stream would be approximately equal to the concentration of that size fraction in the sampled airstream.
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From page 57... ...
The collection system requires very little energy to effect the particle collection—the pressure loss is only about 1 kilopascal. Because the basic Collection concept is cyclonic separation of the aerosol particles, such a system can be scaled to accommodate other air sampling flow rates.
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From page 58... ...
One drop of B-cell hydrosol is added to the tube, which is then centrifuged for 5 seconds prior to detection. A bench-scale prototype sensor that employs this concept utilizes a single-stage virtual impactor that is designed to concentrate 1 to 10 pm AD aerosol particles from a 33 liter per minute airflow into a 3 liter per minute flow 3~ The efficiency of this concentrator is approximately 40 percent for particles 1 pm in size and 60 percent for part ales 3 to 10 pm in size 32 A nonspeciating optical trigger (30 second detection time)
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From page 59... ...
However, at the present bme, area samplers are most relevant to occupied environments where ductwork sampling is not an option, e.g., occupied environments with no central air conditioning systems. Sampling systems for detection of bioaerosol particles in the occupied environment of a building are of relatively straightforward design, as shown by the sample system in Figure 4.11.
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From page 60... ...
The near-real-time radionuclide aerosol detection systems of the nuclear industry have focused on aerosols that will penetrate to the thoracic region of the human respiratory system and not on particles with sizes as large as 30 pm AD. It is anticipated that new inlet designs will need to be developed to accommodate sampling of these larger bioaerosol particles.
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From page 61... ...
Inertially affected aerosol particles can be lost in an inlet as a result of curvature of flhe streamlines when the flow turns the comers to pass through the rain-protective elements and when the aerosol takes on a vortex flow pattern. Because the phenomena that induce losses are related to airstreamline curvature, the losses can be characterized as Stokes number dependent.
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From page 62... ...
A typical system is the SOP Model 1001, which samples air at a flow rate of 1,000 liter per minute and concentrates particles in the range of 2.5 to 10 pm AD into a flow stream of 1 liter per minute. MSP Corporation, also of Minneapolis, developed a virtual impactor that samples at a flow rate of 300 liters per minute and concentrates aerosol particles in the range of 2.5 to 10 pm AD into a coarse aerosol particle exhaust flow rate of 1 liter per minute.49 The JBPDS has a cyclone concentrator that samples air at a flow rate of 780 liters per minute and concentrates the particulate matter into a hydrosol flow of 1 milliliter per minute, which is then subjected to near-real-time bioanalyses.
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From page 63... ...
For a virtual impactor concentrator, the fractional efficiency is the ratio of the particle mass flow rate associated with a given small interval of particle size in the exhaust stream of a concentrator to the particle mass flow rate for the same size interval in the sampled airstream. Inadvertent losses of aerosol particles on the internal walls of a concentrator reduce the fractional penetration.
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From page 64... ...
Also, for virtual impactors with small slit or nozzle dimensions, there is a problem of Meaning, particularly for the latter stages of multistage devices where flhe concentration at the entrance plane is high s' indeed, if the design cutpoint were 0.5 pm AD, flhe slit widths would be comparable in size to the upper end of the 0.5 to 30 pm range, which suggests the nozzle could be plugged by some of the very particles it is designed to transmit. Performance of Cyclonic Concentrators The Aerojet General cyclone is designed to sample air at a rate of 1,000 liters per minute and to concentrate the particulate matter into a liquid flow rate of 1 milliliter per minute, thereby providing an ideal aerosol-to-hydrosol concentration factor of 1 05.
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From page 65... ...
The pressure loss in virtual impactors can be kept low by use of extended slit lengths to accommodate both desired flow rate and cutpoint. In general, the pressure loss varies as the square of the velocity at the exit plane of the acceleration jet, but if the velocity is held approximately constant as the design cutpoint is reduced, the pressure drop will not be significantly affected.
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From page 66... ...
In contrast, virtual impactor concentrators, when used in series with aerosol-to-hydrosol transfer stages, require little extra power to prevent freezing of the liquid in the AHTS. In the case of a system such as the SCP Model 1001 concentrator, which has an airflow rate of 1 liter per minute at the exit port, only 2 to 3 watts would be needed to heat the airstream from -30°C to +5°C.
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From page 67... ...
(c) FIGURE 4.14 An aerosol-to-hydrosol transfer stage: (a)
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From page 68... ...
Recommendation 4-3: The development of integrated, turnkey sampling systems should be supported for aerosol particles that are automatic, robust, and require little maintenance. Studies should be conducted on the advantages and disadvantages of developing systems that would extend the range for extraction, transport, and collection of aerosol particles, from 1 to 10 pm AD to 0.5 to 30 pm AD.
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From page 69... ...
However, it may not be possible to accomplish this, because very small slit widths (or jet diameters for circular jet virtual impactors) are needed.
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From page 70... ...
Use of new materials and fabrication techniques should be considered. Finding 4-10: An aerosol concentrator can require the expenditure of considerable power.
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