Testing of bioaerosol samplers ensures responders are armed for biological threats.
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Figure 1. Bench scale testing is typically the least expensive and shortest duration. In this approach, the unit under test is subjected to well-controlled conditions to determine collection rate, collection efficiency, level of detection, and ability to preserve viable organisms. Source: MRI
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Consider these two scenarios: an accident leaks a pathogen with potential to impact the food supply; a terrorist releases a biological agent in an act of aggression. In both cases, advanced monitoring and sampling systems would stand as the first line of defense in identifying the threat quickly, allowing responses to minimize the impact of the threat. Today, thousands of monitors comprised of passive and active samplers can scan soil, air, and water for threats from chemical, biological, radiological, nuclear, and explosive agents.
What are the monitors capable of detecting? What amounts? What determines the effectiveness and quality of those sampling devices? Midwest Research Institute (MRI), Kansas City, Mo., has tested and validated aerosol particle samplers for inert, biological, and chemical targets.
MRI initially conducted air sampling in the field for the U.S. Environmental Protection Agency (EPA) in the 1970s, developing collection and detection techniques for particle size ranges from 1 to 10 µm in diameter, the same range useful for biological specimens, according to Chat Cowherd, PhD, principal advisor for engineering. MRI expanded its expertise into remote sensing devices, and environmental monitoring at ultra-trace and background levels for chemical agents, biological agents, and other toxic materials.
Bob Barton, MRI’s principal advisor for National Security & Systems Integration, notes that a variety of bioaerosol samplers use inertial collection phenomena: impingers, wet concentrators, and dry filter units.
The collection approaches require that the target particles for collection achieve a high velocity relative to the collection media. The smaller the particle collected, the larger the velocity required. As a result, the collection equipment is large, requires significant energy, and tends to diminish the viability of collections. Impingers are the current “gold standard,” in part because the liquid collection medium tends to keep organisms viable and helps preserve samples for analysis.
In evaluating sampler technologies and product types, MRI compares the performance features of each type in terms of intake air flow rate, particle collection efficiency, sample viability, whether the sample is in liquid form for laboratory analysis, cost, portability, and energy consumption.
The evaluations also assess sampler ruggedness, reproducibility, reliability, and performance in challenging environments to predict incidences of false positives or false negatives. Other tests examine the effects from ambient environmental background as possible interferants, and the sensitivity to multiple biological threats by a single device.
There are three levels of testing for bioaerosol samplers: bench, chamber, and outdoor. Key factors considered within tests at each scale are:
- Dissemination methods for generating an aerosol of known characteristics;
- The transport phenomena responsible for ensuring that the aerosol is conveyed from the dissemination point to the unit under test;
- Ground-truth validation for accurate delivery and collection; and
- Preserving target samples in the aerosol.
Bench-scale testing
In bench-scale testing, the unit under test is subjected to well-controlled conditions to determine collection rate, collection efficiency, level of detection, and ability to preserve viable organisms. Bench testing validates performance parameters, such as detection limits for target samples of viruses, bacteria, and toxins. Typically, this method is the least expensive and shortest duration. The scale is amenable to work with the most dangerous target materials, such as human pathogens.
This testing scale can also determine the impact that selected common interferants and backgrounds will have on the ability of the system to detect target particles. The flow tube, the most common instrumentation used to conduct bench-scale tests, ensures that a well-controlled and characterized suspension is presented to the system under test and the associated monitoring systems (Figure 1).
Air entering and leaving the apparatus is passed through high-efficiency particulate air (HEPA) filters to remove particles and, if necessary, pass them through a carbon absorber to remove chemical vapors. This ensures that a well-defined background is present during the tests.
The choice of an aerosol-generation device is based on the nature of the target materials and the size distribution of the desired aerosol. It is common to use an aqueous suspension as the feed material. In this case, a nebulizer that generates a monodisperse droplet stream is used. The droplets dry quickly in the flow tube to create the desired particulate/aerosol-containing air stream.
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Bioaerosol chamber sampling ports monitor the dynamic state of the aerosol cloud during testing and can maintain temperature ranges and relative humidity levels. Image: MRI
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Typically, reference aerosol concentrations are measured using real-time instrumentation, such as aerodynamic particle sizers. Ground truth samples are normally collected using impingers. The concentration of collected samples is subsequently determined by a suitable analysis, such as enumerating the colonies appearing after growth in culture, by plaque assay, or by enzyme-linked immunosorbent assay (ELISA). For other target collections, gas chromatography with mass spectral detection (GC-MS) or liquid chromatography with mass spectral detection (LC-MS) can be used for analysis.
Chamber testing
Bioaerosol chamber testing offers a higher scale of sampler evaluation by closely replicating expected deployment conditions. A chamber test—using a room-sized containment system—can measure responses to time-duration challenges, environmental variations like temperature and humidity, and realistic spatial concentration variability. Cost and safety concerns often prevent using the most dangerous materials at a chamber scale.
MRI uses a 2,200-ft3 room with HEPA and carbon filters on the inlet and outlet air. Bioaerosol chambers and Bio-Safety Level 3 facilities are key assets to develop, test, and evaluate sampling and monitoring instrumentation in expected-use conditions.
The chamber has sampling ports to monitor the dynamic state of the aerosol cloud during testing, and can maintain temperature ranges from 35°F to 100°F with relative humidity levels from 15% to 90%. Ultraviolet germicidal lamps in the ceiling can decontaminate the chamber.
Key parameters in chamber testing include maintaining controlled exposure concentrations; dispersion and mixing of the aerosol into the chamber air; and verification of exposure concentrations.
Chamber conditions are at slightly lower air pressures than the surrounding area to ensure no outward migration of particles. Sample fluid can be diluted as needed and injected into the chamber at the air inlets by atomizing nozzles or droplet generators. The air must be well-mixed before arrival at the test location inside the chamber. Air concentrations can be measured in real time using a particle counter.
Aerosol generation, dissemination, and detection devices inside the chamber verify performance for the unit under test.
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MRI is working to improve the service of bioaerosol samplers. One approach using electrostatic precipitation technology can be applied for use in very small unmanned aerial vehicles. Image: MRI
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Outdoor field testing
Outdoor testing—typically occurring after bench and chamber testing stages are complete—combine all the expected usage conditions for samplers and other instruments in open air events. This third scale of testing is conducted after laboratory and chamber testing stages are completed to assure that essential performance is ready for more demanding environments. Besides costs and time, outdoor tests encounter uncontrolled variations, such as weather. Operations can demand a uniquely isolated location. Testing may be restricted to environmentally benign target materials.
Outdoor tests with kilometer-scale dimensions pose extraordinary demands. These tests require higher-volume dispensers and more rigorous ground-truth testing and validation capabilities to differentiate introduced materials from the background.
The same basic challenges carry over from laboratory and chamber testing to assure accurate and valid results for dissemination, transport, ground truth events, and preservation of target particles. The introduction of unknown interferants can reveal performance variations that challenge the clarity of the results and limitations in the sampler’s capability to discriminate target materials.
The future of testing
Joe Langle, engineering section manager at MRI, says that after developing and applying disciplined and reliable processes, MRI is aware of what works, what could work, and what may be needed in the future. For example, industry is increasingly looking for more compact packaging and more energy-efficient collection of small particles.
MRI is now working to improve the service potential of bioaerosol samplers. One approach uses electrostatic precipitation technology to offer more collection efficiency and higher sampling volume. This design requires relatively low power and is inherently rugged and adaptable—it can be applied to samplers that range in size from a few grams for very small unmanned aerial vehicles (micro-UAV) to large systems capable of sampling more than 10,000 L of air a minute.
Bioaerosol sampler testing and validation perform a crucial role in serving public safety and national security. As demands grow for biological samplers to monitor work places, to be vigilant during large public gatherings, and to defend war fighters around the world—the value also increases for testing these devices.
Published in R & D magazine: Vol. 52, No. 4, August, 2010, pp. 26-28.
