REAL TIME MONITORING AND WARNINGS
Wyatt-Lorenz is developing a system capable of monitoring, in real time, airborne particles and determining whether a biological and/or chemical attack is present. This information is used to create an alert of the attack while it is occurring.
Multi-Angle Light Scattering
The detection of airborne bioweapons-type materials is based on a technique referred to as “Multiangle light scattering” or MALS. Similar techniques are commonly used in both science and technology for elucidating structure. Examples include x-ray crystallography and electron or neutron scattering to determine structure at the atomic and molecular scales, multi-angle laser scattering at the macromolecular and sub-micron scales, and microwave (radar) scattering at the macroscopic scale. These can also help determine the material composition of the target through measuring the refractive index (e.g., differentiating water from other materials).
Dr. Wyatt was the first to propose the use of scattered light as a means for identifying and characterizing microorganisms in his theoretical paper of 1968 [P.J. Wyatt, Applied Optics 7 10(1968)]. Together with coworkers, he subsequently proceeded to explore, at the behest of the Department of Defense, the concept of identifying airborne bacteria and spores in real time using multi-angle light scattering methods [P.J. Wyatt and V.R. Stull, Project 1W662711A096 USAMR&D Command, Feb. 1972], constructing the first laser-based device for this purpose. The early prototype measured high-resolution polar angle scattering patterns, experimentally confirming theoretical predictions of these patterns and demonstrating
the potential for discriminating bacteria and spore strains via MALS. Later studies [G.M. Quist and P.J. Wyatt, J. Opt. Soc. Am. A 2, 1979 (1985); Y.L. Pan et al., Appl. Phys. Lett. 28, 589 (2003); P.J. Wyatt and C. Jackson, Limnology and Oceanography 34 96(1989)] provided further confirmation of the capabilities of MALS, examining scattering patterns due to characteristic internal structuresof such individual bioaerosol particles in a natural airborne state, as well as studying background aerosols.
Data reduction of MALS signals to a few characteristic Optical Observables (OO’s) was proposed in a 1985 paper which demonstrated a quasi-empirical approach to robustly determine the structure and identity of particles. Some years later, under the aegis of the newly formed Wyatt Technology Corporation, MALS instrumentation was developed for the U. S. Army to make such measurements in real time, sampling single particles in an aerosol stream at rates up to 1000 particles per second. The DAWN-A shown in Fig. 1 comprised a spherical scattering chamber holding up to 36 fiber-coupled photomultiplier detectors subtending discrete collection angle positions on the chamber surface, defined in terms of the polar angles
theta (θ) and phi (φ). In conjunction with these is a fine laser beam passing through the center of the chamber and intersecting the aerosol stream. Two of these systems saw over 10 years of continuous use by the Army and the
demonstrating different applications of MALS for aerosol characterization and classification.
Figure 1. Schematic drawing of the DAWN-A MALS chamber
In MALS, the variation of scattered intensity and polarization with angle depends critically upon the size, shape, material, orientation, and internal structure of the particle. Various studies, such as the ones referenced previously, have shown how MALS data may be used to differentiate spores, bacteria, toxin droplets and fine particulate matter. Examples are shown in Fig. 2 where the scattered light intensity patterns from single aerosol particles of approximately the same size and shape are compared, with the patterns in the plane φ = 0.
Figure 2. Multi-Angle Light Scattering patterns in the plane Φ = 0:
a) Smog particle; b) Cell S. epidermidis; c) Flyash; d) Spore B. sphaericus.
The laser beam propagates in the direction of the polar axis at 0°.
Despite the similarity of these bioaerosols, the scattering pattern from a bacterial B. sphaericus spore is distinct from that of a bacterial S. epidermidis cell, and both of these quite different from those of other droplets and fine particles. These patterns were reduced to a simple set of OO’s describing the key pattern characteristics. The measured OO’s were sufficient to clearly differentiate the aerosols with high purity. Knowledge of these absolute and relative intensities in the form of robust OO’s permits the scattering particle to be correlated with a unique class, such as a bacterial spore or virus-laden droplet. Even if a spore were disguised with a thin coating of aluminum, for example,
certain combinations of the recorded MALS patterns could still be used to determine the OO that classifies it. The extraction of these optical observable descriptors for summarizing the distinguishing features of the MALS patterns is a key and proprietary element of the W-L Immediate Warning System.
In one of the later studies, MALS was shown to discriminate between species of phytoplankton in vitro at a confidence level >99% by comparing measurements of OO’s that describe the ratios of optical scattering amplitudes and depolarizations at specific angular positions.
The Scattering Chamber
The central component of this detection technology is the scattering chamber itself, which allows particles to be examined one-at-a-time as they are constrained to pass through one or more fine laser beams. An integrated aerosol handling system guides the particles through the laser beam by means of a sheath flow of particle-free air. Prior to being drawn into the chamber, the handling system preconditions the aerosols by removing both large (> 3μm) and small (<300nm) particles outside the characteristically targeted respirable size range. This aerosol handling system is integrated into the DS and is capable, as well, of diluting the ambient particle density as necessary to ensure that only a single particle
is in the laser beam at any moment.
The MALS instrumentation, incorporated into the detector units, uses off-the-shelf components such as diode lasers, microprocessors, and photo-detectors. Diode lasers are now extremely inexpensive as their use with such commodity devices such as DVD and CD players has resulted in high volume production and rapidly falling prices. The extremely complex analyses required as each particle is measured are now easily performed at great speed using readily available single board microprocessors of the advanced design such as produced by Intel and AMD. Modestly priced single board high speed telecommunications sets are also readily available. The on-board data-processing requirements are relatively
low since the optical observables of any single particle would be comprised of approximately 20 values. This reduces both processing and bandwidth requirements with consequent cost savings. It also allows for the processing of up to 50,000 particles per minute or more. Note, however, that biological threat particles would be expected to arrive at the DS at very low concentrations, perhaps resulting in only a few detections per hour.
The Distributed System
A distributed system involves two separate elements: detector stations (DS’s) and a central station (CS). Each detector station includes an aerosol handling module, a microprocessor controlled data acquisition and processing board, light scattering chamber with detectors and laser, possible fluorescence detection chamber with UV light source, sample collection electrostatic deflector, and an integrated wireless transceiver able to send and receive both raw and processed data. There are many appropriate manufacturers of much of this equipment. The CS is comprised primarily of a large high speed microprocessor and an associated telemetry link able to communicate with each of its associated DS units as
well as to analyze the data sent to it from each such DS in its network. Depending on the number of units comprising a fully deployed system (for example, a shopping mal may have as many as a few hundred DS units and a dozen distinct CS units), processed data may be transmitted for alarm and warning purposes to a command and control center at the customer’s facility or to off-site locations.
A DS contains the following components:
Aerosol handling system
Laser light scattering chamber plus UV excitation source and detectors
High speed CPU
Controlling and parametrized software and data storage means
“Collect-on-demand” capability to retain specific particles
Sufficient ROM and RAM
A CS contains computer processor hardware able to:
Correlate processed data streams from all detection stations
Instruct specific DS to search for specific particle classes
Modify the software of any DS to perform specific searches
Record and identify locations where DS-collected samples may be retrieved and analyzed
Issue early warning alarms or provide facility command center with such alarms
Sufficient ROM and RAM
On rare occasions, a single DS may be appropriate to monitor a local region (e. g. in the cabin of a passenger plane), although in most cases, multiple DS units will be distributed throughout each protected facility. In these cases, the programmed CS will process the collective data from each of the multiple detection stations to combine, quantify and refine further all such information collected in real-time to predict the severity of the attack and its spatial migration. By monitoring the data collected by the multiple DS units, confidence can be increased by the appearance of unusual and similar respirable particles at multiple detectors, which might typically occur with some temporal lag. With the known
location of each DS, and the local detection variations being continuously transmitted, it should be possible to predict where the threat agents may next appear and at what concentrations. This predictive capability will be extremely helpful in orchestrating appropriate responses throughout large facilities.
Figure 3 illustrates an exemplar of a proposed Immediate Warning Systemnetwork to protect a building complex. With such a distributed network, the speed and accuracy of detecting bioattack agents is greatly enhanced.
Figure 3. Networked system of W-L DS and CS units.
The decisions involving when and how to issue an alarm will vary by institution. The CS itself can be triggered to set off an alarm, or it can be designed to alert those in charge of “alarm conditions”, who will make the decision to issue an alarm. The size of the protected facility and its location are two factors that are taken into consideration when installing and configuring an Immediate Warning System for each venue and determining exactly the type of alarm that should be issued to those at risk.
Remediation and Forensic Tools
In addition to the Immediate Warning System, W-L will manufacture and sell tools derived from these systems to be used for remediation and forensic purposes following bioweapon attacks or industrial accidents involving bioagents. Once an attack has been verified, a portable, handheld detection unit will be used to locate the exact site of the attack. The unit would be programmed to recognize particles of interest and issue an alarm immediately upon detecting such particles.
The location believed to harbor the particle can be queried by loosening particles either ultrasonically or with an air burst, and the device can then be used to analyze the particles and issue an alarm if they match the pre-programmed profile. Clean-up of the area may be monitored in a similar fashion, as well. In addition, this apparatus will collect the particles of interest and store them for future examination, laboratory confirmation, and/or destruction.