Wyatt Aerosol Systems, LLC (“Wyatt Aerosol”) is completing development of instrumentation capable of monitoring, classifying, and detecting in real time, respirable aerosol particles within specific closed environments, and determining whether the targeted environment is within predetermined operational and regulatory ranges.
The detection and classification of aerosol particles is based upon a technique referred to as “Multiangle light scattering” or MALS. An aerosol particle passing through a fine laser beam scatters the light incident upon it into all directions, though with varying intensities and polarizations. The intensity variation with scattering angle depends upon the wavelength and polarization of the laser beam, the specific angular direction, the structure of the particle, and its orientation with respect to the incident beam direction. From collection of such scattering signals at a fixed number of detectors, each placed at a different scattering angle, many of the structural properties of the scattering particle may be deduced. This detector placement at multiple angles and the measurement at each such angle is referred to by the MALS acronym. The deduction of physical/structural properties from these MALS measurements is a consequence of the so-called “inverse scattering problem” by which means physical properties are derived from the measured scattered signals. This is a complex and difficult analytical process whose application to aerosol particles forms the basis of the Wyatt Aerosol instruments.
Dr. Philip 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 co-workers, he subsequently proceeded to explore, under contract with 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 structures of 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 determine robustly 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 100 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 Aerosol and Obscuration Sciences, the U. S. Bureau of Mines, and finally and the University of Minnesota 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 those 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 (spherically symmetric) are compared, with the patterns in the plane φ = π/2.
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 along the polar axis at from 180° to 0°.
Despite the similarity of these aerosols, 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 are a key and proprietary element of the Aris™ instrument.
In one of the later studies, MALS was shown to discriminate between species of phytoplankton in vitro (sea water) 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 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.
The MALS instrumentation, incorporated into the analysis 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, AMD, and many others. Modestly priced single board high speed telecommunications sets are also readily available to permit remote monitoring by telecommunication to a control center. 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.