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
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 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
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 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
-
Telecommunication hardware
-
“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 System network
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.
|
