Bioaerosol Sampling and Detection
Detection of viruses and bacteria is normally a time consuming laboratory process, and is not always guaranteed to be successful unless one knows exactly what one is looking for. Detection of airborne pathogens in an airstream is almost impossible, except that certain new technologies offer some promise. If one is not concerned with discriminating pathogens from each other, or from other bioaerosols, then the current technologies for sampling and detection in airstreams may be adapted to this purpose. Sampling and detection methods fall into four general categories, Filtration, Inertial & Gravitation, Optical & Electrical Mobility.Most methods require isokinetic sampling. This is usually accomplished using sharp edged probes and suction pumps. Sampling points must also be chosen a sufficient distance away from disturbances (or system effects). Some examples are shown in the figure.
The collection of samples by filtration is the simplest method and requires microscopic analysis which can be tedious. Different filter porosities are used in stages to collect different sized particles.
Sedimentation & Inertial Samplers
Elutriators
In an elutriator laminar airflow is introduced into a settling chamber where the particles separate under the influence of gravity according to Stokes Law. The terminal velocity is given by
where rhop = particle density, gm/cm3
rho0 = gas density, gm/cm3
Dv = Volume equivalent dia., cm
X = dynamic shape factorThe size range for this method is 2 to 10 microns.
Cascade Impactors
Cascade impactors are the most common samplers and work by directing laminar airflow into and around a series of impact plates. The air velocity increases at every stage such that large particles are deposited in the first stage and successively smaller particles are collected through the remaining plates. The final stage usually contains a submicron filter.
Virtual Impactors
Particle bounce, dislodgement and overloading can reduce the efficiency of a cascade impactor and this has led to the development of virtual impactors, in which the impact plate is replaced with a large stagnant volume.
The size range for both cascade and virtual impactors is about .08 to 35 microns.
Cascade Cyclones
Gas cyclones can be used in place of impactors in the range of 0.5 to 20 microns. The cyclone uses centrifugal force to isolate and collect particles in a series of stages.
Real-Time Analyzers
Real-time analyzers utilize twin laser beams to measure the velocity of particles after sub-sonic acceleration. The recorded transit times can be related directly to the aerodynamic diameter through the equation of motion of the particles
where dUp/dt is the particle acceleration
The actual effective range of this device is from 0.5 to 10 microns. The results outside this range can be distorted by particle albedo, inertial losses at the inlet, and particle overlap.
Centrifugal Spectrometers
Aerosol spectrometers may be divided into two classes, centrifuges and fixed geometry devices. The centrifuges use a rotating spiral channel to direct flow to a collection surface.
At terminal velocity
where x is the disrance from the axis of rotation to the particle location, dx/dt is the outward velocity of the particle. The particle size range can be as wide as 0.08 to 5 microns.
The fixed geometry spectromers utilize a static bend, such as a 90 degree turn, to accomplish the same end, but with a range of about 0.5 to 10 microns.
Analyzers that depend on the interaction between particles and light fall into one of five categories.
Optical Particle Counter
A variety of optical analyzers are currently in use, and most of these depend upon light scattering theory, which requires that
where i is the scattered light intensity
theta is the scattering angle
phi is the polarization angle
lambda is the wavelength of light
m is particle mass
d is particle diameterThe efficiency of these analyzers is restricted by border errors and coincidence errors, especially at high concentrations, although these can be improved with correction factors. Particle range can be typically 0.3 to 20 microns.
Laser Diffractometer
Laser diffractometers look only at the diffraction component of scattered light (ignoring reflection, refraction and polarization). The diffraction component of the scattered light intensity is given by the Fraunhofer expression:
Phase-Doppler System
Laser (phase)-Doppler systems detect the light scattered as particles traverse a series of interference fringes formed from the intersection of two laser beams. Particle velocity and size can be obtained by this method.
Intensity Deconvolution System
These devices measure the absolute light intensity scattered from a focused laser beam on an optically defined volume. Since the particle sizes may vary the potential ambiguity must be resolved through a deconvolution algorithm in a manner analagous to the way CAT-scan images are obtained. This technique is capable of resolving a size range of 0.2 to 200 microns up to a concentration of 107 particles /cm3, but is very expensive.
Laser-Particle Interaction/Image Analyzer
This device is a hybrid of two of separate devices and determines particle size by light blockage and time-of-transition as well as by determining shape factors by rapid image analysis. The range of this instrument is a remarkable 0.5 to 1200 microns, but this represents 4 separate settings.
Electrical Mobility Techniques
Electrical Aerosol Analyzer (EAA)
An EAA consists of a unipolar diffusion charger, a mobility analyzer and a detector. Particles acquire an electrostatic charge, pass through the analyzer and then collect at the detector filter where the charge drains to ground. Through the use of staged current and deconvolution techniques, it is possible to discriminate particles in the range of 0.013 to 0.75 microns.
Differential Mobility Analyzer
DMAs work on similar principles but have an electrostatic classifier in place of the mobility analyzer, as well as some operational differences. The range is 0.01 to 0.9 microns.
LIDAR System
The newest technology on the market is LIDAR (LIight Detection And Ranging). Lidar uses light waves in the same way that radar uses radio waves. A laser shoots a beam of coherent light at a specific frequency at some target. The light which is back-scattered from objects, including molecules and bioaerosols, is received by mirrors and analyzed, again similar to the way radar signals are interpreted. Such systems are in use for monitoring atmospheric properties. A system is currently being developed for the Army which uses a frequency-shifted (266 nanometer) diode-pumped Nd:YAG laser, with output converted to 289 nm, to monitor and detect bioaerosols in air. It accomplishes this by monitoring the presence of tryptophan, which is present in all biological materials, and which fluoresces in the 300-400 nm range.
Biosensors
The technology of biosensors holds great promise for the future of bioaerosol detection. Biosensors comprise an analyte or immobolized reagent integrated with an electronic tranducer. The chemical reaction that occurs between a bioaerosol and the reagent triggers an electrical response in the transducer. This response is then amplified and relayed to some annunciation device. In theory a large number of these tiny biosensors would form an array capable of detecting any one of the known viruses or bacteria and would provide rapid identification as well as digital output to some comtroller or computer.
Microbial biosensors would be designed with analytes that have a specificity for microbiological components such as Vitamin B or tryptophan. The mechanisms by which the microbes are detected could include electrochemical (amperometric, conductimetric or potentiometric) or electromagnetic (optical or mass).
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