A partnership that includes the Naval Research Laboratory (NRL), MIT Lincoln Laboratories and the Edgewood Chemical and Biological Command is engaged in an effort to develop optical techniques for the rapid detection and classification of biological aerosols. This paper will describe two efforts at NRL: development of an improved UV fluorescence front-end trigger and the use of infrared absorption spectroscopy to classify biological aerosol particles. UV Laser-induced fluorescence (UVLIF) has been demonstrated to provide very high sensitivity for differentiating between biological and inorganic aerosol particles. Unfortunately, current UVLIF systems have unacceptably high false alarm rates due to interferences from man made and naturally occurring organic and biological particulates. We have developed a two-wavelength, UVLIF technique that offers a higher level of discrimination than is possible using single wavelength UVLIF. Infrared absorption spectroscopy coupled with multivariate analysis demonstrates a high potential for differentiation among members of biological and chemical sample classes. Two-wavelength UVLIF in combination with the IR interrogation of collected bioaerosols could provide a rapid, reagentless approach to specific classification of biological particles according to an operational level of discrimination - the degree of particle characterization required in order to signal the presence of pathogenic material.
We examine how aggregation affects the light-scattering signatures, especially the polarization in the near-backward-scattering direction. We use the discrete dipole approximation (DDA) to study the backscatter of agglomerate particles consisting of oblong monomers. We examine the effects of monomer number and packing structure on the resulting negative polarization branch at small phase angle. We find large a dependence on the orientation of the monomers within the agglomerate and a smaller dependence on the number of monomers, suggesting that the mechanism producing the negative polarization minimum depends strongly on the interactions between the individual monomers. We also examine experimental measurements of substrates composed of biological cells. We find that the light-scattering signatures in the backward direction are not only different for different spore species, but for spores that have been prepared using different methodologies. These signatures are reproducible in different substrates composed of the spores from the same batches.
A two-wavelength excitation bioaerosol sensor has been developed and characterized for classifying various types of aerosols, including biological organisms and non-biological interferents. Single aerosols, smaller than 10 μm, are interrogated with 266 nm and 355 nm laser pulses separated in time by 400 ns. Fluorescence signals excited by these pulses are detected in three broad spectral bands centered at 350 nm, 450 nm and 550 nm. The results indicate that bacterial spores, vegetative bacterial cells and proteins can be differentiated based on the two wavelength excitation approach.
We describe an instrument developed to monitor the biological fraction of ambient aerosol. This device simultaneously sizes individual particles in an air stream, and measures their total fluorescence following excitation at 266 nm. Recent results of single blind outdoor tests carried out in Alberta, Canada are described. In these, aerosols were generated containing four different types of biological material: ova albumin, MS-2 phage, Erwinia herbicola vegetative cells and Bacillus subtilis spores. Results indicate a probability of detection of 87% was achieved for target aerosol concentrations as low as a few (1-5) P particles/liter. Absolute quantitative detection efficiencies for individual bioaerosols were at least 70%. During the tests, nonbiological aerosols were also released and found not to generate any significant fluorescent signals.