Bioaerosol detection through simultaneous measurement of particle intrinsic fluorescence and spatial light scattering
Interest in the role and detection of airborne biological micro-organisms has increased dramatically in recent years, in part through heightened fears of bioterrorism. Traditional bio-detection methods have generally slow response times and require the use of reagents. Conversely, techniques based on light scattering phenomena are reagent-free and are able to operate in real-time. Previous research has established that classification of certain types of airborne particles on the basis of shape and size may be achieved through the analysis of the spatial light scattering patterns produced by individual particles. Similarly, other research has shown that the intrinsic fluorescence of particles excited by radiation of an appropriate wavelength can be used to establish the presence of biological particles, provided background particles with similar fluorescence properties are not present. This is often not the case. This thesis, therefore, describes the design, development, and testing of a new type of bioaerosol detection instrument in which the advantages of both particle spatial light scattering analysis and intrinsic fluorescence are exploited. The instrument, referred to as the Mult- Parameter Aerosol Monitor (MPAM), is unique in simultaneously recording data relating to the size, shape, and fluorescence properties of individual airborne particles at rates up to several thousand particles per second. The MPAM uses a continuous-wave frequency quadrupled Nd: YVO4 laser to produce both spatial scattering and fluorescence data from particles carried in single-file through the laser beam. This use of a CW laser leads to opto-mechanical simplicity and reduces fluorescence bleaching effects. A custom-designed multi-pixel Hybrid Photodiode (HPD) detector is used to record the spatial scattering data in forward scattering plane whilst particle fluorescence is recorded via a large solid-angle ellipsoidal reflector and single photomultiplier detector. Calibration tests and experimental trials involving a range of both biological and nonbiological aerosols have shown that the MPAM, when supported by appropriate data analysis algorithms, is capable of achieving enhanced levels of discrimination between biological and non-biological particles down to the submicrometre sizes and, in some cases, enhanced discrimination between classes of biological particle.