We describe a prototype low-cost multi-channel aerosol fluorescence sensor designed for unattended deployment in medium to large area bio-aerosol detection networks. Individual airborne particles down to ~1mum in size are detected and sized by measurement of light scattered from a continuous-wave diode laser (660nm). This scatter signal is then used to trigger the sequential firing of two xenon sources which irradiate the particle with UV pulses at ~280 nm and ~370 nm, optimal for excitation of bio-fluorophores tryptophan and NADH (nicotinamide adenine dinucleotide) respectively. For each excitation wavelength, fluorescence is detected across two bands embracing the peak emissions of the same bio-fluorophores. Current measurement rates are up to ~125 particles/s, corresponding to all particles for concentrations up to 1.3 x 104 particles/l. Developments to increase this to ~500 particles/s are in hand. Device sensitivity is illustrated in preliminary data recorded from aerosols of E.coli, BG spores, and a variety of non-biological materials.
We describe a low-cost prototype bio-aerosol fluorescence sensor designed for unattended deployment in medium to large area networks. The sensor uses two compact xenon flash units to excite fluorescence in an aerosol sample volume drawn continuously from the ambient environment. In operation, the xenons are pulsed alternately at 300ms intervals whilst absorption filters restrict their radiation output to UV bands ~260-290nm and ~340-380nm respectively, optimal for exciting the biological fluorophores tryptophan and NADH. Fluorescence from all particles instantaneously present within a sensing volume is measured using two miniature photomultiplier detectors optically filtered to detect radiation in the bands ~320-600nm and ~410-600nm. The second of these bands covers the principal emission from NADH, whilst the difference between the first and second detector channels yields fluorescence in the 320-410nm band, covering much of the tryptophan emission. Whilst each sensor is clearly limited in specificity, the low sensor cost (<$5k) offers potential for the deployment in large networks that would be prohibitively expensive using particle fluorescence sensors based on currently available UV lasers. Preliminary details are also given of a variant of the sensor, currently under development, in which xenon illumination is used to acquire single particle fluorescence data at rates of up to 200 particles per second.
The use of intrinsic fluorescence to characterise airborne particles is often applied to the detection of biological materials, particularly micro-organisms. However, as a number of particles which are found in the atmosphere also fluoresce (whether natural or artificially generated), simple measures of particle fluorescence alone may not be sufficient to indicate the presence of biological agents in the atmosphere. An instrument has been developed for the real-time measurement of aerosols using UV induced fluorescence emission and elastic scattered light to characterise individual particles in terms of size, shape and fluorescence. Particles are detected as they scatter light from a CW red laser beam, which triggers a pulse of 266nm radiation to induce fluorescence. Elastic scatter from the red laser beam is used to measure particle size and shape parameters, and total fluorescence between ~300 to 500nm is collected. The performance of the instrument has been investigated in laboratory tests and field trials, using a range of biological agent simulants and interferents. An automated classification technique has been applied to assess the ability of the instrument to recognise potential threats against the natural background environment.
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