An ultraviolet (UV) laser induced fluorescence (LIF) light detection and ranging (LIDAR) system has been constructed
and commissioned by Dstl and demonstrated to be an effective technique for discriminating between some common
fluorescent potentially interfering aerosols and biological warfare agent (BWA) simulants at a distance remote from the
release. The Mk 3 UV-LIF LIDAR employs the fundamental wavelength (1064 nm) of a Nd:YAG laser to spatially map
aerosol clouds, and the fourth harmonic (266 nm) to excite fluorescence. The fluorescence emission is spectrally
resolved into ten detection channels between 300-500 nm, permitting classification by a discrimination algorithm. The
UV-LIF LIDAR was trialled in 2007 in the Joint Ambient Breeze Tunnel (JABT) and on the open range, at the US Army
Dugway Proving Ground (DPG), Utah. In the JABT, calibration instruments were used to characterise the BWA
simulant and interferent aerosol releases, permitting calculation of the system's limits of detection (LoD) and
discrimination ability.
Light detection and ranging (LIDAR) has potential to be a successful technique for remote detection of airborne
biological warfare agents (BWA) that pose a health hazard. Potential techniques for detecting BWA often use
spectroscopy to probe molecular structure properties (e.g. UV-fluorescence, Raman and differential absorption
spectroscopy). An alternative approach is to differentiate BWA from background interferents by their differing
morphology; depolarisation offers one such method. Here, we investigate the feasibility of introducing depolarisation
into a short range (approximately 10 m) LIDAR designed to be a simple, inexpensive, low power consumption, portable
instrument.
T-matrix calculations are presented for a randomly oriented, polydisperse size distribution of Bacillus atrophaeus
spheroids. The relationship between backscatter depolarisation and particle aspect ratio is investigated at several incident
wavelengths corresponding to those produced by low cost, commercially available laser sources. Through a series of
simulations, we determine the best combination of wavelengths for a multi-wavelength instrument design that exploits
the concept of normalised depolarisation to determine particle aspect ratio, with the possibility of facilitating BWA
detection.
Detection of biological warfare agents must be achieved as far upwind of their potential target as possible to provide the
time necessary to adopt an effective protective posture. A small-scale fluorescence lidar has been designed and
constructed by Dstl. The active element is a solid state Nd:YAG laser, the frequency of which is quadrupled to yield
266nm excitation of 9ns pulses with 40mJ energy. Fluorescence is collected from 300 to 500nm and is divided into 10
channels to investigate discrimination between common fluorescent interferent aerosols and biosimulants. The UV Laser
Induced Fluorescence (LIF) LIDAR operated in trials to assess standoff biological detection systems at Dugway Proving
Grounds, Utah, USA, participating in both breeze tunnel and open range trials. The collected biological simulant and
interferent data has been utilised to train the discrimination algorithm and to assess the system's limit of detection and
discrimination ability.
Laser diodes and light-emitting diodes capable of continuous sub-300 nm radiation emission will ultimately represent optimal excitation sources for compact and fieldable bio-aerosol monitors. However, until such devices are routinely available and whilst solid-state UV lasers remain relatively expensive, other low-cost sources of UV can offer advantages. This paper describes one such prototype that employs compact xenon discharge UV sources to excite intrinsic fluorescence from individual particles within an ambient aerosol sample. The prototype monitor samples ambient air via a laminar sheathed-flow arrangement such that particles within the sample flow column are rendered in single file as they intersect the beam from a continuous-wave 660nm diode laser. Each individual particle produces a scattered light signal from which an estimate of particle size (down to ~1 um) may be derived. This same signal also initiates the sequential firing (~10 us apart) of two xenon sources which irradiate the particle with UV pulses centred upon ~280 nm and ~370 nm wavelength, optimal for excitation of bio-fluorophores tryptophan and NADH respectively. For each excitation wavelength, fluorescence is detected across two bands embracing the peak emissions of the same bio-fluorophores. Thus, for each particle, a 2-dimensional fluorescence excitation-emission matrix is recorded together with an estimate of particle size. Current measurement rates are up to ~125 particles/s (limited by the xenon recharge time), corresponding to all particles for concentrations up to ~2 x 104 particles/l. Developments to increase this to ~500 particles/s are in hand. Analysis of results from aerosols of E.coli, BG spores, and a variety of non-biological materials are given.
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.
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.
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