Optics: measuring and testing – For size of particles – By particle light scattering
Reexamination Certificate
1999-06-23
2003-08-12
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
For size of particles
By particle light scattering
C356S338000, C356S343000
Reexamination Certificate
active
06606157
ABSTRACT:
FIELD OF THE INVENTION
This invention describes a new instrument by which potentially hazardous individual airborne fibres, such as those of asbestos, may be detected in real-time within an ambient environment. The instrument uses a rapid analysis of the spatial laser scattering profile (i.e: the complex manner in which individual particles scatter laser light) recorded from individual airborne particles, as a means of classifying the particles in terms of their morphological characteristics. The instrument incorporates a dedicated detector array chip to record the spatial scattering profiles from individual particles at high throughput rates and dedicated electronic processing routines to establish the possible presence of hazardous fibres.
BACKGROUND TO THE INVENTION
The in situ detection of potentially hazardous respirable fibres has become a growing concern within industrialised countries as the health risks associated with these fibres have become more fully understood. The most commonly encountered hazardous fibres are of asbestos materials that, despite a wide-spread ban on their use for many years, are still present in vast quantities in public and commercial buildings and plants throughout the world. The most abundant asbestos mineral, Chrysotile (or white) asbestos, is present in over 95% of these installations. The second most commonly found variety is Crocidolite (or blue) asbestos, with Amosite (or brown) asbestos being a third but much rarer form. Crocidolite and Amosite belong to the amphibole class and are characterised by the fine, straight, needlelike fibres produced when the material is fragmented. Chrysotile asbestos belongs to the serpentine class of minerals and is characterised by a natural curvature in the fibres it produces. All three materials produce fibres that are capable of penetrating deep into the lung and that, because of their shape, become entrapped there. Crocidolite and Amosite fibres are known to be far more carcinogenic than those of Chrysotile asbestos, and although the exact reasons for this are still not confirmed, the half-life of the fibres in the lung (a function of the body's ability to chemically dissolve the fibres) is believed to play a major role since this may be measured in decades for amphibole fibres compared with months for Chrysotile fibres.
Airborne asbestos fibre is a significant health hazard. Peto et al (Peto, J., Hodgson, J. T., Matthews, F. E. and Jones, J. R.
The Lancet
. 345, 535-539, Mar. 4, 1995), for example, highlight the continuing increase in mesothelioma mortality in Britain as a result of respirable asbestos fibres generated during clearance operations or routine building maintenance work The unambiguous confirmation of the presence of airborne asbestos fibres within an occupational environment can normally only be achieved by the use of filter cassette sampling of airborne particles followed by electron microscopy and, to determine chemical identity, a technique such as energy dispersive X-ray analysis. These processes are laborious and expensive to perform, and perhaps most importantly, provide results only many hours after the sample acquisition and possible personnel exposure has occurred. Several attempts have therefore been made to develop methods by which real-time or in-situ detection of airborne asbestos may be achieved. Rood et al [AP Rood, E J Walker and D Moore, “Construction of a portable fibre monitor measuring the differential light scattering from aligned fibres”, in
Proceedings of the International Symposium: Clean Air at Work
, R H Brown, M Curtis, K J Saunders, and S Vandrendreissche, eds (
Royal Society of Chemistry
, London, 1992), pp 265-267] for example, have described a low cost portable fibre monitor developed at the UK Health & Safety Executive laboratories. This device is based on the differential light scattering produced by fibrous particles which are deposited electrostatically in uniform alignment onto a glass substrate. The device is capable of detecting fibrous particles but is not designed to detect individual particles, relying on the summation of scattering signals from a substantial number of deposited fibres in order to achieve a detectable signal. Rood states that the UK clearance limit for asbestos in buildings of 10 fibres per liter of air can be detected after about 300 minutes sampling time. This does not therefore constitute a real-time detection technique.
Another example is the comparatively widely used FAM-7400 Fibrous Aerosol Monitor (Mie Inc., Bedford, Mass.) developed originally by Lilienfeld et al. (Lilienfeld, P., Elterman, P., and Baron P.
A. Ind. Hyg. Assoc. J
. 40, 4, 270-282, 1979). This instrument draws air containing the airborne particles into a laser scattering chamber where the particles are carried along a horizontal glass tube coaxial with an illuminating laser beam. The particles remain in the beam for a comparatively long period, approximately 0.1 seconds, and many particles may be illuminated simultaneously. Around the glass tube is a quadruple electrode arrangement. By applying a time varying signal to the electrodes, the electric field within the tube causes electrically conducting fibres present in the air-flow to oscillate. The consequent cyclic variation in light scattered by the fibres to a single light detector at the side of the chamber is used to assess fibre concentration in the air. The FAM-7400 has several limitations (described in, for example ‘Aerosol Measurement’ by Willeke K. and Baron P. A., Van Nostrand Reinhold, 1993, pp 403-408): its sample volume flow rate through the laser beam is very low, resulting in comparatively long response times at low fibre concentrations (typically requiring 10 minutes to count 10 fibres at a concentration of 0.1 fibres/ml); it may classify as fibres non-fibrous particles which happen to oscillate in the applied electric field; since more than one fibre may be present in the beam at a given time, it can only estimate the number of fibres by the magnitude of the oscillation signal, and this requires some assumptions about the sizes of the fibres present; and it has reduced sensitivity for fibres which exhibit a natural curved morphology, such as the most common asbestos form, Chrysotile.
Spatial Laser Scattering Profiles
In theory, the detailed spatial intensity distribution of light scattered by individual particles (the scattering profile) contains information relating to inter alia the particle's size, its shape, and its orientation with respect to the incident illumination. The invention reported here is aimed at exploiting this fact with a view to discriminating, in real-time, individual respirable hazardous fibres, such as asbestos, from other particles within an ambient environment.
Most optical scattering instruments used for particle counting and/or sizing, rely on collecting the scattered light with a single discrete detector. Such instruments cannot provide information on particle shape, and indeed normally assume that all measured particles are spherical when ascribing a size value to them. When a small number of discrete detectors are used, each collecting light over a different solid angle within the sphere of scattering around the particle, some shape as well as size information is obtainable. This principle is embodied in a number of patented instruments which may be considered as prior art: (‘Portable Particle Analysers’. Ludlow, I. K. and Kaye P H. European Patent EP 0 316 172, July 1992; ‘Portable Particle Analysers Having Plural Detectors’. Kaye P H and Ludlow I K U.S. Pat. No. 5,043,591 August 1991; ‘Particle Asymmetry Analyser having Sphericity Detectors’. Kaye, P. H. and Ludlow, I. K. U.S. Pat. No. 5,089,714. February 1992; ‘Particle Asymmetry Analyser’. Ludlow, I. K. and Kaye, P. H. European Patent EP 0 316 171, September 1992; ‘Analysis of Particle Characteristics’. Kaye, P. H., and Hirst, E. UK Patent GB 2278679B).
However, in order to extract more subtle information relating to particle morphology which may aid particle discrimination, the spat
Hirst Edward
Kaye Paul Henry
Buyan Robert D.
Pham Hoa Q.
Stout, Uxa Buyan & Mullins, LLP
University of Hertfordshire
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