Measuring and testing – Sampler – sample handling – etc. – With constituent separation
Reexamination Certificate
2000-04-07
2001-11-27
Raevis, Robert (Department: 2856)
Measuring and testing
Sampler, sample handling, etc.
With constituent separation
Reexamination Certificate
active
06321608
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a miniature, passive device for sampling aerosol particles and dusts. The invention also relates to associated methods for determining the type, size distribution and airborne concentration of particles to which the sampling device was exposed. The device may be deployed as a personal exposure-monitoring device or as a stationary exposure-monitoring device for use in measuring airborne particle hazards.
Aerosol particle samplers (or dust samplers) are devices that may be used to collect airborne particulate matter for subsequent exposure analysis. A goal in aerosol particle sampling is to determine the average airborne mass concentration of particles (mass of particles per unit volume of air) as a function of particle size to which the aerosol sampler was exposed during a sampling period. Typically, such samplers are employed to measure airborne particle hazards for heath-related reasons.
Conventional particle samplers operate by pumping a quantity of the atmosphere containing the particulate matter through the sampler. In one common approach, the particles are collected on a filter and may be assessed by subsequent gravimetric analysis, examination by microscopy, or chemical analysis. Through various calibrations, an assessment of the particles collected with the sampler provides a means for determining an estimate of the average mass concentration of airborne particles to which the sampler was exposed during the sampling period. These types of samplers may be considered active aerosol samplers because the particle-containing atmosphere is drawn into or forced through the sampler by an external pump or vacuum source. Such a sampling device and method are described in U.S. Pat. Nos. 5,954,845 and 5,958,111 issued to Willeke et al.
Aerosol samplers may be either stationary or personal. Stationary samplers are typically used to monitor community exposure in a defined area. However, monitoring community exposure with a few centrally-located aerosol samplers may not adequately monitor exposures in outlying regions. To adequately monitor community exposure in all regions of an area, a large number of samplers may be needed. If active aerosol samplers are being used, the costs involved in providing the necessary pumps and power sources may be substantial. In addition, if long-term exposure monitoring is required, the extended power and maintenance requirements of active samplers can further increase costs.
Personal active samplers are typically smaller than stationary active samplers, but their sampling pumps can be noisy, heavy, or bulky. Further, such samplers require their own power sources, adding to both cost and weight. Considerations of noise, weight and bulk are important because any inconvenience to a person wearing a personal sampler may alter behavior and produce nonrepresentative exposure estimates.
A different approach to aerosol sampling involves the use of passive aerosol samplers which do not operate by drawing or forcing air through the sampler and which do not require sampling pumps. Rather, during sampling, particles passively deposit onto a collection surface by gravity, inertia, diffusion, or through electrostatic interaction if the collection surface is charged. Subsequent analysis of the collected particles is then carried out to determine an estimate of the average mass concentration of airborne particles to which the sampler was exposed during the sampling period.
One type of passive aerosol sampler known in the art is the passive dust sampler described in U.S. Pat. No. 5,607,497 issued to Brown and also described in an article by Brown et al. entitled “Electret-based Passive Dust Sampler: Sampling of Organic Dusts” (
Analyst
Vol. 121, pp. 1241-1246, September 1996). The Brown passive dust sampler collects particles electrostatically with a charged dielectric electret surface. However, calculation of aerosol concentration of particles to which the sampler was exposed requires knowledge of the average aerosol mobility and electret charge, information that is difficult to determine accurately. Alternatively, the electret mass can be correlated with the results of conventional aerosol samplers for different aerosol types. However, the particle sample collected by the Brown et al. sampler is not collected on an electrically conducting surface in a manner convenient for analysis by scanning electron microscopy (SEM).
Another type of passive aerosol sampler is that described by Vinzents in an article entitled “A Passive Personal Dust Monitor” (
Annals of Occupational Hygiene
, Vol. 40, pp 261-28, 1996). The Vinzents passive sampler collects particles onto upward-facing, sideways-facing, and downward-facing sticky substrates. A light extinction measurement technique is used to provide an assessment of the particles deposited on the collection surfaces. However, the Vinzents passive sampler, being approximately 14 cm×6 cm×5 cm in size, is larger and heavier than desired for a personal, passive aerosol sampler. Further, the particle sample collected by the Vinzents sampler is not collected on a collection surface convenient for analysis by SEM.
Accordingly, a need exists for a passive aerosol particle sampler that can be used as a stationary or personal sampling device, that is unobtrusive, that is inexpensive to produce, that provides for convenient SEM analysis, that is easy to use, and that can provide reliable exposure information. The present invention provides a miniature passive aerosol sampler and a passive aerosol sampling and analysis method that achieves these and other goals.
SUMMARY AND OBJECTS OF THE INVENTION
It is an object of the invention to alleviate the deficiencies of the prior art. It is another object of the invention to provide a passive aerosol sampler that provides for convenient SEM analysis and that is easy to use. It is another object of the invention to provide a passive aerosol sampler that provides for convenient analysis by other microscopy techniques including environmental SEM (ESEM), optical microscopy, transmission election microscopy (TEM), and atomic force microscopy (AFM). It is another object of the invention to provide an analysis method for determining an average airborne mass concentration of aerosol particles as a function of particle size from measurements made with the passive aerosol sampler. These and other objects and features of the present invention will be apparent from the written description and drawings presented herein.
In one aspect of the invention there is provided a passive aerosol sampler, comprising:
a body provided with an electrically conducting collection surface; and
a removable mesh cover, the removable mesh cover comprising a mesh sheet having a plurality of apertures therein, wherein a first portion of the removable mesh cover is removably attached to the body and a second portion of the removable mesh cover is disposed adjacent to the collection surface, wherein aerosol particles can deposit through the apertures onto the collection surface.
In another aspect of the invention there is provided a method of determining an average airborne concentration of aerosol particles as a function of particle size in a measurement region from a sample of aerosol particles collected on a collection surface of a passive aerosol sampler, the passive aerosol sampler having a body provided with the collection surface and a removable mesh cover removably attached to the body, the removable mesh cover having a plurality of apertures therein, comprising the steps of:
carrying out microscopy on the sample of aerosol particles collected on the collection surface;
obtaining images of the sample of aerosol particles using said microscopy;
determining a particle-count size distribution using image analysis of the images; and
calculating the average airborne concentration of aerosol particles as a function of particle size from the particle-count size distribution using a mathematical model having a theoretical component and an empirical component, w
Leith David
Wagner Jeff
Burns Doane Swecker & Mathis L.L.P.
Raevis Robert
The University of North Carolina - Chapel Hill
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