Measuring and testing – Sampler – sample handling – etc. – With constituent separation
Reexamination Certificate
2000-03-31
2002-08-20
Williams, Hezron (Department: 2856)
Measuring and testing
Sampler, sample handling, etc.
With constituent separation
C073S865500
Reexamination Certificate
active
06435043
ABSTRACT:
BACKGROUND OF THE INVENTION
Atmospheric particles play an important role in atmospheric processes and human health. Indeed, particles can absorb or reflect solar radiation and can act as cloud condensation nuclei (CCN) (Charlson, R. J., Schwartz, S. E., Hales, J. M., Cess, R. D., Coakley, J. A., Hansen, J. E., and Hofinann D. J.
Science,
255, 423-429 (1992); Novakov, T. and Penner, J. E.
Nature,
365, 823-826 (1993)). Furthermore, particles can become trapped in different regions of the human respiratory tract and have been implicated in premature death, difficult breathing, aggravated asthma, increased hospital admissions, and increased respiratory problems to children (Seaton, A., MacNee, W., Donaldson, K. and Godden, D.,
Lancet,
345, 176-178 (1995)). Because of these undesirable characteristics, measurement of particle concentrations has become increasingly valuable as a regulatory tool as well as for determining health hazards.
The magnitude of the impact of airborne particles on both atmospheric processes and human health is influenced by the size distribution of these particles. Longer lifetimes and higher optical extinction efficiencies of fine aerosol (aerodynamic diameter d
p
<2.5 &mgr;m) compared to coarse particles (2.5<d
p
<10 &mgr;m) are the main reasons for the direct and indirect effects of airborne particles on climate changes (Seinfeld, J. and Pandis, S., Atmospheric Chemistry and Physics: From Air Pollution to Climate Changes, John Wiley & Sons, London, England (1997)). Moreover, exposures to particles with diameters less than 2.5 &mgr;m have special importance, due to the findings of epidemiological and clinical studies, which showed a relationship between ambient particle concentration and increased respiratory problems and mortality rates—(Utell, M and Samet, J., Airborne particles and respiratory disease: Clinical and pathogenetic considerations, in
Particles in our Air
, Wilson R, and Spengler J. D., Eds., Harvard University Press (1996)). Thus, the ability to accurately measure concentrations of particles within specific size ranges is of considerable importance.
Conventional inertial impactors have been used to classify ambient particles according to their diameter (Pierce, R. C. and Katz, M., D,
Environ. Sci. Technol.,
9, 347-353 (1975); Milford, J. B. and Davidson, C. I.,
J Air. Poll. Control. Assoc.,
35, 1249-1260 (1985); Venkataraman, C., Lyons, J. M. and Friedlander, S. K.,
Environ. Sci. Technol.,
28, 555-562 (1994)). The performance of conventional inertial impactors has been studied extensively, and their behavior and characteristics can be predicted quite accurately (Marple, V. A. and Liu, B. Y. H.,
Environ. Sci. Technol.,
8, 648-654 (1974); Marple, V. A., Rubow, K. L., and Olson, B. A.,
Aerosol Measurement
, Willeke, K. and Baron, P. A. Eds., Van Nostrand Reinhold, New York, 106-232 (1993)). The appropriate type of the impaction substrate depends on the species and chemical analysis to be performed.
Other types of samplers have been designed and developed. One such sampler is the virtual inertial impactor. One limitation of virtual impactors is the lack of complete separation of particles for sizes below the cut point, resulting in a mixture of concentrated coarse particles and unconcentrated fine particles in the minor flow of the impactor (Chen, B. T., Yeh, H. C. and Cheng, Y. S.,
Areosl. Sci. Technol.,
5, 369-376 (1986)). A different type of conventional impactor with a rotating stage design, the microorifice uniform deposit impactor, has also been developed, (MOUDI) (Marple, V. A., Rubow, K. L. and Behm, S. M.,
Areosl. Sci. Technol.,
14, 434-446 (1991)). With the MOUDI, it is still possible for bounce-off and re-entrainment losses to occur, since several tens of layers of particles are accumulated during sampling. Furthermore, multiple jet interactions can deteriorate the performance of the impactor, affecting both the cut-off point and internal losses (Fang, C. P., Marple, V. A. and Rubow, K. L.
J. Aerosol.Sci.
22, 403-415 (1991)).
Many of these types of impactors and substrates used for collection of ambient particles (e.g., solid flat plates and thin porous membranes (generally 0.2 min or thinner)) exhibit decreased particle collection efficiency under such conditions. The decreased particle collection efficiency is a result of at least two factors: particles of high momentum impact the substrate and bounce off, and particles which have been previously collected are displaced from the substrate and re-entrained in the airstream (Sehmel, G. A.,
Environ. Intern.,
4, 107-127 (1980); Wall, S., John, W., Wang, H. C. and Coren, S. L.,
Areosl. Sci. Technol.,
12, 926-946 (1990); John, W., Fritter, D. N. and Winklmayr, W.,
J. Aerosol. Sci.,
22, 723-736 (1991); John, W. and Sethi, V.,
Aerosol Sci. Technol.,
19, 57-68 (1993)). In addition, because these two processes typically depend on particle size, the size distribution of the collected particles can be substantially distorted, which can confound the results of later physicochemical and biological tests performed with the collected particles.
To overcome these problems, porous metallic or glass materials have been used as impaction substrates after having been saturated with mineral oil. However, the collected particles were contaminated by substances present in the mineral oil (Reischl, G. P. and John, W.,
Stuab. Reinhalt. Luft,
38-55 (1978); Pak, S. S., Liu, B. Y. H. and Rubow, K. L.,
Aerosol. Sci. Technol.,
16, 141-150 (1992); Tsai, C. J. and Cheng, Y. H.,
Areosl. Sci. Technol.,
23, 96-106 (1995); Biswas, P. and Flagan, R. C.,
J. Aerosol. Sci.,
19, 113-121 (1988)). The use of oil or grease-coated substrates is an important limitation for collection and analysis of ambient particles in two ways. First, the collection efficiency of these surfaces, as a function of particle size, depends on the amount of particles collected. Thus, the collection efficiency changes during the collection of a sample, as the amount of the material collected increases with time. Second, particles collected on the impactor substrate are contaminated by components of the coating material, prohibiting or interfering with certain types of chemical analysis and toxicological testing.
An impaction substrate capable of more selectively and reliably trapping particles of interest for measurement under a wide range of conditions is critical for ensuring accurate regulation, monitoring, and risk calculation.
SUMMARY OF THE INVENTION
The present invention concerns devices and methods which utilize porous substrates for the collection of particles in a gas sample. The porous substrates are useful as impaction substrates in particle collection devices such as conventional inertial impactor systems. Specifically, the substrates of the present invention can be used to collect particles of a particular size (aerodynamic diameter) range from a gas sample for analysis. They can also be used to remove particles above a given size range to allow for analysis of the particles remaining in the gas sample.
Porous substrates of the present invention offer several advantages over other materials used for the collection of particles. For example, porous substrates are capable of highly efficient particle collection, even under conditions of heavy particle loading. The use of porous materials as impaction substrates also eliminates contamination problems associated with mineral oil-coated substrates, as the porous substrates described herein are uncoated. This characteristic simplifies recovery of particles from the substrate for both chemical analysis and toxicological studies. In addition, the significantly higher collection efficiency and capacity of porous substrates allows for longer sampling time periods without significant distortion of the size distribution of collected particles.
Furthermore, because the porous materials are themselves chemically inert, the collected particles are not contaminated by the substrate and are suitable for physico-chemical and biological testing for effects
Ferguson Stephen T.
Kavouras Ilias G.
Koutrakis Petros
Wolfson Jack M.
Foley & Hoag LLP
Garber C D.
Gordon Dana M.
President and Fellows of Harvard College
Williams Hezron
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