Optics: measuring and testing – For size of particles – By particle light scattering
Reexamination Certificate
2002-03-01
2003-10-28
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For size of particles
By particle light scattering
C356S343000, C356S038000, C356S037000, C250S574000, C073S028010
Reexamination Certificate
active
06639671
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for measuring the size distribution of aerosols over a wide particle size range. Specifically, the invention relates to the measurement of particles suspended in a gas, which is referred to as an aerosol. The most common carrier gas is air, but other gases, such as nitrogen, helium, argon, CO
2
, and other gases, may also be the media for particle suspension. The particles can be solid, liquid, or a mixture of both.
In the ambient atmosphere, particles may exist over a size range from about 2 nanometers (nm) to over 50,000 nm in diameter, with particles in the 10 nm to 10,000 nm range being the most important from a health and safety standpoint. No single device currently exists that can measure particles over this range. The wide-range particle counter (WPC) described herein makes this possible.
Particle counters now available have a limited operable range of sizes, and several different particle counters are needed to properly analyze aerosols.
Aerosols occur both in nature and in the human environment. They are important in scientific research and in technical applications. Aerosol particles in the atmosphere can scatter light and affect atmospheric visibility. When inhaled, the suspended particles can deposit in the lungs to cause potential health effects in humans. Aerosol particles often need to be measured so the sources of the particles can be controlled or precautions taken if the sources cannot be controlled.
Aerosols are also generated on purpose for scientific and technical applications. In laboratory studies, for instance, aerosols with controlled size distribution are needed to test filters and other particle collectors to determine their efficiency. In medical applications, drug compounds are frequently generated in aerosol form for delivery to the lungs for disease treatment. The particle size distribution is important because particle size determines the specific regions of the lungs where the inhaled particles will deposit, hence the effectiveness and efficacy of the inhaled drugs. In all cases in this specification, a gas containing suspended particles shall be referred to as an aerosol, with no limitation being made as to the chemical nature of the particles and that of the gas, and their respective physical states.
One of the most widely used aerosol-measuring instruments presently is the optical particle counter (OPC) first described in U.S. Pat. No. 2,732,753 (O'Konski). In an OPC, the aerosol is passed through a beam of light to cause optical scattering. The scattered light signal from each particle is then detected and related to particle size. The OPC is capable of detecting particles to a lower size limit of about 100 nm in diameter, with some special OPCs having been designed to detect particles as small as 60 nm in diameter or a characteristic dimension.
Another particle-measuring instrument is the condensation nucleus counter (CNC), also referred to as a condensation particle counter. The most widely used CNC is that based on U.S. Pat. No. 4,790,650 (Keady). In this CNC, the aerosol is first saturated with the vapor of a working fluid at an elevated temperature. A typical working fluid is butyl alcohol, and a typical saturator temperature is 35° C. The vapor-laden aerosol then passes through a condenser, typically kept at 5° C. to cool the gas and cause the vapor to condense on particles to form droplets. The droplets are then counted by optical scattering, as in a conventional OPC. The CNC is capable of detecting particles below the lower size limit of the OPC, since droplets formed by vapor condensation are considerably larger than the particles themselves, thus making them easier to detector by light scattering.
Since a CNC is only capable of counting particles, but not measuring the particle size, a CNC must be combined with a size-analyzing device, such as a mobility analyzer, in order to both determine the size and the particles count. A differential mobility analyzer (DMA) is usually used for size determination. The DMA method of size classification is based on the electrical mobility of singly charged particles, i.e. particles carrying a single electron unit of charge. Liu and Pui (1974) and Knutson and Whitby (1975) were the developers of the DMA for this application. The publications explaining this DMA method are: “A Submicron Aerosol Standard and the Primary, Absolute Calibration of the Condensation Nuclei Counter,” Benjamin Y. H. Liu, David Y. H. Pui,
Journal of Colloid and Interface Science
, vol. 47, No. 1, Apr. 1974; and “Aerosol Classification by Electric Mobility: Apparatus, Theory, and Applications,”
Journal of Aerosol Science
, 1975 pp. 443-451, W. O. Knutson and K. T. Whitby.
Recent improvements in the DMA are described in the article “Design and Testing of an Aerosol/Sheath Inlet for High Resolution Measurements with a DMA,” Da-Ren Chen, David Y. H. Pui, George W. Mulholland, and Marco Fernandez,
Journal of Aerosol Science
, Vol. 30, No. 8, pp. 983-999, 1999 by Chen et al (1995). The development of the nano-DMA for particle measurement below 50 nm in particle diameter is disclosed by Pui et al in U.S. Pat. No. 6,230,572 B1. These recent developments further improved the accuracy and range of the DMA devices.
The DMA method of size classification relies on the fact that the electrical mobility of a singly charged particle is inversely related to particle size. A polydisburse aerosol containing singly charged particles over a range of sizes can be classified according to size in an electric field and produce a nearly monodisburse aerosol within a narrow range of electrical mobilities and thus the produced aerosol contains particles of substantially the same size. The classified aerosol can then be counted by a CNC. The DMA is generally limited to particles smaller than about 500 nm in diameter.
All aerosol-measuring instruments have certain inherent size limits. In the case of the DMA, the limit is due to the low electrical mobility of large particles. As the particle size increases, the electrical voltage needed to classify the particle by electrical mobility also increases. At the usual flow rate used in differential mobility analysis, voltages as high as 10,000 Volts may be needed to classify particles at a diameter of 500 nm. For this reason, mobility analysis is seldom used beyond an upper size limit of about 500 nm.
On the other hand, the OPC is limited in the particle size it can satisfactorily detect due to the scattered light signal from a particle generally decreasing with decreasing particle size. Below about 100 nm, the scattered light signal begins to enter the so-called Rayleigh scattering regime, where the signal varies approximately as the sixth power of particle size. A factor-of-two reduction in particle size would thus lead to approximately a 64-fold reduction in the scattered light signal. Detecting small particles below 100 nm becomes increasingly more difficult, even when using a high-powered lasers as light sources, collecting optics with a high numerical aperture, and sensitive photo-detectors. Although optical particle counters have been designed to detect particles as small as 60 nm in diameter, the equipment needed is generally large and expensive. For this reason, high sensitivity optical particle counters are not widely used.
In principle, optical particle counters can be further improved to detect particles smaller than 60 nm. With further advance, even smaller particles may be detectable. However, advances in optical particle counting technology have not made the technology more useful for aerosol measurement over a wide size range. Designers of optical particle counters have not recognized the issues related to wide range particle counting and the special requirements that must be met in order to measure particles over a wide size range. A requirement that is illustrated with the following example.
In the ambient atmosphere, the aerosol size distribution generally follows Junge's law, which states th
Font Frank G.
MSP Corporation
Nguyen Sang H.
Westman Champlin & Kelly P.A.
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