Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – With means applying electromagnetic wave energy or...
Reexamination Certificate
2000-06-16
2003-04-08
Seidleck, James J. (Department: 1711)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
With means applying electromagnetic wave energy or...
C422S186000
Reexamination Certificate
active
06544484
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to devices used to alter the electrical charge distributions of aerosols, and more particularly devices that utilize a corona discharge to generate ions, which then are merged with an aerosol to either charge or neutralize the aerosol.
The study of aerosols involves a variety of applications in which it is desired to adjust the charges on the particles or droplets of the aerosol. There are applications in which it is advantageous to provide a charge distribution in which positive and negatively charged particles counterbalance one another, i.e., an equilibrium charge distribution. In other applications, it is considered more important that a predominant number of the particles carry no charge. In yet further applications, researchers skew the charge distribution toward either the positive or the negative side, and in a more specific application of this type attempt to maximize the number of particles that carry a specific non-zero charge. Corona discharge can be used in all of these applications.
To produce a corona discharge, a non-uniform electrostatic field is established between an electrically conductive needle and a conductive structure proximate the needle, e.g., a plate or a tube surrounding the needle. Given a sufficient field strength, air near the needle experiences a breakdown and becomes conductive. In the conductive corona region, accelerated electrons collide with air molecules to create a dense cloud of free electrons and positive ions. If the needle is biased to a positive voltage relative to the surrounding structure, the electrons return to the needle while the positive ions stream away from the needle toward the adjacent structure. When the discharge needle is disposed within a gas stream, many of the ions do not reach the adjacent structure, but instead become entrained in the gas stream and are transported by the gas stream toward the aerosol. When the discharge needle is negatively biased, the free electrons leave the needle, some of them attaching to molecules of the gas to form negative ions, and are transported toward the aerosol by the gas stream.
In an increasing number of aerosol studies, it is desired to generate aerosols in which the particles are monodisperse, i.e., substantially uniform in size. For these applications, an electrospray nebulizer is preferred, due to its ability to generate small and uniform droplets. In an electrospray nebulizer, an electrically conductive liquid is supplied at a controlled rate to a capillary tube. A voltage differential between the capillary tube and a surrounding conductive wall creates an electrostatic field that induces a surface charge in the liquid emerging from the tube. Electrostatic forces disperse the liquid into a fine spray of charged droplets.
To produce the spray, the droplets are charged near the “Rayleigh” limit, i.e., near the charge at which electrostatic repulsion would overcome the surface tension that otherwise holds the droplet together. As each electrospray droplet evaporates, the charge density on its surface increases, eventually exceeding the Rayleigh limit, causing a disintegration of the droplet into smaller droplets. The droplet fragments in turn continue to evaporate and can experience further disintegration. As a result, the distribution of droplet sizes lacks the uniformity desired for analysis of residue within the droplets.
To counteract this tendency, the droplets are charge neutralized, beginning immediately or shortly after their formation. In one approach, disclosed in U.S. Pat. No. 5,247,842 (Kaufman, et al.), radioactive Polonium is placed inside a chamber through which the electrospray generated droplets travel as they evaporate. The Polonium produces radiation that ionizes air molecules, which in turn encounter the droplets and reduce their charge. This enhances uniformity of the droplets by counteracting their tendency to disintegrate due to electrostatic forces.
This approach yields a reproducible charge distribution by exposing the aerosol particles or droplets to a bipolar plasma of gas ions, both positive and negative, allowing the aerosol elements to reach a steady state of charge distribution. This distribution is useful because it is predictable and produces a large proportion of particles having no charge. This approach has disadvantages, however, in that the use of radioactive materials raises safety and regulatory concerns. The cost of radioactive Polonium is relatively high, and its half-life is relatively short. Further, although the level of ion production can be varied by partially shielding the radioactive material, the level of ionization cannot be precisely controlled.
In view of the above, ion generation through use of a corona discharge has been considered as an alternative method of neutralizing electrospray droplets. The corona discharge can generate unipolar (e.g., only negative) ions, and thus be configured to counteract the charge of the electrospray droplets. Alternatively, if both positive and negative ions are desired, corona discharge devices can have oppositely charged corona discharge needles, or a single corona discharge tip can be rapidly alternated between positively and negatively charged states.
A disadvantage of corona discharge devices is their tendency to generate aerosol particles. The problem is thought to arise from the removal of material from the discharge needle, the creation of highly reactive gaseous species at concentrations sufficient to allow their aggregation into particles, or a combination of these factors. In any event, aerosols generated by the corona discharge needle interfere with attempts to measure the aerosol under study. The tendency especially interferes with the analysis of extremely fine particles, i.e., particles having diameters of about ten nanometers or less.
Particle generation by corona discharge devices interferes with their use in semiconductor manufacturing clean rooms, because the particles can be large enough to contaminate silicone wafers during their processing. In recognition of the problem, U.S. Pat. No. 4,967,608 (Yost) a describes a test chamber for measuring particles larger than three nanometers in diameter emitted from a corona discharge device. U.S. Pat. No. 5,447,763 and U.S. Pat. No. 5,650,203, both issued to Gehlke, recommend selecting certain materials for corona discharge tips, e.g., titanium, aluminum, and other metals that form protective oxide layers. Silicone coated tips of these materials were favored. Platinum and tungsten also were considered, but said to show substantial particle production, and thus found unsatisfactory.
Recently, electrostatic generation of droplets has been considered as a source of aerosols subject to analysis by mass spectrometry, given the capability of generating aerosol droplets that are small (submicron) and monodisperse. In addition, the ability to rapidly and efficiently neutralize the aerosol, preferably to the point where the aerosol consists predominantly of singly charged particles, is a key factor when the aerosol is provided to a mass spectrometer. Although the aforementioned Kaufman patent discloses both the droplet generation and neutralizing beneficial in this regard, a more efficient and more controllable neutralizing of the aerosol would considerably enhance the utility of electrospray-ionization mass spectrometry.
Therefore, it is an object of the present invention to provide an aerosol system in which the charged droplets or charged particles are neutralized more rapidly and in a manner that affords more control over the degree of neutralizing.
Another object is to provide a corona discharge device capable of selectively altering the charge distributions of aerosols formed of extremely small droplets and particles, without generating its own detectable particles and thereby interfering with an analysis of the aerosol under study.
A further object is to provide an electrospray-ionization mass spectrometry system in which the electrostatically generated aerosol is effectively n
Dorman Frank D.
Kaufman Stanley L.
Larkin Hoffman Daly & Lindgren Ltd.
Niebuhr, Esq. Frederick W.
Seidleck James J.
Tran Thao
TSI Incorporated
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