Electricity: measuring and testing – Using ionization effects – For analysis of gas – vapor – or particles of matter
Reexamination Certificate
2003-01-22
2004-12-07
Nguyen, Vincent Q. (Department: 2858)
Electricity: measuring and testing
Using ionization effects
For analysis of gas, vapor, or particles of matter
C340S632000, C073S023350, C073S863710
Reexamination Certificate
active
06828795
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to detection of explosives and more particularly to an ion mobility spectrometry instrument that detects chemicals present as vapors in air or other gases, or liberated as vapors from condensed phases such as particles or solutions.
2. Description of Related Art
IMS instruments operate on the basis of the time taken by ionized molecules to move through a gas-filled drift region to a current collector while under the influence of an electric field. The ions are created in a gas-filled region called the ion source, which is connected to the drift region through an orifice or a barrier grid. The ion source may use any of a variety of techniques to ionize atoms and molecules. One or more flowing streams of gas enter the ion source through one or more orifices, and the gas may exit through one or more different orifices. At least one of the flowing gas streams entering the ion source includes gas that has been sampled (the “sample gas”) from the surrounding atmosphere or other source of vapor to be analyzed.
In same cases, the process of taking a sample begins with an operator rubbing an absorbent substance, such as chemical filter paper, onto the surface to be tested. Particles of the chemical of interest may then be transferred and concentrated on the absorber. This intermediate absorber is then brought to the vicinity of the sampling orifice of the IMS. The method of concentrating using an absorbent substance is deficient in that it tends to be relatively slow to implement and is subject to variations in the skill of the operator. Additionally, while the absorber is relatively low in cost, the process of taking a great many samples becomes expensive in that the absorber generally should only be used once to ensure consistent results.
The quantity of particles of the target substance on the target surface is usually very small, often corresponding to only nanograms or even picograms of particles per square centimeter. The IMS must be very sensitive to identify a positive signal from evaporated target molecules when the initial concentration and surface area of target particles is so small.
A sampling method that is employed is to provide a gas pump, which draws the sample gas into the ion source through a tube. For example, the pump may be disposed to provide a partial vacuum at the exit of the ion source. The partial vacuum is transmitted through the confines of the ion source and appears at the entrance orifice of the ion source. A further tubulation may be provided as an extension to a more conveniently disposed sampling orifice external to the IMS. The operator places a sample in the near vicinity of this external sampling orifice, and the ambient vapor is drawn into the gas flow moving towards the ion source.
The ion source of the IMS provides a signal that is approximately proportional to the concentration of target molecule vapor. This concentration is further dependent on the equilibrium vapor pressure of the target molecule, the temperature of the target molecule where it is emitting the vapor, the total flow rate of non-target gas that dilutes the target vapor, and possible adsorption losses on surfaces of the gas sampling system. Existing systems that utilize absorbent surface concentration sometimes employ an oven to greatly warm the absorbent material, often up to 200°, and thereby increase the target vapor concentration.
In some circumstances, it is desirable for IMS instruments to be able to sample vapors at a distance from the external sampling orifice. Examples may include, but not be limited to, sampling of vapor from complex surfaces that contain many holes, crevices, or deep depressions, textured materials such as cloth, people and animals that prefer not to be rubbed by absorbent material, large three dimensional objects, surfaces that must be sampled in a short time, and surfaces in which surface rubbing by human operators is inconvenient or expensive. In addition, it has been observed that the sampling orifice may become contaminated with vapor-emitting particles if the sample inadvertently contacts the orifice. Such contamination is particularly difficult to remove in a short period of time, thus preventing continuous operation of the instrument. Such contamination could be avoided if vapors are sampled at a distance from the sampling orifice.
The distance where vapors may be sampled beyond the sampling orifice may be increased by increasing the sample gas flow rate, i.e., increasing the pumping speed. However, besides the interference with the performance of the ion source of the IMS caused by high velocity flow, this method dilutes the concentration of the desired sample vapor by mixing in a much larger volume of ambient gas. Therefore, the sensitivity of the IMS may decline if the sample gas flow rate is increased excessively.
Warming surfaces at a distance using an oven is generally not very efficient. While warmed gas can be blown onto a distant surface, for example with a “heat gun”, when the target surface is a living person or animal, this may not be an acceptable option. Additionally, many surfaces cannot tolerate excessive heating and may be damaged.
SUMMARY OF THE INVENTION
According to the present invention, an explosive detection system includes a sampling orifice that receives sampled gas, a fluid rotator that creates a cyclonic gas flow beyond the sampling orifice, an ion source, coupled to the sampling orifice, that generates ions corresponding to the sampled gas, a drift tube having the ion source coupled to a first end thereof, and a detector coupled to an other end of the drift tube, where the detector detects in the sampled gas the presence of ions associated with explosives. The cyclonic gas flow may have an outer rotary flow about an axis substantially parallel to the central axis of the sampling orifice and an inner flow substantially parallel to the central axis of the sampling orifice. The drift tube may operate at substantially ambient gas pressure. A gas pump may draw a gas flow through the sampling orifice and generate a vacuum within 50 millimeters of mercury (50 Torr) of the substantially ambient gas pressure. The fluid rotator may include at least one vane. The fluid rotator may include a rotation-inducing orifice surrounding the sampling orifice. The inside surface of the rotation-inducing orifice may deflect a gas flow into a cyclonic gas flow. The explosive detection system may further include a gas pump connected to the rotation-inducing orifice that creates a cyclonic gas flow. The explosive detection system may include a precipitator that removes at least a portion of any entrained particles within the gas flow into the sampling orifice. The precipitator may be an electrostatic precipitator. The electrostatic precipitator may include a cathode disposed on or near the drift tube, the cathode applying a voltage greater than 3000 Volts. The axis of the cyclonic gas flow may rotate about a rotation axis perpendicular to its central axis. The axis of the cyclonic gas flow may rotate about a plurality of rotation axes perpendicular to its central axis.
According further to the present invention, an explosive detection system includes a sampling inlet that receives sampled gas, a heat source, mounted proximal to the gas sampling inlet, the heat source providing photonic emissions to one side of a target proximal to the sampling inlet, an ion source, coupled to the sampling orifice, that generates ions corresponding to the sampled gas, a drift tube having the ion source coupled to a first end thereof, and a detector coupled to an other end of the drift tube, where the detector detects in the sampled gas the presence of ions associated with explosives. The photonic emissions may be substantially in the infrared portion of the spectrum. The source of photon emission may be made to be substantially in the infrared using at least one of a filter, coating, and covering. The source of photon emission may have enhanced emission substantially in the infrared
Belyakov Vladimir V.
Bunker Stephen N.
Kekukh Vladimir B.
Krasnobaev Leonid Y.
Persenkov Vyacheslav S.
Choate Hall & Stewart
Implant Sciences Corporation
Nguyen Vincent Q.
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