Patent Application
20110203931
The invention relates to sampling and concentrative apparatus and methods for collection of trace analytes from surfaces and substrates where the analyte is in the form of a particulate, a particulate combined with a vapor, or a free vapor and particularly to such apparatus and methods as are useful in surveillance for trace explosives residues.
There is a need for inspection and sampling of persons, articles of clothing, buildings, furnishings, vehicles, baggage, cargo containers, dumpsters, packages, mail, and the like for contaminating residues (termed here more generally “trace analytes”) that may indicate chemical, radiological, biological, illicit, or infectious hazards. Applications involve detection of trace materials, both particles and optionally vapors, associated with persons who have handled explosives, detection of toxins in mail, or detection of spores on surfaces, while not limited thereto.
Current methods for surface sampling often involve contacting use of swabs or liquids, but methods for sampling by “sniffing” are preferred. To inspect mail or luggage for example, the sampling method of U.S. Pat. No. 6,887,710 involves first placing the article or articles in a box-like enclosure equipped with airlocks, directing a blast of air onto the exposed surfaces in order to dislodge particles associated with the articles, then sampling the gaseous contents of the box by drawing any resulting aerosol through a sampling port.
However, the process is inherently slow because each article or person must be moved into the box or chamber and the box sealed before sampling, an obvious disadvantage when large numbers of articles or persons must be screened, or when the articles are larger than can be reasonably enclosed, such as a truck, shipping container, or the hallway surfaces of a building. Similar comments may be made regarding the teachings of U.S. Pat. No. 6,324,927 to Omath, where an enclosed shaker is used to dislodge particles.
An approach for sampling persons is seen in U.S. Pat. No. 6,073,499 to Settles, aspects of which are also discussed in “Sniffers: fluid dynamic sampling for olfactory trace detection in nature and homeland security”, J Fluids Eng 127:189-218.
McGown in U.S. Pat. No. 4,909,090 describes a hand-held vapor sampler, optionally with a shroud for enclosing a sampling space, for using low pressure puffs of hot air to vaporize illicit substances on surfaces and trap any vapors on a collector coil. The coil contains ribbon-like windings of metal which have a thin coating of material such as an organic polymer effective in absorbing organic molecules such as cocaine. However, particles are not sampled and would not be successfully aspirated under the conditions described, which relies on a 250 Watt lamp and a spring-actuated plunger for generating a puff of air. Improvements to the collector/desorber device are disclosed in U.S. Pat. No. 5,123,274 to Carroll.
Ishikawa in U.S. Pat. No. 7,275,453 discloses a cover enclosure in contact with a surface, the enclosure with internally directed jet for operatively flushing and ejecting particles from the surface. The particles may be collected by means of an inertial impactor and thermally gasified from the impactor for detection of chemical constituents by mass spectroscopy. Use of a plate-type inertial impactor avoids the need for a fine-mesh filter, such as would become clogged.
Various particle and vapor traps are disclosed in patents to Linker of Sandia Labs, including US RE38,797 and U.S. Pat. Nos. 7,299,711, 6,978,657, 6,604,406, 6,523,393, 6,345,545, 6,085,601 and 5,854,431, by Corrigan in U.S. Pat. Nos. 5,465,607 and 4,987,767, and Syage in U.S. Pat. No. 7,299,710, but implementation has proved difficult because particles have been found to poison commonly used vapor trap materials and means for efficiently separating particles and vapors are not recognized.
Teachings by Hitachi in U.S. Pat. No. 7,275,453 relate to an unusual inertial impactor with central void for discarding particles in excess of the cut size of the impactor. This has the unfortunate effect of dramatically reducing the amount of analyte available for detection. Also disclosed is a heatable rotary trap, as has longstandingly been known in the art.
Detection technologies are known. Of particular interest for detection of explosives are electron capture (often combined with gas chromatography), ion mobility spectroscopy, mass spectroscopy, and chemiluminescence (often combined with gas chromatography).
One common analytical instrument for detection of nitrate-type explosives relies on pyrolysis followed by redox (electron capture) detection of NO2 groups (Scientrex EVD 3000), but is prone to false alarms. Also of interest is differential mobility spectroscopy as described in U.S. Pat. No. 7,605,367 to Miller. Ion mobility spectroscopic (IMS) detectors are in widespread use and typically have picogram sensitivity. IMS requires ionization of the sample, which is typically accomplished by a radioactive source such as Nickel-63 or Americium-241. This technology is found in most commercially available explosive detectors like the GE VaporTracer (GESecurity, Bradenton, Fla.), Sabre 4000 (Smiths Detection, Herts, UK), Barringer IonScan™ 400, and Russian built models.
The luminescence of certain compounds undergoing reaction with electron-rich explosive vapors has been improved with the introduction of amplifying fluorescent polymers as described in U.S. Pat. No. 7,208,122 to Swager (ICx Technologies, Arlington Va.). Typically vapors are introduced into a tubular sensor lined with a conductive quenchable fluorescent polymer by suction. These sensors lack a pre-concentrator and work only for analytes with electron-donating properties. More recent advances have extended work with fluorescent polymers to include boronic peroxide-induced fluorescence, as is useful for detecting certain classes of explosives.
Other analytical modalities are available, and include the MDS Sciex CONDOR, Thermedics EGIS, Ion Track Instruments Model 97, the Sandia Microhound, Smith's Detection Cyranose, FIDO® (FLIR Systems, Arlington Va., formerly ICx Technologies), Gelperin's e-nose (U.S. Pat. No. 5,675,070), Implant Sciences' Quantum Sniffer, and others. However, these technologies are associated with aspiration and analysis of vapors, which are typically in vanishingly small concentrations, either because a) the vapor pressure of the material is inherently small, or b) if vapor pressure is larger, then significant quantities of a more volatile analyte will have been lost due to ageing of the material prior to sampling. Some of these detectors also have had maintenance issues, often related to fouling due to aspiration of particles.
Aerodynamic focusing has been used to produce particle beams or ribbons in a gas stream, process in which the gas streamlines are separated into a particle-depleted sheath flow and a particle-enriched flow. The two flows can then be separated, resulting in particle concentration. An aerodynamic lens particle concentration system typically consists of four parts: a flow control orifice, at least one focusing lenses, an acceleration nozzle, and a skimmer. The choked inlet orifice fixes the mass flow rate through the system and reduces pressure from ambient to the value required to achieve aerodynamic focusing. The focusing lenses are a series of orifices contained in a tube that create a converging-diverging path resulting in flow accelerations and decelerations, through which particles are separated from the carrier gas due to their inertia and focused into a tight particle beam or ribbon. The accelerating nozzle controls the operating pressure within the lens assembly and accelerates particles to downstream destinations. The skimmer is typically a virtual impactor with virtual impactor void for collecting the particle beam or ribbon while diverting the greater mass of the particle-depleted bulk flow, thus concentrating the particle fraction.
Focusing of a range of micron and submicron size aerosol particles has been carried out using aerodynamic forces in periodic aerodynamic lens arrays [see Liu et al, 1995, Generating particle beams of controlled dimensions and divergence, Aerosol Sci. Techn., 22:293-313, Wang, X et al, 2005, A design tool for aerodynamic lens system, Aerosol Sci Techn 39:624-636; U.S. Pat. Appl. Doc. 2006/0102837 to Wang]. Such arrays may be used as inlets to on-line single-particle analyzers [see Wexler and Johnston (2001) in Aerosol Measurement: Principles, Techniques, and Applications, Baron and Willeke eds, Wiley, New York, and U.S. Pat. No. 5,565,677 to Wexler]. As known in the art, a major class of skimmers generally comprise a cone or plate with a hole in the center (i.e., are virtual impactors).
Aerodynamic lenses have been used in particle mass spectrometers and as an adjunct to ion mobility spectroscopy, (for example as described in U.S. Pat. Nos. 7,256,396, 7,260,483, and 6,972,408 and more recently in U.S. Pat. 2010/0252731), where high vacuum is used (0.1 to 30 mTorr). In this system, analyte vapors released from a very well collimated particle beam (typically <0.25 mm diameter) are laser ablated and ionized in flight and the resulting vapors are conveyed in a buffer gas at high vacuum, typically with Einzel lensing, to a mass spectrometer or an ion mobility spectrometer. The downstream analyzer can be badly damaged by the entry of intact particles. Moreover, the particle-by-particle approach taught in the art substantially limits application for high throughput analysis and is not scaleable except by an impractical redundancy of parallel systems.
Related systems are described in PCT Publication WO/2008/049038 to Prather, U.S. Pat. No. 6,906,322 to Berggren, and U.S. Pat. No. 6,664,550 to Rader. However, these devices are readily overloaded when confronted with large amounts of complex mixtures, interferents, and dust, such as are likely to be encountered in routine use.
Thus, strategies are needed to improve analyte collection efficiency and avoid interferences. There is a need for a front end device with directional head for mobilization of particles from substrate to aerosol, a head that can be portably directed to dislodge particles and optionally vapor residues from target surfaces, then efficiently capture and concentrate them before presentation to an analytical instrument of choice, an approach that optimizes sensitivity and can speed deployment because the need to enclose the target surface in a sealed chamber or shroud is overcome. In particular, there is a need for a front end collection system that may be used in environments where a small amount of a target analyte must be detected in the presence of larger amounts of ubiquitous background particulates, for example dust and water with small amounts of target analyte, and with means for regenerating capture surfaces.
The preferred devices, systems and methods overcome the above disadvantages and limitations and are useful in detecting hazardous particles, vapors and volatiles associated with objects, structures, surfaces, cavities, vehicles or persons.
Disclosed is a pneumatic sampler head with “virtual sampling chamber” for sampling hazardous contaminants such as traces of explosives, infectious agents, or toxins on persons, articles of clothing, buildings, furnishings, vehicles, cavities, dumpsters, cargo containers, baggage, packages, mail, and the like.
A first system includes a sampler head with a central collection intake operated under suction and an array of jet nozzles directed convergingly toward the apex of a virtual cone extending from the sampler head. A virtual sampling chamber is formed when streamlines of gas discharged by the jet nozzle array impinge on an external surface. The jets serve to dislodge and mobilize particulate and vapor residues on a surface and the suction intake draws them into the sampler head. Use of the jet-enclosed virtual sampling chamber extends and directs the reach of the suction intake, which would otherwise draw air from behind the intake.
Surprisingly, gas jets operated in a millisecond-scale pulse mode are found to be more effective than gas jets operated continuously in collecting particulate and/or vapor residues with the sampler head. The virtual sampling chamber may be formed and collapsed in less than a second in response to a single synchronized jet pulse while under suction, or may be formed intermittently, such as by a train of synchronized pulses separated by a fraction of a second or longer, during operation. The sampler head may be compact for portable hand-directed operation or scaled up and operated robotically for screening of vehicles, cargo containers, and so forth, while not limited thereto.
In one sampling system, the apparatus is a pneumatic sampler head for sampling residues, including particulate and vapor residues, from an external surface of an object, structure, vehicle or person, which comprises a) a sampler head with forward face and perimeter; b) a suction intake port disposed centrally on the forward face and an array of two or more jet nozzles peripherally disposed on the forward face around the suction intake port, wherein the jet nozzles are directed at a virtual apex of a virtual cone with base resting on the forward face; c) a positive pressure source for firing or propelling a gas sampling jet pulse or stream with streamlines from each nozzle of the array of jet nozzles; d) a suction pressure source for drawing a sampling return stream of gas into the suction intake port, the suction pressure source having an inlet and an outlet; where the streamlines of the gas sampling jet pulses are directed toward the virtual apex of the virtual cone, the streamlines tracing an involuted frustroconical “U-turn” under the attraction of the suction pressure source and converging with the sampling return stream at the suction intake port along a central axis of the virtual cone when impinging on the external surface.
The out-flow of the gas sampling jets and in-flow of the sampling return stream form a “virtual sampling chamber” with the gas sampling jet pulses directed linearly along the walls of the virtual cone toward its apex and the sampling return stream directed along the central axis of the virtual cone toward its base, and further wherein the involuted frustroconical “U” fluidly connects the gas sampling jets and the sampling return stream at a virtual frustrum when impinging on an external surface. In preferred embodiments the device is operative at up to 1 foot from the external surface.
Surprisingly, we have found that pneumatic pulses or streams emitted from a concentric array of gas interrogation jet nozzles directed in trajectories along the walls of a virtual cone will turn inward when directed at a surface and return to a common suction intake port mounted in the sampler head in the center of the jet array. The sampler head may be held at a distance and aimed at the surface to be interrogated. Targetable jet nozzles and laser guidance may be used to shape the pulse geometry if desired. Particles or vapors removed from the interrogated surface are efficiently mobilized in the “virtual sampling chamber” and aspirated through the suction intake, where they may then be concentrated and analyzed by a variety of methods.
In use, pneumatic pulses initially follow directional vectors converging along the virtual “walls” of a “virtual cone”, but upon contact with a surface disposed at a distance from the base of the cone Df which is less than the height of the cone Dc, a virtual frustrum is formed by involution of the streamline vectors so that the streamlines flow back along the central axis of the cone into an intake duct centrally mounted on the face of the sampler head. The virtual cone thus becomes a closed “virtual sampling chamber” where objects or surfaces brought within the cone are stripped of volatiles and loose particulates and carried into the sampler head. Once entrained in the suction intake, particles or vapors in the stream of air may be concentrated for collection or analysis.
Sampling jet and suction intake gas flows may be discontinuous or continuous, balanced or imbalanced, subsonic or sonic in character. In one application, the in-flows and out-flows from the sampler head are equal and opposite and form a closed loop, so that vapors or particles not trapped in the sampler head are recirculated and accumulate in the loop. In a preferred embodiment, the jet pulse out-flow is powered by an independent pressure source and is exceeded by the suction in-flow to achieve a net positive sampling, such as when a millisecond sampling pulse out-flow is followed by a suction in-flow of longer duration to ensure that the sampled air volume is greater than volume of the pulsed air jet:
V(SUCTION)>V(JET PULSE)
In practice, it has proved useful to operate the gas jets in single pulse mode or pulse train mode while under continuous or semi-continuous suction. In single pulse mode, the gas jets fire as a short burst after first activating the suction intake. In pulse train mode, a series of short bursts are emitted from the gas jets while operating the suction intake. A surface, substrate or object may be sampled with a single pulse or with a series of pulses. The sampler head may be moved or stationary between pulses, or a series of pulses may be emitted while the sampler head is moving and suction is engaged.
In another sampling system, the array of interrogation jet nozzles is surrounded by a perimeter of circumferential slits that emit a curtain wall of lower velocity gas forming a virtual shroud, skirt or apron around the virtual cone of the higher velocity convergent jets. This air is conveniently supplied by the exhaust of the suction intake. The exhaust of a blower used to power the suction intake, for example, may also be used to provide the gas flow for the curtain wall.
In yet another aspect, the invention is a method for sampling a residue from an exterior surface of an object, structure or person, which comprises contacting a virtual sampling chamber as described herein with an exterior surface at a distance less than the height Dc of the virtual cone, whereby residues dislodged from the external surface by the gas jets are swept into a sampling return stream by the suction intake. The virtual sampling chamber may be employed intermittently with triggering, or cyclically, or continuously, but is preferentially pulsed with a pulse interval selected so that the jet pulse volume may efficiently be aspirated before firing a second pulse.
In a preferred aspect, one approach to a pneumatic sampler head combines biomimetic “sniffing” and interrogation jets for aerosolizing particles and optionally vapors, the combination serving as an efficient front end particle and/or vapor residue concentrator and capture device for use with a variety of analytical tools and instruments.
With respect to explosives surveillance and detection, the invention is an apparatus for concentration and collection of samples of explosives and explosives-associated materials for analysis, the samples having a particle fraction (including any adsorbed vapors) and a free vapor fraction. The apparatus comprises a) a sampler head with directional nose, the nose having an intake port and upstream channel for receiving a first sample as a suction gas flow having a volume and a velocity and conveying the suction gas flow to an air-to-air particle concentrator, the air-to-air particle concentrator for accelerating and inertially dividing the suction gas flow according to a flow split into a particle-enriched flow in a first downstream channel and a bulk flow in a second downstream channel; b) a particle trap disposed in the first downstream channel for immobilizingly accumulating particles from the particle-enriched flow; c) a vapor trap disposed in the second downstream channel for immobilizingly accumulating free vapors from the bulk flow; d) a means for stripping a first constituent from a particle fraction in the particle trap and an independent means for stripping a second constituent from a vapor fraction in the vapor trap, and optionally e) a means for detecting a first signal from the accumulated particles and a means for detecting a second signal from the accumulated vapors so as to detect an explosive or explosive associated material in the first sample by integrating or comparing the first and the second signal. The apparatus enables independently detecting a first signal from a particle constituent and a second signal from a vapor constituent and integrating or comparing the signals to detect an explosive or explosive associated material in the sample.
Certain improvements in performance are made possible by use of the air-to-air concentrator. Losses of particles in the size range of 5 to 200 microns are reduced by shunting the bulk flow around the particle trap. Particle fouling of the vapor trap is reduced by adjusting the cut size of a virtual impactor or particle separator to 5 to 10 microns, resulting in cleaner signals in the vapor channel detector.
Systems having on-board means for analyzing particle and vapor constituents are termed “fully integrated systems” and may be differentiated from systems for interfacing with remote analytical instrumentation, for example those systems where an insertable cartridge containing the immobilized samples of particle and vapor are conveyed to a stand-alone analytical instrument for analysis.
The air-to-air particle concentrator may be an aerodynamic lens with skimmer, an inlet particle separator with splitter, a vortex particle separator with particle diverter, or an elutriative particle separator with particle diverter. The air-to-air concentrator preferably includes at least one aerodynamic lens or lens array disposed in the upstream channel and fluidly connected to the skimmer. The skimmer typically includes an inlet for receiving a particle beam or ribbon from the aerodynamic lens element, and splits the gas stream so that a bulk flow is diverted to a lateral flow channel and a particle-enriched flow is directed to a collector duct for particle capture and analysis. The skimmer is provided with a skimmer body, a skimmer nose, a lateral flow channel for receiving the bulk flow, and a virtual impactor mouth in fluid communication with a collector duct for receiving the particle-enriched flow. A particle trap is disposed in the collector duct.
The particle trap is typically mounted proximate to and downstream from the skimmer in the collector duct, and may be incorporated in the skimmer body. The skimmer body optionally is provided with a heating means for heating the particle trap. The particle trap may be a centrifugal impactor, a pervious screen, a bluff body impactor, or an electrostatic precipitator. The pervious screen may be selected from a ceramic filter or mesh, a glass filter or mesh, a plastic filter or mesh, or a metal filter or mesh. The vapor trap is generally a sorbent bed or film or a carbon bed or film, but may also be a liquid.
Means for stripping the particle constituent or constituents for analysis from materials accumulated in the particle trap include: a) injecting or circulating a volume of a hot carrier gas through the particle trap; b) directing an infrared emission, a microwave emission, or a laser emission at a particle in the particle trap; c) resistively heating the particle trap; d) injecting a solvent or solvent vapor; or e) any combination of one or more of the above means for analyzing the particle constituent or constituents. Means for stripping and analyzing the free vapor constituent or constituents may include: a) injecting or circulating a volume of a hot carrier gas through the vapor trap; b) injecting or circulating a solvent vapor in a carrier gas into the vapor trap; c) directing an infrared emission or a microwave emission at the vapor trap; d) resistively heating the vapor trap; or e) any combination of one or more of the above means for analyzing the free vapor constituent or constituents.
Means for detecting a particle or a free vapor constituent accumulated in one of the traps further generally comprise a) means for performing a liquid chromatographic step; b) means for performing a gas chromatographic step; c) means for performing an affinity binding step; d) means for performing an ionization step; e) means for performing an electrophoretic step; f) means for performing a spectrometric, fluorometric, or photometric step; g) means for performing a mass spectroscopic step; h) a means for performing an electron capture step; i) means for in situ detection; j) a combination of one or more of the above means; or k) other analysis and detection means known in the art. Analysis means may be shared for particles and for vapors or may be independent. Optionally, particle constituents and vapor constituents may be pooled before analysis.
Advantageously, independent capture of particle and vapor constituents from separate traps improves reliability and robustness of detection, reducing both false positives and false negatives. Using systems of the invention, constituents of the particle trap and constituents of the vapor trap may be stripped and analyzed (or analyzed and stripped) independently, so that analysis and regeneration conditions in each trap are independently optimized. Separate accumulation of free vapors trap yields cleaner vapor signals when present. Separate accumulation of particles is useful because stripping can be performed selectively, eluting selected classes of analytes in one or more solvents, for example. Solvent eluates can be flash evaporated to remove interferents from the sample. Unstable analytes can be subjected to liquid chromatography without thermal degradative losses. And those semi-volatile analytes that are difficult or impossible to detect as free vapors because of their low vapor pressure, can be analyzed without losses to surfaces in the sampling head.
Also included are methods for sampling particulate and vapor residues from an object, structure, surface, cavity, vehicle or person to detect an explosive. A method may comprise steps for a) aspirating a first sample having a volume and a velocity into a suction intake of a sampling head and conveying the volume as a suction gas flow through an upstream channel, the volume containing particles and free vapors; b) inertially dividing the suction gas flow into a particle-enriched gas flow containing a particle concentrate and a bulk gas flow containing the bulk of the free vapors, and directing, according to a flow split, the particle-enriched gas flow to a first downstream channel and the bulk flow to a second downstream channel, wherein the first downstream channel and the second downstream channel bifurcate from the upstream channel; c) immobilizingly accumulating any particles in a particle trap disposed in the first downstream channel and any free vapors in a vapor trap disposed in the second downstream channel; d) analyzing any constituents of the particles or free vapors accumulated in the traps to detect an explosive or explosive-associated residue therein. The step for analyzing may comprise eluting any constituents of interest in the particle trap in a liquid volume, optionally with heat, or volatilizing the constituents in the particle trap in a carrier gas volume, optionally with heat or solvent, or alternatively, an in situ analysis may be performed without elution or desorption of constituents. The step for analyzing also comprises desorbing any constituents of interest in the vapor trap, generally in a hot carrier gas volume, optionally with solvent vapor, optionally with a step for further concentration of the vapors in a secondary focusing trap, and conveying any desorbed constituents to a detector.
A step for cleardown of the sampling system between analyses may also be provided. Cleardown is achieved by convectively, conductively or irradiatively heating the traps; by injecting a purgative solvent; by purging the traps under a forward or reverse flow of a gas stream; by replacing the traps (as with a new or reconditions cartridge), or a combination of the above means.
Interchangeable sampler heads may be configured for sampling surfaces and also for interrogating spaces between surfaces, such as under pallets, between stacks of articles, inside vehicle compartments and trash cans, between boxes, in the nap or pile of a rugs, along floorboards, in bins of vegetables, and so forth, where we have found that combinations of jets with suction, can be optimized to improve overall sampling efficiency. Particulates are aerosolized by this treatment and entrained in the suction intake. Vapor recovery is improved by stripping any unstirred boundary layer in the sample area, such as is useful for detection of landmines. High velocity jets also erode contaminated substrates to yield additional analyte.
Sampler heads may be interfaced with particle and/or vapor collection and analysis systems for detection of trace residues associated with explosives and explosives-associated compounds, detection of landmines, particles associated with biowarfare agents, residues or particles associated with narcotrafficking, smuggling of chemicals, and animals or animal parts, environmental contamination of surfaces with toxins, bacterial or other contamination in food processing facilities, bacteria, fungi, viruses and insects on agricultural and forest products, and so forth. These systems are thus useful as part of larger surveillance systems for surveillance of complex environments, such as traffic at a border crossing, flow of mail, monitoring of ecosystems, ingress and egress of persons to and from secure areas, and in forensic investigations, for example.
The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:
Although the following detailed description contains many specific details for the purposes of illustration, one of skill in the art will appreciate that many variations, substitutions and alterations to the following details are within the scope of the invention. Accordingly, the exemplary embodiments of the invention described below are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.
The invention has applications for surveillance and analysis of particulates and volatile residues borne upon persons, articles of clothing, interior or exterior surfaces of buildings, furnishings, vehicles, baggage, packages, mail, and so forth. The following definitions are provided for convenience.
“Particles” include dust, droplets, mists, explosives residues, chemical agents, biological particulate agents, and toxins, while not limited thereto, and are generally smaller than grains of sand. Before or during sampling, particles may form “agglomerates” that have aerosolization and settling characteristics distinct from the particles themselves. Of particular interest are particles in the range of 1 to 200 microns, more preferentially 5 to 100 microns, where most of the mass is generally found. Adsorbed vapors are frequently found as constituents of particles, including particles such as fibers, dust, soil, clay, hairs, skin cells, mists and so forth. Constituents of particles include analytes of interest, interferents, and matrix materials.
The terms “mobilization”, “re-suspension”, “aerosolization”, and “re-aerosolization”, refer to a phenomenon in which particles, initially resting on a surface (or “substrate”), are advectively entrained in a moving gas volume in contact with the surface.
As use here, particle “aerosolization” can also involve erosion of surfaces such as cardboard, cloth, packing materials, paint, and standing water on surfaces, through the action of aggressive gas jets.
When the term, “air” is used, included as well for the purposes of the present disclosure are other gases and mixtures of gas more generally that may contain particles or vapors in dilute concentrations. For convenience, “air” includes all such gases to the extent that they act as diluents and carriers for target analytes, particles, volatiles, and vapors alike.
“Particle concentrators” include air-to-air concentrators generally, including aerodynamic lens particle concentrators, aerodynamic lens array concentrators, and micro-aerodynamic lens array concentrators when used in conjunction with a virtual impactor, skimmer or other means for inertially separating a gas flow into a particle-enriched flow (also termed “minor flow” or “scavenger flow”) and a “bulk flow”. Also included are cyclone separators, ultrasound concentrators, inlet particle separators, and vortex particle separators. Air-to-air concentrators split an intake flow into two downstream branches at a bifurcation, where the bifurcation may be a “skimmer”, a virtual impactor, a “splitter”, a simple “tee”, or a particle diverter. The ratio of particle-enriched flow rate to bulk flow rate is determined according to a flow split, which is a function of the pressure drop in each of the two downstream arms, the cross-sectional area, and any resistance related to Cv. The particle-enriched gas stream, also sometimes termed a “particle beam” or a “particle ribbon” is delivered to an outlet of the particle concentrator or module and may be conveyed to an aerosol collector module (or “particle trap”, see below). The “cut size” refers to the size of particles that are captured in the particle beam or ribbon, and is generally taken as the apparent aerodynamic size or diameter (D50) for which 50% of the particles are captured.
“Aerodynamic focusing” refers to systems for forming generally collimated beams or ribbons of particles in a flowing gas stream. The systems contain three elements: an intake orifice for receiving a flowing gas stream, one or more focusing lenses disposed along the long axis of the gas stream, and an acceleration nozzle downstream from the aerodynamic lens or lenses. Aerodynamic lenses are constrictions in a channel that create converging and diverging flow accelerations and decelerations through which particle tracks converge by inertia on the center axis of flow, thereby depleting the surrounding gas streamlines of their particle content. Aerodynamic lenses may be of “slit” geometry or of “annular” geometry. Aerodynamic lens or lenses may also be disposed as arrays as described in U.S. Pat. No. 7,704,294 to Ariessohn, which is co-assigned.
“Skimmers” refer to systems for splitting a flowing gas stream at a junction so that a bulk flow and a particle-enriched flow are directed into separate, bifurcating downstream channels. Generally a “virtual impactor” is positioned to receive the minor flow in a collector duct. Skimmers are described for example in U.S. Pat. No. 7,875,095 to Ariessohn, which is co-assigned. Skimmers are related to particle splitters and particle diverters more generally, all operating by similar principles of inertia.
“Inlet particle separators” also use inertia to separate particles from surrounding gas in a moving stream. Air entering through an intake manifold is accelerated and then bent sharply. Clean, particle-depleted air flows around the bend, but particles having inertial mass are not deflected with the streamlines and are captured by a splitter lip, continuing into a “scavenger” bypass channel. The terminology may also refer to an outer bypass stream (herein a “particle-enriched flow”) and a “core engine stream” (here a “particle-depleted bulk flow”). Inlet particle separators may be operated under vaneless conditions equivalent to slot-type aerodynamic lens geometry, or under swirl conditions, where vanes are used to generate a vortex-like flow regime in a cylindrical channel that forces particles to the outer wall of the channel, under and outside an annular splitter lip, and into a particle diverter duct. Clean air at the centerline of the vortex enters a downstream recovery manifold over and into the annular splitter, which can be modeled as an airfoil.
“Particle traps” or “particle collectors” include inertial impactors broadly, particularly centrifugal impactors, and also bluff body impactors and fine meshes or filters capable of capturing particles in a targeted size range. Special classes of impactors include liquid impingers and plate impactors. Also included are wetted wall impactors and rotary vane impactors. Filters for particle removal include membrane filters, depth filters, felts, mesh, mesh layers, and beds, also termed generally, “barrier filters”. Also included are elutriative particle collectors. Particle collectors are described in U.S. patent application Ser. Nos. 12/364672 (titled “Aerosol Collection and Microdroplet Delivery for Analysis”) and 12/833665 (titled “Progressive Cut-Size Particle Trap and Aerosol Collection Apparatus”), which are coassigned and are hereby incorporated in full by reference.
Sensitivity of a trap is in part a function of preconcentration factor PF:
PF=C
f
/C
0
where C0 is the initial concentration of an analyte in a sample and Cf is the post-collection and processing concentration. This experimental ratio may also be used to account for material lost in the trap during desorption.
“Stripping” refers to a process of removing captured materials from a particle trap, as in preparation for analysis or as in regenerating the trap for a next sample. Stripping may be performed with a combination of heat, solvent, gas, or solvent vapor, in combination with ultrasound, for example, and may involve selective extraction of constituents that are analytes of interest, interferents or matrix materials.
“Explosives residues” include 2,4,6-trinitrotoluene (TNT), nitroglycerin (NG), dinitroglycerin (DNG), ethylene glycol dinitrate (EGDN), cyclonite or hexogen (hexahydro-1,3,5-trinitro-1,3,5-triazine, RDX), octogen (HMX), pentaerythritol tetranitrate (PETN), dipicramide (DIPAM), ethylenedinitramine (EDNA), 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), triacetone triperoxide (TATP), acetone peroxide/nitrocellulose (APNC), hexamethylene triperoxide diamine (HMTD), tetryl, ammonium nitrate, urea nitrate, ANFO (ammonium nitrate/fuel oil mixtures), plasticized blends of cyclomethylenetrinitramine (RDX) and PETN (such as Semtex), other polymer bonded explosives (PBX), for example, while not limited thereto. Explosives-associated compounds more generally, particularly volatile molecular analyte species such as ethylene glycol dinitrate (EGDN), dimethyldinitrobutane (DMDNB), mononitroluene, or isotopically labeled explosives used for “tagging” commercial explosives as a means of source identification, are also of use for detection [Steinfeld J I and J Wormhoudt. 1998. Explosives detection: a challenge for physical chemistry. Ann Rev Phys 49:203-32; Singh S. 2007. Sensors—an effective approach for the detection of explosives. J Hazardous Matl 1-2:15-28]. Dogs are very sensitive to DMDNB and can detect as little as 0.5 parts per billion in the air. Also of interest as targets for detection are those agents identified and listed by the Bureau of Alcohol, Tobacco and Firearms as explosives under section 841(d) of Title 18, USC. Firearms residues, both before and after ignition, may also be encountered.
Referring now to the figures, a conventional vacuum sampling device (1) with intake (2) is shown schematically in
As described in U.S. Pat. Nos. 6,861,646 and 6,828,795, application of a cyclonic outer flow regime is reported to improve the ability to sample complex surfaces at a distance from the detector head. This is shown schematically in
Contrastingly, we have directed sonic jet pulses or streams converging toward a virtual apex of a cone behind the surface to be interrogated without cyclonic flow. Cyclonic flow of the incident air stream is not believed relevant to the operation of our invention. We have found that for particle removal the impingement or incidence angle of a jet streamline, i.e. the angle of the streamline relative to a flat surface generally parallel to the sampler head, exhibits improved dislodgement and aspiration efficiency at an incidence angle of about 60 to 85 degrees (i.e., where 90 degrees is perpendicular).
In more detail, for a first embodiment (200) of the invention, sampler head (210) has a forward face (211) and a ring of jet nozzles (212) mounted in a circumferential array around a central axis (214). At the center of the forward face is a suction intake port (213) with conical inlet. Sampling jets (220) propelled from the jet nozzles (212) are directed to converge on an external surface (4), forming the walls of a truncated virtual cone. On striking the surface, the jets are turned inward and are returned under suction to the suction intake port (213). Suction is generated by a vacuum pump (or blower inlet) mounted in or connected to the sampler head. A bundled core of suction return streamlines (230) is shown at the central long axis of what is a “virtual sampling chamber” (250), the virtual sampling chamber having a truncated conical shape with base formed by the forward face (211) of the sampler head and frustrum by out-flow streamlines making an involuted frustroconical “U” turn (221) on the interrogation surface (4). The out-flowing gas jets (220) are connected with the bundled core of in-flowing return streamlines (230) directed into the suction intake by the frustroconical “U-turn” of the streamlines at the surface.
Also shown is a positive pressure source (240), here a diaphragm pump, for charging the gas jets and tubulation (246) for discharging a curtain wall flow through annular slit orifices (245) disposed as an apron around the sampler head, as will be discussed further below.
The geometry of the conical “virtual sampling chamber” is illustrated schematically in
As discussed further below, the sampling jets may be emitted as a single pulse or pulsed burst, and after an interval of a few microseconds, the emitted gas volume is efficiently recovered by application of a strong suction pulse. Thus it can be seen that the gas-walled sampling chamber is formed and then collapses—truly an evanescent manifestation of a virtual sampling chamber having a duty cycle of a few seconds, while not limited thereto. Individual pulse cycles may be repeated at defined pulse intervals, or in response to a triggering event.
Although not shown, the source of pressurized gas for the sampling jets and vacuum for the suction intake may include centrifugal, rotary vane, piston, or diaphragm pumps, or other pumps as known in the art. The exhaust of the suction gas generator may be used to drive the gas jets of the out-flow. A high pressure tank of a gas or pressure reservoir may be charged to a pressure setpoint and gas released using high-speed solenoid valves to generate sampling jet pulses. Pressurized gas may be stored in tubulations (such as elastic hoses) within the sampler head. An outermost peripheral annular curtain wall flow may also be used to further enclose the virtual sampling chamber, as will be described below.
Average jet flow velocities in the range of 20 to 300 m/s have been found useful in studies performed to date. The calculated average jet velocity at the outlet of a nozzle for smaller diameter nozzles approaches 300 m/s, which indicates that the velocity at the nozzle center line is sonic, and that it operates at choked conditions with higher than ambient air density. Supersonic jets may also be used. Modeling studies by computational fluid dynamics show that jet velocities and suction pressure diminish over distance from the sampling nozzle, but are capable of forming a virtual sampling chamber enclosing a distance Df of up to about 12 inches or more from the interrogated surface, where the distance Df is the height of a frustrum of a virtual cone as measured from its base (
Practical illustrations of the force of the jets in eroding residues from surfaces and forming aerosols are seen in
The apex angle “theta” (or “vertex angle”) of convergence of the jets forming the virtual cone may be varied as desired, but is found to be more effective in the range of 10 to 60 degrees, most preferably about 15 degrees. For a jet, the incidence angle is the external angle of the half angle of theta (359) and is 90 degrees for a jet normal to a surface. Incidence angle of a jet pulse is most effective in the range of 60-85 degrees. In some applications, in order to increase the standoff distance Dc, it may be desirable to use a jet that approaches normal (perpendicular) to the forward face of the sampler head. Instead of a virtual cone, a virtual sampling chamber that is generally cylindrical can be formed when the jets are essentially parallel in trajectory.
A computational fluid dynamics (CFD) model (300) of the pneumatic action of a sampler head with four jets (320a,b,c,d) is shown in
The impingement or incidence angle of a linear streamline forming the walls of a virtual sampling chamber is most effective for residue dislodgement and aspiration at about 5 to 30 degrees from normal (i.e. about 60 to 85 degrees from horizontal to the surface), which cannot be achieved in a cyclonic flow regime, where streamlines are essentially perpendicular to the bulk axis of flow and the impingement angle approaches zero. At lower impingement angles, rolling and sliding of particles is favored over lift-off. The higher impingement angle permits the use of higher intensity focused jets and the application of pulsatile sonic and supersonic flow regimes, which results in lift-off and removal of both particulate and volatile materials from irregular and complex surfaces, and in better re-aerosolization and aspiration efficiencies for particles.
Optionally, by balancing the “out-flow” of the jet nozzles and the “in-flow” of the suction intake, a closed loop may be formed in which sample residues are concentrated over multiple passes through a vapor or particle trap. The sampling device is intended for particle and vapor removal and for aspiration of dislodged particles and vapors into the sample head from surfaces or objects from a distance Df of up to about 1 foot, for example a vehicle driven between stanchions supporting sampling devices directed at intervals onto the surfaces of the vehicle (
While configurations with four jets, six jets and eight jets are shown, other configurations and numbers of jets are envisaged. In selected geometries, a three-jet or a two jet sampler head, where the jets are fan shaped, is directed at a surface and a mated central suction intake is configured to capture materials ejected from the surface by the impinging jets, optionally with a curtain wall or apron of flowing air improve containment. Other variants for establishing a virtual sampling chamber are possible and are not enumerated here.
The effect of pulse duration and pulse separation is analyzed in
ηS=ηR·ηA,
where ηS is the product of two efficiencies, the removal efficiency ηR and the aspiration efficiency ηA.
In
However, when corrected for removal efficiency, overall efficiency is shown in
Supplemental means for dislodging particles and volatile residues in the sampling cone include pulsatile flow regimes as described by Ziskind (Gutfinger C and G Ziskind, 1999, Particle resuspension by air jets—application to clean rooms. J Aerosol Sci 30:S537-38; Ziskind G et al, 2002, Experimental investigation of particle removal from surfaces by pulsed air jets. Aerosol Sci Tech 36:652-59), ionized plasmas directed through the sampling jets, liquid or solvent directed through the sampling jets, or shock waves directed from the sampler head. The gas in the loop may also be heated, chilled or humidified to improve performance, although caution is taken to avoid losses of volatile particles due to heating. If desired, the jet nozzle array may be operated in repetitive pulse mode, for example for sampling of a continuously moving belt.
A flow split is established whereby part of the gas flow, the “minor flow” (461) enriched for particles, is directed to the particle collector or trap (470). The particle-depleted “bulk” or “major” flow (462) is diverted, typically by use of a skimmer, and is ducted instead directly to the suction pressure pump. All the gas exhausted from the concentrator (462) and the gas exhausted from the particle trap (471) are returned to a common suction pressure source for recirculation through the sampler head. As shown in this example, the pressurized exhaust from the vacuum pump or blower (430) is used to drive sampling jets (420) forming the virtual sampling chamber (450). Particles resident on the interrogated surface (4) are dislodged and drawn into the sampler head. Material in the particle trap is periodically analyzed in situ by methods known in the art, or archived for example by removal of a filter cartridge for later analysis by chemical, biochemical or physical methods. Separate pumps may be used for out-flow and suction in-flows if asymmetric flow rates are desired. Gas flows may be filtered or purified before re-use if desired.
An apparatus with one or more combinations of particle and/or vapor analytical capability is also envisaged. Detection means for analysis and identification of particles or vapors are known in the art and may be selected for physical, chemical or biological analysis.
Conceived is an apparatus combining functional elements for separating particles and vapors in an air-to-air concentrator followed by particle and vapor trapping for analysis.
Surprisingly, one or more jet pulses of several milliseconds can be superimposed on curtain flow and suction cycles of one to several seconds, during which the flow regime conforms to the conditions required for use of stacked aerodynamic lenses as shown.
The virtual impactor (808) is comprised of a skimmer mouth (808a), a central collector duct (808b), a discoid chimney duct (808c) for routing the bulk flow (809) to nipples (808d) adapted, as shown here, for a hose connection to a vacuum source. Aerosolized particulate material is collected in a trap associated with the minor flow. Explosives materials for example are frequently crystalline or solid and are detected when aerosolized by a pressurized jet. Flow splits of greater than 100× are readily achieved with annular devices of this type, dramatically leveraging detection sensitivity by several orders of magnitude.
Multiple aerodynamic lenses may be used. For example by stacking four lenses, concentration of particles over a broad range of particle sizes can be achieved. Beginning with the first lens, which acts on larger particles, the remaining lenses in the stack progressively act on smaller particles in steps of 2× to 4×. Thus by example, a four lens stacks may focus particles of 100, 30, 10, and 5 microns respectively, while not limited thereto.
In order to increase particle velocities in the central collector duct and reduce elutriative effects, the intake duct or “bell” geometry may be aerodynamically shaped to minimize particle impact, for example as per a NACA duct, Laval duct, elliptical duct intake, bell shaped duct intake, parabolic horn intake, exponential horn intake, quadratic convergent duct intake, power series convergent duct intake, or other tapered geometry of the intake. Fins or airfoils for minimizing turbulence, reducing deadspace and increasing linear velocities of the streamlines may also be used. As the lenses are improved by contouring to relieve eddy separation and particle wall impaction, performance is also seen to improve significantly, particularly in the collection of larger particles, which problematically are otherwise lost to sedimentation and rebound following wall impaction in the sampler head and concentrator.
Devices and systems of the invention have applications for sampling and detection of explosives residues. A wide range of analytes must be detected. Surveillance systems for selective sampling and detection of only a few explosives-associated analytes or families of analytes would have significant vulnerabilities. Nitro- and nitrate-based materials are the most numerous, but materials such as perchlorates, peroxides, azides, incendiaries, propellants, and hydrocarbons must also be considered. Mixtures and combinations, such as of fuel oil and ammonium nitrate, are also of interest. Detection of crystalline ammonium nitrate in combination with fuel oil vapor is significantly more conclusive than detection of either a nitrate (such as from a prescription tablet) or a fuel oil vapor (such as from dirty shoes) alone. Also of particular interest are mixtures including taggants and other explosives-associated materials (XAM) indicative of processed explosives.
Equilibrium vapor pressures of explosive materials range widely, from over 4.4×10−4 Torr for nitroglycerin (NG), 7.1×10−6 Torr for TNT, to 1.4×10−8 Torr for PETN and 4.6×10−9 Torr for RDX at 25° C. [Conrad F J 1984 Nucl Mater Manag 13:212]. Also to be considered, however, is the affinity of the vapor molecules for solid surfaces, which may suppress free vapor concentrations, thus reducing detectable thresholds. We find that detection of volatile compounds such a dinitrotoluene, a degradant of TNT which has an affinity for solid surfaces, can be improved by collecting particles that have equilibrated with vapors of the explosive. These particles are typically endogenous materials that are exposed to the explosive residues in the environment, for example road dust, silica, ceramic, clay, squamous epithelial cells, hairs, fibers, and so forth. By collection of exogenous particulate materials, explosives residues associated with the particulate debris are found to be more reliably detected.
A sampling system for collection of particles and vapors is depicted conceptually in
The sampling device comprises a jet-suction head 1002 with a pair of forward facing jet nozzles 1003 and central suction intake 1004, a wand with handle and control interface, a suction blower 1005 for pulling a bulk flow, and internal pneumatics as described schematically in
The internal workings of a wand or sampler head 1000 generally include (
Also provided are control circuits 1019 for powering and controlling operation of the apparatus. Control elements may include a microprocessor or microprocessors, RAM memory, complex logic instructions stored in non-volatile memory (such as EEPROM), optional firmware, and I/O systems with A/D conversion for collecting data and D/A conversion for transmitting instructions to analog subsystems such as pumps and valves and for controlling the flow of power to component systems of the pneumatics and any on-board analytic module(s). Logic circuits may be configured for comparing or integrating detection signals from a particle channel and a vapor channel.
Three pumps are shown and arrows represent gas flows; black arrows indicating positive pressure, open arrows indicating suction pressure. System timing is provided by a controller 1019 which optionally also supplies power to the component subsystems. For purposes of illustration, only two jets and paired solenoids are shown. During sampling, jet pulse outflows from the nose of the device are deflected by collision with an external surface and are aspirated, at least in part, as a suction intake flow 1015 through a suction intake in the forward face or “nose” of the sampling head and into the air-to-air concentrator 1014. A skimmer 1018 is used to separate the particle-enriched flow 1016 and the particle-depleted bulk flow 1017 at a flow split that is determined by the relative capacity of suction blower 1005 used to pull the bulk flow and vacuum source 1013 used to pull the particle ribbon or beam. The bulk flow contains the majority of the free vapors in the sample. The flow split between bulk flow and central core flow is typically configured to be greater than 50:1 and may approach or exceed 250:1. Pressure drops on the particle and vapor sides of the skimmer may be controlled separately.
Bulk flow 1017 is drawn through a vapor trap 1007 to capture any entrained free vapors of interest. The particle ribbon or beam flow 1016 is drawn through particle trap 1006 to capture any entrained particulate matter and adsorbed vapors. The particle and vapor constituents of the suction intake flow are thus not collected in series, but are instead separated so as to independently optimize their respective conditions for accumulation, extraction, and analysis.
Analytical systems may be supplied on board (not shown) or may be provided at a remote workstation. Thus the particle and vapor traps are optionally cartridges that are placed in the gas flows and removed for analysis. Optionally, the skimmer nose may also be supplied as part of the cartridge. In integrated systems, a common analytic system may be used to analyze both particle and vapor trap constituents; or the analytic systems may be independent.
The capacity of a representative suction blower 1005 is typically in the range of 300 to 1500 liters/min at a suction head pressure of 5 inches of water, while not limited thereto. The required flow rates may be achieved with a centrifugal blower such as a Windjammer Model 116630E or a 5.7″ regenerative blower (AMTEK Part No. 116638-08, Kent Ohio). The capacity is designed to be effective in aspiration of solid from up to about 1 foot (>30 cm) from the sampler head, typically with jet assist. For portable operation on DC power, a Microjammer 3.3″ BLDC low-voltage blower (AMETEK Part No. 119497) may be used. Fans may also be used.
Particle ribbon or beam flow may be powered for example by a diaphragm vacuum pump 1013 such as a BTC-IIS Vacuum Diaphragm Pump obtained from Parker-Hargraves (Model No. C.1C60G1.1C60N1.A12VDC, Mooresville N.C.). Flow rate for the particle-enriched flow downstream from the skimmer is typically in the range of 10 to 15 L/min or less at a suction head pressure of about 20 to 30 inches of water.
Exhaust from the suction blower 1005 optionally may be used to power a curtain air flow through slits mounted peripherally on the sampler head, although not shown here.
Jet pressure is provided by a compressor 1008, typically a diaphragm pump such as a Parker-Hargraves D737-23-01 double diaphragm pressure pump or a Thomas (Part No. 11580C56, Sheboygan Wis.). Optionally, any 100-120 psi air pressure source such as compressed air can be used. Pressure is typically accumulated in a pressure reservoir 1009, which may be a tubulation or an in-line tank and is distributed through a manifold 1010 to an array of jets; the manifold is configured to equalize pressurized gas feed to the individual jets.
Solenoids 1011 include Gem Sensors (Plainville Conn.) Part Nos. B2017-V-VO-C111 with a Cv flow factor of 0.43 and 7 Watt coil; D2014-589 (D2014-SB1-V-VO-C111) with 0.21 Cv body and 10 Watt coil; and A2016-V-VO-C111 with 0.24 Cv body and 6 Watt coil operable at 100 psi. Also tested was an ASCO Part No. 8262H112 with a Cv of 0.52 which is also available in DC configuration. These valves were selected for their fast reaction times in order to generate pulses of about 2 to 20 millisecond duration. For general purposes, a 10 ms pulse is useful.
Individual jet pulse outflows in the range of 5 to 20 ms duration have a volume at STP of about 2 to 6 cc3. Because jet arrays can contain multiple nozzles, total jet volume is typically a multiple of that, for example 12 to 36 cc3 for a 6 jet array. Jets are typically operated at choke or near-choke conditions, and at the nozzle, jet out-flow linear velocity approaches the supersonic threshold of 320 m/s. Pulses are thus pressurized at up to about 10 Atm or higher, typically at least 30 to 150 psi, and are underexpanded when released. Jet velocity stagnation (as measured by centerline velocity) is not seen at distances of up to 30 cm, as shown in
Under choked flow conditions with fast valve actuation (solenoids 1011), jet pulse 1012 energy may be varied by selecting nozzle size or critical dimension. Jet nozzles may be circular or may have asymmetrical shapes, such as fan or chisel shapes. Nozzles may be arranged in various configurations on the sampler head and the pulse volume emitted by each nozzle is generally summed to determine the total pulse volume. Jet velocity at the nozzle is graphed in
Following jet-assisted suction aspiration, any analyte captured in the particle trap is stripped from the trap and conveyed 1023 to an analytic module. Analyte captured in the vapor trap is also stripped from the trap for 1024 for conveyance to an analytic module. Both traps will be pneumatically (or hydraulically) coupled so that a volume of a carrier gas (or liquid) can be passed through each trap, concentrating the analyte from each trap in a smaller volume for analysis. In the analytic module, analysis and detection of any signal from one or more constituents or analytes is by conventional means. Optionally, all or part of the volume from each trap may be directed to a focusing trap for further concentration before analysis or may be captured on a sorbent for archiving if desired. In situ detection technologies may also be used.
Once any entrapped analyte or analytes have been extracted, a purge step 1025 is initiated so that the traps are regenerated in preparation for a second analytical cycle. Where in situ detection is practiced, negative samples are discarded without further analysis and a second cycle of jet-assisted suction may be initiated immediately. Alternatively, regeneration is accomplished by cartridge replacement. Stripping means are useful to extract “strippates” for analysis and also to purge the traps.
Thus a single analytical cycle may have a duration of a few seconds to perhaps a minute. First, a jet pulse or pulse burst is actuated to dislodge a sample, suction is continued for several seconds to a minute or more to aspirate the sample; the contents of particle and vapor traps are then examined, and the traps are then purged or replaced and the electronics cleared so that a next sampling cycle may be initiated with a clean trap and no alarms pending (the process of purging the traps and resetting the electronics is termed “cleardown”).
Referring to
In a first step (
Analysis is then initiated. This process is depicted in
The particle trap will typically contain explosives residues having higher boiling points in particulate form, whereas the vapor trap will contain lower boiling point materials. Thus the stripping operation and analytic module 1032 used with the particle trap may be operated independently or at different conditions than the stripping operation and analytic module 1031 used with the vapor trap. Because vapor-related and particle-related analytes frequently benefit from different analytical conditions, separately optimized analytic modules (1031, 1032) are shown. Stripping of low vapor pressure explosives from the particle trap for delivery to an analytic module, for example, is more efficient when performed by liquid elution rather than by evaporation, but stripping of higher vapor pressure analytes from the vapor trap is more efficiently performed using thermal desorption in most cases.
However, if desired, a single common analytic module may be used for both channels, either by performing sequential analysis or by pooling the particle and vapor samples. Particles can also carry adherent volatiles and themselves may be volatilized in full or in part by heat so that both the high boiling point volatiles and any associated vapor constituents associated with the particle concentrate may be analyzed together.
Analysis generates detection signals, a first signal for any particle constituent from the particle trap and a second signal for any vapor constituent from the vapor trap, if present. The two signals may be integrated and/or compared for additional information of use in detection of explosives. Confirmatory information is obtained. Information is also obtained if an interferent disables one analysis channel but not the other. Thus false positives and false negatives are reduced. Particles can be associated with a large amount of interferents, but by adjusting the cut size to essentially eliminate particle mass from the vapor channel, very clean vapor signals result.
The analytic module or modules may contain hydraulics or pneumatics and one or more conventional means for detecting one or more analytes/constituents of interest. The kinds of analytical instruments that may be adapted for explosives detection from particles and vapors are those that are known in the art. The analytic module may also contain focusing traps which function essentially as second stage preconcentrators in series with the particle and/or vapor traps and may also be used to prepare samples for archiving.
In a third stage of a sampling/analytical cycle, the particle trap 1006 and vapor trap 1007 are regenerated if necessary as depicted in
There is a need for systems capable of detecting both particles and vapors, yielding complementary information. As can be seen from
Conversely, certain explosives and explosives associated materials occur with vapor pressures in excess of parts per million and are relatively straightforward to detect as free vapor. They are sometimes smelled with the human nose and are targets for canine detection. These include nitroglycerin, fuel oil, ammonium nitrate, ANFO mixtures, and taggants, for example. Taggants have been proposed to facilitate detection of low vapor pressure explosives. Taggants include 2,3-dimethyl-2,3-dinitrobutane (DMDNB), ethylene glycol dinitrate (EGDN), and 4-nitrotoluene (para-NT). These compounds were chosen because they do not occur in nature, they do not tightly adhere to common substrates, and because they continue to release their vapors for 5 to 10 years [J. Yinon. 1995. Forensic Applications of Mass Spectrometry, CRC Press, Boca Raton, Fla.]. Other odiferous “fingerprint compounds” such as cyclohexanone (CXO—used in recrystallization of RDX), benzoquinone, 2-ethyl hexanol (2-EH, used in manufacture of plasticizers), triacetin, and diphenylamine also may be present in significant amounts for detection [Williams et al, 1998, Canine detection odor signatures for explosives, Proc SPIE 35:291-301; WIPO Doc. No. 2010/095123]. These odor fingerprint compounds can be captured for example using gas phase SPME and detected by IMS [U.S. Pat. Doc. 2009/0309016; Perr et al, 2005, Solid phase microextraction ion mobility spectrometer interface for explosive and taggant detection, J Sep Sci 28:177-183; Lai et al, 2008,
Analysis of volatile components of drugs and explosives by solid phase microextraction-ion mobility spectrometry. J Sep Sci 31: 402-412]. However, taggants are generally not used by illicit explosives manufacturers and a negative vapor detection event must always be viewed with uncertainty.
Use of upstream air-to-air concentrators has unexpected benefits when both particles and vapors are to be detected. A synergy is achieved when the sample is split into a particle-rich fraction and a particle depleted fraction. When particles are directed to a particle trap and vapors are directed to a vapor trap downstream from an air-to-air particle concentrator, the following benefits accrue:
These synergies have not been anticipated in the art. The prior art taught particle traps having large surface areas and deadspace (generally employing a particle trap to collect both particles and vapors or a particle trap in series with a vapor trap). The smaller the particle to be collected, the larger the pressure drop per unit filter area, and thus pressure drop dictates the surface area-to-cut size ratio of particle filters. While it would be useful to sample hundreds of liters of air for trace vapors, passing such a volume of air through a fine particle filter would be prohibitive in a small unit. As shown here, use of an in-line air-to-air particle concentrator overcomes this problem. Air-to-air concentrators may be operated a flow split of 30:1, 50:1, 100:1 or even 250:1 and at particle cut sizes (in the concentrator) of 5 to 10 microns (or even 1 micron if desired), thus shunting very large amounts of particle-depleted air around the particle trap and permitting miniaturization of the particle trap. Happily, stripping operations for harvesting particle constituents from a very small particle trap can be conducted with a correspondingly small volume of stripping agent, with geometric increases in preconcentration factor and sensitivity.
With air-to-air concentrators, operational systems have been achieved at more than 1000 sLpm in portable units and are readily scaled for higher throughputs. Miniaturization of the particle trap increases detection sensitivity by increasing the preconcentration factor; the hollow trap volume of the particle traps (i.e., the deadspace volume of the trap) may be reduced to sub-milliliter dimensions in this way.
Correspondingly, very large quantities of air may be sampled for free vapors. Particles are not allowed to impact the vapor trap. Vapor trap signals are cleaner without this interference. As pointed out perhaps first by Corrigan (U.S. Pat. No. 5,465,607, Col 20 lines 3-14), semi-volatile materials can overwhelm and degrade performance of GC/MS and MS/MS instruments. Thus by freeing the vapor signal from particle-derived interferents, more sensitive and refined analytical techniques may be applied.
Particles can rapidly foul vapor sorbent beds, poisoning the sorbent and preventing regeneration and cleardown. The excess heat required to fully bake off or incinerate particulates on a sorbent bed can exceed the thermal stability of the resin. Sorbents are likely to bleed particle-associated interferents for long periods of time, degrading the effectiveness of subsequent sampling.
The challenge for particle and vapor collection systems is made more difficult because sampling and detection conditions are not necessarily copacetic. Vapor analyte stripping from a vapor trap is inherently best performed by desorption, but stripping of analytes from a particle trap may be best performed with a solvent, for example. Heating of HMTD, for example, is likely to yield CO2, ammonia and trimethylamine, but with solvent elution, intact HMTD will be recovered, greatly aiding interpretation of the resulting spectrograms. PETN has an extremely low vapor pressure, a tendency to adhere to surfaces, and is unstable at temperatures of even 100° C., making gas chromatographic detection difficult. Lower nitrate esters of pentaerythritol are more readily detected under conditions that would not favor vapor detection, such as with liquid chromatography. Conversely, DNT has a higher vapor pressure than TNT, and is a favored analyte for vapor detection, but TATP or EGDN would not likely be detected by thermal desorption under conditions suitable for desorbing DNT. Thus the use of a single stripping technique for both the vapor trap and the particle trap, as proposed by Syage for example in U.S. Pat. No. 7,299,710, can result in significant blind spots in surveillance.
Independent detection of the contents of vapor and particle traps can also yield patterns that are more definitive than single channel analysis. Given the significant differences between the kinds of materials likely to be directed to the particle trap versus the vapor trap, a physical separation of the two traps results in a unique opportunity to apply different analytical techniques to each.
Finally, by diverting the bulk flow to the vapor trap, higher velocities in the suction intake may be achieved. A lower pressure drop in the vapor trap is readily achieved, and higher flow rates more easily accommodated. By increasing velocity of the suction intake, particles that would otherwise settle out and be lost may be successfully aspirated with the sample. A single larger particle can have more informational value than thousands of liters of vapor. Because, as indicated by the data of
Particles are the primary information-rich content of any sample, and include not only explosives crystals and residues, but also fibers, dust and skin cells saturated with adsorbed vapors from contact with explosives and explosives associated materials (XAM), including taggants. A rigorous, jet-assisted sampling apparatus, with capacity for accumulating particles in a particle trap from a larger volume of aspirated air, will improve surveillance and reduce false negatives. A single particle of diameter of 10 microns may have a mass of about 1 picogram; a particle of diameter 25 microns a mass of about 13 picograms; a particle of diameter 50 microns a mass of about 105 picograms: thus a single particle may be sufficient for detection of an explosive having 200-400 MW, even an explosive having negligible vapor pressure.
The relevance of particles in detecting explosives is thus readily apparent. For example, in a fingerprint containing 100 ng of crystalline explosive (as shown in
One comprehensive solution uses an air-to-air particle concentrator for focusing and concentrating particles from a high volume throughput suction intake, accumulating particles (and any adsorbed vapors) from a particle-enriched flow in a particle trap, accumulating any free vapors in a vapor trap in a bulk flow, and analyzing the contents of the particle trap and the vapor trap, either independently or after pooling any analytes stripped from both traps. A real time dual detection platform for both vapor and particulate explosives residues at high throughput is achieved by combining jet-assisted aspiration with skimmer-assisted separation of particles and vapors prior to capture and analysis, and advantageously overcomes technical problems that occur where separation of a particle-enriched flow and a particle-depleted bulk flow is not provided.
Top and bottom aluminum mounting plates (1107, 1108) are fastened together so that the cartridge can be inserted and removed from the housing as a single unit. The air column being sampled flows from the bottom of the assembly through a central passage and out the top.
From bottom to top, the moving gas sample encounters pervious supporting layers (1104, 1103) which sandwich a ceramic layer with central cutout, the ceramic layer 1102. The central well is for receiving a bed of vapor adsorbent beads and the two surrounding layers hold the beads in place during operation. The depth of the bead bed can be seen to be relatively shallow so that pressure drop across the vapor trap is low. Resting on the uppermost support layer is a stainless steel plate 1106 with central open grid for supporting the resistive heating coil 1105. A ceramic cuff 1109 that fits over the coil is notched 1110 for the passage of electrical wires to the heating coil.
The heating coil is actuated during desorption only, generally during the analytic step (IV) and purge step (V) of
As shown in
An ideal vapor preconcentrator has only one theoretical plate, and an adsorbate species is thus adsorbed or desorbed in essentially one fully reversible “on/off” process. However, in practical application, efficient vapor trapping necessarily relies on more complex free paths and binding site affinities to ensure capture of a variety of analytes. In our experience, useful vapor adsorption efficiency, acceptable breakthrough volume VB, and shorter desorption time to cleardown, can be achieved at a low pressure drop and high throughput rate for light (C2-C5) and mid-range (C5-C12) volatiles typical of explosives-associated compounds by reducing bed thickness to that having a pressure drop of 5 inches of water or less at 1000 L/min through a reasonable surface area. These conditions describe a thin plate, disk or layer suspended across the gas stream and having a thickness of at most 2 mm.
In
Thus the interest in exchangeable cartridges. Exchangeable cartridges containing a vapor trap or a vapor/particle trap combination may be used. Disposable cartridges permit suspicious samples to be archived or transferred for more extensive analysis, and also eliminate the need for expensive maintenance if the vapor trap becomes contaminated with a “sticky substance”. Off line analysis of vapor sorbent filters is described for example in WIPO Doc. No. 2010/095123 and in U.S. Pat. Appl. Doc. 2009/008421).
Sorbent bed life is also increased by avoiding exposure of the sorbent bed to higher molecular weight adsorbates, those considered “semi-volatile”, by supplying particle-depleted air to the vapor trap, high boiling point “sticky” volatiles in the bulk flow are largely avoided. Thus the cut size of the particle concentrator is generally configured so that particulates are directed away from the vapor trap and to the particle trap, advantageously reducing vapor trap fouling.
Jet operation is as earlier described: jet pulses 1251′ are emitted intermittently by the action of high speed solenoid valves 1252 at near sonic velocity and have kinetic effects at up to a foot away, collisionally dislodging, mobilizing and eroding materials from substrates. Multiple jet nozzles ring a central suction intake or are used in pairs. The jet pulses may form an intermittent, instantaneous virtual sampling cone in which particles and vapors are mobilized and directed as a suction intake flow 1253 into the suction intake and central intake duct 1254.
Within the central intake duct 1254, particles are concentrated as a central particle beam or ribbon of flowing gas by the focusing action of one or more aerodynamic lens elements 1257. The gas stream 1253 is accelerated as the duct narrows and encounters a virtual impactor 1258 with skimmer body 1259, skimmer nose 1260, and collector duct 1261. The bulk flow 1262 streamlines are deflected on the skimmer nose 1260 into lateral flow channels 1263 while the particle-enriched flow 1264 with particles flows into the mouth of the nose, also termed the mouth or void of the virtual impactor 1258. As shown here, the particle-enriched stream is then stripped of particles by a mesh-type impactor 1256 mounted within the nose before exhausting through collector duct 1265. Downstream pumps for pulling the bulk flow 1262 and the particle- exhausted flow 1264 are separately controllable and are used to establish a flow split between the two flows and also the overall suction intake volume per unit time.
Flow 1262 of particle-depleted gas is directed through a low pressure drop vapor trap 1268 containing an adsorbent material with affinity for the desired vapor analyte(s). Vapor-depleted air 1262′ exits the vapor trap housing (drawn to suction blower 1005). In this way both vapor and particulate fractions of interest may be captured at a higher overall sampling flow rate and velocity then would be possible if the flows were not separated according to the flow split.
The particle trap 1256 shown here is built as a cartridge assembly 1266 constructed to be withdrawn from the apparatus. The cartridge comprises a cylindrical sleeve around the collector duct and the particle trap member 1256 and inserts into a receiving port 1267 at the base of the collector duct. The receiving port is co-axial with the long axis of flow of the particle beam. The cartridge is removable for remote analysis or archiving.
Pervious filter or mesh members 1256 generally are heat resistant and are selected from glass or ceramic where electrical interference is to be avoided, such as for certain in-situ detectors. However, conductive stainless steel or carbon materials may also be used if desired. Encased carbon fiber materials may also be used as a coarser supporting matrix to improve heat transfer. In special circumstances, such as disposable cartridges, plastic filters or meshes may be used, and analytes may be stripped with vapor or solvent rather than heat.
Under choked flow conditions with fast valve actuation, jet pulse energy may be varied by selecting nozzle size (or critical dimension). Nozzles may be circular or may have asymmetrical shapes, such as fan or chisel shapes. Data shown is for a series of circular nozzles. The distance ratio is defined as L/d, where L is the distance between the jet nozzle orifice and the substrate and d is the critical dimension of the jet nozzle. The ratio is found to have a correlation with particle removal efficiency and can be seen to scale linearly. At length/diameter ratios of 30×, recovery is still sufficient to detect all three explosives. At 10× jet length to diameter ratios, recovery (1271, solid line) approaches unity for more crystalline materials such as TNT, but is less for C-4 (1273), which is a plastic explosive and is more clay-like, containing aliphatic oils which are sticky. RDX, the active crystalline component of C-4, is shown to be more readily aerosolized (1272). Studies by others have shown that fingerprints of persons handling RDX and C-4, for example, typically contain crystals larger than 10 microns, and these crystals contain most of the total mass, underlining the value of collecting particulate solids.
Effects of number of layers of filter or mesh on particle capture efficiency are shown in
Because the bulk of the air volume aspirated has been diverted in the skimmer, lower pressure drops, smaller particle traps, and higher particle capture efficiencies are achieved.
Particle capture efficiency is negatively impacted by particle scattering and elutriative losses.
For portable surveillance systems, it would be common for a sampler head to be held at a somewhat horizontal orientation. The data indicate the need for higher linear flow velocities in the intake nose to minimize settling dropout. In heads with bell size maximal diameter of greater than about 5 inches, for example, a linear in-flow velocity is at least 0.8 m/is deemed sufficient to efficiently aspirate the majority of particles of 5 to 100 microns aerodynamic diameter without major settling losses. Higher linear intake velocity with acceptable pressure drops across a smaller cross-sectioned particle trap is realized, happily, by inserting an air-to-air particle concentrator between the suction intake and the particle trap. Further increase in volume throughput may be achieved by reducing the pressure drop for the bulk flow and by increasing the flow split.
Efficiency data are useful in optimizing jet and suction configurations for efficient particle resuspension and aspiration.
As jet diameter increases under choked flow conditions, particle removal efficiency ηR is seen to increase, indicating greater kinetic energy of the jet pulse; however, aspiration efficiency ηA, indicating particle capture, decreases almost inversely, indicating that particles are scattered outside the sampling zone. In this example there is an optimum balance, as seen by a peak in overall sampling efficiency ηS is apparent at a jet diameter of about 3 mm. This result has been repeated under a number of experimental conditions and represents a useful approach for optimization of sampler head configuration.
The force of the jets in eroding materials from a surface is illustrated in
Interchangeable detector heads are provided, as is useful to increase flexibility in use.
The narrow elongate nose 1283 depicted rightmost in
Nose attachments with four jets and two jets are shown, but the number of jet nozzles (1251) may be varied as indicated in
The sampler head body 1280 generally also includes any control mechanisms for pulsatile emission of jets (here a pair of solenoids 1285 are shown), any pressure reservoir and manifold useful for supplying and distributing pressurized gas feed to the jets, an air-to-air particle concentrator, a collector duct, pumps and any power supplies as required. Thus any wiring connections need not extend into the sampling nose attachments. The nose attachments include jet nozzles 1251 for directing jet pulses onto a substrate and a central suction intake for aspirating a gas volume and any associated vapors and particles. Tubulations are not shown for simplicity. The body is provided with a generic interconnect mechanism (here three nipples 1286a,b,c) so that each of the nose tools are engaged with a sealed and air-tight connection. Other sealable connectors are known in the art.
For enclosed spaces, two jets are typically sufficient although it may be desirable to control or vary jet incidence angle to better sample the walls of any crevice or cranny that is being interrogated. For larger surfaces and for situations where a sampler head traverses a surface (or a surface is moved beneath a sampler head) four, six or eight jets may provide additional efficiency in particle removal.
Because the jet pulses have a kinetic energy, any flexible walls or wrappings of parcels, letters, luggage and boxes are readily collapsed by the propulsive force of the jet and then reflated under vacuum, causing fractions of air to be expelled from inside the package or bag. Serial pulse trains are particularly useful in exploiting this percussive effect. The jet-suction head thus is superior to plain suction in mobilizing residues from inside parcels. In this way, false negatives are more readily avoided.
In one embodiment, the widemouth bell has an internal diameter of about 5.5 inches at the inlet end and a conical profile, terminating in central intake duct 1290 with an internal open diameter of about 1.77 inches. The suction velocity at the wide end (of the sampler cone is about 1 m/s at 1000 L/min. The suction velocity at the narrow end (1.77 inch diameter) of the cone, at the point of entry into the particle concentrator, is about 10 m/s under these conditions.
Aaberg lateral flows may be employed to extend the forward reach of the suction low pressure zone and more parallelly align in-flow streamlines. Since a large-volume regenerative air flow is readily available for feeding lateral flows (the bulk flow exhaust from the sampler head), the Aaberg effect can be readily achieved at little to no energetic cost for device operation.
As suggested by
Functionality of skimmers having concavoconvexedly reverse curved lateral channels 1308 for the bulk flow is described in more detail in U.S. Pat. No. 7,875,095 and co-pending U.S. patent application Ser. No. 12/964700, which are co-assigned and are incorporated in full by reference. Briefly, the downstream walls of the lateral channels are shown to support the bending streamlines of the bulk flow in turning more than 90 degrees from the long axis of flow of the gas streamlines in the suction intake, the downstream wall support serving to reduce eddies and wall separation instabilities so as to promote a cleaner separation of the bulk flow from the particle-enriched flow. The bulk flow and particle-enriched flow streams diverge above the virtual impactor mouth, shown here with a generally “cross-tee” configuration 1306′ with four channel arms in section. This geometry is useful for both slit-type and annular (axisymmetrical) skimmers.
Also occupying the skimmer body 1320 is an injection circuit or loop with inlet 1330 and outlet 1331. The injection circuit is a pneumatic (or hydraulic) injection channel or loop and interfaces with rotatable cylinder 1321 that houses the particle trap. In
The two views thus correspond to two steps of a sampling and analysis cycle. In a first, “normal” position, the trap hollow volume 1327 and particle trap 1314 are aligned with the long axis of the suction intake 1322 and are positioned to capture any particle concentrate in the intake flow for a defined period of time, for example one minute. In a second “orthogonal” position, the trap hollow volume is aligned crosswise as is convenient for stripping volatiles (or solutes) from the particle trap into the injection duct circuit within the skimmer body.
Advantageously, no separate valving is needed and, in both positions, flow is through the particle trap mesh, not crosswise over it. The particle “cut size” of the mesh or filter is generally about 5 microns. Reliable collection of particulates in the range of 5 to 100 microns is associated with a higher degree of detection sensitivity at a reasonable energy cost. The system has been shown to be operable at suction intake flow rates of 500 to 1500 sLpm , while not limited thereto, but the flow of particle concentrate through the particle trap is substantially less (as dictated by the flow split) and may be 5 to 15 sLpm or less, for illustration. The overall preconcentration factor on a volume basis can thus be about 750,000× or more. The preconcentration factor is equal to the total aspirated volume 1322 (which can be up to 1500 liters or more per minute) divided by the hollow trap volume 1327 of injectate plus any volume in the injection loop. For slit-type traps trap deadspace will be perhaps 1-3 cc3, but for annular traps, sub-milliliter traps are possible (see U.S. Pat. Doc. No. 2010/0186524, which is coassigned). The small volume achieves significant improvement in preconcentration over systems lacking an air-to-air particle concentrator. Since only a single particle of sufficient mass is required for detection, the lower limit of detection is the limit of the analytical detector itself per person, container, pallet, vehicle, and so on. Thus a limit of detection by mass spectrometry is conservatively 100 picograms or less per sample [Committee on Assessment of Security Technologies for Transportation, 2004, Mass Spectrometry for Trace Detection of Threat Agents, In, Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. The National Academies Press, Washington, D.C, pp 15-28.] Importantly, reduction of interferents by selective stripping (either selectively stripping analytes of interest or selectively stripping interferents, such as by solvent elution or thermal ramping) may improve sensitivity by eliminating or reducing background signals.
In
Following desorption, the mesh can be returned to the first “normal” position and heated more aggressively to incinerate or char remaining particulate materials. The ash and residues can then be blown from the system, either with suction or more preferably by reversing the pump so as to blow the material out the front end of the apparatus.
Other particle trap configurations may also be used, such as an electrostatic trap, a liquid impinger, a bluff body, or an inertial impactor plate mounted in a repositionable body that intersects the collector duct. Optionally, the cylindrical body is a disposable cartridge and can be removed from the particle concentrator assembly for off-line analysis or archiving.
In one explosives detection system, the particle concentrator assembly 1335 may include a centrifugal impactor 1336 as shown in
With suitable detectors, particulate material can be analyzed directly in the trap by spectrometric means. Or constituents that are stripped from the particle trap are conveyed to an analytic module for analysis. In a preferred system, the particle traps of
A coating of carbon in the particle trap may be used to enhance capture of volatiles and vapors associated with the particle-enriched stream. While carbon has a very high affinity for many vapors, hot solvents are generally more effective in releasing adsorbed vapors than heat alone.
In
In
In a fully integrated system, the system combines a jet-suction nose for drawing a suction flow, an air-to-air particle concentrator for separating a bulk flow from a particle-enriched flow, a particle trap with integrated mechanism in the skimmer nose for collecting explosives-associated residues, and valveless means for conveying captive volatiles or vapors from the particle trap to a detection means. Yet more compact systems with detection means for screening particulate residues incorporated in situ in the particle trap, such as described in WIPO Pat. Doc 2004/027386 or for in situ spectroscopy, are also conceived.
In
In
The system is thus capable of essentially continuous operation by alternating collection and analysis modes between the two particle traps. Conditions during the dissolution or volatilization part of the cycle may be intensified so that regeneration of each trap is accomplished before the trap is returned to the collection station. As required, the body surrounding the trap and the cylindrical sliding body may be heated. When not in use, the injector pathways are blocked by the body of the reciprocating member, thus there are only two passages through the reciprocating body, each constituting a trap hollow volume. This feature eliminates the need for supplementary valving. Not shown is a cavity in the sampling head for receiving the reciprocating member. O-rings, gaskets, and registration features as would be useful in operation of the device are well known and are not shown for simplicity of illustration.
Because vapor analysis frequently involves thermal desorption, EGDN (which can decompose at 115° C.) may be more readily detected in a particle trap that uses liquid elution or cold detection; and similarly DMDNB is sticky and likely to cling to particulate materials it comes in contact with. But many industrial solvents are very volatile and are less likely to be retained with a particle fraction under high throughput sampling conditions. These chemicals include materials not always associated with explosive manufacture but when detected in a vapor trap along with any simultaneous detection of a nitrate, perchlorate, or plasticizer in the particle trap, for example, an alarm is triggered. The systems thus have learning capability to recognize and distinguish innocent and suspicious chemical signatures based on dual channel detection, where the vapor channel is optimized for lighter molecular weight materials and the particle trap is optimized for heavier and stickier materials. Taken together, substantial confidence in detection across a wider range of known and as yet unknown explosives is achieved.
A number of methods may be used to augment the capacity of the sampler head to strip off particles and vapor residues from substrates. One such technique is a jet gas feed ionized by contact with a source of ions, such sources including but not limited to a “corona wire,” a source of ionizing radiation, a glow discharge ionization source, or a radio-frequency discharge. The ionized gas stream is used to neutralize electrostatic associations of particles with surfaces and improve lift off of particles.
Collisions of higher molecular weight gas atoms or molecules results in improved desorption of particulate and vapor residues. The carrier is typically air, argon or nitrogen and the gas or solvent is a high molecular weight molecule sufficient to aid in dissociation of particles and volatile residues from the object or environmental surface of interest. Pressurized gas tanks eliminate the need for an on-board compressor, thus reducing power requirements for portable applications. The presence of organic vapors also can aid in volatilizing chemical residues such as explosives and will compete with organic molecules for binding to solid substrates. Heated jet pulses or infrared lamps directed from the sampling head improve sampling efficiency for vapors, however, it should be recognized that premature heating can reduce particle collection; and contrary to the teachings of others, near sonic jet pulses are preferable to hot air for aerosolizing particles from substrates.
Hot solvent vapor also increases the specific heat capacity of a hot carrier gas stream and can improve convective heating of sorbent beds, aiding in desorption of constituents of interest and in cleardown.
In one study, 20 nanograms of TNT trace explosive was deposited on a glass surface using a dry transfer technique from a Teflon® Bytek strip and interrogated with a surface sampler of the invention. The dry transfer technique was performed essentially as described by Chamberlain (U.S. Pat. No. 6,470,730). Particle size distribution (crystal size distribution) was about 10-200 microns. The apparatus was operated with a 3 mm jet array at 80 psig back pressure. The dislodged TNT particles were aspirated at a 1000 sLpm flow rate into a high flow air-to-air particle concentrator with aerodynamic lenses and skimmer and captured in a particle trap formed of a 13 mm pervious member. Explosives constituents of captive particles were dissolved into 100 μL of acetonitrile of which 10 μL was injected into an IMS detector. A measurable TNT signal was observed. The experiment demonstrates detection of trace explosive residues at a nanogram detection level using a jet-assisted non-contact sampling head of the invention.
While the above is a complete description of selected embodiments of the present invention, it is possible to practice the invention use various alternatives, modifications, combinations and equivalents. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification are incorporated herein by reference in their entirety. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
This application is a continuation-in-part of U.S. patent application Ser. No. 12/834860, filed 12 Jul. 2010, which claims the benefit of priority under 35 U.S.C. §119(e) from U.S. Provisional Patent Application No. 61/318313 filed Mar. 27, 2010 and from U.S. Provisional Patent Application No. 61/225007 filed Jul. 13, 2009; said patent documents being incorporated herein in entirety for all purposes by reference.
The United States Government may have certain rights in this invention pursuant to Grant Nos. HSHQDC-08-C-00076 and HSHQDC-09-C-00131 awarded by the Department of Homeland Security.
| Number | Date | Country | |
|---|---|---|---|
| 61225007 | Jul 2009 | US | |
| 61318313 | Mar 2010 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12834860 | Jul 2010 | US |
| Child | 13078997 | US |
