WO2004027386A2 - Thermal analysis of energetic materials - Google Patents

Abstract

The invention relates to improvements in methods of processing thermal profiles, and systems (200) for rapidly and precisely collecting and detecting energetic materials, such as explosives, and non-energetic materials such as drugs and other contraband.

Description

THERMAL ANALYSIS OF ENERGETIC MATERIALS

FIELD OF THE INVENTION

The invention relates to the detection and identification of energetic materials such as explosives.

BACKGROUND OF THE INVENTION

To protect public safety and prevent terrorist activity, it is important to detect hidden explosives. For example, many airports routinely use x-ray scanning systems to identify explosives or other violent weapons hidden within baggage. Also, airports and bomb squads routinely use "sniffing" detection devices that absorb particulate or vapor matter and analyze the matter for the presence of explosives. Analytical techniques used by such detection devices include ion mobility spectrometry (IMS) and gas chromatography.

Explosives can be made from a wide range of energetic materials including, e.g., organic nitrates, organonitro compounds, ketone and acyl peroxides, inorganic chlorates, perchlorates, nitrates, fulminates, and acetylides. Unfortunately, because of the wide range of energetic materials and the many differences in their physical properties, several detection devices detect only certain types of explosives and fail to detect others. For example, many detection devices readily detect conventional explosives made of organic nitro and rfitrate compounds, but fail to detect explosives made of inorganic nitrates or non-nitrogeneous compounds. In particular, many nitrogen-based detection devices fail to detect explosives such as ' ANFO (ammonium nitrate in fuel oil), Black Powder ("gun powder" formed from potassium nitrate, sulfur, and charcoal), and triacetone triperoxide (TATP). As a result, such explosives are sometimes referred to as "transparent." Moreover, TATP, for example, can be easily prepared in a basement lab using commercially available starting materials obtained from, e.g., hardware stores, pharmacies, and stores selling cosmetics, and can be as or more powerful than military analogs.

In addition to detecting hidden explosives, it is also desirable to identify the particular type of explosive once it is detected to assess its danger, deactivate it, and/or provide forensic evidence.

SUMMARY OF THE INVENTION The invention features very sensitive methods and systems for detecting the presence of an energetic material, such as an explosive, in a sample, e.g., of air or water, or taken from a swab of a surface of an object, e.g., a piece of luggage. The methods and systems are based in part on the recognition that all self-contained explosives decompose and release significant amounts of energy upon thermal excitation, whereas most other materials absorb energy upon thermal decomposition. ■ 0 Thus, the presence of an explosive in an unknown sample can be detected by thermally analyzing the unknown sample. In addition, both energetic and nonenergetic materials can be detected and identified by their particular thermal signature.

In particular embodiments the sample is collected using new electrostatic L 5 devices, the signal is enhanced using new methods, and the thermal analysis devices include micro-electro-mechanical system (MEMS) microthermal sensors.

In one embodiment, the invention features methods for detecting the presence of an energetic material, e.g., an explosive, in a sample by obtaining a thermal profile of the sample; computing a first derivative of the thermal profile; 0 computing a second derivative of the thermal profile; and generating a graph of a ratio of the second derivative to the first derivative; wherein the presence of a peak, e.g., two peaks, in the graph of the ratio indicates the presence of an energetic material in the sample. Some materials are "energetic materials," but are "explosive" only when there is a sufficient mass of the material. 5 In these methods, the thermal profile of the sample can be obtained by obtaining a first thermal profile of the sample under modulated temperature control; deconvoluting the first thermal profile into two or more separate thermal profiles; and selecting one or more of the separated thermal profiles for use as the thermal profile of the sample. The thermal profile can be obtained under aerobic or substantially 0 anaerobic conditions.

In another aspect, the invention features devices for detecting the presence of an energetic material in a sample. These devices include a hollow cylindrical housing; an aperture at one end of the housing for entry of the sample; one or more thermal sensors arranged within the housing, each thermal sensor comprising a cavity; 5 a temperature controller that controls heating of the one or more thermal sensors and material in proximity to the thermal sensors; a conductor for transmitting an electrical charge to the cavity; and a measurement circuit for thermal analysis of particles and vapors contacting the one or more thermal sensors. In another aspect, the invention features devices for detecting the presence of an energetic material in a sample that include a hollow cylindrical housing; an aperture at one end of the housing for entry of the sample; an ionizer for charging particles in the sample; one or more thermal sensors arranged within the housing, each unit comprising a cavity; a temperature controller that controls heating of the one or more thermal sensors and material in proximity to the thermal sensors; a conductor for electrically grounding the cavity; and a measurement circuit for thermal analysis of particles and vapors contacting the one or more thermal sensors.

The invention also features devices for detecting the presence of an energetic material in a sample that include a hollow cylindrical housing; an entrance aperture at one end of the housing for entry of the sample; an exit aperture at an end of the housing opposite the entrance aperture; and a plurality of thermal sensors (e.g., up to 2, 3, 5, 10, 15, 20 or more sensors) arranged in series within the housing between the entrance and exit apertures, whereby the sample flows into the housing, through each of the thermal sensors, and out of the housing; wherein each thermal sensor comprises a mesh sensor element that comprises mesh openings, wherein each sensor in the housing comprises a mesh sensor element with different size mesh openings, and wherein the mesh sensor elements act as particle traps; and further wherein the sensors are arranged within the housing such that larger particles in the sample are trapped before smaller particles as the sample passes through the mesh openings.

In another aspect, the invention also features other devices for detecting the presence of an energetic material in a sample that include an array of detector columns arranged in parallel, wherein each detector column in the array comprises one or more thermal sensors and conductors that carry electrical signals from the thermal sensors; a sample collection chamber arranged to distribute the sample into detector columns in the array; a temperature controller that controls heat of the thermal sensors and material in proximity to the thermal sensors; and a thermal analysis system that provides thermal analysis of electrical signals from the conductors indicating particles passing between the thermal sensors in each detector column. These devices can further include an exhaust chamber arranged to collect the sample that has passed through the detector columns.

Any of these devices can further include a heater located external to the housing, wherein the temperature controller controls the heater to heat the one or more thermal sensors and material in proximity to the thermal sensors. In other embodiments, the thermal sensors are heated by rmining electrical current through the thermal sensors, and wherein the temperature controller controls the electrical current. In various embodiments, one or more of the thermal sensors can include or be a micro-electro-mechanical system (MEMS) sensor. The thermal sensors can be thermopiles, resistance thermometers, thermo-acoustic sensors, infrared sensors, ultraviolet sensors, or any combination of these types of thermal sensors, h certain embodiments, the devices can further include an ionizer that charges the particles of the sample with the opposite charge of the cavity prior to entering the cavity. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

The new improvements described herein enable the rapid detection of explosives and other energetic materials in minute quantities of samples. Thus, the new methods and devices can be used to monitor samples for explosives in a wide variety of environments and fixed locations, such as at airports, train stations, and the like. In addition, the new methods and devices can also be used in portable systems that can be transported and operated by a person on land, air or water vehicles for field evaluations. The new devices and methods also provide very rapid determinations of the presence of energetic materials in samples, thus enabling a high throughput of samples.

Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a differential scanning calorimeter. FIG. 2 is a schematic of an explosives detector. FIG. 3 A is a schematic of a design for a tiered mesh system with MEMS microthermal sensor units.

FIG. 3B is a side view of a the design illustrated in FIG. 3 A

FIG. 3C shows a cross-sectional view of a sensor unit in the system of FIGs. 3 A and 3B.

FIG. 3D is a schematic illustrating the use of the collection mechanism within the tiered mesh system of FIGs. 3 A and 3B.

FIGs. 4A-D are schematics of a design for a flow-through proximity sensor for use in an explosives detection system. FIG. 5 is a schematic of a collection system for use with an explosives detector, e.g., as shown in FIG. 2.

FIGs. 6A-B are schematics of an explosives detection system with electrostatic collection and MEMS sensor detection.

FIGs. 7A-C are schematics of a metallic chamber for an electrostatic collection and detection system.

FIGs. 8 - 10 are thermograms relating to the detection and identification of various explosives using an electrostatic collection system illustrated in FIGs. 7A-C.

FIG. 11 is a series of graphs that illustrate a method of analysis of the thermograms relating to the detection of various explosives. FIG. 12 is a schematic of an electrostatic particle collection system.

FIG. 13 is a thermogram relating to the detection and identification of various explosives using an electrostatic collection system illustrated in FIG.12.

DETAILED DESCRIPTION

The invention features methods and systems for determining the presence of energetic materials, such as explosives, in an unknown sample using thermal analysis.

Thermal Analysis

Generally, thermal analysis monitors heat flow to, or from, a sample. In one example, thermal analysis involves measuring the temperature of a sample as a function of time as the sample is allowed to warm (or cool) to the temperature of its environment. The rate at which the sample warms (or cools) can change abruptly at particular temperatures when the sample undergoes a phase transition (e.g., melting or thermal decomposition of one or more components of the sample). Such phase transition temperatures are characteristics of the sample and a curve illustrating at least a portion of the time-dependent temperature of the sample can be used as a signature of the sample for a particular measurement protocol. In another example, thermal analysis involves measuring the heat flow required to heat (or cool) a sample at a steady or prescribed rate as a function of temperature. Once again, when the sample undergoes phase transitions, the rate of heat flow required to heat (or cool) a sample can change abruptly. The temperatures at which such abrupt changes occur (e.g., the phase transition temperatures) and the magnitudes of the changes are characteristic signatures of the sample. In many examples, the thermal measurements involve a differential analysis in which changes in heat flow to, or from, a measurement sample is compared to corresponding changes in a reference sample. For a general discussion of thermal analysis, see, e.g., R.C. Mackenzie, Differential Thermal Analysis (Academic Press, London, 1972).

In general, thermal analysis can be done by a number of techniques including differential thermal analysis (DTA), quantitative differential thermal analysis (QDTA), dynamic differential calorimetry (DDC), dynamic enthalpic analysis (DEA), thermogravimetric analysis (TGA), and differential scanning calorimetry

(DSC).

DSC is a technique in which the difference in energy input (e.g., heat flow) into a measurement sample and a reference sample is measured as a function of temperature while the measurement and reference samples are subjected to a controlled temperature program. The resulting energy versus temperature curve is an example of a thermogram. Alternatively, the thermogram can be energy versus time curve, where the temperature is changing as a function of time in response to a prescribed heating or cooling program.

A schematic of one example of a differential scanning calorimeter 100 is shown in Fig. 1. The calorimeter includes a sample chamber 102 having a reference pan 104 and a measurement pan 106 for the reference and measurement samples, respectively. A heating block 108 surrounds sample chamber 102 and connects to a thermoelectric disk 110, which supports pans 104 and 106. A power source 112 provides electrical energy to disk 110 that causes disk 110 to equally heat pans 104 and 106. A controller 114 controls power source 112 such that the heat generated at pans 104 and 106 can be user-specified and can follow a prescribed program. Thermocouples 116 and 118 monitor the temperature of the reference and measurement pans 104 and 106, respectively, and sends signals 120 and 122 indicative of those temperatures to controller 114. A purge gas source 124 (e.g., nitrogen) connects to sample chamber 102 through inlet 126 to purge, during operation, oxygen from the sample chamber and thereby create an anaerobic environment within the sample chamber. A lid 132 permits introduction and removal of the reference and measurement samples from pans 104 and 106. In many applications, the reference pan is left empty so that the reference sample is simply the gas in the ambient environment, e.g., nitrogen or air.

During operation, the measurement sample is placed in the measurement pan and the sample chamber is purged with nitrogen. Then, controller 114 executes a prescribed heating program, e.g., constant heating, and records the overall temperature T of sample chamber 102 based on the average of the temperatures measured by thermocouples 116 and 118. Controller 114 also records the temperature differential AT between the measurement and reference samples based on the difference between the temperatures measured by thermocouples 116 and 118. Although the thermoelectric disk heats the samples evenly, thermally activated physical changes

(e.g., a phase transition) in one, but not the other, of the two samples can release or absorb additional heat, thereby producing a non-zero temperature differential between the samples. These types of thermally activated changes can be characterized by DSC. For example, if the measurement sample includes a component not present in the reference sample, as the overall temperature of the sample chamber passes through the melting point of the component, the temperature of the measurement sample lags that of the reference sample as the component melts. Thus, the recorded thermogram (i.e., the plot of ΔT versus T recorded by the controller) will include a peak at the melting temperature of the component.

As is well known, melting and vaporization are phase transitions that absorb energy from the environment, i.e., they are endothermic. Other types of physical change, e.g., freezing, can release energy to the environment, i.e., they are exothermic. As noted in Bannister et al., U.S. Patent No. 6,406,918, all energetic materials generally decompose exothermically, whereas other materials commonly found in the environment decompose endothermally.

In contrast to thermal decomposition, other chemical reactions of non- energetic materials can, however, be either exothermic or endothermic. For example, materials that burn in the presence of oxygen, i.e., combustion reactions, are exothermic. In addition, many oxidation reactions are exothermic. To limit thermal contributions to the thermogram caused by reactions between the sample and oxygen, the sample chamber can be purged with an inert gas such as nitrogen to produce a substantially anaerobic environment. Other methods can be used to depress the combustion process including reduction of air pressure or adding materials that act as oxygen-scrubbers by preferentially reacting with the oxygen present at lower temperatures than the ignition of combustion processes. Alternatively, the temperature range of the thermal analysis can be kept below minimum temperatures required for such exothermal reactions involving oxygen. For example, the maximum temperature can be kept below about 600°C, 500°C, 450°C, 400°C, or even below about 350°C, to limit thermal contributions to the thermogram caused by reactions between various types of samples and oxygen in the chamber. The maximum temperature for the thermal analysis scan should be high enough that the sample decomposes, but low enough that the sample does not combust. Another method for discriminating between thermal decomposition and other chemical reactions is to use a quickly modulating heating program such as Modulated DSC (MDSC), available from TA Instruments Inc. (New Castle, DE).

MDSC uses a modulating or sinusoidal change in heating rate (rather than the linear heating rate used in DSC) to automatically separate the heat capacity baseline from the total heat flow signal. This allows one to identify, measure, and quantify kinetic processes, such as crystallization and crystal perfection in a single experiment. See, e.g., U.S. Patent Nos. 5,224,775 and 5,3446306, and Thomas, "Use of Multiple Heating Rate DSC and Modulated Temperature DSC to Detect and Analyze Temperature-Time-Dependent Transitions in Materials, American

Laboratory, Vol. 1, 7-10, January 2001. Subsequent analyses of the measured signals can separate overlapping thermal events based on the kinetics of each reaction.

Differential scanning calorimeters can differ from the schematic of Fig. 1. For example, in other embodiments, the reference and measurement pans are heated by separate heaters that are operated by a servo-control system to minimize the temperature differential between the pans. A controller measures the difference in energy provided by the heaters to minimize the temperature differential as a function of the overall average temperature.

Defection of Explosives

A general property of all self-contained explosives is that they release considerable amounts of energy, even in the absence of oxygen, upon excitation by an external source of energy such as heat, friction, or impact. In contrast, non-explosives absorb energy when thermally decomposed. The contrast is striking when one considers that when burnt in the presence of oxygen (at temperatures greater than about 600°C), sugar, a non-explosive, releases more heat (heat of combustion equal to -3,900 kcal/kg) than that of trinitrotoluene (TNT) (heat of combustion equal to -3,600 kcal/kg). However, TNT is much less stable than sugar and will spontaneously release large amounts of its internal energy upon excitation by a relatively low input of initiation energy, even in the absence of oxygen in external air. In other words, TNT and other explosives undergo strong exothermal decomposition in an anaerobic environment when thermally excited. In contrast, and like other stable compounds, sugar will decompose endothermally when thermally excited in an anaerobic environment.

Even in sample chambers containing oxygen, stable compounds will typically exhibit only endothermic transitions provided that the temperature is kept below a minimum threshold value required for exothermal reactions with oxygen. For example, many exothermal reactions with oxygen require temperatures in excess of about 500°C, or at least in excess of about 350°C or 400°C. Thus, the explosives detection can also be performed in the presence of oxygen, however, in many cases the temperature should be less than about 500°C, and in some cases, less than about 350°C or 400°C. Samples of interest can thus be divided into two general categories: 1) energetic materials such as explosives - high energy, low stability compounds that exothermically decompose with heat; and 2) stable materials that endothermally decompose with heat. Such stable compounds include volatile alcohols, ketones, acids, esters, aldehydes, amines, and other compounds that are commonly found in, e.g., perfumes, cosmetics, beverages, and condiments.

When trying to detect explosives in public places such as airports, products such as cosmetics, beverages, and condiments are examples of the types of innocuous materials that must be differentiated from explosive agents that would be used in a terrorist attack. Without exception, all of these innocuous species thermally decompose endothermically in the absence of oxygen. The temperature at which the decomposition is initiated can vary greatly among different materials.

Fig. 2 is a schematic of an explosives detector 200 that exploits this property. A collection system 210 gathers particles or vapors from objects that potentially contact or contain explosives. For example, the collection apparatus can be a hand-held vacuum that collects particles, e.g., from luggage or passenger's clothing, on a filter and condenses the particles on the filter into a sample 212, whose composition is unknown. Collection system 210 delivers sample 212 to an anaerobic chamber 216 in a thermal analysis apparatus 214, such as a differential scanning calorimeter. Thermal analysis apparatus 214 generates a thermogram 218 of sample 212 under anaerobic conditions sends a signal 219 indicative of the thermogram to an analyzer 220. Based on signal 219, analyzer 220 determines whether thermogram 218 includes a strong exothermal peak. If analyzer 220 finds such a peak, it signals that an explosive is present in the unknown sample 212 using, e.g., an alarm 222 or visual display 224. Thereafter, if necessary, the operator can conduct a more thorough search of the articles from which the sample was collected.

Alternatively, as described above, chamber 216 need not be maintained under anaerobic conditions provided that the temperature in the chamber is kept lower than the temperature at which exothermal oxidative or combustive reaction occurs for a given material.

FIG. 3 A illustrates a tiered mesh design for a micro-electro-mechanical system (MEMS) microthermal sensor unit. A tiered mesh design 700 comprises a plurality of mesh layers 710, arranged in such a manner that the sample passes serially through each of the mesh layers 710. In one embodiment, the mesh layers 710 are arranged in order of decreasing mesh opening size, such that a sample, e.g., of air or water, would encounter the larger mesh opening layer first and progressively encounter mesh layers of decreasing mesh opening size as it passes through the tiered mesh design. Each mesh layer includes a plurality of MEMS microthermal sensors 720, the sensors 720 in each layer being optionally arranged to be in contact with each other along their edges. There can be anywhere from 2 to 20 or more layers, each having a different size of mesh opening. The "mesh" can be formed, as shown in FIG. 3A by a single wire in a "snaking" or "S" configuration. The "mesh" can also be formed by arranging wires in other configurations, as long as there is a mesh opening with a consistent size for each layer in the device.

FIG. 3B is an expanded view of the MEMS microthermal sensors 720, in a side view of the tiered mesh design, comprising individual sensor units 730, arranged within a tubular or cylindrical member 740.

FIG. 3 C is an enlarged, cross-sectional view of the sensor units 730, showing a top which comprises the sensor element 720, and a bottom 760, which has been etched or cut through, for example with potassium hydroxide, to form an opening 770 that allows the sample to pass through the sensor. The tiered mesh design functions as a heater/sensor to sense particles that contact it, and also serves as a net that physically traps particles. Further, since the mesh sensor elements 720 in the units 730 are arranged in order of decreasing mesh opening size with respect to point of sample entry, particles are trapped in decreasing order of size. FIG. 3D illustrates the MEMS microthermal sensor units 730 in use. As a sample, e.g., of air, enters and passes through the device, larger particles in the sample contact and are sensed by the sensor element 720 in the first unit 730 A, while smaller particles pass through the openings 770 and contact the sensor element 720 of the next microthermal sensor 730B. The steps are repeated such that the smallest captured particles are cauglit in the sensor units 730 that are furthest away from the point of sample entry.

Once the collection is completed, the heating scans of all the sensors can be carried out for the detection of energetic compounds. One advantage of such a collection mechanism is that the capture of the sample particles of interest is based on the physical dimension of the particles. Capturing sample particles by size minimizes interference that causes problems with other sample collection methods such as the problems due to humidity on electrostatic collection or the problems due to solubility differences on solvent condensation and extraction. Additional sensor/detector configurations are possible. For example, FIGs. 4A-D illustrate a schematic for a flow-through or proximity sensor system. FIG. 4A shows a proximity sensor system 600, comprising a chamber 610 for sample collection in inert gas, which is connected via a column exit aperture 615, through a conduit 620, which serves as a pretreatment zone to heat the sample, to a sensing zone 630 through a column entry aperture 640. The sensing zone 630 comprises an exit aperture 660 for exhaust of carrier gas and a cylindrical sensor column 650 arranged in a way such that it can receive the sample entering through the entry aperture 640. The cylindrical sensor column 650 is connected to a sensor electronic system 670 for processing the information received from the sensor column 650 to generate thermal profiles for the samples.

FIG. 4B shows an expanded view of the cylindrical sensor column 650 which comprises a plurality of proximity sensor units 680 stacked within the sensor column 650 in a manner such that the sample enters the sensor column 650 and flows through the individual proximity sensor units 680 for sensing and detection of the presence of explosives. The structure for the sensor column 650 provides a defined pathway for the carrier gas to bring the sample into proximity of the sensors 680.

FIGS. 4C and 4D are expanded views of the individual proximity sensor units 680. Proximity sensor units 680 each comprise an aperture 690 for entry of sample, and one or more thermal sensors 695 contained within 680, for example lining the wall of the proximity sensor unit 680. Thermal sensor 695 may be, for example, a thermopile or a resistance thermometer.

In use, the sample, which may be concentrated prior to entry or may be loaded onto a carrier gas, travels from the sample chamber 610 through the chamber exit aperture 615, through the pretreatment zone, conduit 620, where the sample is heated, for example to 180°C, through the column entry aperture 640, and reaches the sensing zone 630. Within the sensing zone, the sample enters the individual proximity sensor units 680 through their entry apertures 690. The sensor units 680 may in some embodiments function as heating and sensing units or may in other embodiments function only as sensing units, wherein a separate heat source would be required. The gas delivering the sample to the sensing area can flow essentially linearly over or through the sensor column 650, or it may be purposely agitated by the design of the structure 650 to provide turbulent flow near the sensors 680. The laminar flow has the advantage of more quickly delivering the sample to the sensing area, more quickly purging the sample from the sensing area, and thereby minimizing the overall testing time for detection for a single target. The turbulent flow has the advantage of increasing the likelihood of bringing the particles in the sample into closer proximity to the sensor and the heating element at some time during their passage between the sensors. As the heated sample passes through the sensor unit 680, the sensor unit detects the presence of an energetic material. The information is processed and a thermal profile for the sample is generated in the sensor electronic system 670. In another embodiment of the proximity sensor, the individual sensor unit

680 further comprises infrared and ultraviolet detectors to detect the infrared (IR) and ultraviolet (UV) radiation emitted during the thermal decomposition of an energetic material, resulting in a spectrum with features in the UV and IR regions. The presence of features in the ultraviolet region is diagnostic of the presence of an energetic material, because ultraviolet energy is not given off by non-energetic material, and also because there is no spectral interference in the ultraviolet region from the thermal energy radiated by the sample. Furthermore, the thermal energy radiated by the sample will interfere with the spectrum in the infrared region.

The advantages of such a proximity flow-through sensor system 600 is the ability to rapidly detect the presence of explosives particularly in situations where contact sensing is unreliable due to mechanical or environmental interference, time consuming, and or too expensive.

As described in greater detail below, suitable collection systems and thermal analyzers that can be used in conjunction with the devices and systems described herein are commercially available. The analyzer can be a computer connected to the thermal analysis system and operating appropriate software, a dedicated electronic circuit embedded in the thermal analysis system, or some other similar electronic component. For example, in one embodiment the thermal analysis apparatus could measure the thermogram and send a signal carrying the thermogram information to a computer, which stores software, e.g., on a hard-disk or CD-ROM, for analyzing the thermogram. The thermogram typically consists of a series of data correlating heat transfer, e.g., as measured by a temperature difference between test and reference samples, to the programmed temperature. The software causes the processor in the computer to analyze the series of data and identify whether the data includes an exothermal peak. Such software is commercially available from TA Instruments, Inc. (New Castle, DE).

Alternatively, suitable software can be programmed by those skilled in the

.0 art using, e.g., standard programming languages such as C, C++, or Visual Basic, a general purpose interfaces bus (GPIB) with standard IEEE-488 software, and packaged software routines, such as those found in Numerical Reci e in C: The art of Scientific Computing by William H. Press et al. (Cambridge University Press, 1993). For example, the presence of an exothermal peak may be identified as, e.g., at

L5 least a 1% increase in heat flow followed by at least a 1% decrease in heat flow, or at least a 10% increase in heat flow followed by at least a 5% decrease in heat flow, over a span of, e.g., about 5 to 20°C. In practice, however, the presence of an exothermal peak may depend on the particular parameters (e.g., heating rate) of the thermal analysis apparatus and the particular explosives of interest. If the computer identifies 0 an exothermal peak, it confirms the presence of an energetic material in the test sample, otherwise it confirms that no energetic material is present in the test sample.

Many of the explosives that maybe anticipated in potential terrorist encounters release large amounts of thermal energy during thermal decomposition. Thus, relatively small quantities of an explosive in a sample collected by collection 5 system 210 can produce a detectable exotherm. For example, C4 is a plastic explosive formed largely (about 90%) of Trimethylenetrinitramin, a military plastic explosive given the acronym RDX, and about 10% plastic binder with other species in small quantities. Anaerobic heating of less than 1 milligram of C4 will release sufficient heat as to be readily detected with inexpensive, conventional differential scanning 0 calorimeters or other differential thermal analysis equipment. Furthermore, as described in greater detail below, recently developed micro-differential scanning calorimeters (microDSCs) such as Model 2990 from TA Instruments, Inc., (New Castle, DE) can reduce the required quantity of detectable explosive to picogram amounts with rapid (about 4 seconds) detection times. See, e.g., U.S. Patent No. 5, 5 248,199.

Detection Limits

The previous DSC scans were based on modest quantities of explosive materials, e.g., hundreds of micrograms. Using more advanced DSC technology, such as the commercially available microDSCs from TA Instruments, described previously, much smaller quantities, as low as one picogram can be detected. Furthermore, using methods that can concentrate the sample of interest up to a million times (described infra), microthermal detection can be used to detect very small traces of explosives obtained from air vacuumed from luggage or directly from passengers as they walk through a detector. It can also quickly detect small traces of explosives in environmental samples taken to evaluate the safety of locations or in forensic investigations.

Collection and Collection-Detention Systems

The collection system 210 in the explosives detector of Fig. 2 can take many forms. For example, an airstream can be driven through a filter, such as a charcoal filter, to collect airborne particles in the airstream. Alternatively, a gauze tissue can be used to wipe the surfaces of articles that potentially contact explosive agents. Particles trapped by the filter or tissue can be extracted therefrom and condensed by using an air stream, an extraction fluid, electrostatic techniques, or some combination thereof. In general, suitable collection systems are known in the art and many are used in conjunction with alternative explosives detectors. See, for example, the collection systems in U.S. Patents 5,092,218 and 5,345,809, which describe suitable collection systems.

Another suitable collection system is a space charged atomizing electrostatic precipitation (SCAEP) air sampler such as the SCAEP Air Sampler/Concentrator commercially available from Team Technologies (Newton, MA). The instrument concentrates minute quantities of airborne particles in large quantities of air into relatively small quantities of water, e.g., up to a million parts of air into 1 part of water, by combining conventional air scrubbing with electrostatic precipitation. Contaminated air particles are sampled countercurrent to a charged liquid aerosol spray that intercepts particles, vapors, and gases, and delivers them to the collection fluid.

Another suitable collection system 300 is shown in FIG. 5. A pump 310 draws an air stream 312 into a collection chamber 314. Air stream 312 first passes through the electrodes of an electrostatic voltage source 316, which electrostatically charges the particles in the air stream. Thereafter, air stream 312 passes above a conveyor belt 318 having a series of conical collection cups 320, which have an electrostatic charge opposite to that of the particles. Attractive forces between the opposite charges force the particles to precipitate into cups 320, which concentrate them. Thereafter, conveyor belt 318 delivers the concentrated sample to a thermal analysis instrument, such as a differential scanning calorimeter.

Alternatively, organic solvent extraction can be used in place of electrostatic precipitation to concentrate the sample. In such a case, collection cups 320 carry a volatile organic solvent, such as methylene chloride, perflouroalkanes, hydrofluorochloroalkanes, substituted ethers, or esters. The organic solvent collects and dissolves the airborne vapors and particles. As conveyor belt 318 moves, it passes over heating elements 322 that evaporate the volatile collection fluid, thereby concentrating the collected particles. h some embodiments, the collection and detection systems are combined in micron scale devices. For example, FIG. 6A illustrates a system 400 comprising a combined electrostatic collection and MEMS detection system. System 400 comprises a cylindrical or tubular body 410 comprising within it a charging device 420, such as a wire, held in place by being secured to the wall and is also connected to a voltage source, and MEMS microthermal sensors 430. In some embodiments, system 400 does not include a charging device. The microthermal sensors 430 comprise a sensor element 470 and a well or cavity 480. The MEMS sensor element 470 may be a thermistor, made of a metal such as platinum, with a flat surface to maximize contact with the particles. MEMS sensors have high sensitivity, for example on the order of 3.262°C/mW, and a response time of less than 5 ms. The sensors can operate in either the constant voltage or in the constant current mode. In some embodiments, the length of the microthermal sensors 430 is about 180 μm and the width of the well region 480 of the particle collector is from about 100 to 1000 μm. Sample enters system 400 through aperture 450. The sample can include neutral particles or charged particles, the charged particles being charged by charger 420. In one embodiment, the microthermal MEMS sensors are located along the wall 415 of tubular body 410, the wall 415 can be made of a polymeric material such as plastic or TEFLON.

FIG. 6B illustrates an expanded view of the electrostatic collection and MEMS detection system of FIG. 6A. The MEMS sensors 430 are charged by means of metal needles 460, winch can be metal wires, for example copper wires, placed directly beneath the wells of the sensor units, h this embodiment, a voltage is applied to the needles such that the sensors become positively charged, which results in an electrostatic field 485 enveloping the sensors, which causes the negatively ionized particles to be attracted towards the sensors.

Electrostatic collection coupled with MEMS microthermal sensor detection is desirable for the following reasons. With electrostatic collection, heat loss due to contact with surrounding media is minimized, which increases sensitivity of measurement. Further, since the collection and detection systems are coupled into one device, efficiency of detection is greatly improved. These factors can be highly advantageous in certain environments, for example in airports.

Another suitable collection and detection system 500, comprising a Wollaston wire thermal probe-based microthermal analyzer, is illustrated in FIGs. 7A- C. A Wollaston probe 510 consists of a resistance thermometer sensor 520 mounted on a piezoelectric device 530. The probe functions as a sensitive temperature sensor and an atomic force microscope (AFM) probe when positioned on a specific area of the sample. For a reference on Wollaston probes, see, e.g., R.E. Dinwiddie et. al., Thermal Conductivity 222ed, 668-669, 1994 and A. Hammiche et. al., Meas. Sci. Technol. 7:142, 1996. See also, U.S. Patent Nos. 5,441,343 and 5,469,734. For a reference on microthermal analysis, see, e.g., U.S. Patent No. 5,248,199.

In this embodiment, the sample 540 is placed in an aluminum cup 550. A Wollaston wire thermal probe 510 is fixed to the lid 512 and the metal areas (excluding the Wollaston wire area) are electrically insulated. For the electrostatic collection process, a Hi-Z Megmeter 514, (Ross Engineering Corp, Campbell, CA) capable of delivering D.C. charges in the range of 0.1 - 10 kV, is used for charging the sample.

The sample is collected in the bottom cup 516 and the lid 512 (equipped with the probe 510) is placed securely to cover the sample chamber 518. While the probe is grounded, a high voltage (2000 to 3000 Volts) is applied to the sample chamber causing the sample 540 to charge up and move towards the grounded Wollaston wire 508. The voltage source 514 and the ground wire 508 are removed before the probe is connected to the microthermal analyzer (μTA 2990®). In a typical micro-thermal analysis, the probes are heated at the rate of 25°C per second from room temperature to 450°C. Alternately, such a system can be configured to perform thermal measurements with the probe suspended in air.

The power curves obtained in microthermal analysis (μTA) represent the energy required to maintain the prescribed temperature profile for each thermal probe, which is typically a linear or modulated ramp. Therefore, the power will depend on the thermal conductivity and the heat capacity of the mixture of materials in close proximity to the probe. In the case of the measurement of electrostatically collected samples, the heat capacities dominate heat consumption, because the heat conduction through surrounding air is relatively small. Therefore, the total power consumed to maintain the temperature profile is determined by the mass, heat capacity, and mean thermal path of material that collects on the probe by contact. For a given composition of the bulk of the sample and similar deposition, the power is proportional to the mass of the sample. FIGS. 8 and 9, show the thermal profiles for sugar powder (a non-energetic sample) and PETN powder (an energetic sample) that were electrostatically collected and detected using a microthermal analyzer (μTA 2990® illustrated in FIGS. 7A-C). Temperature calibration of the instrument was performed using the melting point of a standard sample of polyethyleneterephthalate (PET). Referring to FIG. 8, the melting of sugar on the μTA probe is not accompanied by any change of the total mass. However there is a certain amount of increase in the heat capacity, as well as in the interfacial thermal conductance due to the slight increase of the contact area between liquid sample and the Wollaston wire 510. A clear and permanent shift in the baseline is noticed during the melting process. An increase in heat capacity when the solid sugar particle melts is also observed.

In contrast, in FIG. 9 the thermal profile of PETN powder shows an exothermal response with the power curve showing a shift to the opposite (positive) direction relative to that of the sugar sample. Further there is a loss in mass accompanying the thermal decomposition of the PETN sample. FIG. 10 schematically illustrates the two processes occurring during the thermal decomposition of a PETN sample. A rapid exothermic release of energy occurs, causing a rebound of the power curve from the "solid sample" curve to the "no sample" curve. Also, the reduction in mass causes a further shift of the power curve towards the "no sample" curve. The amount of released energy absorbed by the probe can be estimated by integrating the area between the power curve and baseline curve. hi the PETN sample graph in FIG. 9, the amount of the released energy that was absorbed by the probe is estimated to be 0.031 mJ, which is about 13.6 ng of PETN based on the typical energy of deflagration for PETN of 2291 J/g.

FIG. 11 illustrates graphs that are used in and result from a new method of analyzing thermal profiles that enables the rapid and precise detection of energetic material. In FIG. 11, curve A is a plot of the change in power (in milliwatts) required to heat the sample to achieve the programmed change in temperature (in °C). Curve B refers to the first derivative of the power curve A with respect to time in seconds.

Curve C is the second derivative of the power curve (curve A) with respect to time in seconds. Curve D plots the ratio of the second derivative curve (curve C) to the first derivative curve (curve B) (i.e., the second derivative curve divided by the first derivative curve). Curve E is an expanded view of the peak region of curve D. The experimental data were recorded at the rate of approximately 30 points per second. Most transitions appear more prominently in the first derivative curve (curve B) compared to the power curve (curve A), and even more prominently in the second derivative curve (curve C,) thereby providing much higher sensitivity.

Due to the power rebound at the rapid release of energy, two zero values are expected in the first derivative curve and are observed. No other thermal responses with these characteristics have been observed in μTA. This phenomenon is a clear indicator of rapid energy release. Further, the ratio of the second derivative to first derivative curves yields peaks corresponding to the zero points in the first derivative curve. This ratio curve dramatically reduces background noise and highlights the peaks resulting from an energetic material. Curve E is an enlarged portion of Curve D showing the two peaks that correspond to the zero points of the first derivative curve.

This new method of processing thermal profiles from explosives detectors, such as those described herein, enables the quick, accurate, and highly sensitive detection of the presence of explosive materials, by significantly improving the signal to noise ratio of standard or first derivative thermal profiles. The result of using this data evaluation method is particularly amenable for use for electronic or computer- generated determination of a Yes/No response to the presence of explosives in the sample. Note that although there are typically two peaks in the ratio curve, even a single peak indicates the presence of an energetic material. Any non-energetic material will not produce any peaks in the ratio curve. The second peak typically present can be the same size as, larger, or smaller, than the first peak, and may be

.0 missing in the event a very small sample is taken or there is considerable interfering material in the sample that causes noise. Thus, either one or two peaks in the ratio curve indicate the presence of an energetic material in a sample.

Another suitable collection system 800 is illustrated in FIG. 12. System 800 comprises a T-shaped body with an entry port for air towards one end at the top of

L5 the T, which also contains a charging rod 810 to charge an external electrode 820, such as a tungsten electrode. Another electrode 860 is located at the opposite end of the device, with a stop collar 862 for connecting to a high voltage source 870 to apply voltages, for example in the range of 1.0-1.5 kV. h some embodiments, the positively charged electrode 860 may in addition be a detector probe. Insulating 0 material, for example boron nitride ceramic, is present at both ends containing the electrodes.

The sample 840 enters the system 800 through a sample entry port 850 located perpendicular to the two electrodes. The system includes a Venturi block 880, which can be made of clear polycarbonate. Air, under low pressure (10-15 psi), enters 5 the collection system through entry port 810 and passes over the negatively charged electrode 820, which may be a Tungsten electrode, which may be charged by means of a charge rod 830. Sample 840 (e.g., in some examples described herein the sample is about 1 mg of test particles prepared by mixing a small amount, 3.8 wt%, of ammonium perchlorate with talcum powder) enters the system, in a manner 0 perpendicular to the electrode 820, through the sample entry port 850. The negatively charged air moving forwards, encounters the sample coming downward, and a charged sample results. The negatively charged sample is attracted towards the positively charged electrode 860. The sample thus collected can then be taken to a detection and analysis system, such as a microthermal analysis system, to test for the presence of an 5 energetic material, hi some embodiments, the electrode 860 may itself be the detector probe, or may have a series of sensors attached, wherein it can be connected to a data analyzer, which would yield information regarding the presence of energetic material in the sample. FIG. 13 is a thermal profile of a mixture of ammonium perchlorate (an explosive) and talcum powder (a non explosive) analyzed using the collection and detection system described in FIG. 12. FIG. 13 shows a strong exothermic signal around 340 °C in close agreement with conventional DSC data. Other collection systems can also be used with the new explosives detectors and methods. For example, particles suspended in water, such as those electrostatically precipitated on a running water film or aerosol, can be removed from the water and concentrated by extraction using a volatile organic solvent, such as methylene chloride, perflouroalkanes, hydrofluorochloroalkanes, substituted ethers, or esters. Also, depending on the end-use application, the collection system can be implemented in various ways, such as a hand-held device or wand, which would be useful to scan luggage or passengers, a continuous air sampling system, which would be useful in an airport or airplane cargo bay, or as a laboratory device for forensic testing.

Identification of Explosives and Drugs

As described above, thermal analysis of an unknown sample can be used to detect the presence of explosive or energetic agents in a test sample of unknown composition. Thus, if combustion is suppressed, either by reducing the presence of oxygen in the test environment, or by limiting the temperature to below the ignition temperature for the combustion of other materials in the sample, then an exothermic peak is a positive and reliable indication that an energetic material is present in the sample. Conversely, a thermogram having only endothermic peaks is a reliable indication that an energetic material is not present in the sample. Further, if the power data used for these calculations is obtained under a modulated temperature heatmg program, then, only that portion of the power measurements that correspond to the kinetic parameters of the thermal decomposition of the explosive will contribute to the presence or absence of the exothermal peak used to detect the presence of an explosive. When the detection system of Fig. 2 detects an energetic agent or explosive in a sample, it may also be useful to determine which particular energetic material has been detected, i.e., to identify the energetic material since different materials have characteristic thermograms. hi particular, the thermograms of each explosive and energetic material have one or more exotherms at temperatures that are characteristic for that particular material. Thus, not only can explosives be generally detected, particular explosives can be identified. In addition, other features of the thermogram, such as its endotherms, can be used to further distinguish one energetic material from another.

During operation, the analyzer would first determine whether the measured thermogram for a particular sample contained an exotherm. If so, the analyzer would recognize the presence of an explosive or energetic material in the sample. Thereafter, the analyzer would determine the temperature ranges of the exotherms and endotherms in the thermogram and compare those temperatures with temperatures in a reference library. If the measured transition temperatures agree with those for a particular material in the reference library, the analyzer would identify that energetic or explosive material as the one present in the test sample. In addition, the analyzer can compare other features of the exotherms and endotherms such as their shape, or the amount of heat released (ΔH_exo) or absorbed (ΔHendo), respectively. Alternatively, the analyzer can compare the entire thermogram of the test sample with reference thermograms stored in the analyzer, and determine the best match. As described previously, the analyzer can be, e.g., a computer storing software that causes a processor in the computer to carry out the steps described above or a dedicated electronic circuit programmed to operate similarly. Reference library 228 can be stored in any type of standard data storage, e.g., on a hard disk of a computer or a CD ROM.

As described previously, the precise features of the thermogram can vary with the thermal analysis protocol, e.g., heat rates and sample size. Thus, the thermogram for the test sample should be measured using the same protocol as that used to obtain the reference thermograms. In addition, the precise features of the thermogram can depend on what combination of materials is present in the sample. Thus, the reference library can also include reference thermograms of explosives or other controlled substances mixed with other materials, in varying amounts. For example, the reference library can include reference thermograms for mixtures of explosives such as C4, Black Powder, and ANFO, with other materials.

In practice, the circumstances for identification would differ from those of detection. The detection method and system described above provides rapid detection (e.g., within a matter of seconds or a few minutes) of the presence of an energetic material in an unknown sample, as may be necessary, e.g., for airport security or field evaluation. The identification method and system, however, would typically be used under circumstances where forensic identification of the composition of a sample is

.0 required, h such cases, possible compositions for the sample may already be known and, relative to the time available for, e.g., an airport security check (to detect explosives), there may be a significant amount of time available for the forensic identification. Thus, a suitable set of reference thermograms can be selected and compared with a high resolution DSC scan of the test sample. If necessary, additional i- 5 reference thermograms can be measured to confirm a positive identification. Both a rapid detection system, and a somewhat less rapid identification system can of course be combined into one unit.

The thermal analysis techniques described above can also be used to identify drugs, e.g., cocaine, heroin, marijuana, and hashish, and other controlled 0 substances, i such cases and unlike energetic materials, the drugs do not exhibit an anaerobic exotherm that permits them to be generally distinguished from non-drugs. However, the thermogram features of each particular drug enable the detection and identification of that drug in a sample of unknown composition in the same way as was described in the previous section with regard to identifying particular explosives. 5

OTHER EMBODIMENTS It is to be understood that while the invention has been described in conjunction with the detailed description thereof, that the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the 0 scope of the appended claims.

For example, additional analytical techniques can aid the identification of the specific energetic material or drug. For example, the microDSC from TA Instruments described previously also includes an atomic force microscopy (AFM) head with a thermal probe that permits measurement of sample topography, thermal 5 conductivity, and thermal diffusivity. Thus, such properties can be measured for a test sample (in addition to the thermogram) and compared with the corresponding properties of reference samples measured by the same instrument. The microDSC also includes a CCD camera that can record images of the test sample to indicate its morphology, which once again can be compared to the morphologies of reference samples to aid identification of the test sample.

Additionally, the devices described herein can be combined with other detection, collection, observation, or measurement equipment to perform these functions in concert with other security or analysis functions. For example, such a device can be combined with a personnel portal sampling system, an X-ray luggage analyzer, or an analytical instrument. These devices can be packaged to be carried on a person or on commercial, emergency, or military vehicles.

OTHER EMBODIMENTS

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.

What is claimed is:

Claims

L A device for detecting the presence of an energetic material in a sample, the device comprising: a hollow cylindrical housing; an aperture at one end of the housing for entry of the sample; one or more thermal sensors arranged within the housing, each thermal sensor comprising a cavity; a temperature controller that controls heating of the one or more thermal sensors and material in proximity to the thermal sensors; a conductor for transmitting an electrical charge to the cavity; and a measurement circuit for thermal analysis of particles and vapors contacting the one or more thermal sensors.

2. A device for detecting the presence of an energetic material in a sample, the device comprising: a hollow cylindrical housing; an aperture at one end of the housing for entry of the sample; an ionizer for charging particles in the sample; one or more thermal sensors arranged within the housing, each unit comprising a cavity; a temperature controller that controls heating of the one or more thermal sensors and material in proximity to the thermal sensors; a conductor for electrically grounding the cavity; and a measurement circuit for thermal analysis of particles and vapors contacting the one or more thermal sensors.

3. A device for detecting the presence of an energetic material in a sample, the device comprising: a hollow cylindrical housing; an entrance aperture at one end of the housing for entry of the sample; an exit aperture at an end of the housing opposite the entrance aperture; and a plurality of thermal sensors arranged in series within the housing between the entrance and exit apertures, whereby the sample flows into the housing, through each of the thermal sensors, and out of the housing; wherein each thermal sensor comprises a mesh sensor element that comprises mesh openings, wherein each sensor in the housing comprises a mesh sensor element with different size mesh openings, and wherein the mesh sensor elements act as particle traps; and further wherein the sensors are arranged within the housing such that larger particles in the sample are trapped before smaller particles as the sample passes through the mesh openings.

4. A device for detecting the presence of an energetic material in a sample, the device comprising: an array of detector columns arranged in parallel, wherein each detector column in the array comprises one or more thermal sensors and conductors that carry electrical signals from the thermal sensors; a sample collection chamber arranged to distribute the sample into detector columns in the array; a temperature controller that controls heat of the thermal sensors and material in proximity to the thermal sensors; and a thermal analysis system that provides thermal analysis of electrical signals from the conductors indicating particles passing between the thermal sensors in each detector column.

5. A device of any of claims 1 to 4, further comprising a heater located external to the housing, wherein the temperature controller controls the heater to heat one or more of the thermal sensors and material in proximity to the thermal sensors.

6. A device of any of claims 1 to 4, wherein one or more of the thermal sensors are heated by running electrical current through the thermal sensors, and wherein the temperature controller controls the electrical current.

7. A device of any of claims 1 to 6, wherein one or more of the thermal sensors comprises a micro-electro-mechanical system (MEMS) sensor.

8. A device of any of claims 1 or 3 to 7, further comprising an ionizer that charges the particles of the sample with the opposite charge of the cavity prior to entering the cavity.

9. A device of any of claims 1 to 8, wherein the housing comprises up to 20 thermal sensors.

10. A device of any of claims 1 to 9, wherein the thermal sensors are thermopiles, resistance thermometers, thermo-acoustic sensors, infrared sensors, ultraviolet sensors, or any combination thereof.

11. A device of claim 4, further comprising an exhaust chamber arranged to collect the sample that has passed through the detector columns

12. A method for detecting the presence of an energetic material in a sample, the method comprising; obtaining a thermal profile of the sample; computing a first derivative of the theπnal profile; computing a second derivative of the thermal profile; generating a graph of a ratio of the second derivative to the first derivative; wherein the presence of a peak in the graph of the ratio indicates the presence of an energetic material in the sample.

13. The method of claim 12, wherein the presence of two peaks in the graph of the ratio indicates the presence of an energetic material in the sample.

14. The method of claim 12, wherein the energetic material is an explosive.

15. The method of claim 12, wherein obtaining a thermal profile of the sample comprises: obtaining a first thermal profile of the sample under modulated temperature control; deconvoluting the first thermal profile into two or more separate thermal profiles; and selecting one or more of the separated thermal profiles for use as the thermal profile of the sample.

16. The method of claim 15, wherein the presence of two peaks in the graph of the ratio indicates the presence of an energetic material in the sample.

17. The method of claim 12, wherein the thermal profile is obtained under substantially anaerobic conditions.

18. The method of claim 15, wherein the thermal profile is obtained under substantially anaerobic conditions.