Detection of nitrogen containing and nitrogen free explosives

Abstract

A compact explosive detecting system collects explosive residues in the form of vapor powder. The residues are accumulated on a desorber which is subjected to pyrolysis to release a gaseous sample. The sample is pumped to a detecting system through a metering valve. A luminol cell reacts with the gaseous sample to create chemiluminescence, the light output of which is measured by a photo multiplier tube. The light intensity is indicative of the amount of explosive present. Based on the amount of explosive present, a metering valve is adjusted to pass the gaseous sample into a highly sensitive metal oxide sensor array to detect NO.sub.2 from nitrogen containing explosive and CO/CO.sub.2 from non nitrogen containing explosive. The metal oxide sensor array reliably selects explosives from those compounds indicating chemiluminescence.

Claims

1. A system for screening for the presence of explosives, the system comprising: a. a chamber for collecting explosive residues; b. a desorber; c. a pyrolysis tube for pyrolyzing said explosive residue to generate a gaseous sample from decomposition of the explosive residues comprising CO/CO.sub.2; d. a pump for pumping the gaseous sample sequentially into a luminol reaction cell followed by transferring a smaller portion of the gaseous sample to a metal oxide sensor (MOS) array or pumping the gaseous sample partially into a luminol cell and partially to an MOS array; e. said luminol producing chemiluminescence by a chemical reaction between said luminol and said gaseous sample within said reaction cell; f. measuring means comprising photomultiplier tube or photodiode array for measuring an intensity of chemiluminescence and for outputting a first electrical signal indicative of an amount of explosive detected; g. a reduced quantity of said gaseous sample configured to be pumped to said metal oxide sensor array to prevent saturation of metal oxide detector; h. the MOS array comprising a conductivity sensor having a thin ceramic or fused silica sheet with an oxide sensing layer or oxide film and electrodes for measuring electrical resistance of the oxide sensing layer or oxide film, said electrical resistance being changed by adsorption of said pumped gaseous sample indicating the presence of said explosive; whereby the chemiluminescence is refined by the metal oxide sensor array to eliminate an interference composition from detection.

2. The system for screening for the presence of explosives as recited by claim 1, wherein at least one of said explosives is a nitrogen free explosive, a pyrolysis of which releases CO/CO.sub.2.

3. The system for screening for the presence of explosives as recited by claim 2, wherein at least one of said explosives is a nitrogen containing explosive, a pyrolysis of which releases NO.sub.2.

4. The system for screening for the presence of explosives as recited by claim 1, wherein said metal oxide sensor array includes metal oxide sensors for detecting NO.sub.2 and metal oxide sensors for detecting CO/CO.sub.2 operating in dual mode.

5. The system for screening for the presence of explosives as recited by claim 3, wherein the electrical resistance of said metal oxide sensor array for detecting NO.sub.2 increases when exposed to a gaseous sample containing oxidized NO.sub.2.

6. The system for screening for the presence of explosives as recited by claim 2, wherein the electrical resistance of said metal oxide sensor array for detecting CO/CO.sub.2 decreases when exposed to a gaseous sample containing reduced CO/CO.sub.2.

7. A system as recited by claim 3, wherein said metal oxide sensor array for detecting NO.sub.2 is made by coating a thin sheet of ceramic or a thin sheet of fused silica onto an underside with a nickel or nickel chromium electrode heater having a top surface coated with sensing metal oxide film.

8. A system as recited by claim 7, wherein said metal oxide sensor array is operated at a temperature range of 300 to 800° C.

9. A system as recited by claim 7, wherein said metal oxide sensor array is operated at a temperature of 700° C.

10. A system as recited by claim 7, wherein said metal oxide film is a member selected from the group consisting of tin oxide, indium tin oxide, and tungsten oxide semiconductor.

11. A system for screening for the presence of explosives, the system comprising: a. a chamber for collecting explosive residues; b. a desorber; c. a pyrolysis tube for pyrolyzing said explosive residue to generate a gaseous sample from decomposition of the explosive residues; d. a pump for pumping the gaseous sample to a metal oxide sensor (MOS) array; e. a metering valve for reducing the quantity of said gaseous sample to said metal oxide sensor array, to prevent saturation of metal oxide detector; f. the MOS array comprising a conductivity sensor having a thin ceramic sheet or fused silica sheet with an oxide sensing layer or oxide film and electrodes for measuring electrical resistance of the oxide sensing layer or oxide film, said electrical resistance being changed by adsorption of said pumped gaseous sample indicating the presence of at least one of said explosives that contains NO.sub.2 and, optionally, at least one of said explosives that contains CO/CO.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be more fully understood and further advantages will become apparent when reference is had to the following detailed description of the preferred embodiments of the invention and the accompanying drawings, wherein like reference numerals denote similar elements throughout the several views, and in which:

(2) FIG. 1 is a schematic view of an explosive detection system of the invention using two parallel systems one using a luminol detector and the other using a MOS detector;

(3) FIG. 2 is a perspective view of the luminol explosive detection system of the invention;

(4) FIG. 3 is a schematic representation of chemiluminescence of luminol under different conditions;

(5) FIG. 4A is a schematic representation of the constructional details of the MOS sensor;

(6) FIGS. 4B and 4C illustrate the detection mechanism detection capability of NO.sub.2 and CO/CO.sub.2;

(7) FIG. 5 is a graph showing resistivity change of the MOS detector for various explosive compounds;

(8) FIG. 6 is a graph depicting the signal intensity of the MOS sensor and recovery time as a function of amount of PETN in the pyrolysis tube;

(9) FIG. 7 is a graph depicting the detection of explosives of nitrogen containing explosive with dual channel metal oxide semiconductor;

(10) FIG. 8 is a graph depicting the detection of explosives of nitrogen free explosive TATP with dual channel metal oxide semiconductor;

(11) FIG. 9 is a graph depicting the detection of explosives of interference 1 composition with dual channel metal oxide semiconductor;

(12) FIG. 10 is a graph depicting the detection of explosives of interference 2 composition with dual channel metal oxide semiconductor;

(13) FIG. 11 is a graph depicting the detection of explosives of interference 3 composition with dual channel metal oxide semiconductor; and

(14) FIG. 12 shows comparison of MOS detector response for NO.sub.2 (channel 2) and CO/CO.sub.2 (channel 1) for nitrogen containing explosive, nitrogen free explosive and interference compositions 1, 2 ND 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(15) The present invention provides an apparatus and method for analyzing surface explosive residue suspected of containing one or more explosive agents using the chemiluminescent, gas-liquid phase reaction of luminol and MOS sensors detecting NO.sub.2 and CO/CO.sub.2.

(16) A number of explosive compositions are known. Trinitrotoluene (TNT) has the following structure. TNT is nitrated from toluene in three steps, employing nitric acid and sulphuric acid mixtures as nitrating agent. TNT has a negative OB, meaning it contains insufficient oxygen to give complete combustion of the carbon on detonation. It is therefore, usefully is mixed with ammonium nitrate, which has an excess of oxygen.

(17) ##STR00001##

(18) Another explosive pentaerythritol tetranitrate (PETN) has the following molecular structure. Pentaerythritol is made commercially by the reaction of formaldehyde and acetaldehyde in the presence of alkali. It is then nitrated by adding it to strong nitric acid at temperatures below about 30° C. PETN is very stable both chemically and thermally, but is sensitive to shock and friction. Upon ignition, the oxygen atoms in the nitrate sub-group terminals oxidize the carbon and hydrogen atoms in the interior of the molecule.

(19) ##STR00002##

(20) Another explosive cyclotrimethylenetrinitramine (RDX) has the following molecular structure. It is a white crystalline powder made by the nitration of hexamine (hexamethylenetetramine) with nitric acid and ammonium nitrate below 30° C. RDX is widely used as in both military and industries as a base component of explosives compounds, such as plastic explosives, and is considered one of the most stable and powerful high explosives.

(21) ##STR00003##

(22) Another nitrogen containing explosive is Hexamethylene triperoxide diamine (HMTD). HMTD is another common homemade primary explosives used by terrorists in suicide bombings and other attacks. It is made from hydrogen peroxide (H.sub.2O.sub.2) and Hexamethylenetetramine ((CH.sub.2).sub.6N.sub.4) in presence of citric acid or dilute sulfuric acid as catalyst. HMTD is extremely sensitive to shock, heat and friction, making it dangerous to manufacture but ideal as a detonator. HMTD has been used in a large number of suicide bombing and other terrorist attacks all over the world.

(23) ##STR00004##

(24) Another nitrogen free explosive is triacetone triperoxide (TATP). TATP is an organic peroxide explosive which is extremely sensitive to heat, shock and friction. It is produced from hydrogen peroxide (H.sub.2O.sub.2) and acetone (C.sub.3H.sub.6O) in presence of strong acid (sulfuric acid). In mildly acid or neutral condition, the reaction will produce more diacetone diperoxide (DADP) or even acetone peroxide monomer than TATP. TATP has a relatively high vapor pressure, which allows it sublime under room temperature. Also, it is a very powerful explosive which produces approximately 80% of outward force that TNT produces with the same amount of explosive. Due to its readily available material and easy to synthesize, TATP has been used as the initiator in many terrorist attacks.

(25) ##STR00005##

(26) Virtually all common explosive types, including organo-nitro explosives, as well as nitrogen free explosives may be decomposed under suitable conditions to produce NO.sub.2 and CO/CO.sub.2. In one aspect of the invention, the decomposition comprises pyrolysis of nitrogen containing explosives according to reactions of the following type:

(27) ##STR00006##

(28) Nitrogen free explosive such as TATP undergoes similar reaction releasing CO and CO.sub.2. The pyrolysis reaction ordinarily requires a suitable heating means for efficient production of NO.sub.2 or CO and CO.sub.2. It has been found that heated Nichrome or Pt, while a heated Pt—Rh alloy is preferred. The heating temperature for pyrolysis is typically in the range of about 300 to 800° C., and more preferably, the temperature ranges from about 500 to 700° C.

(29) The first detector uses luminol chemiluminescence to detect the presence of explosive in the sample being subject to pyrolysis. Luminol (5-amino-2,3-dihydro-1,4-pthalazine dione) is known to react with NO.sub.2 in the presence of oxygen to produce light at a wavelength centered at about 425 nm according to the following reaction:

(30) ##STR00007##

(31) The selectivity of the present system for NO.sub.2 is enhanced by providing luminol in an aqueous, alkaline solution. Use of potassium hydroxide (KOH) as the base is preferred, as it surprisingly has been found to increase the chemiluminescent light output over that resulting from solutions comprising other bases such as sodium hydroxide (NaOH). The light thereby produced can readily be detected by a light detector, such as a photomultiplier tube (PMT). Advantageously, conventional PMT's are quite sensitive to light of this wavelength.

(32) Referring now to FIG. 1 of the drawings, there is depicted at 100 a schematic view of an explosive detection system of the invention using two parallel systems one using a luminol detector 104 and other detector 105 using a MOS detector. The explosive residue is collected by wiping or vacuum suction means and is desorbed in a carrier at 101, which is then subject to pyrolysis at 102 releasing NO.sub.2 or CO/CO.sub.2. It is then pumped at 103 sequentially to separate station one containing a luminol solution and a second detector containing MOS sensors. Since the MOS detectors are very sensitive, the inlet of the pumped NO.sub.2 or CO/CO.sub.2 gases is reduced to prevent saturation of the MOS detectors.

(33) FIG. 2. is a perspective view of the luminol explosive detection system of the invention at 200. The detection technology relies on thermal decomposition of explosives such as RDX, PETN, AN, UN, TNT, NG, TATP, Chlorate, perchlorate, (H.sub.2O.sub.2) and Taggants yielding to the formation of chemicals which specifically react with luminol to generate light. The explosive powder detection portion is shown at 201, Explosive vapor detection is shown at 202. The mode selection valve 203 selects which pyrolized explosive is sent to the pump 204. The luminol reaction cell is shown at 205. The luminol solution is packaged into a scintillation vial equipped with a semi-permeable membrane, which allows NO.sub.2 to react with the luminol and at the same time prevent any possible leakage from the cell.

(34) FIG. 3 is a schematic representation of chemiluminescence of luminol under different conditions. Clearly explosives show a very large chemiluminescence response. A number of interference species also produce significant chemiluminescence. The explosive response is maximum at about 29 seconds with a broad peak extending to several seconds as luminol is consumed by the chemical reaction of luminol with NO.sub.2 and CO/CO.sub.2.

(35) FIG. 4A is a schematic representation of the constructional details of the MOS sensor. A conductivity sensor is a sensing system that measures the change in conductivity due to the interaction between sensing material and explosive molecules. The substrate is a thin ceramic or fused silica sheet with heater on nickel or nickel chromium or platinum heating elements to heat the substrate to about 700° C. The top surface of the sensor has oxide layer sensing material and electrodes are provided to measure the conductivity.

(36) FIGS. 4B and 4C illustrate the detection mechanism detection capability of NO.sub.2 and CO/CO.sub.2. NO.sub.2 oxidizes the surface of the oxide sensing layer increasing electrical resistance at the grain boundaries. CO/CO.sub.2 reduces the surface of the oxide sensing layer decreasing electrical resistance at the grain boundaries.

(37) The operation of the metal oxide conductivity sensor is illustrated at FIG. 4B and FIG. 4C. When NO.sub.2 gas from pyrolysis reacts with the metal oxide surface as shown at FIG. 4B, it oxidizes the surface layer to form a depletion layer that creates negatively charged electrons. The grain boundaries of the polycrystalline metal oxide are strengthened by the oxidation and the overall resistance of the film increases. Accordingly, the resistance of the oxide layer increases and the measurement system registers this as a reduction in voltage. When NO.sub.2/CO/CO.sub.2 gas from pyrolysis reacts with the metal oxide surface as shown at FIG. 4C, it reduces the surface layer, creating negatively charged electrons. The grain boundaries of the polycrystalline metal oxide are weakened by the reducing action and the overall resistance of the film decreases. Accordingly, the resistance of the oxide layer decreases and the measurement system registers this as an increase in voltage.

(38) Conducting metal oxides are mostly semiconductors, which are the most commonly utilized classes of sensing materials in conductivity sensors, and this figure demonstrates typical schematics of a conductometric sensor. Metal oxide gas sensors based on measuring the conductivity change of the semiconducting materials, or known as conductometric sensors, are one of the most investigated groups of gas sensors due to their low cost and flexibility associated to their production, simplicity of their use and wide possible application fields. The cause of change of sensor conductivity can be traced down to two major causes: physical process and chemical process. One is that when the chemicals are physically adsorbed to the surface of metal oxide, transfer of charge carriers (electrons or holes) between semiconductor and adsorbed species or their catalytic decomposition products leads to the conductivity changes. The other reason is that the chemicals or their catalytic decomposition products is very reactive, either oxidizing or reducing the metal oxide to a different oxidation state which will then alter the electrical property of the semiconducting material.

(39) FIG. 5 shows graphs the resistivity change of the MOS detector for various explosive compounds. All the explosives except TATP are nitrogen containing explosives. All the explosives show a strong reduction in measured voltage across conductors on the oxide film, the minimum occurring at about 5 seconds. When absorption of the NO.sub.2 is small, the sensor resistance recovers within a short time (typically about 12 seconds), while some explosives do not recover well for a prolonged time period.

(40) FIG. 6 is a graph depicting the signal intensity of the MOS sensor and recovery time as a function of the amount of PETN in the pyrolysis tube. The minimum still occurs at about 5 seconds. When the absorption of the NO.sub.2 is small, the sensor resistance recovers within a short time (about 12 seconds). When explosives quantity is high, for example 100 ng of PETN, the resistance of the oxide film is not readily recovered even after 20 seconds.

(41) FIG. 7 is a graph depicting the detection of nitrogen containing explosive with a dual channel metal oxide semiconductor. Channel 1 detects CO/CO.sub.2 while channel 2 detects NO.sub.2. When channel 1 detects CO/CO.sub.2, the voltage goes up indicating that the resistance of the oxide film has decreased by the reducing action of CO/CO.sub.2. Since this is a nitrogen containing explosive, channel I response is minimal. When channel 2 detects NO.sub.2, the voltage decreases, indicating that the resistance of the oxide film has increased by the oxidizing action of NO.sub.2.

(42) FIG. 8 is a graph depicting the detection of explosives of nitrogen free explosive TATP with a dual channel metal oxide semiconductor. TATP is a non nitrogen containing explosive. Channel 1 detects CO/CO.sub.2 while channel 2 detects NO.sub.2. When channel 1 detects CO/CO.sub.2, the voltage goes up indicating that the resistance of the oxide film has decreased by the reducing action of CO/CO.sub.2. Since this is a non nitrogen containing explosive, channel 2 response is minimal. When channel 1 detects CO/CO.sub.2, the voltage increases, indicating that the resistance of the oxide film has decreased by the reducing action of CO/CO.sub.2.

(43) FIG. 9 is a graph depicting the detection of explosives of interference 1 composition with dual channel metal oxide semiconductor. The interference 1 composition is leather. Leather contains both carbon and nitrogen and the pyrolysis products contain both NO.sub.2 and CO/CO.sub.2. Accordingly, both channel 1 and channel 2 show a strong response, indicating that interference 2 composition is not an explosive.

(44) FIG. 10 is a graph depicting the detection of explosives of interference 2 composition with dual channel metal oxide semiconductor. The interference 2 composition is a protective glove. The protective glove contains both carbon and nitrogen and the pyrolysis products contain both NO.sub.2 and CO/CO.sub.2. Accordingly, both channel 1 and channel 2 show a strong response indicating that interference 2 composition is not an explosive.

(45) FIG. 11 is a graph depicting the detection of explosives of interference 3 composition with dual channel metal oxide semiconductor. The interference 2 composition is a finger pint residue. The finger print residue contains both carbon and nitrogen and the pyrolysis products contain both NO.sub.2 and CO/CO.sub.2. Accordingly, both channel 1 and channel 2 show a strong response indicating that interference 3 composition is not an explosive.

(46) FIG. 12 is a table that shows a comparison of MOS detector response for NO.sub.2 (channel 2) and CO/CO.sub.2 (channel 1) for nitrogen containing explosive, nitrogen free explosive and interference compositions 1, 2 and 3. In this table ‘+’ means that a positive response is observed from the initial peak, ‘−’ means that a negative response is observed from the initial peak, ‘0’ means that nearly a flat response is observed from the initial peak. Also shown in the last column is the chemiluminescent response. Thus the MOS sensors identify the explosives even when chemiluminescent response is absent.

(47) Having thus described the invention in rather full detail, it will be understood that such detail need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the present invention as defined by the subjoined claims.