US3920987A - Method and system for detecting explosives - Google Patents

Abstract

A method and system for detecting extremely low concentrations of polynitro vapors such as tri-nitrotoluene (TNT) or nitroglycerine (NG) in which an air sample under investigation is introduced into a reaction chamber and mixed with a negatively ionized gas in the reaction chamber to transfer electrons to NG and TNT molecules in the air sample. A drift field is applied to the mixture in the reaction chamber to transport negative ions therein in a desired direction out of the reaction chamber. The thus transported ions are then mass filtered to separate out NG and TNT ions. The NG and TNT ions impinge upon means such as an electron multiplier for producing an electrical current, with the electrical current providing an indication of the NG and TNT concentrations in the air sample.

Description

[ Nov. 18, 1975 METHOD AND SYSTEM FOR DETECTING EXPLOSIVES Inventors: Michael Anbar, Palo Alto; John T.

Moseley, Sunnyvale, both of Calif.

Stanford Research Institute, Menlo Park, Calif.

Filed: Aug. 6, 1974 Appl. No.: 495,206

Related US. Application Data Continuation of Ser. No. 288,960, Sept. 14, 1972. abandoned.

[73] Assignee:

US. Cl. 250/282; 250/281; 250/423 Int. Cl. H01J 39/34 Field of Search 250/281, 282, 283, 292,

{5 6] References Cited UNITED STATES PATENTS OTHER PUBLICATIONS Ion-Molecular Reactions in Gases, by V. L. Talrose from Pure and Applied Chemistry, Vol. 5, 1962. Butterworths, London, pp. 457-462, 476 & 477.

Primary E.\'aminer.Archie R. Borchelt Assistant E.\'amin erB. C. Anderson [57] ABSTRACT A method and system for detecting extremely low concentrations of polynitro vapors such as trinitrotoluene (TNT) or nitroglycerine (NC) in which an air sample under investigation is introduced into a reaction chamber and mixed with a negatively ionized gas in the reaction chamber to transfer electrons to NO and TNT molecules in the air sample. A drift field is applied to the mixture in the reaction chamber to transport negative ions therein in a desired direction out of the reaction chamber. The thus transported ions are then mass filtered to separate out NG and TNT ions. The NO and TNT ions impinge upon means such as an electron multiplier for producing an electrical current, with the electrical current providing an indication of the NG and TNT concentrations in the air sample.

3 Claims, 1 Drawing Figure AIR SAMPLE lN ION SOURCE I L m p/2l /26 2 23 24 28 ELECTRON sF- I ATT E srg N66 5' iii-5.1.1135 AND ALARM PUM US. Patent Nov. 18, 1975 mmi HHH IQZ mum mum E m mm wumDOw 20 wmPCcZ Z wmw METHOD AND SYSTEM FOR DETECTING EXPLOSIVES This application is a continuation of Ser. No. 288,960

filed Sept. 14, 1972 now abandoned. I

BACKGROUND or THE-INVENTION This invention relates to a method and system for detecting explosives and particularly pertains to a method and system for detecting polynitro compoundssu'ch as tri-nitrotoluene (TNT) and nitroglycerine (NG).

As airplane bombings, hijackings and threats have grown in intensity and sophistication, the need for devices to detect concealed explosives and weapons has become crucial. Detection of such materials is also of obvious importance in other civilian areas, such as banks and public buildings, as well as to the military.

An explosives detector should have high sensitivity, not

be susceptible to false alarms, and be reasonably compact and operate with a minimum of inconvenience to persons utilizing the service which the device is de signed to protect.

The great majority of explosives utilized in nonmilitary incidents contain NG, TNT or one of the other polynitro compounds.

The prior art contains many methods for attempting to detect such explosives. Such techniques include, for example, tagging explosives with a radioactive tracer as a means of detecting them. Another possible technique is gas chromatographic concentration and separation .of nitro compounds using sensitive electron capture sensors. To make such a device sensitive enough, however, requires a long response time since the sensitivity is directlyrelated to time spent in passage of a sample through a chromatographic column. Furthermore, the operation of a sensitive gas chromatograph is fairly complicated. Another method proposed in the prior art is based on the decomposition 'of nitro compounds to yield NO and the detection of the NO by fluoresence or by chemiluminescence on reaction with oxygen atoms. These methods are not specific since they give a positive reading for any organic compound that yields NOon decomposition. i

A mass spectrometric gas analyzer can be considered as a technique to observe NG or TNT directly. How- 'ever, to distinguish between these explosives and all other compounds of similar molecular weight, a high resolution mass spectrometer is required. Even so, such an instrument does not achieve a sensitivity better than ppb. Prior separation by gas chromatography can increase the sensitivity by two orders of magnitude, but

with a substantial increase in the response time. Operation of a mass spectrometer in tandem with the gas chromatograph results in a device only marginally better than the gas chromatograph alone, and at a substantial increase in both cost and complication.-

The method and apparatus of the present invention avoids the shortcomings of the prior art techniques discussed above.

BACKGROUND OF THE INVENTION It is an object of this invention to provide a method and system for detecting explosives in an air sample.

It is a more specific object of this invention to provide a method and system for detection of NG or TNT vapors in an air sample which utilizes an electron transfer reaction in combination with a low resolution mass filter. I

, Briefly, in accordance with one embodiment of the invention, an air sample under investigation is introduced into'a reaction chamber. A negatively ionized gas is introduced into the reaction chamber sothat it mixes with the air sample and transfers electrons to NO and TNT molecules in the air sample to form negative ions thereof. A drift field is applied to the mixture in the reaction chamber to transport the negative ions therein in a desired direction out of the reaction chamber. The thus transported ions are then mass filtered to separate out NG and TNT ions. The NG and TNT ions generate an electrical current which provides an indication of the NG and TNT concentration in the air samples.

BRIEF DESCRIPTION OF THE DRAWING The drawing illustrates in schematic form apparatus in accordance with the invention for practicing the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method and apparatus of this invention utilizes an electron transfer reaction in combination with a low resolution mass separator for fast detection of NG and TNT vapors. When a stable negative ion such as SF], encounters an NG or TNT molecule, there is a high probability that the electron transfer reaction will occur owing to the fact that NG and TNT both have larger electron affinities than SF The electron affinity of NO or TNT is greater than 1.7 electron volts (eV) whereas the electron affinity of SF is 1.29 eV. However, the major constituents of the normal atmo sphere all have electron affinities lower than 1.29 cV. Therefore, the reaction set forth in equation 1 will not occur between SF} and most atmospheric constituents. In fact. if negative ions of any of these species were formed, they would also undergo a reaction similar to equation 1, thus leading to NG or TNT.

The details of the method and apparatus in accordance with this invention can be described by reference to the FIGURE. A mixture of SF and N 11 is introduced into an electron attachment ion source 12. The N is merely used to dilute the SFgand the electron attachment ion source 12 is of a conventional type where electrons emitted from the filament attach to the SF forming SF ions. These SF] ions are introduced into a reaction chamber 13.

As. discussed above, SF does not react significantly with major atmospheric components since the SF has an electron affinity substantially higher than most atmospheric components. On encountering an NG or TNT molecule, however, the reaction described above in equation 1 proceeds very rapidly.

The reaction chamber 13 has an air inlet 14 where an air sample to be analyzed is introduced and an air outlet 16 for exhausting air samples. The reaction chamber 13 includes means for establishing a weak drift field therein, illustrated schematically in the FIGURE by electrodes 17.

In the reaction chamber 13 the drift field maintained therein causes ions to drift slowly from the entrance 19 of the reaction chamber to the exit 20 thereof. A typical time for an ion to traverse the reaction chamber 13 is 0.5 seconds. During this drift the ion typically undergoes about 10' collisions. Therefore, if the NG or TNT concentration were 1 ppb. each SF molecule would have a probability of about 0.1 of reacting to form a negative NG or TNT ion.

The result of the process takingplace in the reaction chamber 13 is that a large effective amplification of the TNT or N6 is achieved. For example. if the entering air sample contains 1 part in 10 of TNT or N6 molecules. the exiting ion swarm at the exit 20 of reaction chamber 13 will contain up to 1 part in 10 of TNT or NG ions. Thus. an effective amplification of up to 10 can be readily achieved.

The ions leaving exit 20 of the reaction chamber 13 enter a differential pumping chamber 21 having a pumping outlet 22 for connection to a pump to remove most of the gas that exits the chamber 13 through the aperture 20. A skimmer 23 is utilized in a conventional manner for sampling the negative ions. The skimmer 23 operates to separate the negative SF} and NO or TNT ions from most of the neutral molecules leaving the exit 20.

Following the electron transfer reactions and the sampling. the negative ions are introduced into a low resolution mass separator. generally indicated by reference numeral 24 in the FIGURE. Several low resolution mass spectrometers are suitable for use in the apparatus and according to the method of this invention. including for example. a Wein filter or a quadrapole filter. It is very easy to separate NG and TNT from SF. and ions of atmospheric constituents. NG and TNT both have a molecular weight of 227 whereas SF has a molecular weight of 146 and the ions of atmospheric constituents have a molecular weight less than 100. There are practically no interfering organic compounds with molecular weights in the range 200 to 250 that have sufficiently high electron affinities to efficiently extract electrons from SF... The mass filter chamber 26 has a pumping outlet 27 to keep the chamber 26 at a sufficiently low pressure to facilitate ion travel therethrough.

The mass separator or filter 24 is. according to one embodiment. a mass selective velocity filter. That is. all the ions have the same energy but will have different velocities depending upon their mass. A Wein filter balances crossed electric andmagnetic fields for one mass so that one mass of ions passing through the filter 24 assumes a particular trajectory whereas ions of other masses assume different trajectories. in accordance with one specific embodiment of the invention. an electron multiplier 28 is provided situated in a position such that NG* or TNT ions selected by the mass filter 24 impinge thereon. The electron multiplier 28 generates an electrical current in response to this impingement. The electrical current drives level discriminators and alarms for signaling the presence of significant concentrations of NG or TNT in the air sample. A 1 ppb concentration of NG or TNT yields a current of about 10' ampere from the electron multiplier 28 so that a sensitivity greater than 0.001 ppb isattainable.

As previously discussed. the high electron affinity of SE; (1.29 eV) greatly inhibits the formation of negative ions from the major atmospheric constituents and from heavier organic compounds. Only molecules with electron affinities greater than 1.3 eV can efficiently extract an electron from SF-a Polynitro compounds are known to have such a high electron affinity. The gas SF has been found to be particularly useful in the electron transfer reaction to NO and TNT molecules. However. other gases are suitable. The requirements are that the gas utilized have an electron affinity greater than that of the normal atmospheric constituents but less than that of the polynitro compounds. such as the 1.7 eV electron affinity of TNT and NG.

Thus what has been described is an apparatus and method for detecting small concentrations of polynitro vapors in an air sample. In accordance with one specific embodiment. an electron transfer reaction is utilized'between SF and polynitro molecules such as TNT and NG to selectively ionize the polynitro molecules, with a low resolution mass filter utilized in conjunction with an electron multiplier to generate an electrical current representative of the concentration of polynitro molecules in the air sample. Although the invention has been discussed with respect to this specific embodiment. it will be obvious to those skilled in the art that various modifications may be made to this specific embodiment without departing from the true spiritand scope of the invention.

What is claimed is:

l. A method for detecting concentrations of polynitro molecules in an air sample comprising the steps of introducing an air sample which is not ionized into a reaction chamber. providing a single negatively ionized gas in the reaction chamber whereby the ionized gas mixes with the air sample. said negatively ionized gas having an electron affinity greater than that of normal atmospheric constituents and less than that of polynitro compounds of NO or TNT whereby said ionized gas selectively transfers electrons whereby the polynitro molecules in the air sample to form negative ions thereof, applying a drift field to the mixture in the reaction chamber to transport the negative ions therein in a desired direction out of the reaction chamber, mass filtering the thus transported ions to separate out polynitro ions. generating an electrical current by the polynitro ions. and measuring the electrical current to provide an indication of the polynitro concentration in the air sample.

2. A method in accordance with claim 1 wherein the negatively ionized gas has an electron affinity greater than one electron volt but less than the electron affinity of NG and TNT.

3. A method in accordance with claim 2 wherein the negatively ionized gas is SF

Claims (3)

1. A METHOD FOR DETECTING CONCENTRATIONS OF POLYNITRO MOLECULES IN AN AIR SAMPLE COMPRISING THE STEPS OF INTRODUCING AN AIR SAMPLE WHICH IS NOT IONIZED INTO A REACTION CHAMBER, PROVIDING A SINGLE NEGATIVELY IONIZED GAS IN THE REACTION CHAMBER WHEREBY THE IONIZED GAS MIXES WITH THE AIR SAMPLE, SAID NEGATIVELY IONIZED GAS HAVING AN ELECTROL AFFINITY GREATER THAN THAT OF NORMAL ATMOSPHERIC CONSTITUENTS AND LESS THAN THAT OF POLYNITRO COMPOUNDS OF NG OR TNT WHEREBY SAID INOIZED GAS SELECTIVELY TRANSFERS ELECTRONS WHEREBY THE POLYNITRO MOLECULES IN THE AIR SAMPLE TO FORM NEGATIVE IONS THEREOF, APPLYING A DRIFT FIELD TO THE MIXTURE IN THE REACTION CHAMBER TO TRANSPORT THE NEGATIVE IONS THEREIN IN A DESIRED DIRECTION OUT OF THE REACTION CHAMBER, MASS FILTERING THE THUS TRANSPORTED IONS TO SEPARATE OUT POLYNITRO IONS, GENERATING AN ELECTRICAL CURRENT BY THE POLYNITRO IONS, AND MEASURING THE ELECTRICAL CURRENT TO PROVIDE AN INDICATION OF THE POLYNITRO CONCENTRATION IN THE AIR SAMPLE.

2. A method in accordance with claim 1 wherein the negatively ionized gas has an electron affinity greater than one electron volt but less than the electron affinity of NG and TNT.

3. A method in accordance with claim 2 wherein the negatively ionized gas is SF6.

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