US3430482A - Automatic bomb detector - Google Patents

Description

March 4, 1969 A. DRAVNIEKS E'II'AL 3,

AUTOMATIC BOMB DETECTOR Filed July 20, 1967 Sheet of 2 VENT ROTAMETER VENT DRY MR v mg 6 INVENTORS ANDREW DRAVNIEKS 8: 2 MARTIN J. SALKOWSKI BY 71 M March 4, 1969 DRAVNlEKs ETAL 3,430,482

AUTOMATIC BOMB DETECTOR Sheet g of2 Filed July 20, 1967 FIG.4

FIG

ma WS E S VU W K L A DS m E m DT. M A M 5 F BY M 1 011 ATTORNEY United States Patent O 3,430,482 AUTOMATIC BOMB DETECTOR Andrew Dravnieks, Park Forest, and Martin J. Salkowski, Downers Grove, Ill., assignors to the United States of America as represented by the Secretary of the Army and/or the Administrator of the Federal Aviation Administration Filed July 20, 1967, Ser. No. 655,267 US. Cl. 73-23.1 Claims Int. Cl. G01n 31/06, 31/08, 33/22 ABSTRACT OF THE DISCLOSURE This invention relates to a bomb detector for identity ing the presence of dynamite, for example, in an airplane. The basic method detects the presence of ethylene glycol dinitrate vapor in air. The essential steps are selective adsorption of the vapor on a surface, desorption of the vapor and a timed passage through a short, chromatographic partition column, followed by passage through a vapor detector which emits an identifying signal.

BACKGROUND OF THE INVENTION Field of the invention The invention describes a particular method and apparatus for qualitative gas analysis utilizing the properties of a particular vapor to identify minute quantities thereof in air. The method may be modified depending on the particular vapor to be identified. The vapor is characteristically evolved from a particular organic explosive material. For instance, ethylene glycol dinitrate (hereinafter referred to as EGDN) vapor evolves from dynamite and self-made nitroglycerine. Nitrobenzene, which can originate from various solvents, is separable from EGDN vapor and is detectable from plastic explosives by a similar process. No method is herein disclosed to detect the presence of trinitrotoluene and dinitrotoluene containing explosives or black powder.

Description of the prior art The prior art contains no rapid means for detecting the presence of a bomb in an airplane short of a physical search.

SUMMARY It has been discovered that trace quantities of EGDN vapor will be readily adsorbed on a gold surface, and desorbed with an increase in temperature. This property is utilized together with a partition column to identify the vapor. The process and apparatus of this invention separate the EGDN vapor from air and pass is through a partition column for subsequent identification based upon the time taken for the vapor to travel through the column and its subsequent ability to change the electric current flow in a tritium containing electron-capture detector.

Accordingly, it is an object of this invention to detect the presence of explosive materials in a closed area such as an airplane rapidly and with a minimum of passenger discomfort.

It is another object to provide a compact vapor analysis apparatus capable of installation on an airplane which will reliably indicate the presence of explosives in a few minutes of operation.

It is a further object to provide a vapor analysis apparatus which will analyze the air exhausted from an airplane to detect minute quantities of EGDN vapor.

These and other objects will become more readily ap parent with reference to the drawings and following description.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the analysis apparatus of this invention.

FIG. 2 shows the primary adsorber in longitudinal cross-section.

FIG. 3 shows an end view of the rotary disk valve used at either end of the primary adsorber of FIG. 2.

FIG. 4 shows the electron-capture detector in partial section.

FIG. 5 shows the secondary adsorber DESCRIPTION OF THE PREFERRED EMBODIMENT To detect the presence of EGDN vapor the analyzer of this invention may be miniaturized for installation on board and airplane or it may be installed on the same truck used to heat and air-condition aircraft on the ramp. Cabin air at ambient temperature is circulated by a blower .1 through the primary adsorber, 2.

The adsorber, 2, consists of three concentric metal cylinders, 3. The metal internal to the adsorber is the only exposed metal internal to the system. Stainless steel, copper, polytetrafiuoroethylene (hereinafter referred to as Teflon), platinum, and gold were used in experimentation. Since EGDN is an electron-accepting, polar molecule, it may be held on a metal surface by a partial transfer of electrons to the adsorbed molecule. This process would then be partially inhibited in metals having the usual oxide layer from reaction with air and adsorbed moisture films. It was therefore found that gold, a relatively chemically inactive metal, displayed a great affinity for EGDN molecules, and the cylinders of the preferred embodiment are constructed of gold foil.

As the cabin air passes longitudinally through the primary adsorber, FIG. 2, the EGDN vapors present are adsorbed on the gold surfaces. When the air has passed through adsorber, 2, rotary disk valves, 4, at either end are closed.

The cylinders, 3, are in reality one turn shorts in a transformer, 5, having primary coils, 6, and a laminated, insulated core, 7.

The rotary disk valve, 4, comprises a stationary disk, 8, and a rotating disk, 9, having ports 8 and 9 respectively. These disks may be Teflon coated aluminum.

When the valves are closed current is applied to the system and cylinders, 3, are heated to C. At this temperature, EGDN molecules are desorbed. An inert carrier gas such as argon is admitted during the heating at manifold, 40, to carry the vapor from exit manifold, 41, to the secondary adsorber, 10.

The secondary adsorber, 10, consists of a gold tube, 11, wrapped with asbestos covered Nichrome wire heating element, 12. A thermo couple, 13, is used to regulate the internal temperature, and a cooling means, 14, such as a Vortex tube is used to cool the system.

When the vapor containing argon is admitted through the inlet valve of the gold tube, 15, the cooling means, 14, has pro-cooled the tube to 0 C. At this temperature EGDN molecules are adsorbed on the gold surface. The tube, 11, is then heated by wire, 12, to 80 C. at which point the EGDN molecules are desorbed.

The outlet valve, 16, is then opened and argon gas is injected to carry the vapors to the partition column, 20'.

Column, 20, is a chromatographic column used to differentially separate gases based upon the time taken to travel the length of the column. The column is a tube having a packing and an inner surface coated with an absorbent material. The gases are dissolved and evaporate until equilibrium is reached. Because each chemical is absorbed and evaporated at a different rate, each has a 3 difierent rate of travel down the length of the tube so that each emerges at a different time.

The column, 20, is of conventional design. The length is important because too great a length will allow the vapors to decompose. Accordingly, a Teflon tube 4; inch outside diameter and approximately 1214 inches long was selected. The column was packed with 0.690 gram of 10% Apiezon-L on Flourpak 80 which is commercially available shredded Teflon coated with Apiezon-L, a hydrocar bon grease with a molecular weight of about 1400. The internal wall of the column was also coated with the same material.

Various other absorbent materials were experimentally substituted as coatings for the packing, Apiezon-N, a grease with a molecular weight of about 1200,

Carotene, a vitamin having the formula C s', Biliverden, a bile pigment having a formula of C H O N Carbowax 4000, a polyethylene glycol with a structure of HOCH (CH OCH CH OH and a molecular weight of about 4000; Squalene, C H Silicon Ester-52, a proprietary silicon rubber; Dinonylpht-halate, C H O and Kel-F Grease, Cl- (CF2'CFC1) 'CL As the vapors exit the column they enter the electroncapture detector, 30, of FIG. 4. It is essentially an ionization device in which vapors that have an aflinity for electrons decrease the electrical conductivity of gases. Because gases are poor conductors a means, 31, to introduce ions is placed between the cathode, 32, and the anode, 33. The means selected was a foil of titanium metal that has been retracted with the radio active hydrogen isotope, tritium. The tritium is a source of high energy electrons (beta particles) which interact with the incoming gas atoms to lose energy and produce lower-energy, slow electrons. Under the electrical potential provided by a battery, these electrons are collected at the positive electrode, 33, the inlet tube. This causes a current to flow.

When any material that is capable of forming stable negative ions (anions) enters the detector some of the electrons flowing toward the anode react to form such anions. Since the mobility of an anion is some or 10 times less than that of free electrons, the anions are carried away by the flowing carrier gas and react with positively charged gas molecules instead of being discharged at the anode.

The process in the detector may be summarized as follows. A tritium atom disintegrates to form a helium-3 positive ion and a beta particle with an energy of about 1800 ev. By collisional interaction with the argon carrier gas, n+1 slow electrons are produced. The value of n is about 500 to 1000, since about ev. is required to ionize argon. The slow electrons either recombine with the positive argon ions or are collected by the anode. When a gas that forms stable negative ions as EGDN is admitted to the detector with the argon carrier the slow electron moving toward the anode is captured to form the stable negative ions which flow out with the carrier gas. This capture effect causes a decrease in current flow which may be amplified to produce a measurable signal.

The change in the current flow through the detector may be recorded as peaks on a strip chart, or it may be coordinated with a time signal to ring an alarm bell when a peak appears at a preselected time interval. These recording devices are conventional and modificaitons would be obvious to one of ordinary skill in the art.

Because of the nature of the processes in an electron capture detector, such parameters as the applied voltage, type of carrier gas, temperature, pressure, gas flow rate, and type of recording device affect the instrument performance, but the calibration of such an instrument is within the skill of an ordinary practitioner in the art.

After the sample of air passes through each of the chambers described above, it is necessary to clean the vacated chamber. This can be readily accomplished by heating to a temperature of from 150200 C., to assure that all residual vapors are released. The chamber is then purged with the inert carrier gas. This decontamination is necessary to assure that explosive material vapors detected in one aircraft do not give a false alarm in subsequent tests.

Because EGDN is a strong electron acceptor readily adsorbed on a metal surface the internal parts of the system, vapor sampling device, transport lines, valves (denoted by an in FIG. 1), connections, and chromatographic column are constructed of a material which adsorbs as little as possible. Accordingly, Teflon was used, and the gold tube and cylinders are the only exposed metal surfaces internal to the system.

The engineering model constructed had the following dimensional characteristics. The gold cylinders 3, in the primary adsorber 2, have an exposed area of 364 square inches. The air flow rate through the primary adsorber was up to 200 c.f.m. Since desorbtion must be uniform and complete the thickness of the cylinders 3, should be adjusted so that each has approximately the same electrical resistance. It was determined that the middle cylinder should be 6.6 mills thick, the outside cylinder 8.4, and the inner tube 5.0 mills thick. The argon is fed into the adsorber 2, at distribution collar 40, at a flow rate of 8 liters per minute. The secondary adsorber used a 3 inch long by inch inside diameter gold tube and had an effective adsorbing area of approximately 3.7 cm.

With the above approximate dimensions, a flow rate to the column of 50 cc./min., and a column temperature of C. the retention time in the column is 1 minute and 30 seconds.

It was also found that Teflon at room temperature adsorbed a measurable amount of EDGN. Accordingly, the overall system temperature was raised to 74-80 C.

The argon used for the detector step was certified to be less than .5 p.p.m. hydrocarbon, and that used in the adsorbers 2 and 10 was 99.996% pure.

On actual tests with an engineering model as described above, a test could be completely run and the system purged for another test in as little as 6 minutes 50 seconds. This model was designed so that a peak in current change would read out between 1 and 2 minutes after sample injection into the column.

It will be obvious that the system design may be varied greatly within the basic inventive concept. The invention is not intended to be limited by the apparatus described as a preferred embodiment.

We claim:

1. Apparatus for the detection of ethylene glycol dinitrate vapor in air comprising:

(a) a metallic adsorber, having an inlet and an out let, for selectively adsorbing electron-accepting, polar, gaseous molecules;

(b) means electrically coupled with said adsorber for raising and lowering the temperature thereof;

(c) means for directing said air and vapor through said adsorber;

(d) a gas chromatographic column, having an inlet and an outlet, said column containing a coating of a viscous, absorbent liquid, the inlet of said column communicating with the outlet of said adsorber;

(e) means for directing electron-accepting, polar, gaseous molecules from said adsorber through said column;

(f) an electron capture detector communicating with the outlet of said column, said detector indicating when ethylene glycol dinitrate vapor leaves said column and enter said detector; and

(g) recording means for recording the indication of said detector and the time taken for said molecules to pass through said column.

2. The apparatus of claim 1 wherein said adsorber further comprises:

(a) a primary adsorber having an inlet and an outlet; and an internal metallic adsorbtion surface, the inlet receiving the air and vapor;

(b) a secondary adsorber having an internal metallic, readsorption surface, an inlet communicating with the outlet of said primary adsorber and an outlet communicating with the inlet to said column; said primary adsorber having an internal heating element to heat the adsorption surface, and the secondary adsorber having an internal heating element and an internal cooling element to heat and cool the readsorption surface.

3. The adsorber of claim 1 wherein said metallic surface is a metal selected from the group consisting of copper, gold, stainless steel, and platinum.

4. The chromatographic column of claim 1 wherein the column is packed and said column and packing are of inert, inactive, non-metallic material, and the internal surface of said column and said packing are coated with a hydrocarbon grease.

5. The apparatus of claim 1 wherein said means for directing molecules from said adsorber through said column includes an inert carrier gas.

6. In the apparatus of claim 1 the electron capture detector further comprising:

(a) a housing having an inlet and an outlet, the inlet communicating with the outlet of said column;

(b) a cathode disposed internally to said housing and adjacent the outlet;

(c) an anode disposed internally to said housing and adjacent the inlet;

(d) a source of high energy electrons disposed internally to said housing and at said cathode; and

(e) electrical energizing means connected to said anode and cathode.

7. The detector of claim 6 wherein the source of high energy electrons is tritium carried by titanium foil.

8. The method of detecting the presence of dynamite in a confined area by identification of the presence of ethylene glycol dinitrate vapor in air comprising:

(a) sampling the vapor in said area;

(b) applying said vapor sample to a selective metallic adsorption surface to concentrate said ethylene glycol dinitrate;

(c) desorbing said vapor from the metallic surface;

(d) passing said vapor from the metallic surface;

(e) passing said concentrated vapor through a chromatograph column; and

(f) measuring the retention time of said vapor in the chromatograph column.

9. The method of detecting the presence of a dynamite bomb in an airplane comprising the steps of successively:

(a) taking a sample of the air internal to the airplane;

(b) directing said sample over a first clean metallic surface at ambient temperature to adsorb any molecules of ethylene glycol dinitrate vapor present in said sample on said metallic surface;

(c) heating said surface to approximately C. to

desorb any of said vapor molecules;

(d) directing said desorbed vapor over a second clean metallic surface, said surface at approximately 0 C., to readsorb said vapor molecules on said surface;

(e) heating said second surface to a temperature of approximately 80 C. to desorb said vapor molecules;

(f) directing said vapor molecules through a gas chromatographic column to separate said vapor from any impurities;

(g) measuring the time taken for said molecules to travel through said column; and

(h) identifying the presence of said vapor in said sample by comparing the time taken for said vapor to travel through said column and the time required for a known sample of said vapor to travel through said column.

10. The method of claim 9 wherein the step of measuring includes:

(a) recording the time said vapor is admitted to said column;

(b) continuously feeding the vapor exiting from the column into an electron capture detector; and

(c) recording the time at which said detector registers a peak decrease in current.

References Cited UNITED STATES PATENTS 3,358,140 12/1967 Curran et a1. 25043.5

FOREIGN PATENTS 876,072 Great Britain. 1,087,652 10/1967 Great Britain.

OTHER REFERENCES Chem Abstracts, vol. 64, Nos. 1889c and 7373a; vol. 66, Nos. 30634q and 117538k.

RICHARD C. QUEISSER, Primary Examiner.

VICTOR I. TOTH, Assistant Examiner.

US. Cl. X.R. 340237

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