Method of explosives detection

Details Method of explosives detection Method of explosives detection

1998-11-18

2001-06-12

Snay, Jeffrey (Department: 1743)

Chemistry: analytical and immunological testing

Nitrogen containing

Nitrite or nitrate

C436S135000, C436S172000

Reexamination Certificate

active

06245576

ABSTRACT:

This application relates to a method of explosives detection which is based on the technique of chemiluminescence and to an apparatus suitable for carrying out the method.
The technique of chemiluminescence has been employed in determining the concentration of constituents in gaseous sample mixtures. The technique depends on measuring the chemiluminescence generated during reaction of the constituent and an introduced reactant. For example the concentrations of both nitrogen oxide and ozone in gaseous mixtures have been determined by measuring the chemiluminescence produced by the reaction between these two compounds. To do this the mixture containing one of these target species is blended with a known quantity of the other reactant in a well-stirred reactor at relatively low pressures of one Torr or less and the emitted light is detected by, for example a suitable photomultiplier tube and associated current measuring device.
In a particular application of the above general methodology, the phenomenon of chemiluminescence has also been used in the detection of explosives through analysis of the vapours which are given off by such compounds. For example in the method of U.S. Pat. No. 5,092,220, vapours of explosives materials are collected on surfaces coated with gas chromatograph material which trap the explosives vapours but repel nitric oxide. In this way extraneous nitric oxide is eliminated from the sample. The sample vapours are then desorbed and concentrated in one or more cold spot concentrators, after which different vapours are separated by high speed gas chromatography. The individual sample vapours are then decomposed in a pyrolyzer to generate nitric oxide which is fed to a chemiluminescence NO detector where it is mixed with ozone and the resultant radiation detected by a photodetector. The detector is operated at a pressure of 1-2 Torr.
This chemiluminescent equipment is known on the market as a Thermal Energy Analyser (TEA) and the combined system with the gas chromatograph used to give separation of mixtures whose components are to be detected is termed a GC/TEA system.
It will be appreciated that the GC/TEA system described here is quite elaborate and costly to implement. Moreover the chemiluminescent emission for the NO/O
3
reaction is in the very near IR region of the spectrum and a number of other chemiluminescent reactions occurring between small molecules which may be present, for example CO, give rise to similar emissions. Consequently spectral filtering has to be employed and the sensitivity of the equipment is reduced.
The aim of the present invention is to provide a method of explosives detection which utilises chemiluminescence while avoiding some of the disadvantages of the prior art systems, in particular their limited selectivity and the complexity of the apparatus involved.
To achieve this, the method of this invention makes use of the earlier observation of Gray and Yoffe (Proc. Royal Soc. A, 200, 1949, pp 114-124) that very dilute mixtures of alkyl nitrate vapours with an inert gas such as argon, emit a blue glow when heated under reduced pressure. The effect was observed over the temperature range 300-500° C. and at pressures of up to about 30 kPa.
The applicant has now appreciated that, employing the phenomenon described by Gray and Yoffe, it is possible to devise a method of detection which relies on the direct chemiluminescent emission of heated molecules of energetic materials such as explosives and propellants without the need to provide any external energetic species to aid the chemiluminescent emission. In consequence of this the method of the invention demonstrates a higher degree of selectivity than prior methods with the possibility of eliminating altogether the need for chromatographic separation prior to admission of samples to the chemiluminescent detector. Additional advantages over the indirect methods which have been previously employed are, firstly, that with emission in the blue region of the spectrum, detection of emission is less affected by thermal noise than is the near IR emission from the NO/O
3
reaction. Secondly, the highly specific nature of the effect greatly reduces the need to filter light passing into the photomultiplier and thirdly, a detector employing this principle of operation can be made smaller, more robust and much less expensively than the current type of GC/TEA equipments.
Accordingly, the present invention provides a method for the detection of an energetic material in a sample to be tested which comprises introducing the sample into a chamber wherein the sample is heated while being maintained under a reduced pressure of less than 100 mbar, and detecting any light emitted.
In a preferred method of operation the sample is introduced into the heated chamber together with a carrier gas, conveniently as the output from a gas chromatography apparatus.
In the process according to the invention the temperature of the material in the chamber will be at least about 200° C. dependent upon the substance or substances to be detected. Preferably the temperature is of the order of 300° C. to 500° C., most preferably around 400° C. but it should be observed that the satisfactory temperature operating range varies with the material to be detected, ie according to its responsiveness and, at the higher end of the temperature range, its chemical stability. The above suggested operating temperatures are therefore given by way only of guidance. For example, pentaerithritol tetranitrate (PETN) may be destroyed if it is contained for any significant length of time at temperatures as high as 400° C. whereas it is thought that a higher temperature (approaching 500° C.) is needed to stimulate chemiluminescent emission from trinitrotoluene (TNT) perhaps due to the great strength of the aromatic C—NO
2
bond which has to be broken. The skilled addressee will readily understand the requirement to determine an optimum temperature of operation for the detection of any particular substance and the means of so doing.
Likewise the optimum operating pressure again varies according to the nature of the material to be detected. Thus, for example, it has been generally observed that PETN produces a more intense emission at lower pressures (below 10 mbar) within the defined range whilst both cyclotrimethylene trinitramine (RDX) and nitroglycerine (NG) generally give more intense emissions at pressures towards the higher end of the range. Higher pressures are also required for ready observation of emission from ethylene glycol dinitrate (EGDN) and TNT.
Apart from the effects of the temperature and pressure under which the present process is operated in relation to the responsiveness of different materials which it may be desired to detect, the amount of light generated by different substances has been found to vary with the amount of substance introduced into the chamber of the apparatus and also the sensitivity of the detector is found to be to some extent substance dependent. For example generally lower responses have been found for RDX and TNT than for PETN. The responses for particular explosives vary also with the nature of any carrier gas which is present in the chamber.
Thus, although an inert gas such as those in Group 8 of the Periodic Table (helium, neon, argon etc.) or most conveniently nitrogen, may be used as the carrier gas, specific gases may have the ability to enhance the chemiluminescent reaction for a particular sample material. For example, it has been found that the use of methane instead of helium as the carrier gas enhances the response in the case of NG.
Accordingly, the skilled person will readily appreciate that some trialling will be required to determine, for each substance which is to be detected, the preferred operating conditions and, in the case where mixtures of substances are expected to arise, conditions which are a compromise between those most appropriate to the various substances anticipated in the mixture may have to be selected.
The samples may be introduced either neat into the chamber or may be f

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