Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Patent
1997-02-20
1998-10-06
Glick, Edward J.
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
25033911, 356301, G01N 2165, G01N 3322
Patent
active
058180478
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to apparatus for detecting explosive substances. In some embodiments, at least, the invention relates to the detection of the explosive components of the plastic explosive Semtex.
DESCRIPTION OF PRIOR ART
European Patent Application No. EP 455516 (British Aerospace) describes a security procedure for use in airports and the like. Each passenger boarding an aircraft is given a boarding card in the normal way, which he or she subsequently hands in at a checkpoint, prior to boarding the aircraft. If that person has been handling explosives or other contraband material, traces of that material will be present on the boarding card after he or she has handled it. At the checkpoint, the boarding card is analysed in order to detect these traces. EP 455516 discloses several possible methods for chemical analysis in order to detect the traces of material. These include mass spectrometry, gas chromatography, and ion mobility spectrometry. None of these techniques has proved practicable, however, for various reasons. For example, the explosive components of the plastic explosive Semtex have an extremely low vapour pressure which makes them extremely difficult to detect by "sniffer" technology. Furthermore, the techniques suggested tend to destroy the sample, so that the boarding card is no longer available as evidence against the passenger concerned.
Raman spectroscopy is a known technique for analysing and identifying materials. See for example International Patent Application WO90/07108 (Renishaw) and European Patent Application No. EP 543578 (Renishaw).
The plastic explosive Semtex has two active chemical ingredients: cyclotrimethylene-trinitramine or RDX, and pentaerythritol-tetranitrate or PETN. RDX and PETN are present in the explosive as transparent micrometer sized crystals bound together by an amorphous waxy material. Several workers have previously reported the Raman spectra of RDX and PETN, for example obtained by Fourier Transform Raman spectroscopy. See for example J. Akhavan, "Analysis of High-Explosive Samples by Fourier Transform Raman Spectroscopy", Spectrochimica Acta, Vol 47A, No.9/10 1991, pages 1247-1250. However, such studies have been conducted under laboratory conditions, and do not address the practicalities of attempting to produce apparatus which can detect the presence of a plastic explosive such as Semtex in a relatively short space of time, and possibly contaminated by other substances, such as required in the field.
SUMMARY OF THE INVENTION
The invention follows from the results of further work which we have performed, analysing Semtex samples using the Raman analysis apparatus described in EP 543578. It should be noted that the RDX and PETN components of Semtex occur in varying proportions in commercial samples of Semtex. Moreover, many of the peaks in the Raman spectra of RDX and PETN are polarisation dependent. Since most of the microscopic particles of Semtex which might be found in a field sample such as a fingerprint consist of individual single crystals, we find that the intensities of the polarised Raman bands depend upon the angle differences between the polarisation plane of the laser illumination and the crystalline orientation. Our studies show that the frequencies remain the same, however.
In particular, from our studies we have found that RDX has a strong peak at 885 cm.sup.-1 and PETN has a strong peak at 874 cm.sup.-1, irrespective of polarisation.
According to the present invention, a method for detecting an explosive substance in a field sample comprises illuminating the sample, thereby producing a spectrum of Raman scattered light, and filtering the Raman spectrum thus produced using a narrow bandpass filter covering both 885 cm.sup.-1 and 874 cm.sup.-1, and detecting the light thus filtered. From the above discussion, it will be appreciated that if a sample containing Semtex is analysed, then the method may detect either or both of the RDX band at 885 cm.sup.-1 and/or the PETN band at 874 cm.sup.-1, irrespective of
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Hodges, Colin M. and Jacqueline Akhavan. "The use of Fourier Transform Raman spectroscopy in the forensic identification of illicit drugs and explosives." Spectrochimica Acta, vol. 46A, pp. 303-307, 1990.
Cheng, C. et al., "In Situ Detection and Identification of Trace Explosives by Raman Microscopy," Journal of Forensic Sciences, Jun. 1994, pp. 31-37.
Akhavan, J., "Analysis of high-explosive samples by Fourier transform Raman spectroscopy," Spectrochimica Acta, vol. 47A, No. 9/10, 1991, pp. 1246-1250.
Carver, F.W.S. et al., "Detection of Nitro Compounds on Silica Gel and Carbon by Non-Resonant Raman Spectroscopy," Journal of Raman Spectroscopy, vol. 14, No. 6, 1983, pp. 410-414.
Vo-Dunh et al., "Surface. Enhanced Raman Spectrometry For Trace Organic Analysis", Anal. Chem., vol. 56, No. 9, Aug. 1984, pp. 1667-1670.
Batchelder David N
Chaney Raymond J
Lacey Richard J
Glick Edward J.
Renishaw plc
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