GB2262990A

GB2262990A - Explosives detector

Info

Publication number: GB2262990A
Application number: GB9226244A
Authority: GB
Inventors: Kenneth Leddingham; Archibold Marshall
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1991-12-31
Filing date: 1992-12-16
Publication date: 1993-07-07
Also published as: GB9226244D0; GB9127557D0

Abstract

An explosives detector for detecting very small amounts of nitro-organic compound vapour in a sample of air or other gas selectively ionises the compounds with electro-magnetic radiation tuned to one or more specific wavelengths in the range 200nm to 260nm and uses an ion detector to detect the ions. The sample 46 is fed through membrane 48 into an ionisation chamber 42 into which laser light 14 of the specific wavelength or wavelengths is directed from lasers 52-56 to ionise the selected nitro-organic compound or compounds to NO<+> by a two photon ionisation process. The resulting ions are detected by detector such as a time of flight mass spectrometer 44 or an electron multiplier. At the specific wavelength for selectively ionising a particular nitro-organic compound it is unlikely that any other chemical species in the sample will be ionised. This selective ionisation increases the sensitivity of the explosives detector over known detectors. The laser sources 52-56 supply light pulses in a sequence and grid 62 is energised with a negative pulse in synchronism with the light pulses to attract the resulting ions to the collector electrode 66. <IMAGE>

Description

Explosives Detector This invention relates to an explosives detector and in particular to an explosives detector for detecting very small amounts of vapour of a nitro-organic compound in a sample of air or other gas. The nitro-organic based explosives can be, for example PETN (pentaerythritol tetranitrate) and RDX (cyclotrimethylenetrinitramine) and in particular nitro-aromatic based explosives, for example nitrobenzene and trinitrotoluene.

In the past vapour from explosive compositions has been detected using gas chromatography and ion mobility spectroscopy methods and electron capture methods.

Known gas chromatography methods comprise passing the sample of air to be tested through a tube having an internal coating which selectively adsorbs and desorbs certain chemical species and delays their progress through the tube by predetermined times. The chemical species are detected when they exit the tube using, for example, thermal energy analysis or electron capture devices and the delay associated with a detected species is used to identify the species.

Known ion mobility spectroscopy methods comprise exposing a sample of air to a source of ionising radiation in order to ionise chemical species in the sample and then passing either positively or negatively ionised species (generally negatively ionised) into a drift chamber. Inside the drift chamber the ionised species are accelerated towards a collector electrode by an applied voltage. The ionised species travel across the drift chamber with a velocity dependent on their mass, charge and size (if the drift chamber is not evacuated) and the time at which a particular ionised species arrives at the collector electrode can be used to identify that species. The main disadvantage with this method is that the ionised species tend to react chemically with one another before they are passed into the drift chamber thus effecting the results given by this method.This disadvantage can be alleviated to some extent by ensuring that the ionised species enter the drift chamber as soon as possible after being ionised.

Both methods suffer from the disadvantage that they are unable to detect vapours of explosive compounds at very low concentrations and so are ineffective at detecting explosives which are present in the vicinity of the atmosphere under examination in trace amounts or in sealed containers.

The present invention seeks to overcome at least some of the aforementioned disadvantages by providing an explosives detector that can detect the presence of nitro-organic compounds in gaseous samples at lower concentrations than has hitherto been possible using the aforementioned techniques.

According to the present invention there is provided an explosives detector comprising an ionisation chamber for receiving a gaseous sample, means for producing electro-magnetic radiation of sufficient intensity to ionise molecules of nitro-organic compounds in the gaseous sample and means for directing said radiation into the ionisation chamber and an ion detector arranged to detect the resulting ions, wherein the electro-magnetic radiation is tuned to at least one wavelength in the range 200nm to 260nm.

It has been discovered that at specific wavelengths of electro-magnetic radiation in the range of 200nm to 260nm the yield of positively charged ions resulting from the interaction of the radiation with specific nitro-organic molecules are highly distinctive and can be analysed to identify the nitro-organic molecules. This discovery has been applied to the field of explosives detection to produce an explosives detector that can detect concentrations of explosive compounds in gaseous samples of between 10 and 100 times lower than those detectable using conventional explosives detectors.

A tuned wavelength of the electro-magnetic radiation is chosen for each nitro-organic compound that the explosives detector is designed to detect. Preferably the tuned wavelength is the optimum wavelength at which the compound concerned produces the greatest ion yield from a two-photon ionisation process, as this will optimise the sensitivity of the explosives detector. The increase in sensitivity of the present invention over known explosives detectors is achieved because any species in the sample other than molecules of the selected nitro-organic compound will have a very low probability of being ionised in the ionisation chamber. Therefore if any ions are detected by the ion detector when radiation of a tuned wavelength is directed through a sample then it is highly likely that molecules of the associated nitro-organic compound are present in the sample.It is not necessary to distinguish between the ions of other chemical species and the ions of interest.

This ability to ionise selectively specific molecules enables the 10 to 100 fold increase in sensitivity.

Preferably the ion detector is switchable between two modes, a first mode for detecting ions with a mass of 30 atomic mass units (amu) and a second mode for detecting ions over a broad range of atomic masses. It has been found that at specific wavelengths of electro-magnetic radiation NO+ ions with masses of 30amu are generally produced from nitro-organic molecules.

When analysed in a mass dependant spectrometer the N0+ ions produce a large signal at mass 30amu in a region of the mass spectrum where little other background activity is normally present. Therefore in order to detect nitro-organic compounds with an increased sensitively, it is advantageous to detect the presence of the N0+ ions first and then in order to distinguish between different compounds the other ions produced can be analysed.

At the optimum wavelength for a particular compound it is very unlikely that any other species in the sample will be ionised so if any ions are produced at that wavelength the sample most probably contains that compound and so ion detectors that give a measurement of the number of ions produced can be used to identify the compound. The advantage of using such an ion detector, for example, an electron multiplier is that it is highly sensitive and enables the detection of very low concentrations.

Alternatively a mass spectrometer type of ion detector can be used. The term "mass spectrometer type of ion detector" covers ion detectors that can distinguish between different ions when they have different masses and/or other differences in their characteristics, for example charge or size. If a mass spectrometer type of ion detector is used more information is available about the ions detected and this enables the ions detected to be identified more reliably which in turn makes the identification of the ionised molecule more-reliable. However, the mass spectrometer type of detectors are generally less sensitive than ion detectors that give a measure of the number of ions produced. More preferably the ion detector is an ion mobility chamber because they can be compact and robust and thus can be incorporated in a hand-portable explosives detector.Another alternative would be to use an ion-trap because compact and robust ion-traps have been developed recently.

Preferably the means for producing electro-magnatic radiation and directing it into the ionisation chamber comprises a laser directed at the chamber. More preferably at least one frequency doubling means lies in the path of the electro-magnetic radiation output from the laser between the laser and the ionisation chamber for doubling the frequency of the said electro-magnetic radiation. The frequency doubling means would generally be a frequency doubling crystal which can be located either inside or outside of the laser cavity. In order to produce more than one wavelength of radiation the laser may be tuneable or alternatively more than one laser and associated frequency doubling means can be used.

Preferably the means for producing electro-magnetic radiation is arranged to produce a short duration pulse of tuned electro-magnetic radiation and the ions are pulled into the ion detector by a short duration electric or magnetic field shortly after being ionised. This separates the ions from the other species in the sample very quickly in order to prevent the ions reacting chemically with the said other species which would effect the reading of the ion detector.

Embodiments of the present invention will now be described by way of example only with reference to the following drawings in which: Figure la is a schematic representation of an explosives detector according to a first embodiment of the present invention in which the ion detector is a time of flight mass spectrometer.

Figure 1b is a schematic representation of alternative apparatus which could replace the apparatus inside the box in Figure la to form an explosives detector according to a second embodiment of the present invention.

Figure 2 is a close up of the region of interaction between the sample and the laser beam in the first embodiment shown in Figure la.

Figure 3 is a graph showing how the ion signal intensity produced by N0' ions obtained from the apparatus in Figure la varies with the wavelength of laser light when a samples of nitrobenzene are introduced into the ionisation chamber.

Figure 4 is a series of 7 graphs showing how the ion signal intensity produced by 7 different fragment ions obtained from the apparatus in Figure la varies with the wavelength of laser light when a samples of nitrobenzene are introduced into the ionisation chamber.

Figure 5a shows a mass spectrum produced from the apparatus shown in Figure la when a sample of nitrobenzene is introduced into the ionisation chamber and laser light with a wavelengths of 245.3nm is directed into the ionisation chamber.

Figure 5b shows a mass spectrum produced from the apparatus shown in Figure la when a sample of nitrobenzene is introduced into the ionisation chamber and laser light with a wavelengths of 247.2nm is directed into the ionisation chamber.

Figure 5c shows a mass spectrum produced from the apparatus shown in Figure la when a sample of nitrobenzene is introduced into the ionisation chamber and laser light with a wavelengths of 249.8nm is directed into the ionisation chamber.

Figure 6 shows a schematic representation of an explosives detector according to a third embodiment of the present invention.

Referring first to Figure la. In order to produce a tuneable source of pulsed electro-magnetic radiation of the desired wavelength range a Lumonics TEM 860E3 Excimer XeCl laser 2 is used to pump a Lumonics EPD 330 dye laser 4 operating with Coumarin 500 dye and the output of the dye laser 4 is frequency doubled using a BB0 crystal mounted in an Inrad 510 auto tracker 5 to maximise the W output and to allow tuning of the electro-magnetic radiation. Typical pulses are of approximately 6ns in duration with a maximum energy of approximately 108 Wcm-2 and a variable repetition rate from Hz to kHz. Laser fluence could be varied using a Newport attenuator 6.

The laser beam 8 output from the tuneable source is directed into a high vacuum chamber 10 through a 30cm focal length lens 12. The high vacuum chamber 10 is kept at a pressure of approximately 10-8 Torr and forms the ionisation chamber. The ringed area 16 in the vacuum chamber 10 in Figure la is reproduced and enlarged in Figure 2. The laser beam 8 intersects the sample in the region 14 of the high vacuum chamber 10.

The sample is introduced into the vacuum chamber 10 via a needle valve controlled stainless-steel capillary tube 18. When a samples introduced the pressure in vacuum chamber 10 rises to approximately 10-6 Torr but the sample density in the region 14 would be higher than this. Any positive ions formed in the region 14 when the laser beam 8 interacts with the sample are extracted from the region 14 by the extract optics 22 and are detected in a 1.20m time-of-flight mass spectrometer 20. In the spectrometer 20 an electron multiplier 24 is used to record the arrival of various ion species.

In operation the sample is introduced into the region 14 shortly before a laser pulse enters the region 14 and then shortly after the laser light has interacted with the sample any positive ions produced are extracted and accelerated towards the electron multiplier by the extract optics 22.

The vacuum pump that is used to keep the vacuum chamber 10 evacuated is in operation all the time and so can draw most of the remaining species in the sample out of the region 14 before the next sample is introduced.

In order to design an explosives detector for a particular nitro-organic compound according to the present invention it is necessary to locate the associated tuned wavelength that will ionise the molecules of the compound in a two-photon ionisation process. A method that can be used to locate the tuned wavelength will now be described for the nitro-organic compound nitrobenzene.

The ionisation potential of nitrobenzene is 9.93eV. Therefore, as an example, in order for a two-photon ionisation of nitrobenzene to be possible, photons which have an energy of at least 4.97eV must be must be incident on the nitrobenzene sample. Therefore the photons incident on the nitrobenzene sample must have a wavelength less than 249.2nm. The next step is to measure the wavelength dependence of the ion yield from a sample of nitrobenzene using the apparatus in Figure la. The wavelength range that was scanned was between 245nm and 250nm because of the 249.2nm limit discussed above and the laser pulse energy was approximately 660 J.

Figure 3 shows the wavelength dependence spectrum of the yield of N0+ ions from samples of nitrobenzene over this wavelength range. Figure 4 shows the wavelength dependence spectrum of the yield of several positive hydrocarbon ions from samples of nitrobenzene over this wavelength range. To obtain the results of Figures 3 and 4 the peak corresponding to the particular ion of interest is selected using a timing gate and the ion intensity is recorded using an ll-bit analogue to digital convertor on a data acquisition system.

In Figure 3, the N0+ wavelength dependence is very distinctive and bears no resemblance to the hydrocarbon spectra in Figure 4. The closely spaced peaks in the N0+ wavelength dependence are characteristic of rotational and vibrational spectra of small molecules. In Figure 4, which shows the wavelength dependences of several positive hydrocarbon ions, there is a very striking similarity in the wavelength behaviour of the various CnHrn ions (n=l to 6, m=0,1,2....). From Figures 3 and 4 it is clear that the greatest ion yield occurs at a wavelength around 247.2nm and so the tuned wavelength for an explosive device designed to detect nitrobenzene could be 247.2nm.

Figures 5a to 5c show time-of-flight mass spectra recorded by the device in Figure la by the time-of-flight mass spectrometer with the output of the electron multiplier 24 being fed directly into a digital oscilloscope 26. The spectra were obtained by averaging over approximately 600 laser light pulses. The spectrum in Figure 5a for optical radiation of wavelength 245.3nm corresponds to a maximum yield of N0+ ions. The spectrum in Figure 5b for optical radiation of wavelength 247.2 corresponds to a high ion yield common to all the ion fragments as discussed above. The spectrum in Figure 5c for optical radiation of wavelength 249.8nm does not correspond a high ion yield for any of the ion fragments. These mass spectra further demonstrate the suitability of a wavelength of 247.2nm as the tuned frequency for nitrobenzene.

Therefore the device shown in Figure la can be arranged to detect very low concentrations of nitrobenzene if the laser beam 8 produced has a wavelength of 247.2nm. When a sample of air that contains nitrobenzene is introduced into the region 14 at least some of the nitrobenzene molecules will be ionised and the distinctive mass spectrum of Figure 5b will be produced and displayed on the digital oscilloscope 26. When a sample of air that does not contain nitrobenzene is introduced into the region 14 very few ions, if any, will be produced and detected by the electron multiplier 24 and the mass spectrum produced will have a small amplitude and will not resemble the mass spectrum of Figure 5b.Clearly, electronic circuitry or a micro-processor can be used to identify the mass spectrum of nitrobenzene and produce a signal of some kind to the operator of the device when such a mass spectrum is produced.

Referring now to Figure ib the apparatus shown therein can be used in place of the apparatus inside the box in Figure la. The focussed laser beam 8 is directed through a quartz window 9 into a chamber 10 operated at atmospheric pressure of inert gas. The sample is introduced into the region 14 via a gas flow line 18 and the laser beam 8 intersects the sample in the region 14 of the chamber 10. Parallel metal plates 28 and 30 are positioned inside the chamber 10 and electrically insulated from it by the spacers 32. A potential difference is maintained across the plates 28 and 30 and therefore any ions produced from the interaction between the laser beam 8 and the sample will be accelerated towards one or other of the plates and cause an electric current to be produced which can be amplified by the amplifier 34.The output of the amplifier 34 can then be fed into a data acquisition system 26 for processing and storage.

The explosives detector including the apparatus in Figure 1b can be used to detect nitrobenzene if the laser beam 8 is operated at an optimum wavelength, for example 247.2nm. When a sample of air containing nitrobenzene is introduced into the region 14 at least some of the nitrobenzene molecules will be ionised and an electric current will be produced which will be amplified by the amplifier 34 and detected by the data acquisition system 26. When a sample of air that does not contain nitrobenzene is introduced into the region 14 very few ions, if any, will be produced and so the amplitude of the signal detected by the data acquisition system 26 will be very small, typically of the order of ten times less than the amplitude of the signal produced when nitrobenzene is present.Clearly, very simple electronic circuitry can be used to produce a signal of some kind for the operator of the explosives detector when a signal with an amplitude over a selected threshold level is output from the amplifier 34.

Although the above description relates exclusively to nitrobenzene, other nitro-aromatic compounds can be analysed to find the tuned frequency in the same way as described above. Then the explosives detectors described above in relation to Figures la and ib can be used for other nitro-aromatic compounds simply by altering the tuned wavelength.

Referring now to Figure 6 which shows an explosives detector according to a further embodiment of the present invention. The detector comprises three regions which are a sample chamber 40, an ionisation chamber 42 and an ion mobility spectrometer 44.

A continuous sample of air from the neighbourhood of the detector is pulled through the sample chamber 40 as indicated by arrows 46 by a pump (not shown). A semi-permeable membrane 48 separates the sample chamber 40 from the ionisation chamber 42. The primary purpose of the membrane 48 is to prevent water molecules from entering the ionisation chamber 42 but to allow the other molecules in the sample and in particular molecules of the explosive compound into the ionisation chamber 42.

The membrane 48 is not however essential. A stream of a dry gas, for example air from the neighbourhood of the detector that has had water molecules removed from it, is passed through the ionisation chamber as indicated by the arrows 50 by a pump (not shown). The stream of dry gas pulls molecules of the sample that pass through the membrane 48 through the region 14 and then out of the ionisation chamber 42. Three lasers 52, 54 and 56 which also comprise frequency doubling means, for example a frequency doubling crystal located within the laser cavity, are arranged to have outputs of three different selected wavelengths 1AW 1B and lc respectively. The lasers 52, 54 and 56 are arranged to emit pulses of laser light alternately.Mirrors 58 are arranged so that the laser light from each of the lasers 52, 54 and 56 will be directed into the ionisation chamber 42 through a quartz window 60. The laser light outputs are directed by the mirrors 58 towards the region 14 where they can interact with the sample molecules. The laser light is directed through a second quartz window 61 and onto a photo diode 63. The output from the photo-diode is fed into a digital processor 72 which controls an impulse generator 64 and an analogue-to-digital convertor 68. A metal grid 62 separates the ionisation chamber 42 from the ion mobility spectrometer 44 and is electrically insulated from the surface of the chambers 42 and 44.The digital processor 72 causes the electrical pulse generator 64 to supply an electrical pulse of negative voltage to the grid 62 a predetermined time after the photo-diode 63 detects a laser pulse. In this way the grid 62 can be held at a negative electrical potential for short periods of time. At the opposite end of the ion mobility spectrometer 44 from the grid 62 is a collector electrode 66.

When ions arrive at the electrode 66 an electric current is produced which is amplified by the amplifier 34. The output of the amplifier 34 is fed to an analogue-to-digital convertor 68, the output of which is fed to a micro-processor 70. The digital processor 72 triggers the analogue-to-digital convertor 68 to begin an acquisition cycle a predetermined time after the photo-diode 63 detects a laser pulse.

The device in Figure 6 is designed to detect three different explosive compounds A, B and C. Initially the compound A, B and C have to be analysed as discussed above in relation to nitrobenzene to determine the tuned wavelength of radiation for each of the compounds which gives the maximum ion yield. Then a laser 52 is constructed so that 1A is the tuned wavelength for compounds A, a laser 54 is constructed so that 1B is the tuned wavelength for compound B and a laser 56 is constructed so that is is the tuned wavelength for compound C.

In operation the sample flows through the sample chamber 40 and some of the molecules in the sample pass into the ionisation chamber 42 through the membrane 48. The molecules that enter the ionisation chamber 42 are drawn towards the region 14 and are then expelled from the ionisation chamber. This flow of sample molecules is continuous. The lasers 52, 54 and 56 are arranged to emit pulses of light alternately. When the laser 52 emits a pulse of light at wavelength 1A if there are any molecules of compound A in the region 14 some molecules of these molecules will be ionised. The laser pulse is then detected by the photo-diode 63 the output of which is fed into the digital processor 72. The digital processor 72 then causes the pulse generator 64 to generate a short negative voltage pulse which is applied to the grid 62.Any positive ions produced from the interaction between the laser pulse of wavelength 1A and the sample molecules in the region 14 will thus be accelerated towards the grid 62 shortly after being ionised. The ions will travel towards the collector electrode at a characteristic speed dependent on the mass, charge and size of the ion. Thus identical ions will arrive at the collector electrode at substantially the same characteristic time.

The ions arriving at the electrode produce an electrical current the amplitude of which is dependent on the number of ions reaching the electrode at the same time, ie. if there are a lot of identical ions a large current will be produced. This current is amplified by the amplifier 34 and input into the analogue-to-digital convertor 68. The analogue-to-digital convertor 68 starts an acquisition cycle a predetermined time after the laser pulse is detected by the digital processor. In this way the ions can be identified by the temporal position of a peak in the sampled signal relative to the beginning of the sampled signal. The sampled signal is then fed to a microprocessor 70 which is programmed to compare the first (and following 3n+lth, n=1,2,3....) sampled signal it receives to a the signal that would be produced if compound A were present in the sample.

If compound A is present in the sample then a relatively large number of ions will be detected and the mass spectrum detected will be characteristic of compound A. If compound A is not present then very few ions will be detected and the mass spectrum will have a very low amplitude and will not be characteristic of compound A.

Next laser 54 emits a pulse of laser light and ions are produced and detected as described above. The microprocessor 70 is programmed to compare the second (and following 3n+2th, n=1,2,3....) sampled signal it receives to a signal that would be produced if compound B were present in the sample. Next laser 56 emits a pulse of laser light and ions are produced and detected as described above. The micro processor 70 is programmed to compare the third (and following 3n+3th, n=1,2,3....) signal it receives to a signal that would be produced if compound C were present in the sample. The microprocessor 70 is programmed to give a signal to the operator of the explosive device if any of compounds A, B or C are detected, indicating which compound is actually detected.

The explosives detector in Figure 6 can be modified to detect 2 different explosive compounds at lower concentrations than the arrangement discussed above. This can be done by arranging the detector in a first mode to detect NO+ ions only and then if NO+ ions are detected the detector is located in the position where the maximum number of NO+ are detected and is then switched to a second mode to detect compounds B and C as described above. In the mass spectrum in Figure 5a corresponding to a wavelength that gives a maximum yield of NO+ ions for nitrobenzene the strength of the NO+ peak is very great compared to the other peaks in the spectra in Figures Sa to 5c.Therefore at very low concentrations the NO+ peak will be detected before the other peaks in the mass spectrum. Furthermore, in nitrobenzene there is only one NO2 group per benzene ring but in other nitro-organic based explosive compounds there are more than one, promising even greater relative strengths of the NO+ peak.

To modify the device in Figure 6 it would first be necessary to obtain results for compounds B and C similar to those shown in Figure 3 for nitrobenzene. Then comparing the results a wavelength 1NO could be chosen which gives large yields of NO+ ions from both compounds. Then the laser 52 could be constructed to emit laser pulses of wavelength 1NOE In the first mode of operation only laser 52 would emit pulses and the micro processor would compare the signals it receives with the signal that would be received when N0+ ions are produced in the region 14. The micro processor would be programmed to give a display that would indicate the concentration of N0+ ions detected. The operator of the device would then position the detector in a position where the micro processor indicates that the largest concentration is present. The operator would then switch the detector to the second mode which is the same as that described above for the unmodified device in Figure 6 for detecting compounds B and C.

Claims

1. An explosives detector comprising an ionisation chamber for receiving a gaseous sample, means for producing electro-magnetic radiation of sufficient intensity to ionise molecules of nitro-organic compounds in the gaseous sample and means for directing said radiation into the ionisation chamber and an ion detector arranged to detect the resulting ions, wherein the electro-magnetic radiation is tuned to at least one wavelength in the range 200nm to 260nm.

2. An explosives detector according to claim 1 wherein the the ion detector is switchable between two modes, a first mode for detecting ions with a mass of 30 atomic mass units (amu) and a second mode for detecting ions over a broad range of atomic masses.

3. An explosives detector according to claim 1 or claim 2 wherein the ion detector is a mass spectrometer type of ion detector.

4. An explosives detector according to claim 3 wherein the ion detector is an ion mobility detector.

5. An explosives detector according to claim 1 or claim 2 wherein the ion detector is an ion counter detector.

6. An explosives detector according to any one of the preceding claims wherein the means for producing electro-magnatic radiation comprises one or more lasers directed at the ionisation chamber.

7. An explosives detector according to claim 6 wherein at least one frequency doubling means lies in the path of the electro-magnetic radiation output from each laser between each laser and the ionisation chamber for doubling the frequency of the said electro-magnetic radiation.

8. An explosives detector according to claim 6 or claim 7 wherein the lasers are tuneable.