PTIOPTIX: Op T I
x
tical echnologies for the dentification of
E plosivesAlison J Hobro , Bernhard Zachhuber , Bernhard Lendl and other members of the OPTIX Project Consortium
Vienna University of Technology, Getreidemarkt 9/164 AC, 1060 Vienna, Austria. See www.fp7-optix.eu for more details.
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OPTIX: Optical Technologies for the Identification of Explosives is an EU Framework 7 project that aims to develop an instrument capable of detecting
and identifying explosives at a stand-off distance of 20 meters, reducing the risk of exposure to the end user. The OPTIX project seeks to make use of three
complimentary laser-based optical technologies, Raman spectroscopy, laser induced breakdown spectroscopy (LIBS) and infrared spectroscopy (IR) for
detection and, in combination with chemometric analysis techniques, the identification of explosives. Here we present an overview of the three
techniques, highlighting their complimentarity and potential for integration into a single device.
Raman Spectroscopy Laser Induced Breakdown Spectroscopy
(LIBS)
Infrared Spectroscopy (IR) OPTIX Prototype
OPTIX Partners Funding
OPTIX is funded by the European Communitys Seventh Framework
Programme (FP7/2007-2013) under grant agreement n [218037]
Computer
iCCD
Spectrograph
Fiber OpticCable
Notch orEdge Filter
Laser
Telescope Sample
Mirror
Lens
Computer
iCCD
Spectrograph
Fiber OpticCable
Laser
Telescope Sample
Mirror
Gold Coated Mirror
Computer Mid-IRDetector
Mirror
Quantum CascadeLasers (QCL)
Fragmentation Laser
Mirror
Sample
LIBS Spectrometersand iCCD
Raman Spectrometerand iCCD
Laser head containingNd:YAG and QCL lasers
Telescope
Sample
Mid-IRDetector
DichroicMirror
MovingMirror
Focusinglenses andedge filter
Figure 1 - Diagrammatic representation of a simple stand-off Raman system.A laser is arranged co-axially to a telescopefor the collection of Raman scattered light. The collected light is passed through a notch or edge filter, to remove Rayleighscattered light, and then guided into a spectrograph via a fibre optic cable. The use of an intensified CCD camera allowsfor gated detection reducing the contribution of fluorescence and ambient light to the collected spectra. Figure 3 - Diagrammatic representation of a simple stand-off LIBS system. A relatively high energy laser pulse is
directed at the sample generating a plasma at the sample surface. The atomic emission lines generated by this eventare collected by a telescope and guided into a spectrograph via a fibre optic cable. The use of an intensified CCDdetector allows for the reduction of the white-light background generated immediately after plasma formation.
Plasma
Figure 5 - Diagrammatic representation of a simple stand-off Infrared system. A fragmentation laser is directed to thesample, creating a plume of fragmented molecules. This plume is then probed using two QCLs at different wavelengths.The back-scattered QCLbeams are collected by a series of mirrors and focussed onto a mid-IR detector.
Figure 7 - Diagrammatic representation of the OPTIX Prototype. Light froma Nd:YAG laser, operating at 532 nm, and from two QCLs, operating at 5.3
and 6.3 m, is directed on the sample. The returning scattered light iscollected by a telescope to a dichroic mirror. Here, the IR light is directed tothe mid-IR detector and the remaining light directed to a moving mirror. Themoving mirror can direct the light to the LIBS spectrometers and detector orthrough a notch filter and on to the Raman spectrometer and iCCD. Theresulting spectra are compared to an on-board spectral library by computerand an explosive/non-explosive result, along with potential substanceidentification, posted on-screen.
Raman spectroscopy measures vibrational transitions in a molecule or sample through the analysis ofinelastically scattered light and, as this information is specific to the particular chemical structure in amolecule, Raman spect a sample. Raman spectroscopy usuallyemploys a laser in the visible, near ultraviolet or near infrared range and a spectrum is usually describedin terms of a Raman shift from the excitation wavelength, allowing comparisons between spectracollected at different excitation wavelengths.
ra can be thought of as a fingerprint of
The technique is non-invasive, non-destructive and can beused to analyse a wide range of substances in gaseous, liquid and solid forms.
Laser induced breakdown spectroscopy (LIBS) is based on atomic emission spectroscopy. A laser isfocused onto a small area on the sample where it ablates a small amount of the sample surface creating aplasma. As the plasma cools there is a short timeframe where characteristic atomic emission lines of theelements can be observed. LIBS is usually performed using a laser operating at 1064 or 532 nm and the fullspectral range runs from 200 to 980 nm. Although LIBS relies on ablation, the ablated area is small and sothe technique can be considered as minimally destructive and the small ablation spot size allows formeasurements with a high spatial resolution. LIBS can also be used for depth profiling by repeated firingof the laser onto the same spot, effectively allowing for the removal of surface contaminants.
In terms of explosives analysis, Ramanspectroscopy has been used to study bothnitrogen and peroxide based explosives in bulkand trace quantities. Spectra can be obtainedfrom explosives on a wide range of surfaces,such as metals or plastics, and in differentcontainers, such as glass bottles. Stand-offdistances range from 3 to 533 m and somereported spectra have been obtained underadverse conditionsincluding rain and fog.Figure 2 shows typical Raman spectra for someexplosives and precursors. Figure 2 - Raman spectra of selected explosives/ precursors
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6x108
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Ra
ma
nIn
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sity
Wavenumber (cm-1)
Ammonium Nitrate
Sodium Chlorate
2,4-Dinitrotoluene
2,6-Dinitrotoluene
Regarding explosives analysis, LIBS has been used tostudy a range of different explosives, both nitrogen andperoxide based, at stand-off distances of up to 130 m.Recent studies have shown that it is possible to performL I B S o n t r a c e e x p l o s i v e s w i t h apolymethylmethacrylate or glass barrier between thespectrometer and the sample, showing the potential forperforming LIBS analysis through windows. Otherstudies have highlighted the potential of LIBS forlandmine identification.Figure 4 - LIBS spectra of DNT
Infrared spectroscopy is a form of absorption spectroscopy and, like Raman spectroscopy, gives rise toinformation specific to a particular chemical structure, creating a spectral fingerprint of the sample.Stand-off IR spectroscopy relies on the fact that most molecules exhibit distinct absorption bands
between 3 and 10 m. The use of pulsed laser fragmentation (PLF) generates volatile fragmentationproducts that can be used for detection. The plume formed during this process is probed by two quantumcascade lasers (QCLs) operating at different wavelengths to analyse specific absorption bands, e.g. 5.3
and 6.3 m for probing NO and NO , respectively. The ratio of the two detector signals is used to
distinguish between different substances.
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Figure 6 - Ratio of the NO/NO production by PLF for
TNT on a PMMAsurface2
Stand-off IR analysis of explosives hasconcentrated on direct sensing of nitrogen basedexplosives such as TNT and HMX through thesimultaneous detection of NO and NO in the
sample and calculating the ratio between the two.Studies have also been performed on peroxidebased explosives such as TATP, relying on theirrelatively vapour pressure in conjunction withcareful selection of the laser wavelength in orderto record spectra using a small hand-held probe.Current stand-off distances are on the order of 5 mbut with improvements in collection optics it isenvisaged that 20 m is possible.
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All three techniques have a number of common components that facilitate integration:
Laser
Collection optics
Spectrographs
: Raman and LIBS spectrometers can both operate using a frequency doubled Nd:YAG,although the power density requirements are different meaning the analyses cannot beperformed simultaneously. The Nd:YAG operating at 532 nm can also provide the pulsed laserfragmentation required for IR whilst IR spectroscopy also requires two QCLs.
: The integration of the three techniques requires collection optics optimisedfor light in several different regions; 350 - 974 nm for LIBS and Raman, and 5.2 - 5.3 and 6.2 - 6.3
m for the IR QCLs. The returning light will be collected by a telescope and then the signalsdivided into different paths through a series of mirrors and fibre optic cables to three separatedetectors.
: Although other combined Raman / LIBS systems utilise a single spectrographand detector the OPTIX system will contain separate spectrographs for LIBS and Raman toincrease the spectral resolution obtainable compared to that of a combined spectrograph and
The OPTIX system will allow alternative or sequential analysis by the three different optical technologiesat a stand-off distance of 20 m. The advantages of using the three complementary technologies include:
Higher probability of detecting the presence of explosive over a range of potential threats.More difficult to avoid, confuse, or defeat the system.Increased sensitivity, speci