Introduction:

Explosive detection is an application of advanced spectroscopic technologies which impacts an increasingly prominent global issue. Explosives, such as gunpowder, have been in use for centuries, but it is only very recently that technology has allowed for the rapid identification of explosive materials in a range of situations.

Detecting explosives can be a dangerous task, due to the potential presence of toxic materials, and the imminent threat of explosion, which can hamper traditional forensic investigation. It is clear, therefore, that any analysis techniques which are capable of working quickly, efficiently, and at a safe distance, will be highly desirable in this application.

Of course, explosive incidents are not always intentional. Accidental industrial explosions, for example, are a major risk factor in several sectors, and understanding how and why they occur is crucial when trying to prevent future incidents. Civilian agencies such as the U.S. Chemical Safety and Hazard Investigations Board regulate the analysis of post-explosive residues, in order to determine the cause of accidents. In these investigations, identification of these residues is a highly important task, which can benefit from many of the same technologies.

The 2017 Pittcon Conference & Expo (March 5-9, 2017, McCormick Place, Chicago, IL. USA) is an ideal place for researchers to learn about the latest trends in explosive material analysis. Pittcon (The Pittsburgh Conference & Exposition) began life back in the 1950’s as a conference on Analytical Chemistry and Applied Spectroscopy. Over the years it has evolved to encompass all laboratory based scientists testing or analyzing the chemical / biological properties of compounds or molecules.

There are several talks relevant to this topic at various symposia of this year’s event, such as ‘Eye-Safe Near-Infrared Trace Explosives Detection and Imaging’ delivered by Marcos Dantus of Michigan State University, ‘Discriminating Power of Volatiles from Forensic Specimens in the Field Using Innovative Sampling and Analysis’ delivered by Kenneth G Furton of Florida international University and ‘Mass Spectral Tools for Characterization of Synthetic Phenethylamines’ delivered by Ruth Smith of Michigan State University.

This year’s Expo will feature several leading companies relevant to explosive material detection. Companies such as Thermo Scientific, Metrohm and Rigaku will demonstrate their latest optical detection technologies ranging from lasers to spectrometers and optics all helping to miniaturize detection technology. Recent technological developments have led to a rapid rise in handheld devices capable of tackling explosive detection, removing the need for extensive sample preparation in a laboratory environment and allowing quick, accurate analysis at a suitable standoff distance directly in the field.

Chapter 1 – Recent challenges in Identifying Explosive Materials

Explosive Materials

Explosive materials are generally composed of a hydrocarbon-based fuel component, and a nitrogen- or oxygen-based trigger, such as a nitrate or a peroxide. Explosives are classified as high or low energy, depending upon the speed of propagation of the combustion reaction – in high explosives, the reaction propagates faster than the speed of sound, creating a shockwave. It is estimated there are at least 150 separate materials in use today.

As well as the vast array of explosives available today, many have multiple uses – for example, industrial chemicals like nitric acid, or fertilizers like ammonium nitrate combined with a hydrocarbon like diesel fuel. Detection of these more common materials can be difficult, as their presence does not necessarily indicate the intent to detonate an explosive device.

Response to the Global IED Threat

A notable concern in today’s world is improvised explosive devices (IEDs). These are particularly tricky to identify, as they take many forms, and use numerous activation methods. By definition they are non-traditional and often contain non-military components. They are often used indiscriminately, causing both military and civilian casualties. The adaptability of these devices is both why they are employed, and also why they can be so difficult to counter. They can be deployed in a wide range of ways, typically disguised as part of the environment or as an everyday object. Detonation methods include via trip wire, cell phone or by hand.

To tackle this ever changing threat, NATO developed a counter IED action plan in 2010 with three focal points: Defeating the device, attacking the network and preparing the forces. The first point looks at how to detect and neutralize an IED, which is the primary focus in this article. The ‘Defense Against Terrorism Program of Work’ has several projects specifically relating to developing sensors and information technology for the detection of IEDs. The other two points of NATO’s C-IED plan require organizational cooperation across multiple agencies and often across international borders.

The response to IEDs is mainly driven by various global militaries, who understandably have a large stake in developing portable sensing devices for use in the field. Homeland security maintains slightly different requirements in that detection equipment does not necessarily need to be portable, but it still needs to be adaptable to different situations, including airports, vehicle scanning, and cargo ships.

Techniques for pre/post Explosion Residue Detection

The requirements for a useful detection technique for explosive devices in the field can be summarised by three points:

• High sensitivity, with the ability to detect trace quantities of material
• Can operate from a safe distance
• High specificity for explosives, in the presence of other common materials (sometimes termed ‘chemical clutter’)

Unfortunately, in practice these three requirements often conflict with each other.
It can be seen that there is a need to develop a range of solutions, to cater to multiple possible situations. A recent report supported by the Office of Naval Research has suggested research is necessary in a diverse set of scientific topics including ‘chemical, environmental, and electrical engineering; chemistry and analytical chemistry; applied physics; forensic science; spectroscopy; and optics’. This is to counter the threat not only from the explosive material itself but also arming and firing systems.

An important area of development is the improvement of rapid detection in the field. It is one thing being able to detect trace amounts of material in a lab environment, but entirely different in more challenging and less controlled conditions. Hence, the report continues to recommend research in ‘plume and aerosol dynamics; x-ray, microwave, infrared, and terahertz imaging and spectroscopy; neutron, gamma-ray, magnetic resonance, and magnetic-field systems; optical absorption and fluorescence; light detection and ranging (LIDAR), differential-absorption LIDAR (DIAL), and differential-reflectance LIDAR (DIRL); biosensors and biomimetic sensors; and microelectromechanical systems (MEMS). Researchers in chemical, mechanical, nuclear, and electrical engineering, bioengineering, chemistry, spectroscopy, applied physics, and optics should be involved in these efforts.’

References

1. http://www.nato.int/cps/en/natohq/topics_72809.htm

2. http://www.newyorker.com/news/news-desk/the-limits-of-detection
www.jieddo.mil

3. https://www.nap.edu/read/11953/chapter/2#8

4. http://www.ndu.edu/inss/press/QDR_2001/sdcasch03.html

5. https://www.whitehouse.gov/sites/default/files/docs/cied_1.pdf

6. http://www.smallarmssurvey.org/fileadmin/docs/D-Book-series/book-05-Conventional-Ammo/SAS-Conventional-Ammunition-in-Surplus-Book-16-Chapter-14.pdf

Pittcon Tracks

Chapter 2 – The Rise in Handheld Devices

The detection of explosive materials is challenging for several reasons. The need for fast identification in the field by a wide range of organizations (government agencies and military in particular) has driven a number of recent advances.

Optical spectroscopy techniques are today seen as the most promising for remote detection capability. In general, determining the amounts of oxygen and nitrogen in a sample relative to the other elements present is a good indicator of whether the compound will be energetic or non-energetic. While the full chemical structures may be complex and difficult to distinguish by traditional chemical analysis methods, explosive materials tend to have an abundance of oxygen and nitrogen, whereas diesel, for example, is a mixture of hydrocarbon chains with no oxygen or nitrogen, so it is not an explosive material.

Optical spectroscopy techniques can exploit this by determining the ratio of hydrocarbon to nitrogen/oxygen content. Optical spectroscopy can also distinguish known explosive materials within a cluttered background signal, by looking for more specific chemical signatures from fluorescence or vibrational energy modes.

These optical methods must be simple to operate in a non-laboratory setting by a non-expert user. In field-based applications, it is unlikely that there will be a trained chemist present, especially in more hostile environments. The analysis must also be fast, as there will often be an element of time sensitivity to the situation.

There is a wide array of techniques under development for detecting bulk qualities of explosives, such as terahertz imaging or x-ray analysis, which have seen use at airports and important government buildings. However, the equipment required for these techniques is bulky, expensive, and of limited use for detecting trace quantities of material. X-rays, in particular, are a highly energetic ionizing form of radiation, making bulk x-ray analysis impractical for safe use in the field. Terahertz radiation is non-ionizing, but is susceptible to atmospheric contaminants, with a particularly unfortunate strong absorption band present for water vapor, which has a tendency to mask other signals.

Lower energy optical techniques, including Raman, Laser-Induced Breakdown Spectroscopy (LIBS), Laser-Induced Fluorescence (LIF), and Fourier Transform Infrared Spectroscopy (FTIR), have all seen advances towards handheld use in recent years, as have mass spectrometry techniques such as Ion Mobility Spectrometry (IMS) and Differential Mobility Spectroscopy (DMS).

Mass Spectrometry (IMS, DMS)

These techniques make use of the ability to separate ions based on their mass and charge. In mass spectrometry, a sample is bombarded with electrons so that its molecules will break into fragments, which can be separated by their mass to charge ratio. The specific separation technique will vary depending on the phase of the ions (solid, liquid or gas), amongst other factors. One specific technique, which has seen growing use in airports in recent years, is IMS, a mass spectrometry technique for the gas phase which relies on the mobility of ions in a carrier gas. DMS is a slight variation on this concept, where high electric fields are used to filter out certain types of ions. In both of these techniques, the choice of carrier gas is crucial for an effective analysis.

One reason that mass spectrometry based techniques are popular for explosive detection is their speed and accuracy. They can measure trace amounts of material, as low as picograms (10-15 kg), putting them on a par with optical techniques such as LIBS. However, proximity is an issue; they require samples of materials to be inserted into the spectrometer unit. This limits the applications to more controlled situations, such as airport security, rather than broader field applications where detection at a distance is highly desirable.

Optical Spectroscopy (Raman, LIF, LIBS, FTIR)

To date, many optical techniques have been adapted into portable form factors suitable for standoff explosive material detection. Some examples include Laser-Induced Breakdown Spectroscopy (LIBS), Raman Spectroscopy, Laser-Induced Fluorescence (LIF), Cavity Ringdown Spectroscopy (CRDS), and Photofragmentation followed by Resonance-Enhanced Multiphoton Ionization (PF-REMPI), amongst many more. We shall look in detail at case studies using some of these techniques in the next chapter. They each have their own advantages and disadvantages; Raman for example may be suitable at stand-off distances of over 10 m, although long integration times hamper the technique.

The progress towards advanced handheld devices with the detection capability of full-sized lab instruments is accelerating, and Pittcon 2017 is an ideal place to observe the latest trends. For those interested in the latest advancements in X-Ray detection, Rigaku are exhibiting and will no doubt show their latest X-Ray detectors and optics at the Expo. There are symposia and posters across the entire expo related to both mass spectroscopy and optical techniques as well as several short courses.

References

1. J. Akhavan, “The Chemistry of Explosives”, RSC Publishing, 2011

2. J. I. Steinfeld et al, “Explosives detection: A challenge for physical chemistry,” Annual Review of Physical Chemistry, 49, pp. 203-232, 1998

3. J. F. Federici et al, THz imaging and sensing for security applications-Explosives, weapons and drugs,” Semiconductor Science Technology, 20, pp. S266-S280, 2005

4. http://www.chemguide.co.uk/analysis/masspecmenu.html#top

5. https://www.agilent.com/labs/features/2011_101_spectroscopy.pdf

Chapter 3 – – Experimental Case Studies

Laser-Induced Breakdown Spectroscopy (LIBS)

In this first spectroscopy technique, a highly energetic laser pulse is employed to excite sample residues to the point of breakdown into their constituent parts, forming a microplasma. The light emitted from this can be compared to standard elemental and molecular spectra to determine makeup of the residue.

LIBS is a surface characterization technique, i.e. just a very small amount of material is ablated from the surface. Measurement can take place very quickly – there is an initial broad continuum of emitted radiation lasting a few milliseconds, which needs to be gated out, but the characteristic elemental spectra begin to appear very rapidly, allowing strong signals to be gathered in typically just a few seconds.

Historically, LIBS has been used primarily to test for the presence and relative quantity of individual elements, i.e. metals. Today however, high-resolution broadband spectrometers have enabled detection of multiple elements and compounds from the same sample. In theory, any material may be detected; not just explosives, but any form of matter, and LIBS is now used in a diverse range of applications, from metals to plastics and even biological samples. It is envisaged, with suitable optics or a mechanized setup, LIBS could scan over large areas of a sample to create an elemental map.

The benefits of LIBS are numerous. It is a straightforward technique, with no prior preparation of samples required. This also makes the equipment relatively cheap to setup and operate. The system can be miniaturized, as there is little in the way of complex optics, making it easily adaptable to handheld instruments. The sensitivity of LIBS is high enough to detect trace quantities in a short time (< 1 second, a single laser shot is employed with no long integration times). The small amount of ablated material means the technique is minimally destructive. A drawback associated with this is a lack of depth analysis and the technique being prone to surface contamination, however this may be mitigated by firing several ‘test shots’ of the laser prior to analysis. Work by Jennifer Gottfried and colleagues at the Army Research Laboratory has focused on optimizing LIBS for detecting explosive residue. They suggest that the ability of LIBS to breakdown and analyze a sample with a single laser shot is highly beneficial for explosive detection. As it is a purely optical technique, LIBS is ideal for standoff detection, with the potential for very long distance ranges with the incorporation of telescopic optics. Building on work by researchers for organic material detection, Gottfried’s team took the ability to identify carbon, hydrogen, oxygen and nitrogen atomic emission lines from complex compounds and applied it to military grade explosives. In explosive materials, the ratio of nitrogen and/or oxygen to hydrocarbons is high, which is a useful measurable characteristic to distinguish between a dangerous organic compound such as RDX or TNT and something more benign such as nylon. However, it should be noted that the contribution from the atmosphere cannot be ignored (air is largely made up of nitrogen and oxygen). To overcome this in a lab environment, an inert atmosphere such as argon may be used, although this approach is far from ideal for standoff detection in the field. Again, several laser shots in quick succession may be the best way to overcome this issue. A modified method known as double pulse LIBS was employed by these researchers, using collinear nanosecond pulses to first impact the material and displace the surrounding gas, creating an area of reduced pressure. By the time the second pulse hits, the contribution from atmospheric gases is minimized. In this study, a LIBS system was utilized with a 1064 nm, neodymium–doped yttrium aluminum garnet (Nd:YAG) laser, emitting 320 mJ, 8 nanosecond pulses. Power at the focal point was under 1 GW/cm2 to produce the microplasma, with the double-pulse setup allowing for a complete measurement to be taken in less than one second. Lenses were used to defocus the resulting plasma spark onto a fiber optic bundle of seven fibers, for complete analysis by a broadband spectrometer over the 200-980 nm spectral range. A 1.5 microsecond gating delay was used to allow the plasma to begin cooling. A wide array of explosive materials was surveyed via standard LIBS: RDX, HMX, TNT, PETN, NC, C-4, M-43, LX-14, JA2 and A-5. Subsequently, double pulsed LIBS (each pulse 160mJ separated by 1-10 microseconds) analyzed RDX, mixtures of RDX and TNT as well as RDX contaminated with diesel fuel to try and distinguish these materials. It is worth noting that the laser-induced microplasmas never initiated detonation of the materials, as the short-lived spark is an insufficient ignition source. This is something to be aware of when working with energetic materials, however, and laser intensities must be managed sensibly. Initial results of the single-pulse LIBS analysis showed successful identification of materials such as charcoal over 95% of the time. Double pulse tested LIBS also distinguished the explosive materials well, and is perhaps the more promising technique for standoff detection, thanks to its ability to reject atmospheric background signals without the need for argon flow. The Army Research Laboratory have been working on a promising advancement of this technology known as ‘suitcase LIBS’, with the laser and detector contained within a hand-held wand and laptop/spectrometer fitting in to a suitcase. Weighing only 9 kg (20 lbs), this shows much promise for employing LIBS for standoff detection in the field. At Pittcon this year, there will be several relevant talks, as well as live demonstrations by world leading spectrometer and optics companies. For example, Ibsen Photonics will be giving a live demonstration on ‘How to select the right spectrometer’ a valuable insight especially for broadband applications such as LIBS. There will be an entire symposium dedicated to LIBS, as well as a half-day course for those wishing to learn more about this technique, entitled ‘Elemental Analysis via Laser Induced Breakdown Spectroscopy and X-ray fluorescence’.

References

1. J. L. Gottfried et al. “Detection of energetic materials and explosive residues with laser-induced breakdown spectroscopy: I. Laboratory measurements,” Army Research Laboratory Documentation, 2007

2. R. Noll, “Laser-induced breakdown spectroscopy fundamentals and applications” Springer, 2012

3. K. L. McNesby et al, “Applications of vibrational spectroscopy in the study of explosives,” Wiley, 2002

4. J. L. Gottfried et al, “Double-pulse standoff laser-induced breakdown spectroscopy (ST-LIBS) for versatile hazardous materials detection,” Spectorcimicha Acta, Part B, 62, 12, pp. 1405-1411, 2007

Raman Spectroscopy

A second optical based detection method, Raman Spectroscopy, works on the principle of the Raman effect, where incident light is inelasticity scattered and shifted in frequency due to specific molecular vibrations. Light may be scattered in three ways: elastically (known as Rayleigh scattering), or through two inelastic methods whereby the incident photon induces a dipole within the molecule, causing a transfer of energy. If the molecule absorbs energy, the scattered photon ends up with less energy – this is known as Stokes Raman scattering. Conversely, Anti-Stokes scattering is when the molecule loses energy to the scattered photon.

Raman should not be confused with light emission from an excited energy state (fluorescence or phosphorescence). The light is merely scattered, rather than absorbed and re-emitted.

The benefits of Raman spectroscopy include the ability to measure at a safe standoff distance, and the ability to detect small concentrations of material at low vapor pressures with significant optical and chemical interference. It is non-invasive, can analyze solids, liquids or gases without sample preparation and can yield direct molecular structure information for identification of potential explosive materials.

Many Raman signatures of explosives are well known. The challenge comes in obtaining discernable spectra of trace amounts of material at a safe standoff distance of a few to hundreds of meters, quickly and in a contaminated and changing environment. Often for long standoff measurements pulsed lasers are used as an excitation source with large scale optics to detect the scattered signal.

Work by Diana Smirnova and colleagues at Caltech has investigated explosive materials through Raman Spectroscopy, utilizing the unique quality of these materials of decomposition during heating. Although this sounds like a dangerous prospect, the researchers were careful to select a suitable temperature range – particularly for a heat sensitive explosive like TNT, caution is always recommended! As a bonus, any surrounding material should not interfere in the analysis, as it should be relatively unaffected by heating. This type of spectroscopy is known as ‘two-dimensional correlation spectroscopy’ where a variable is shifted and two dynamic Raman spectra are correlated. They proposed that during heating, Raman peaks for explosive materials would decrease, while decomposition products would increase.

Various experiments were undertaken to analyze Stokes lines (these have a much higher intensity than Anti-Stokes at room temperature as it is far more likely a molecule will be in its ground state rather than excited) of these explosive materials. For standoff measurements, a frequency doubled 532 nm Nd:YAG laser was used at a distance of 9 m from the samples. This wavelength is carefully chosen; Raman can be performed in the infrared, visible or UV range, with corresponding trade-offs between sensitivity of scattered signal, spatial resolution and fluorescence suppression. Thermal modulation is achieved by successive infrared pulses from a CO2 laser. Military grade explosive materials (RDX, HMX, PETN and TNT) were investigated, first by measuring the 1D spectra, followed by 2D analysis. To make the study realistic, a contamination study was performed, mixing samples with saliva and urine amongst other potential contaminants.

The results of this study showed that this 2D correlation spectroscopy technique is indeed successful, perturbing the spectra of the measured explosive residues, and distinguishing it from the static background contaminants, hence reducing the chance of false positive measurements in Raman analysis of explosive materials.

One of the drawbacks of Raman spectroscopy is that the signals can be relatively weak, compared to other spectroscopy techniques based on fluorescence or absorbance. The synchronous spectra from the 2D correlation spectroscopy technique proved useful to help combat this, due to the higher signal-to-noise ratio for strong peaks. It was not expected that the asynchronous spectra would yield much interesting information – however, due to the crystalline nature of the explosive materials under study, lattice separation did produce a Raman signal which contributed to distinguishing the samples from the non-crystalline contaminants.

Further work is also being undertaken by the researchers to optimize the optics and reduce long integration times, to make this technique truly portable and rapid in the field. Large area analysis is still a concern with the pulse repetition rate (10 Hz) not enough currently to make this practical. Further studies also aim to analyze other non-traditional explosives similar to fertilizer, which would be particularly significant as the lack of characteristic chemical structures in these materials pose a great challenge for spectroscopic techniques.

Exhibiting at this year’s Pittcon are Biotools, Inc, who will be performing a live demonstration of high sensitivity and fast results with a handheld Raman device. B&W Tek will be demonstrating their i-Raman Pro technology for portable real time monitoring. Metrohm USA will also demonstrate their Raman capabilities at the Expo. Renishaw, the manufacturer of the spectrometer used in this work, will also be exhibiting at Pittcon 2017.

References

1. http://thesis.library.caltech.edu/3476/4/Ch2_explosives.pdf

2. www.jieddo.mil

3. C. Bauer et al, “Potentials and limits of mid-infrared laser spectroscopy for the detection of explosives,” Applied Physics B-Lasers and Optics, 92, 3, pp. 327-333, 2008

4. N. F. Fell et al, “Characterization of Raman spectral changes in energetic materials and propellants during heating,” Journal of Raman Spectroscopy, 29, 3, pp. 165-172, 1998

5. M. A. Czarnecki, “Interpretation of two-dimensional correlation spectra: Science or art?,” Applied Spectroscopy, 52, 12, pp. 1583-1590, 1998

Laser-Induced Fluorescence (LIF)

Laser-Induced Fluorescence (LIF) is a spectroscopic technique whereby a molecule is excited to a higher energy state by the absorption of laser light. Spontaneous light emission will follow when the molecule drops back down to its ground state. The difference between fluorescence and Raman spectroscopy is that in LIF, light is absorbed by the molecule and a unique photon is emitted, rather than the interaction and scattering due to vibrational energy which Raman spectroscopy makes use of.

The preparation of explosives, particularly non-professional or improvised devices, can often leave measurable quantities of residue on surfaces during preparation, transport or other forms of handling. Even if a person is not directly handling explosive material, there is enough transferred material, for example from one person to a door handle, and then a second person touching that residue, for a quantifiable trace to be identified.

Work by Charles M. Wynn and colleagues from MIT aims to develop a specific adapted LIF technique capable of detecting these residues rapidly and at a standoff distance of at least ten meters. They suggest that transfer mechanisms across a variety of conditions usually yield at least 1 ug/cm2 of material, and set out to detect explosive material in this quantity. It is clearly preferable to discover explosives during the preparation of material, before there is an actual bomb in a dangerous and volatile situation. Wynn’s team therefore aimed to develop a technique that could be used to forensically detect explosive making activities within a relatively large area, negating the need for higher-risk activity by security of law enforcement forces further down the line.

The proposed technique involves photodissociation followed by Laser-Induced Fluorescence (PD-LIF), whereby the first step of the analysis is dissociating polyatomic material into diatomic molecules. As discussed in the previous example, high explosives such as TNT and RDX tend to be nitrogen-rich molecules with similar structures, which will dissociate when illuminated with UV light. The dissociation products are very characteristic of these materials – one key fragment being nitric oxide (NO) which has very well defined spectra. Although NO can be present in background quantities as an atmospheric pollutant, the difference here is that it dissociates with excess vibrational energy (rather than being in its ground state), and hence is clearly distinguishable from surrounding contaminants.

After dissociation, a second photon (from the same laser pulse) of the same wavelength is absorbed. Importantly, this happens within a few nanoseconds, within the lifetime of a vibrationally excited NO molecule. This will pump the NO to an excited electronic state. Finally, a photo-detector is setup to detect what should be a slightly shorter wavelength (higher energy) UV beam returning from the sample (if it is indeed dissociated rather than surrounding contamination), indicating the presence of a nitrogen based explosive.

This technique differs subtly from LIBS (subject of a previous case study) in that LIBS dissociates material into constituent atoms by creating a microplasma from a high energy laser pulse, whereas PD-LIF utilizes lower energies to break the sample into molecular components. This provides several advantages over LIBS, in that a lower power eye-safe laser may be used. Also, LIBS is susceptible to significant contamination from nitrogen and oxygen present in samples from contaminants or simply the surface it sits on.

Other benefits of this technique include a strong returning signal and strong specificity. In this work, a laser pulse of 236.2 nm is used, and returning light from the NO has a wavelength of 226 nm. Even if there is surrounding contamination that absorbs in the UV range, it is extremely unlikely for it to fluoresce a precise 226 nm photon. It is unusual for a fluorescence process to emit higher energy radiation than the excitation energy. This means that the availability of vibrationally excited NO molecules immediately prior to further electronic excitation is extremely unlikely to occur, unless nitrogen-rich explosive materials are present.

LIF also benefits from very low ‘optical clutter’, as it operates in the UV spectrum below 300 nm known as the ‘solar-blind region’, where absorption due to atmospheric ozone prevents light reaching earth. This is a distinct advantage over infrared spectroscopy techniques, such as Raman, where filtering of this clutter is paramount for good signal to noise. The only filter needed for LIF is an optical narrowband filter to filter reflected light of the same wavelength.

Experiments were performed on DNT, TNT, PETN and RDX (all nitrogen-based explosives). When comparing to a spectrum for NO, it is clear for all samples that the returning signal is from the vibrationally excited (and hence dissociated) NO. Further experiments were then performed on RDX and TNT dissolved in acetone to yield a concentration of 2 ug/cm2 to emulate trace level detection. This too was successful, and showed a good signal to noise ratio when compared to a control sample of a bare silicon wafer.

Work is currently being undertaken to scale up the laser and optics for large area observation. The authors estimate at least 5 mJ laser pulses are required, with a much higher pulse repetition rate than the current 30 Hz, to enable large area scanning. Colleagues at the Lincoln Laboratory’s Laser Technology and Applications group are designing a laser specifically for this purpose.

There will be many companies exhibiting at Pittcon this year who make high quality lasers and spectrometers. Avantes will be exhibiting their range of spectrometers, lasers, and other components for fluorescence analysis, and high-performance OEM laser supplier Cobolt will also be an exhibitor at this year’s event. There is a specific oral session at the conference dedicated to various fluorescence and luminescence techniques as well as a poster session. A short course entitled ‘Analytical Spectroscopy Methods: Absorption, Fluorescence, Raman and SERS’. Will surely be of interest to participants wanting to learn more about this developing technology.

References

1. https://www.ll.mit.edu/publications/journal/pdf/vol17_no2/17_2_1Wynn.pdf

2. P. Andersen, “Laser induced fluorescence,” book chapter in “Optical Measurements” Springer, 2001

3. Steinfeld et al, “Explosives detection: A challenge for physical chemistry,” Annual Review of Physical Chemistry, 49, pp. 203-232, 1998

4. Nagli et al, “Absolute Raman cross-section of some explosives: Trend to UV,” Optical Materials, 30, 11, pp. 1747-1754, 2008

Conclusion

Laser-Induced Breakdown Spectroscopy (LIBS)

In summary, identifying explosive material is a challenging task which is not getting any easier. As soon as new techniques are developed, it seems new ways to disguise explosives from them come along. However, advances in spectroscopy techniques and the rise in handheld devices shows promise for a safer future in this field.
There is clearly not a ‘one size fits all’ approach for explosive detection. Techniques such as X-Ray imaging and mass spectrometry have found a niche in airport security, where proximal sampling is reasonably safe to carry out. Optical techniques are finding many ingenious applications in more hazardous environments, where standoff detection is required.

In the case studies above, researchers have shown it is possible to adapt and modify well known techniques to improve accuracy and portability. For example, a double pulsed LIBS system is shown to mitigate some of the traditional issues with surface contamination, Raman is enhanced using 2D analysis, and by exploiting the decomposition of explosive materials during controlled heating. Finally, LIF can be enhanced by an additional laser pulse to photodissociate molecules prior to analysis.

Work is continuing both in academic and industrial environments to miniaturize these techniques further and increase the standoff detection distances, and Pittcon 2017 will feature many experts in this field. Dozens of companies will be exhibiting, including Thermo Scientific, Rigaku and Metrohm. There will be many symposia and short courses to enhance knowledge and network with like-minded colleagues, making Pittcon 2017 a must attend event for researchers all across the scientific spectrum.