Military personnel face a number of threats on the battlefield. Chemical and biological attacks are some of the most insidious threats since they are silent and hard to detect and can have devastating effects. Chemical and biological weapons have been referred to as "the poor man's atomic bomb" because of the low cost and relative ease of fabrication using materials and supplies that are used extensively for civilian purposes and thus easily accessible. Despite the Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on their Destruction (1) signed by many nations in 1993, there is evidence that some countries are fabricating and stockpiling such weapons.  

Because of their ease of fabrication and use, chemical weapons might also be used by terrorist cells. The tools to detect chemical attacks are thus very relevant for homeland security. In fact, the threat of chemical attacks is more likely to come from nonmilitary-related toxic industrial chemicals (TIC) because of their availability and low cost. As a consequence, the requirements for modern chemical agent detectors now also include a long list of TICs.  

The ideal chemical agent sensor must detect, identify, and ultimately quantify several chemical species (chemical warfare agents [CWA] or TICs) in the areas interrogated. Because of the lethality of the target species, the ideal chemical sensor must operate remotely, up to 5 km from the threat and in near real time, to provide the alert in about 1 min. Chemical agent analysis requires identification capability, to select the course of action (for example, to identify what type of suit to don) and spatial localization capability, to determine where the threat is present. With the localization and meteorological information, one can predict when the threat may reach populated areas, what actions to take, and where not to send troops (contamination avoidance).  

Given this great need, various technologies are already in existence for the detection of CWA and TICs. The most developed and deployed solutions are point sensors (2) that operate using several techniques, including ion mobility spectrometry, conventional infrared absorption spectroscopy, colorimetric chemistry, and flame ionization. These sensors can be characterized as sniffers and operate on the principle of sampling the surrounding air to analyze its constituents, searching for chemical agents. While most of these techniques can be very sensitive and good at differentiating various chemical species, they lack the capability to detect chemicals remotely. This leads to deployment problems when large areas need to be protected and to difficulties of decontamination after an event has occurred. One can thus understand why the development of remote sensors has been supported for decades to complement point sensors.  

The most mature operational chemical remote sensor is the U.S. Army Joint Service Lightweight Standoff Chemical Agent Detector (JSLSCAD) (3). According to General Dynamics, "The JSLSCAD is a small, fully automatic, passive standoff chemical, nerve, blister, and blood agent vapor detector. The sensor is a portable sensor system capable of detecting chemical clouds from a fixed site (tripod) as well as 'on-the-move' mounted on a vehicle."  

The JSLSCAD sensor is a spectroradiometer operating in the thermal infrared region of the electromagnetic spectrum. This passive method of chemical analysis relies on the unbalance of emission and absorption of chemical species in the field of view of the instrument. If the gas is warmer than the temperature of the background, the spectral bands of the molecules in the gas appear in emission, while the same bands are seen in absorption when the gas is colder (4). This technique is analogous to the conventional laboratory infrared absorption spectroscopy except that in the case of passive spectroradiometry, the source of radiation is the background or the gas itself. The JSLSCAD sensor features a single field of view that is scanned 360° in the surrounding environment using an optical–mechanical scanner. When a gas is detected, the sensor applies verification algorithms based upon spectral information to identify the chemical species and confirm the presence of the threat.  

Although current chemical remote sensors have proven their value, there is still room for improvement given the critical need for detection. There exists a consensus that more or better spectral–spatial–temporal information would lead to better detection. In particular, it is believed that the gathering of several fields of view (that is, imaging) will improve the quality of the detection. Imaging is now included in the roadmaps of future generations of chemical remote sensors (5). In theory, a single-pixel sensor can be configured to do imaging, but because of the overhead of the dead time between spectra, the use of single-pixel sensors is quite inefficient at collecting high-resolution images. Imaging spectrometers with large focal plane arrays operated in the staring mode are approximately 10–100 times more efficient at this task.  

Requirements for Remote Chemical Agent Detectors

The requirements for chemical agent detectors usually focus on the list of agents, threshold concentrations, probability of detection, time for detection, and probability of false positives. See for example the specification for the "Commercial joint services lightweight standoff chemical agent detector" (6) issued by Joint Program Executive Office for Chemical and Biological Defense.  

System modeling is necessary to flow down these agent requirements to instrument requirements. This operation usually is performed by the provider of the solution and requires fairly complex radiative transfer models. The light reaching the sensor is governed by the emission and absorption of all gas layers in front of the target gas as well as the emission from the background behind the target (4).  

The emission of any material is given by its emissivity times the Planck function, while absorption in a gas is modeled by Beer's law (7). Both Planck's relation and Beer's law are nonlinear functions. It is possible to implement numerical algorithms (8,9) to solve this radiative system by coding the full nonlinear physical model and an iterative solving procedure.  

To simplify the calculations, it is also possible to linearize the problem. The brightness temperature difference between the spectrum measured in the presence of a gas and the spectrum measured in the absence of the gas can be expressed by equation 1, assuming the gas concentration is low, which is expected for detection close to threshold (10).  

			α is the gas absorption coefficient (m

Once this linear assumption is made it becomes possible to exploit the wealth of existing methods developed in the field of chemometrics and multivariate analysis. Several researchers have developed efficient methods (11,12) in order to retrieve simultaneously varying quantities from complex observables such as spectra data. The choice of a detection algorithm among the various methods is generally based on three key factors: the composition of the analyzed pixel, the type of model used to estimate the variability of the target and background spaces, and the model used to describe the mixed pixels. Several examples of retrieval can be found in the literature (10,13,14).  

The spectral resolution is an important instrumental parameter because it defines the capability of the instrument to resolve the various spectral features of the chemical species of interest. For gases around room temperature and pressure, the FWHM of individual gas lines is dominated by pressure broadening and is on the order of 0.1 cm

  (15). However, it is not necessary to resolve individual lines but rather to choose a spectral resolution in which most species can be distinguished from one another. Figure 1 shows the spectral signatures of nine gases at a spectral resolution of 4 cm

  (7.7–1.8 μm) spectral range. These gases are spectrally representative of CWA signatures. For example, it is clear that the TEP and DMMP signatures are quite similar around 1050 cm

. A certain spectral resolution must be utilized for the DMMP shoulder at 1075 cm

  to be resolved and usable to discriminate against the TEP measurements. Similarly, a certain spectral resolution must be utilized to correctly represent the sharp CH

  to distinguish this gas from the ozone, which otherwise displays a very similar spectrum.  

The motivation for utilizing a coarser spectral resolution is to improve the signal-to-noise ratio (S/N) of the measurement. Equation 2 describes how the noise equivalent spectral radiance (NESR) varies with spectral resolution Δσ. For an equal observation time 

, the NESR is inversely proportional to the spectral resolution Δσ. For example, the NESR at 4 cm

  should be four times higher than at 16 cm

. Consequently, choosing an unnecessarily fine spectral resolution adversely affects the sensitivity of the instrument.  

				where NEP(σ) is the noise equivalent power (W/ √Hz)

 			AΩ is the instrument optical throughput (cm

The spectral resolution chosen for most passive FTS chemical agent detectors ranges from 4 cm

. The coarser spectral resolution is typically used for searching and detecting without definite identification. The finer spectral resolution is typically used for more detailed spectral analysis and identification of gases on interest.  

The spectral range of chemical agent detectors is chosen with three criteria in mind: 					

To be a remote sensor and detect over long distances, the instrument must use a spectral range in which the atmosphere is transparent. The atmosphere absorbs in several regions of the electromagnetic spectrum, mainly because of water vapor and carbon dioxide molecules. The chemical agent detector must then operate in the so-called atmospheric windows where the absorption is low and the transmission is high. Figure 2 shows the conventional 3–5 μm MWIR window and the 8–12 μm LWIR window over 1-km and 5-km path lengths.

The spectral range obviously must coincide with the location of spectral features of the chemical species of interest. In analytical chemistry, the region from about 850 to 1300 cm

  (7.7–11.8 μm) is called the fingerprint region. The bands in this region originate from vibrational modes, resulting in a rich but unique absorption pattern. It is thus possible to differentiate between most compounds by measuring this region. Figure 1 illustrates this statement well: each gas is quite distinguishable from the others. Other spectra regions, including the 3–5 μm atmospheric window, feature less usable bands and are used much less often. 

The spectral range also must be chosen to maximize the S/N. Because the driver in a passive chemical measurement is the temperature contrast between the gas cloud and the background, it is more appropriate to express the noise of the sensor using the noise equivalent temperature difference (NeDT) instead of the noise equivalent spectral radiance (NESR). Assuming typical values of NESR = 2 nW/cm

  for a sensor operating in the MWIR (3–5 μm) and NESR = 10 nW/cm

  for a sensor operating in the LWIR (8–12 μm), we calculate the corresponding NeDT for a reference emitter at room temperature (300 K) as shown in Figure 3. The MWIR range rapidly degrades toward the shorter wavelengths and requires a larger temperature contrast to yield the same S/N. The temperature contrast varies greatly according to the environment and the viewing geometry. On sunny days, the objects are heated while the sky (viewed directly or through diffuse reflection) is very cold, yielding good contrast. On cloudy days or at night, the gradients are smaller. In normal environments the temperature gradients vary from 0.2 to 2 K. For such environments, Figure 3 demonstrates that the MWIR S/N would be insufficient except for a very narrow spectral range. The situation is even worse for other atmospheric windows at shorter wavelength than the MWIR. Atmospheric windows at wavelengths longer than the LWIR also exist, but good detectors for these ranges are not practical to use in the field.  

The published requirements for detection response time range from a fraction of a minute to 2 minutes (6). These specified times must be shared among all of the sensor views performed to cover the field of regard.  

Figure 4 shows the temporal evolution of a field release of SF6 (purple cloud) at a rate of 1.25 kg/min and demonstrates that the temporal evolution of a gas cloud is relatively slow on a field of regard shown (6.4° horizontal). The time interval between frames is 6.2 s. In the horizontal direction, the cloud is moving with the wind at a speed of approximately 5 m/s. The diffusion of the cloud in the vertical direction is approximately 1.3 m/s. It is interesting to note the presence of gas swirls indicative of turbulence.  

Although the clouds move relatively slowly, the movement is fast enough and the shape of the plume changes so that any raster scanning must be extremely rapid to adequately build an accurate representation (imaging) of the plume.  

The sensitivity is the cardinal requirement for any passive remote chemical agent detector and is usually represented by the NESR. The NESR, described in equation 2, represents the minimum radiance detectable by the sensor (that is, the radiance for which the S/N is equal to 1). In typical passive chemical agent sensing scenarios, an NESR of 5–10 nW/cm

  meets the detection and identification requirements for most chemical agents. This is also visible in Figure 3, considering that the typical temperature gradients are above 0.2 K.  

The clouds generated during chemical attacks are relatively dense and produce fairly large absorption/emission features. The concentration levels specified are integrated column densities in mg/m

Equation 2 shows that the sensitivity variation is inversely proportional to the square root of the observation time. Sensitivity comparison between various instruments must be performed for an equal field of regard (spatial range), for an equal instantaneous field of view (spatial resolution), for an equal spectral resolution, and for an equal observation time.  

Field of Regard and Instantaneous Field of View Requirements

The field of regard is the angular region that the sensor must interrogate. The instantaneous field of view (IFOV) is the angular resolution or the extent of a single pixel. Ideally, the instrument IFOV should be twice as small as the dimension of the cloud. If the instrument IFOV is larger than twice this value, the measurement is impure (a mixture of background and target). In the case of a mixture, the signal is diluted, leading to lowered sensitivity, and the retrieval algorithm is complicated by the fact that it has to perform spectral "unmixing" (12).  

Articles

In this article, the authors discuss the need for protection against chemical attacks and the role of passive imaging spectroradiometers in the detection of remote chemical agents.

Chemical Warfare Agent Spectral Imaging for Real-Time Identification and Localization

I'm sure every person reading this letter can remember exactly where they were and what they were doing on September 11, 2001. The details and specifics of that day have been the subject of many movies, documentaries, and television specials at this point, as have the events that evolved from that day. However, one aspect of 9/11 that does not often make its way into mainstream pop culture is the impact it has had on the scientific community, particularly the field of analytical chemistry. 

Advances in airport-screening technology have exploded, as have advances in portable instrumentation for explosives detection, benchtop instrumentation for chemical weapons identification, and more. The scientific community has made technological leaps at a greatly accelerated pace, and as you may have seen during the media coverage of the new risqué X-ray technology now being implemented in airports worldwide, some outsiders can be downright startled to see some of the technologies that now exist as a result. In many ways, it is as though we have collectively jumped ahead in time, and we are now seeing technological advances that would not have existed if not for that tragic day. 

Which brings us to the reason for this supplement. As always, 

's goal is to bring the latest advances and the most cutting-edge technologies to members of the analytical chemistry community, and once again, we believe we have achieved this. Here in one issue are all of the research leaders in the field of homeland security. From handheld devices that have been compared to the famous 

 "Tricorder" to benchtop instruments right out of an episode of CSI, all of the latest advances in this area are here for your perusal. 

It is a certainty that homeland security technologies will continue to advance at their current staggering pace, and 

 will be there to cover those advances as they happen. So please feel free to contact us, submit a technical article, or simply send an e-mail with your feedback. The next technological leap is always right around the corner. 

From the Editor

Editor-in-Chief, David Walsh, weighs in on the impact of homeland security.

Chemical warfare is defined as "Warfare and associated military operations involving the employment of lethal and incapacitating munitions and agents, typically poisons, contaminants, and irritants." This definition only covers deliberate use of agents and excludes most of those used legally, such as herbicides, smoke, and flame. An exception to this is the use of riot-control agents during warfare, which is now forbidden (1,2). 

Chemical warfare has been used since ancient times. The "modern" era of chemical weaponry began in World War I (WWI) with blister agents such as mustard gas and continued with the advances in synthetic organophosphate chemistry during the development of insecticides in the 1930s. 

Since then, thousands of chemicals have been screened as weapons. The current list of chemical warfare agents (CWAs) is divided into five groups, based roughly on mode of action: nerve, blister, choking, blood, and lachrymatory. 

Typically, CWAs are liquids, or vapors dispersed as aerosols or gas. They can enter the body via the respiratory system or through skin contact. Nerve agents can kill within minutes of inhalation and in under an hour after skin contact. VX (Figure 1) and the Russian equivalent, RVX, are among the most toxic compounds in existence. A 1-L volume of VX can theoretically kill one million people, and less than one tenth of the volume of an average swimming pool could wipe out the entire human population (3). Moreover, VX persists on material, equipment, and terrain, remaining deadly for up to three weeks. Table I shows the maximum exposure limits for particular agents and illustrates their extreme toxicity.  

It was quickly realized that the only real prospect for defense against chemical weapons was to ban their use. This prompted international action on a chemical disarmament regime under the 1925 Geneva Protocol. This protocol has limitations, however, including the inability to enforce policing and the fact that it does not forbid retaliation in the case of chemical attack. The Chemical Weapons Convention (CWC) addressed these shortfalls and has now been adopted by over 120 countries. The agreement involves not using or making (or encouraging others to make) chemical weapons. To prove this good intention, all parties agree to inspection of any site where agents might have been used. Another condition was the disposal of all large stockpiles by 2007. While this target was not met, international efforts to destroy agent stockpiles are proceeding at pace, with extensions until 2012 for substantial stockpiles and longer in other cases (4).  

Table I: Maximum limits of human exposure to select CW agents 

Russia and the U.S. possess more than 95% of the Cold War stockpiles of chemical arms. Albania, Libya, South Korea, and India have declared much smaller quantities, and Japan has started elimination of the CW stockpile abandoned in China after World War II.  

Because CWAs are no longer in development, current research is focused on civil defense, personal protection, agent destruction, and antidotes. Many ongoing CWA-related research projects and processes require monitoring of airborne CWA concentrations at the lowest levels, often in highly contaminated environments such as subway stations. Accurate identification can also be critical — for example, in civil defense situations — so that remedial action and antidotes can be deployed rapidly to minimize casualties. Conventional handheld devices are not useful in these situations, as they are prone to false positives and signal the cue for evacuation at levels that are already lethal. Trace-level monitoring of CWAs in complex real-world environments requires speciated and sensitive chemical analysis.  

Thermal desorption was introduced in the early 1980s as a front-end technology for gas chromatography (GC) and GC–mass spectrometry (MS). It involves the thermal desorption/extraction of organic vapors from sorbent tubes using heat and a flow of pure inert gas. After primary desorption, analytes are refocused before rapid secondary desorption and injection into the GC–MS analytical system as a concentrated "band" of vapor. Initial applications for TD-GC–MS included workplace and environmental air monitoring for conventional toxic chemicals. The technique was quickly adapted for agent analysis, both for personal exposure assessment (such as in monitoring personnel manning agent destruction facilities) and for checking airborne concentrations.  

At first, however, TD systems were not compatible with the most demanding CWA, such as VX, nor did they offer sufficient sensitivity to allow detection at the lowest general population limit (GPL) levels. Traditional TD-GC–MS methods for VX, for example, involved derivatization to the "G-analog," using silver fluoride pads in front of the sampling line or sorbent tube. The derivatized compound is more volatile, which improves recovery, but sensitivity and repeatability are compromised because derivatization efficiency can be variable and is less than 100%. 

In contrast to this approach, the latest chemically inert TD technology can be used for direct analysis of free-VX. Figure 2 shows the range of CWAs that can be analyzed simultaneously using one such system. 

Figure 2: Analysis of mixed CWA standards (5 ng level) using TD-GCâMS. 

On-line and off-line TD-based air monitoring protocols have now been developed for CWA analysis. On-line (near real-time) monitoring is performed at fixed installations such as storage bunkers and in mobile installations such as first-responder vehicles (Figure 3). Mobile laboratories can also be positioned near key public buildings for surveillance. 

TD platforms for on-line monitoring typically comprise two reciprocally operated focusing traps. One samples airborne contaminants while the other is desorbed and analyzed, and vice versa (Figure 4). This "twin trapping" feature allows continuous near-real-time monitoring without data gaps. 

Figure 4: Twin reciprocally operated focusing traps for continuous monitoring. 

On-line monitoring is needed at military installations, destruction facilities, and key public buildings as first-line defense against a potential terrorist attack. Example data obtained on-line are shown in Figure 5. 

Figure 5: CWA (sarin) monitoring using online thermal desorption with GCâflame photometric detection. 

Off-line monitoring is used for personal exposure assessment (for example, of military personnel guarding storage bunkers), confirming on-line data, and for research into protective equipment, decontamination procedures, and so forth. Off-line TD systems operate in two stages, with an initial larger sorbent trap/tube being used for maximizing air sampling flows and volumes and a secondary electrically cooled refocusing trap capable of fast desorption and injection of analytes into the GC–MS analyzer for optimum sensitivity. Figure 6 illustrates off-line detection of free-VX at typical GPL levels using an off-line TD-GC–MS system. 

Figure 6: Off-line analysis of free VX from sorbent tubes. Inset shows linearity over a wide concentration range. 

Figure 6 shows free-VX linearity over a wide concentration range (inset) combined with excellent repeatability at low picogram levels (see overlaid chromatograms). This illustrates the reproducibility and effectiveness of modern TD-GC–MS systems for this very challenging application. 

Technological Developments for Enhanced TD-GC–MS Analysis of CWAs

Many of the most recent developments in TD and associated technologies complement the on-line and off-line monitoring protocols discussed above and offer significant benefits to trace detection and identification of CWAs. Relevant innovations include 

Electronic tagging of sorbent tubes and personal exposure monitors for improved traceability. The RFID technology shown in Figure 7, for example, can be automatically written to during field monitoring to minimize transcription errors and is automatically updated during laboratory analysis. 

Figure 7: Electronic tags fitted to tubes as direct replacement for bar codes and manual etching and recording of tube identification numbers.  

Time-of-flight (TOF) mass spectrometers for GC, capable of providing full mass spectral information at traditional selected ion monitoring (SIM) detection limits. This is critical for the elimination of false positives at trace levels in complex sample matrices. The latest compact, benchtop systems offer exceptional mass stability for extended periods (Figure 8). 

Figure 8: Mass stability over extended time periods of a bench-top compatible TOF-MS system for GC.  

A novel approach to GC–MS background compensation that responds dynamically as the analytical background changes. As described elsewhere (5), the new software can accurately distinguish between background and peak-related information even if the same mass ions are present in both. Its use dramatically improves signal to noise (sensitivity) and spectral purity (identification) for trace components (Figure 9) without compromising the original data file. 

Figure 9: Illustration of spectral dynamic background compensation of thiophene. Raw data are shown in the upper panel, and reprocessed data is shown in the lower panel.   

New GC–MS "data mining" software is also under development for enhanced detection of trace target analytes in complex chromatograms. One such novel software approach combines dynamic baseline compensation (5) with spectral deconvolution and pattern recognition. While this approach is not yet available commercially, early results have been encouraging.

Specialized equipment has also been developed for simplifying and extending the vapor sampling process. One example is a micro-chamber/thermal extractor (Figure 10) for testing material emissions and permeation. Multisample units like this interface to standard sorbent tubes and can be used to test the performance of safety equipment and the efficacy of decontamination procedures — for example, to investigate CWA residues on treated and untreated surfaces. 

Figure 10: Example micro-chamber system in use for permeation testing. Inset shows breakthrough of a CWA simulant through a nitrile rubber membrane.   

Mechanically simple approaches to the automation of TD and associated sorbent tube sampling have also recently enabled the development of robust, high-throughput analytical systems and field sampling units. 

The challenge for TD automation always has been to keep sampled and desorbed tubes sealed except during desorption and analysis but without the use of complex robotic procedures such as capping and uncapping, because these can compromise system reliability. The latest automated TD systems employ tube end caps incorporating a "diffusion locking" mechanism. This valve-free innovation prevents ingress of contaminants or outgassing of analytes via diffusion when the tubes are in standby but allows gas to pass when pressure or a vacuum is applied. Figure 11 shows the diffusion locking end caps deployed on sorbent tubes installed in an automated tube sampling device. 

Figure 11: Field portable automated sampling device. 

Quantitative sample re-collection for repeat analysis has also been implemented on some of the latest commercial TD systems and provides a powerful and convenient approach to validating analyte recovery through the thermal desorber (Figure 12). 

Figure 12: Re-collection to illustrate reproducibility in analyte recovery through a TD system.  

Depending on the method of implementation, some modern thermal desorbers allow all or part of any sample split to be quantitatively re-collected on a conditioned sorbent tube after passing through the TD system. A sequence of repeat analyses can thus be performed on a single sample, allowing users to readily monitor analyte recovery relative to split ratio and to other components in the mix. 

The innovations in TD-GC–MS technology for CWA monitoring described in this article have all been implemented over the last decade and have made a significant contribution to improving the performance and reliability of monitoring technology in this challenging application area. 

On-line and off-line monitoring systems are now able to offer much improved data quality at ultratrace levels, which provides military and civilian authorities alike with the tools required to monitor demilitarization processes effectively and to keep us all that little bit safer. 

 are with Markes International, Ltd., Llantrisant, UK.

This article discusses the analysis of a wide range of CWAs at current exposure limits and describes a number of recent beneficial developments in TD and associated analytical technologies for the identification and quantification of CWAs at these levels.

Chemical Warfare Agents and Use of Thermal Desorption–GC–MS to Achieve Improved Trace-Level Detection

Explosives ordnance disposal (EOD) specialists, bomb technicians, forensic scientists, hazardous material (HazMat) technicians, and first responders working in the realm of CBRNE (chemical, biological, radiological, nuclear, and explosives) meet a broad set of challenges in the course of their missions, and consequently require a broad set of tools to assist them in the completion of their duties. Two especially useful technologies in the battle against improvised explosive devices (IEDs) are Raman spectroscopy and Fourier transform–infrared spectroscopy (FT-IR). Recent advances in miniaturization, optical technologies, software development, and packaging have changed the landscape of laboratory-quality analytical devices for explosive identification and put the knowledge — and the instruments — literally in the hands of the first responder who needs them most. 

Tools in the Fight Against Improvised Explosive Devices

IEDs are gaining traction as the weapon of choice for many terrorist organizations worldwide, due in part to the relative ease of production and widespread availability of raw materials. Bombings were responsible for more than half the fatalities and 85% of terrorism-related injuries in 2006 alone. The United States Department of Defense (DoD) spent nearly $4.0 billion in 2008 for the development and fielding of measures to defeat IEDs.  

A key strategy to stop IED production is to locate the toxic industrial chemicals (TICs) and toxic industrial materials (TIMs) that serve as precursors as they are transported through checkpoints. TICs and TIMs are common industrial products that can be mixed, dissolved, blended, or disguised to obscure their true identity. IEDs are not fueled by traditional familiar explosive materials such as TNT, but are made in crude chemical laboratories using industrial chemicals and common household items. One method to stop the trafficking of these precursor materials is to use handheld chemical identification instrumentation in the field at checkpoints and borders. Security coalition partners and law enforcement personnel have demanded accurate, rugged, and portable tools to aid them in the fight against the growing global IED threat.  

Safety and security personnel with a need for field-deployable analytical instruments include the military and civilian first responders responsible for evaluating and countering chemical, natural, and biological threats. These individuals work for U.S. and foreign government agencies as well as state and local agencies, such as fire departments, law enforcement agencies, and forensic laboratories.  

Coalition partners and civilian agencies are working quickly to develop systems to counter the IED threat, equipping the first responders with the right tools to protect themselves and the community. This includes new detection technologies, training, and forensics to quickly identify the threat, find illicit labs, and disable insurgents' capabilities.  

A standard set of instruments used by first responders includes trace detection (vapor and particulates), gas, biological, radiological, nuclear, and chemical identification (Figure 1). The focus of this article centers on solid and liquid chemical identification and recent innovations and advances in this area.  

Figure 1: Common tools in a first responders toolbox. 

In the safety and security market, a fundamental need exists for analytical instruments that can identify unknown chemical substances quickly and precisely at the point of use so that the appropriate course of action can be taken. For these applications, response time and accuracy are critical to save lives, prevent major economic and public disruptions, and aid in resolving emergencies. Instruments located at remote laboratories, while accurate, do not adequately meet the need for a timely response. Portable instruments are sought after to quickly verify the contents of tankers, drums, bags, and bottles at checkpoints.  

First-generation portable chemical identification solutions have been developed during the past 10 years to address the growing need for onsite analysis and have primarily used FT-IR spectroscopy for field identification of dangerous chemical and biological substances. While more suitable than laboratory instruments for these onsite applications, first-generation portable products have some important limitations. Although portable, these instruments are still too large to be handheld, they are not certified for use in the hazard zone, and they require a sample of the unknown substance to be collected by first responders and brought to the instrument. When dealing with an unknown, first responders avoid touching or transporting the substance as much as possible because this creates additional safety risks for the responder and the community. 

Raman and FT-IR spectroscopy are ideal technologies for the accurate identification of liquid and solid TICs and TIMs, as well as numerous explosives and their precursors.  

Analytical instruments are used to detect, identify, authenticate, and quantify chemical and biological substances. One key form of analytical instruments utilizes molecular spectroscopy.   

Molecular spectroscopy involves projecting electromagnetic energy onto a substance and analyzing the resulting interaction. The two major classes of molecular spectroscopy are nuclear magnetic resonance (NMR) and optical. NMR involves directing radio waves at a substance while subjecting it to a magnetic field and measuring the response. Located in laboratories and medical institutions, instruments using NMR are generally large, high-cost instruments used to detect and quantify trace substances.  

On the other hand, optical instruments fall into four categories: ultraviolet–visible (UV–vis), near-infrared (NIR), mid-infrared (MIR), and Raman. Whereas NMR employs radio waves, optical spectroscopy employs different types of energy or light waves. Two forms of molecular spectroscopy well-suited for the identification of solid or liquid explosives are Raman and FT-IR.  

Both Raman spectroscopy and FT-IR spectroscopy measure the interaction of energy with the molecular bonds in a sample of an unknown liquid or solid material. FT-IR measures how much light is absorbed by the bonds of a vibrating molecule (or the remaining energy from the original light source that can be measured after being passed through the substance). In comparison, Raman measures the energy that is emitted or scattered after being excited by a laser. Both techniques are illustrated in Figure 2.  

Figure 2: Comparison of how spectra are measured by FT-IR and Raman spectroscopy technologies. 

FT-IR and Raman spectroscopy are the most precise of the optical techniques, and each offers distinct advantages in specific applications. However, these techniques are distinctly different but complementary methods of spectral analysis. The response of an unknown substance to each technique is dictated by the substance's unique molecular structure, with certain substances responding extremely well to FT-IR and others responding better to Raman spectroscopy. Used together, FT-IR and Raman spectroscopy provide a broader range of unknown substance identification — and better protection for the responder and the community. Figure 3 shows the complementary nature of Raman and FT-IR spectroscopy techniques that are explored in subsequent sections. 

Figure 3: Complementary capabilities of FT-IR and Raman spectroscopy. 

Raman Spectroscopy for Explosive Identification

Raman is a vibrational spectroscopy technique in which a single-wavelength laser is directed toward a sample. The laser energy, which excites the bonds of the molecule, generates or "scatters" measurable light and can be used to identify the material in question. Raman is highly effective at extracting a reliable and accurate identification of an unknown substance based on the underlying chemistry of an unknown substance. Raman is best used in the following scenarios:  

Sealed containers. Raman spectroscopy enables users to collect data through semitransparent containers for both solids and liquids without disturbing the sample.

Aqueous solutions. Water has a very weak Raman signal, which allows Raman instrumentation to virtually disregard water in samples and concentrate on potentially more threatening materials. This makes samples diluted in water easily identifiable with Raman.

White or light-colored powders. White powders with nonpolar covalent chemical bonds have very strong Raman signals. Handheld Raman instruments (such as Ahura Scientific's FirstDefender) make onsite rapid explosive identification a reality. Users are able to quickly and positively identify unknown liquids, solids, and mixtures — usually in less than 30 s. Using a laser to interrogate the sample and a sensitive detector to measure the scattered light, a Raman spectrum is created with a wavelength and intensity that is specific to the sample molecule. 

Instruments collect the molecular spectrum of the substance in question, and onboard chemometric algorithms subsequently identify the chemical composition of the material by matching the acquired spectrum against those stored in the instrument. Some instruments are further able to identify explosive precursors in mixtures. 

Figure 4: Unique Raman spectra of TNP, TNT, and RDX. 

Figure 4 shows the Raman spectra of the explosive threats TNP, TNT, and cyclonite (commonly referred to as RDX). Each material can be uniquely identified based on its Raman spectrum. Another example of Raman explosive spectra is illustrated in Figure 5. 

Figure 5: Raman spectra of TATP, HMTD, and baking soda.  

FT-IR Spectroscopy for Explosive Identification

FT-IR is an absorption spectroscopy technique in which an infrared light is passed through the sample. Some wavelengths may be absorbed, while others merely pass through the sample unaffected. Specific molecular bonds absorb a specific amount of energy, and these losses of energy correspond to the peaks returned during an analysis. 

FT-IR absorptions are strong and provide outstanding and easily interpretable results for substances that contain polar covalent bonds. FT-IR is best used as a potential explosive identification technique in the following scenarios:  

Fluorescent materials. FT-IR is ideal for fluorescent samples.

An illustration of collected FT-IR spectra for Semtex 1H, Detasheet, and black powder can be seen in Figure 6. The sharp peaks make for clear and easily interpretable results. 

Figure 6: FT-IR spectra for the explosive materials Semtex 1H, Detasheet, and black powder. 

In the Real World . . . Mixtures, Precursors, Solvents, and Impurities

Obtaining accurate and actionable information is what first responders and bomb technicians value most. 

Traditional laboratory-grade spectroscopy software commonly employs library searching methods in which a multi-wavelength measure of similarity such as correlation or Euclidean distance is used to rank library items. From a long list of ranked library items, an expert user can more quickly compare "hits" against the unknown and make a final assessment. 

The primary difficulty for field use by non-experts is that the measures of similarity that are most commonly used as a hit-quality index have no probabilistic correspondence. A correlation of 0.96 certainly does not mean a listed library item is 96% likely. To convert any such similarity index into a probabilistic measure would require a definition for both the expectation and the sampling distribution of the statistic, neither of which can be analytically defined for traditional similarity metrics such as correlation or Euclidean distance in spectroscopy. Thus, field use of traditional library search software is hampered by very high "false-identification" rates. 

The embedded analytics in contemporary chemical identification instruments create probabilistic models from their onboard library. The devices are situationally and environmentally aware so that through analytic modeling of the device performance characteristics, probabilistic models are constructed in real time and real probabilities for the basis for the results. The devices directly test for statistical equivalence between the measured material and each library material. The finesse of the calculations results in extremely low false-identification rates (1 in 10,000 [1]) with strong positive identification. These capabilities allow the instruments to present meaningful information that can be acted upon without extensive chemical knowledge. 

Rapid, precise identification of explosives is one of the key tasks for homeland security and public safety personnel, especially with the marked increase of IED usage worldwide. Portable X-ray explosive detection and trained canines add critical capabilities for first responders; however, additional tools are also available to further assist with identification of unknown materials. Instruments that can be used in the field to rapidly and accurately identify various explosives — and their precursors — are essential tools for first responders responsible for the safety and security of the community. Two complementary response techniques for the identification of unknown solid or liquid chemicals are Raman and FT-IR spectroscopy. Each technique offers distinct advantages in specific explosive applications, and when used together, they extend the capabilities of the first responder's toolkit and provide rapid and accurate results on which a responder can rely, enabling more rapid incident response times. 

 is Vice President, Product Management and Marketing, Ahura Scientific, Inc., Wilmington, MA.  

(1) C.D. Brown and R.L. Green, "Performance Characterization of Material Identification Systems." SPIE Proceedings Optics East 2006, Boston. 

This article discusses instruments that can be used in the field to rapidly and accurately identify various explosives and their precursors.

Portable FT-IR and Raman Spectroscopy for Explosives Identification

The ability to detect chemical warfare agents (CWAs) and toxic industrial chemical (TIC) threats in near real-time in the field can be extremely beneficial and is finding broader utilization for a number of applications by military personnel, emergency responders, and environmental scientists. To be effective, such detection technologies must address the criteria shown in Table I for making field measurements with portable instrumentation. Along with advances toward better sensitivity, faster analysis times, and affordability, detection technology is being miniaturized for hand-portability and reliable detection under harsh field environments. Faster speeds of analysis are key to providing the user with instantaneous data to answer time-sensitive questions immediately on-site.  

Table I: Criteria for field detection of CWAs and TICs			

Technologies that are currently used for field detection of CWAs and TICs include dye solubility (detection paper), enzymatic reaction, gas–solid phase reaction (Draeger tubes), surface acoustic wave (SAW), infrared spectrophotometry, Raman spectroscopy, flame photometry, and ion mobility spectrometry (IMS) (1–4). Chemical detectors based upon these technologies, while meeting the criteria of being portable and relatively easy to use in the field, offer only limited chemical specificity and sensitivity and are prone to false positive responses (3). They often are referred to as one-dimensional sensors that can only confirm what is already believed to be present, but cannot provide information about other possible harmful agents (1,5).

Miniaturization of mass spectrometry (MS) for field measurements has been pursued intensively over the past decade. Efforts to develop small, portable mass analyzers have focused on various types and formats of devices, including time-of-flight MS (6), electrostatic-magnetic sector MS-MS (7), cylindrical ion traps (8,9), rectilinear ion traps (10,11), toroidal ion traps (12,13), and halo ion traps (14). Ion traps are emerging quickly as the preferred mass-based detection systems due to their ability to operate at higher pressures and their overall ruggedness. 

Orthogonal pairing of different analytical techniques can be particularly advantageous for detection of chemical threats (12,13,15). For example, while MS is a sensitive and selective detection technology and is valuable for identifying unknown compounds from their characteristic mass spectral fragmentation patterns, its singular use for analysis of complex matrices found in real-world samples might not provide optimal results. However, MS coupled with a separation technique such as gas chromatography (GC) enables two-dimensional analysis, which provides significantly greater capability for identification of compounds in complex mixtures. Analytical advantages of high sensitivity, high selectivity, and rapid response time make GC–MS a preferred detection technique for CWAs and TICs (14,16) as long as it can be packaged for field use in a truly portable, durable, self-supported format that is relatively easy to operate by nonscientific personnel. 

In this article, methods developed for rapid, automated detection of CWAs and TICs using a new low thermal mass capillary gas chromatograph coupled to a toroidal ion trap mass spectrometer are presented. This GC–toroidal ion trap mass spectrometry (TMS) instrument is completely self-contained with onboard helium carrier gas and battery power. A user-defined, embedded library of target analytes is used to provide identification of chemical compounds of interest automatically based upon their characteristic retention times and mass spectral ions. Results are reported in a tabular format that is displayed on the LCD screen. The advantages of reliably detecting CWA and TIC analytes in air, headspace, liquids, and dissolved solids from trace to neat concentrations with GC–TMS are demonstrated. 

Analyte mixtures were spiked in either water or isopropanol at concentrations ranging from 1 to 2000 ppm (w/v). Solid-phase microextraction (SPME) was used as a convenient approach for sampling gaseous, liquid, and dissolved solid samples, concentrating the analytes, and transferring them into the GC–TMS system. The target CWA and TIC analytes shown in this article were extracted and collected by direct immersion of an SPME fiber in the sample matrix for 5–30 s. The SPME fiber was a coated 65-μm 

polydimethylsiloxane–divinylbenzene (PDMS–DVB) phase (Supelco Analytical, Bellefonte, Pennsylvania) and was housed in a syringe holder (CUSTODION SPME syringe, Torion Technologies Inc., American Fork, Utah). Thermal desorption of the SPME fiber was achieved by insertion of the fiber into the heated injection port of the GC–TMS system. The injection port was Sulfinert-treated to ensure inertness of the stainless steel components. 

A hand-portable GUARDION-7 (Torion Technologies Inc.) GC–TMS system was used for rapid separation and detection of the CWA and TIC analytes studied in this work. Fast separations were achieved using a low thermal mass (LTM) capillary GC system. Analytes desorbed from the SPME fiber into the injection port were separated quickly on a resistively heated LTM capillary GC column (MXT-5, 5 m × 0.1 mm, 0.4 μm 

, Restek, Bellefonte, Pennsylvania) coupled directly to the TMS system. A rapid temperature program of 50–270 °C at 2 °C/s was employed. Helium carrier gas was supplied from an onboard disposable cartridge that is capable of supporting ~300 analyses. A septum purge of ~1 mL/min and a split ratio 1:20 were used in conjunction with the split injection method. 

Detection of separated analytes was achieved by the miniaturized TMS system. The RF trapping field of the system was set at 4.0 MHz at an amplitude of ~1200 (max) V

. Resonance ejection was achieved by applying 3.5 to –1.5 V to the filament end cap and scanning the ejection frequency from ~1.6 MHz to ~110 kHz, resulting in a scanned mass range from approximately 50 to 500 

. The dynamic ionization function of the TMS detector resulted in scan rates of 10–15 Hz. Measured mass resolution of 0.55 at 

 222 (diethylphthalate) was typical for these experiments. 

The GC–TMS system is totally self-contained, weighs less than 28 lbs, is battery (24 V) operated, has an onboard helium GC carrier gas supply cartridge (2500 psig, 90 cc), and is hand-portable. The system is packaged in a hardened case with dimensions of 18.5 in. (wide) × 14 in. (deep) × 7 in. (high). The average power consumption of the instrument is ~75 W per sample analysis. When operating under battery power, the system is capable of analyzing ~50 to 75 consecutive samples on a single set of batteries. The system also can be operated with line voltage using an AC-to-DC power converter. Using the temperature program above, the GC–TMS sample analysis cycle time was ~5 min, including GC cool-down time and data processing. 

A PC-based operating software application, CHROMION-1 (Torion Technologies Inc.) was used to operate the GC–TMS system. This software application enables the user to edit the GC–TMS method and calibration parameters and develop analyte libraries for automated detection and identification of targeted chemical compounds based upon their characteristic retention times and mass spectra. Embedded peak deconvolution software (Ion Signature, North Smithfield, Rhode Island) was used to identify CWA and TIC target analytes, including coeluted peaks. A user-defined CWA/TIC library was developed and used by the GC–TMS system for automated detection and compound identification. Results were reported in an easy-to-understand tabular format that is displayed on the LCD screen.  

GC–TMS data have been obtained for several groups of compounds, including chemical warfare agents, explosives, and environmental contaminants, using the GC–TMS instrument. Figure 1 shows the GC–TMS analysis of several TICs and CWA simulants after sampling with SPME. Figure 2 shows the LCD screen display table demonstrating the automated library detection results of the 12 target analytes in this sample. Excellent chromatographic resolution for this separation of TICs and CWA simulants is observed even though the analysis is completed in less than 150 s. Note that although tetrachloroethylene (3) and 1,2-dibromoethane (4) are coeluted under the rapid temperature program LTM GC conditions, their identification is achieved and reported (Figure 2) using the embedded deconvolution–identification software.  

Figures 3 and 4 show the detection and identification of four CWAs at concentrations in isopropanol below standard surety levels used for research, development, testing, and evaluation. Figure 2 shows the results of the auto-identification of VX from this analysis. The mass spectrum for each compound is shown in the inset box. The LTM GC capillary column provided sufficient retention for all of the CWAs with adequate separation from the isopropanol solvent peak. For these compounds, neither the TMS soft or hard solvent delay functions were used. Even at low concentrations and with short SPME sampling times (10–30 s), all four CWAs were detected with signal-to-noise ratios well above the lower detection threshold for the sampling protocol used in this study.  

Although SPME sample introduction into a GC is not generally considered a quantitative technique by many, it offers important advantages for field sampling and analysis. Early detection of chemical threats is the first priority for the types of field measurement considered here. SPME provides adequate capability in terms of its ability to absorb CWA and TIC analytes at low enough levels to provide warning at concentrations hazardous to human health. SPME injection also affords applicability to both volatile (VOC) and semivolatile (SVOC) organic compounds, unlike other membrane-based sample introduction technologies used in field GC–MS instrumentation. Another major advantage of applying this type of injection technology in field monitoring is that the SPME fiber has a finite absorption capacity and, therefore, protects the GC–TMS system from being overloaded and fouled to a point of inoperability. This protection assures the nontechnical user of continued operation in critical conditions.  

Although a majority of compounds demonstrate excellent spectral comparison, the TMS mass spectra of some compounds can differ slightly in relative fragment ion abundances when compared with spectra from other types of mass spectrometers (for example, quadrupole or magnetic sector instruments) that are found in reference libraries. For example, some compounds demonstrate increased molecular ion abundance in their mass spectrum on the TMS. This molecular ion information can be very useful for unknown analyte identification in situations in which a compound's breakdown products have similar mass spectra (such as VX). Together with mass spectral fragment information, the molecular ion information can be used to help identify an unknown. Using retention time and the mass spectra, the compound can then be included in the target compound library. Although the spectral fingerprints for some compounds differ slightly from standard libraries, the same principal fragment ions present in EI spectra are typically observed, which affords comparison to mass spectral reference libraries for unknown compound identification purposes when appropriate search algorithms are used.  

Figures 5–8 compare the mass spectra from the GC–TMS system with those from a widely used reference library for four CWAs and CWA simulants. The comparison of a VX mass spectrum (Figure 5) from the TMS system with the National Institute of Standards and Technology (NIST) library spectrum demonstrates that the TMS system can produce near-identical NIST spectra. Of course, this is an advantage when searching for compound identifications against reference libraries.  

Three examples of different levels of variations for mass spectra from the TMS system and the NIST library can be seen in Figures 6–8. Figure 6 is a spectral comparison for the chemical nerve agent Soman or GD and demonstrates that nearly all of the major ions in the NIST spectrum are present in the TMS spectrum. There are, however, two additional major ions (

 117 and 85) present in the TMS spectrum that are not present in the NIST spectrum. It is beyond the scope of this article to provide interpretation of the molecular structures responsible for these two ions, but it is sufficient to note that these are additional characteristic ions that can be used in the identification library for Soman using this field-portable MS device.  

Figure 7 is a mass spectral comparison between NIST and the TMS spectra for bis(2-chloroethyl) ether. It is apparent from the isotopic ion clusters in both spectra that this is a chlorinated compound. While both spectra exhibit isotopic clusters at 

 63 and 93, there are also isotopic ion clusters at 

 107 and 143 in only the TMS spectrum. The cluster at 

 143 is from the molecular ion. The presence of the molecular ion cluster for this compound can provide supplemental and useful confirmatory data to the user, as mentioned earlier.  

A TMS–NIST mass spectral comparison for pinacolyl alcohol is shown in Figure 8. This example shows that fairly significant differences in the relative abundances of ions in the TMS system as compared with the NIST spectra are observed. While all of the key identifier ions are present in both spectra, they vary in abundance. This type of phenomenon is demonstrated most frequently in relatively polar compounds, such as the low molecular weight alcohol shown in this example.  

These variations in mass fragmentation patterns discussed here are not problematic, however, because the hand-portable GC–TMS system utilizes a TMS instrument-specific mass spectral library. In other words, when standard compounds are analyzed on the GC–TMS system, a library file is built using the characteristic ions from the TMS mass spectra. Coupled with the retention time information for the target analyte, compound identifications are achieved with a high degree of confidence.  

The combination of SPME sampling methodology with GC–TMS for analyte detection and identification provides rapid and accurate determination of chemical threats in a hand-portable design. The SPME–GC–TMS instrument has been shown to be a reliable, selective, and sensitive method for detection of CWAs, CWA simulants, and TICs in less than 150 s. The application of SPME–GC–TMS to a field environment has been demonstrated. The ability to analyze a wide range of sample types from gases to dissolved solids positions this technology as an ideal methodology for the analysis of both VOC and SVOC target analytes in environments that exclude laboratory-based GC–MS instruments. A user's ability to create custom target compound libraries using TMS spectral fragment information together with deconvolution technology expands the capabilities of GC–TMS beyond typical NIST-type library matching.  

Douglas W. Later, Tiffany C. Wirth, Christopher R. Bowerbank,

 are with Torion Technologies Inc., American Fork, Utah. 

 is with Smiths Detection, Danbury, Connecticut. 

(3)   Y. Seto, M. Kanamori-Kataoka, K. Tsuge, I. Ohsawa, K. Matsushita, H. Sekiguchi, T. Itoi, K. Iura, Y. Sano, and S. Yamashiro, 

(4)   W.E. Steiner, S.J. Klopsch, W.A. English, B.H. Clowers, and H.H. Hill, 

(6)   J.A. Syage, B.J. Nies, M.D. Evans, and K.A. Hanold, 

(7)   J.A. Diaz, P. Daley, R. Miles, H. Rohrs, and D. Polla, 

(8)  G.E. Patterson, A.J. Guymon, L.S. Riter, M. Everly, J. Griep-Raming, B.C. Laughlin, Z. Ouyang, and R.G. Cooks, 

(9)   M.G. Blain, L.S. Riter, D. Cruz, D.E. Austin, G. Wu, W.R. Plass, and R.G. Cooks, 

(10) L. Gao, Q. Song, G.E. Patterson, R.G. Cooks, and Z. Ouyang, 

(11) M. Fico, M. Yu, Z. Ouyang, R.G. Cooks, and W.J. Chappell, 

In this article, methods developed for rapid, automated detection of CWAs and TICs using a low thermal mass capillary gas chromatograph coupled to a toroidal ion trap mass spectrometer (TMS) are presented.

Special Issues-04-02-2009 | LCGC Supplements

Chemical Warfare Agent Spectral Imaging for Real-Time Identification and Localization

From the Editor

Chemical Warfare Agents and Use of Thermal Desorption–GC–MS to Achieve Improved Trace-Level Detection

Portable FT-IR and Raman Spectroscopy for Explosives Identification

Rapid Field Detection of Chemical Warfare Agents and Toxic Industrial Chemicals Using a Hand-Portable GC–TMS System