Chemical-specific detection techniques are being applied to explosives screening at airports.

Always a concern, since the September 11 terrorist attacks, airport security has entered the public spotlight and will likely remain there for a long time. An essential aspect of any effective aviation security program—and certainly one that warrants scrutiny—is the ability to detect the presence of explosives in luggage and cargo, as well as on personnel entering an aircraft.

Automated computer tomography (CT) X-ray systems, the focal point of the current weapons and explosives security program in U.S. airports, are increasingly being deployed for screening checked luggage as a result of demands made by the aviation security bill signed last November. These instruments provide geometrical images along with density and effective atomic number information to probe the inside of luggage for the possible presence of an explosives device. According to Lyle Malotky, the scientific adviser for civil aviation security at the Federal Aviation Administration (FAA), “the CT technology is effective for meeting our rather rigorous specifications for the automated detection of explosives.” However, some limitations exist with this approach. “The bad news,” says Malotky, is that “it is not particularly fast, it is expensive [about a million dollars per system], [and] it is heavy.” The somewhat slow speed and large size makes it difficult for this method to fulfill the aviation bill’s goal of screening every checked bag and also doesn’t make it very feasible for use at carry-on security checkpoints. In addition, the effectiveness of this technique, in the end, relies on accurate operator interpretation of suspicious shapes highlighted by the equipment—are they dangerous or not? With the increasing sophistication of the shapes in which plastic explosives can be formulated, the opportunity for false-negative or false-positive results is not trivial.

Thus, the FAA, says Malotky, is investigating other technologies to supplement overall airport security systems in the United States.

Definitive Detection
Naturally, the desire to provide definitive “yes or no” answers about the presence of explosives has led to interest in using tools that offer chemical-specific identification of explosive materials. Toward this end, the FAA has turned its attention to spectroscopic techniques.

FIGURE 1: Three common explosive compounds RDX and PETN are the active ingredients in the plastic explosive Semtex, which was behind the 1988 explosion of Pan Am Flight 103.

There are hundreds of different formulations of explosives, but most are just different kinds of physical mixtures—plastic or otherwise—from a more limited pool of active ingredients, most or all of which contain some nitrogen species (see Figure 1 for several common explosive compounds). Therefore, an effective spectroscopic method for explosives detection would be able to screen, in a targeted manner, for a set of specific molecules. The challenge is to provide the closest thing possible to a precise and accurate “fingerprint” indication of the molecules of interest—and to do so consistently in a crowded airport environment with security system operators who are not scientists.

But how is a usable analytical sample obtained in an airport? One approach—that of trace detection—is to actually retrieve small amounts of particles or vapors from the exterior of an item. Luggage, for example, will inevitably contain trace amounts of explosive material if it was handled in the bag’s vicinity. A particular trace detection technique called ion mobility spectrometry (IMS) has taken on a significant role in U.S. airport screening strategies over the past few years.

Another answer to the sampling question is to follow the example of the bulk detection strategy of X-rays, where the measurement finds the sample. The most promising bulk chemical identification technique being developed for airport use is called nuclear quadrupole resonance (NQR) spectroscopy.

IMS
According to Malotky, there are approximately 700 IMS systems deployed throughout the nation’s airports that have “looked at millions and millions of people’s belongings.”

FIGURE 2: Schematic of IMS experiment in negative ion mode

IMS detects samples based on the rates of movement of their ionized vapors through an oppositely flowing drift gas (see Figure 2). Explosives, for the most part, contain electron-capturing nitro functional groups. So for IMS detection, high-energy electrons are emitted from a ⁶³Ni beta-ray source in an ionization chamber to convert the vaporized sample into negative ions (as opposed to narcotics detection, where IMS works in the positive ion mode). The ions are gated into a drift tube with an electric field. Basically, the drift time of an ion (range from about 5–25 ms) depends on the degree and rate of collision with the drift gas—the larger the ion, the greater the number of the interactions, and, thus, the slower it moves through the tube. The ions are captured on an electrode, and their signals are amplified for detection. It turns out that most explosive compounds of interest have very distinctive drift times, and when such information is programmed into the instrument’s computer for recognition, a simple “pass-fail” light indicator is displayed, all in a matter of about six seconds and with no need for operator interpretation.

In many cases, a special paper disk is used to wipe down an item, such as a suitcase, to capture trace particles. Then, the disk is placed into a sample chamber for vaporization. In addition, several instruments on the market use a vacuum to sample the vapors of more volatile compounds. IMS is typically used at carry-on baggage security checkpoints following baggage X-ray, where articles can be screened quickly. The technique is also being used to a lesser extent for checked baggage. Furthermore, the major IMS suppliers—Barringer Technologies (Warren, NJ) and Ion Track Instruments (Wilmington, MA)—make handheld versions that have applications, for instance, in detecting particles or vapor on unattended baggage or loose cargo. They have also developed human-sized IMS portals that can use airflow to dislodge and sample particles off the clothes and bodies of passengers themselves.

IMS is so effective for explosives detection in airports because it operates under negative ion conditions. “Most of the grunge in our environment”, says Malotky, “is not good negative ion-capturing species, whereas the explosives . . . are happy to operate in the negative ion mode. So you don’t have a very high background.” Therefore, subnanogram levels of material can be detected, and the likelihood of a false-positive result from a nonexplosive compound is low. Another advantage of IMS is its robustness and easy upkeep—vital for any effective security device. Specifically, it functions at standard atmospheric pressure, so it does not require vacuum pumps or other bulky ancillary equipment, as does mass spectrometry (MS), another potential trace method.

Trace Possibilities
Research and development of IMS continues, including miniaturization research at Oak Ridge National Laboratory, so the technology is likely to remain an important element of aviation security in the future. “At the same time,” points out Malotky, “we are funding research in a variety of other trace detection methods.” Raman spectroscopy is one such example. Specifically, scientists in the United Kingdom, along with the FAA, are developing surface-enhanced Raman spectroscopy, which detects molecules attached to nanometer-sized metal particles, as a trace detection method for luggage. Another trace technique of interest is mass spectrometry. “There are certainly some advantages of mass spec over IMS,” notes Malotky, “in that you get . . . certainly more specificity and probably more sensitivity.” Limitations in operational robustness, due to the need for high-vacuum pumps, however, make MS difficult to implement in airports. But MS-on-a-chip research being conducted at Sandia National Laboratories presents future opportunities for a feasible, user-friendly, and sensitive trace-detection system.

A Role for Quadrupole
To directly screen the inside of luggage (without opening it), the bulk detection approach is necessary. NQR has undergone significant development for this endeavor at the U.S. Naval Research Laboratory under the direction of Allen Garroway, head of the Polymer Diagnostics Section, Chemistry Division.

“The technique,” says Garroway, “is related to NMR [nuclear magnetic resonance]. The main thing [in NQR] is that the magnet is not used.” So in NQR, as in NMR, radio frequency incites transitions between polarized nuclear spin states. However, unlike NMR, these polarized states are not created via an externally applied magnetic field, but rather, they are a result of purely intramolecular electronic forces. Specifically, a nuclear physics parameter called the nuclear quadrupole moment, which only arises in nuclei that are nonspherical (spin quantum number I1), interacts with the electric field gradient from the molecule’s electron cloud, forming nondegenerate spin orientations. Since Nitrogen-14 is a quadrupole-active nucleus, this technique is feasible for explosives detection applications.

“The electric field gradient,” states Garroway, “depends excruciatingly on the sight-symmetry the nucleus finds itself in with respect to the valence electrons.” Therefore, the exact resonance frequency is extremely reliant on molecular geometry and, thus, peak positions can occur across a very wide range. Nitrogen-14 frequencies, for instance, show up anywhere from 0 to 6 MHz (compared with a chemical shift separation of 10 ppm in 600 MHz ¹H NMR, which is equivalent to only 6 kHz). Even chemically equivalent nuclei can have distinct peak positions due to crystal packing geometries—the three ring nitrogens in RDX (see Figure 1 above) show three distinct NQR frequencies about 100 kHz apart from each other. According to Garroway, NQR spectra are “arguably unique” for each material of interest, providing what can basically be seen as a fingerprint for most kinds of explosives.

NQR’s considerable selectivity, along with the fact that it does not require a large ionizing magnet, is of primary importance to its practical application in an airport environment. These advantages, however, also carry potential drawbacks. For example, the specificity can be so extreme that for polymeric material, in which local symmetries can vary from sample to sample, a molecule could conceivably have a different NQR spectrum every time it is sampled, making automated and accurate “yes or no” responses difficult. Luckily, says Garroway, “It turns out that most, but not all, explosives are crystalline materials.” Another potential limitation is the signal-to-noise ratio, particularly with the interference that exists in a field environment, because of the fact that NQR polarization is much weaker than that from an external magnetic field. To address this concern, the Naval Research Laboratory has developed a specially designed detection coil with a geometry that causes electrical and magnetic noise to be canceled out without the use of RF shielding.

Quantum Magnetics (QM, San Diego), under NRL license, is manufacturing equipment using the special detection coil, and a small number have been deployed in U.S. airports for field-testing. Currently, QM has two different sized instruments on the market—one appropriate for checked luggage and one for carry-on items—that can purportedly screen 300–400 bags per hour. Basically, the machines bombard bags with RF waves of specified frequencies, corresponding only to the compounds of interest. Therefore, any return signal indicates the presence (and its strength will indicate the amount) of an explosive and will set off an alarm for the operator’s attention. Importantly, this approach is not dependent on the geometry of an explosive device in the baggage. “So if you have sticks of the explosive, if you have thin sheets of the explosive, or a big chunk of it, it all winds up in the same signal,” says Garroway.

According to Lowell Burnett, president and CEO of QM, the company is working to extend the reach of NQR in the airport by developing other products, such as a handheld “wand” and a walk-through portal for passenger and airport personnel screening.

Working Together
IMS and NQR have a combination of beneficial characteristics for airport security, including clearcut operator feedback, operational robustness, high selectivity for the compounds of interest, and great flexibility (these techniques allow newly found explosive material to be programmed into the alarm detection systems, which can also detect narcotics). In the development of these and other approaches, such characteristics are exploited for maximum performance. The goal, however, is not to find one save-all method.

“It’s rather interesting,” says Garroway, “that almost any technique you pick is orthogonal to any other technique you pick. They tend to complement each other in a very nice fashion.” Thus, the strengths and weaknesses of each potential security device must be weighed in order to design a comprehensive strategy that is compatible with the overall law enforcement and customer service considerations in the modern airport.

Further Reading

• Assessment of Technologies Deployed to Improve Aviation Security: First Report (1999), National Academy Press, www.nap.edu/books/0309067871/html/index.html.
• For cost comparisons, see 1997 U.S. General Accounting Office report on commercially available explosives detection devices, www.securitymanagement.com/library/000391.html.

IMS

• Barringer Technologies, Inc.; www.barringer.com.
• Eiceman, G. A., Karpas, Z. Ion Mobility Spectrometry; CRC Press: Boca Raton, FL, 1994.
• Ion Track Instruments; www.iontrack.com.

NQR

• Garraway, A. N.; et al. IEEE Transactions on Geoscience and Remote Sensing 2001, 396, 1108–1118.
• Quantum Magnetics; www.qm.com.