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NQR for Bomb Detection Solution to the Plastic explosives can slip by almost any airport security system, but NQR may help stop them before they reach the plane
T
error on the airlines has been a high-priority international problem since the early 1970s. By now, some form of weapons detector is installed in every commercial airport in the United States and in most foreign countries. But are these detectors good enough to catch the kinds of bombs and weapons terrorists are using today? The increasing use of plastic explosives has experts at the Federal Aviation Administration (FAA), the Department of Defense (DOD), and workers at a number of other agencies and research facilities scrambling for new detection methods to find nonmetallic explosive devices under the realistic operating conditions of an average airport. One of these "new" methods is nuclear quadrupole resonance (NQR) spectrometry, a little-known method that at one time was hailed as a brother to NMR. NQR was first discovered in the early 1950s. Explorations of its potential as an analytical tool continued into the 1960s and early 1970s, but after that it fell out of use in chemical applications. However, in the past five years or so, a number of researchers around the world have independently begun to reconsider NQR as a possible solution to the plastic explosives problem, and they have started developing it specifically for bomb and narcotics detection as part of airline security. Allen Garroway of the Naval Research Laboratory (NRL) in Washington, DC, and Marvin Kroll of Martin Marietta in Catonsville, MD, are two of these re-
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searchers. Garroway and his group of researchers at NRL started to develop a pulsed-radio frequency (rf) spectrometer in the mid-1980s, and when Kroll began to consider NQR about two years ago, he chose to develop the same type of spectrometer after talking with Garroway. In the mid-1970s continuous-wave NQR methods using super-regenerative and marginal oscillators had also been in use, but the pulsed-signal method had analytical advantages over the others and was most analogous to the pulsed-signal methods used in NMR Several other groups around the world, including one at British Technology Group Ltd., have also filed patent applications for NQR in the past two years. Pulsed-rf NQR produces single- or nearly single-peak signals at specific frequencies that depend on the specific bond environment surrounding an element in a given compound, usually a crystalline solid. Although NQR resembles NMR, the signal frequency depends on bond relationships beyond the level of individual functional moieties in a compound, so each compound that produces an NQR signal—and not all do—bears a unique rf signature. "NQR has facetiously been called NMR without the magnet," Garroway says. "In solids, the electric field gradient is there for free, so you don't need a magnet to polarize the nucleus. That cuts your expenses and means that you won't cause damage to magnetic media." The equipment used in the early spectrometers of
Plastics Problem? ·.
the 1950s was based on common components for ham radio transmission and reception, so these instruments were relatively cheap to build. NQR had attracted enough interest by the early 1970s to warrant an international conference, as physicists and chemists tried to understand the connection between the rf signal and the composition of a sample. However, the single-frequency peaks obtained by NQR were so compound-specific that they gave almost no structural information that could be extrapolated to unknown compounds, and the technique could not be used with liquid samples because molecules tumbled about in solution and caused the electric field gradient to average to zero. In the end, most chemists looking for a flexible analytical tool went for NMR, which worked with analytes in solution and provided signal peaks for individual functional groups that could be used to solve the structures of unknown compounds. After the mid-1970s, NQR
dropped out of sight as an analytical tool for chemistry, although since then it has maintained a small following (mostly among physicists) as a tool for observing solid-state phase transitions. Parallel to the analytical history of NQR is the story of its use in the U.S. military. According to Garroway, in the late 1960s and early 1970s the DOD provided funding for an NQR land mine detector that could be used in Vietnam and elsewhere. By the mid-1970s, however, all American troops had pulled out of Vietnam and Cambodia, and the Pentagon's interest in the project flagged. "A colleague of mine in the former USSR tells me the same thing happened there," Garroway relates. "The Soviets were developing an NQR mine detector in the mid1980s for use in Afghanistan, but when the war ended, interest ended." Try, try again Why attempt to revive what looked like a dead analytical technique for something
as crucial as bomb detection? "Basically, it was a method of last resort," Kroll quips, but he's not really joking. Thermal neutron activation (TNA), another scanning method used to detect the high concentrations of nitrogen found in high explosives, was introduced in the mid-1980s with sponsorship from the FAA and tested extensively in airport field trials between 1989 and 1991. But the neutron-generating equipment, the gamma detector, and the shielding needed to protect the operators from harmful radiation made the TNA instruments extremely large and expensive— some instruments reportedly cost more than $1 million. After the bombing of Pan Am flight 103 over Lockerbie, Scotland, in December 1988, it became clear that terrorists were using bombs smaller than the 1-2 kg of explosive TNA was initially designed to detect. TNA scanners modified to detect smaller amounts of explosive resulted in a high rate of false alarms. By the begin-
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ning of 1993, Lee Grodzins, an explosives researcher at the Massachusetts Institute of Technology, was quoted as saying "TNA is dead." Neither TNA nor the new generation of multiple-wavelength X-ray detectors can be used to scan passengers who might be carrying explosives, and both methods are vulnerable to shielding interferences. Bomb "sniffer" methods based on GC/MS or ion mobility spectrometry (IMS) have also been developed and commercialized for airport security systems in the past five years. Although the sniffer methods don't harm baggage or passengers, they rely on the availability of trace vapors of characteristic solvents used in processing or a swab of passengers' hands or luggage to detect the presence of explosives or contraband drugs. Unlike dynamite, however, plastic explosives don't exude volatiles in appreciable concentrations, and a carrier's hands and luggage surfaces are not always reliably tainted with the explosive or drug compounds. What does NQR have to offer that these methods don't? The noninvasive nature of NQR gives it some advantages over X-ray and TNA methods from the standpoint of not harming luggage or passengers, and it doesn't require a physical sample the way the sniffer methods do.
On the other hand, its compound specificity allows it to be used to differentiate between explosives and benign nitrogencontaining compounds. False alarms from polyurethane foam or nylon, both of which have relatively high nitrogen contents, aren't a problem for NQR as they might be for TNA or other generic nitrogen-detecting methods. Because the resonance frequency is almost unique to each compound, NQR exhibits great specificity for analytes such as explosives and contraband narcotics. For security applications, the three most useful elements to monitor by NQR are 14 N, 35C1, and 37C1. Most high explosives contain 30-40% N, and a large number of contraband drugs such as cocaine and heroin are prepared as chloride salts. The polarization of nuclei is caused by valence electrons, which create an electric field within the crystal. Liquids and polymers are too disordered to give an NQR signal, says Garroway, although he notes that some monomers have shown detectable resonances. The disorder comes from molecules tumbling in Brownian motion—the position and orientation of the bonds around target nuclei average out as they spin. For bomb and narcotics detection, any immobilized and relatively ordered compound should work with NQR. In addition
to solid explosives such as RDX, NQR can be used to detect the military plastic explosive C-4. Most pure explosives are crystalline, Garroway explains, and plastic explosives are actually physical mixtures of the crystalline and semicrystalline compounds embedded in a polymer matrix, rather than pure polymeric compounds themselves. "As long as the molecules are immobilized, they produce a unique NQR signature," he says. Commercial and military explosives are physical mixtures of pure explosive compounds with some additive, plasticizer, or binder. Because NQR is so compound-specific, physical additives don't interfere with the signal for a target compound, so NQR can be used to identify explosives that are not in pure form. C-4 contains about 90% RDX (illegal variants may contain less), but the NQR resonance from the RDX in C-4 should be identical to that for pure RDX.
An airport-scale instrument According to Kroll, the idea of scaling up to a practical size for airport luggage may have deterred other researchers from considering NQR earlier. "It was widely assumed that the sensitivity of NQR with a large-volume coil would be so much poorer than for the fist-sized coils used in laboratory work that NQR wouldn't work for baggage inspection. Garroway and his colleagues seem to have been the first group to recognize that this wasn't true," Kroll says. In the lab, convenient sample sizes for NQR are grams and tens of grams or lowmilliliter volumes. But the rf coil for in-lab samples is pretty small—it may be only a few centimeters in diameter. In the airport, says Garroway, "There's no nice sample tube." To accommodate objects such as standard suitcases, the coil needs to be several feet wide. Garroway says that increasing it to a reasonable size for a field model has meant increasing the space inside the coil by a factor of ~ 104—a 300-L (about 10 ft3) volume inside the coil is needed to pass suitcases, backpacks, skis, wine crates, and other typical items through it. With a mean diameter of - 2.5 ft (Garroway's coil is rectangular), the prototype Allen Garroway with the NQR luggage scanner. The rf coil, left, accommodates a standard coil is roughly the size of a whole-body medical NMR imager coil (see photo). suitcase. Source: Naval Research Laboratory.
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content using an integration time of 10 s. The versions of NQR spectrometers However, he says, "I have come to believe designed by Garroway and his colleagues that there is no NQR 'explosive simulant.' at NRL are similar to the ones used until Each material has its specific properties, the mid-1970s, at least as far as the basic so detecting a small quantity of our com hardware is concerned. The sequence of pound is useful only to demonstrate that operation is basically the same: The spec NQR will work with small samples in a trometers transmit and receive rf signals large-volume coil. It doesn't imply that that are then demodulated by radio tech similar sensitivity should be expected for niques at audio frequency and digitized any other compound." for a computer readout. But the new prototypes incorporate adaptations of ad vanced pulse techniques and computer Getting personal capabilities developed since the 1980s for Garroway and his colleagues are also NMR. starting to test a modified coil designed to scan the passengers themselves for explo "We now have a better understanding sives or contraband. The "meander line" of spin dynamics in complicated rf pulse or surface rf coil is flattened out in a ser sequences, in terms of both the theoreti pentine shape. That permits the instru cal aspects and the parameters for optimi ment to scan a wide zation," Garroway surface area at a says. "Understandι controllable limited ing the require depth to avoid ments for the rf ex harming the person citing field is crucial being scanned. The for scaling up to peak signal power very large systems used with the mean like the luggage der coil will be 10 W scanner." or less and, as with The bigger the the luggage scan coil, the bigger the ners, each excita transmitter needed tion pulse lasts less to get a reasonable than a millisecond. signal. Garroway's prototype operates The group at in the range of 1-5 NRL has envisioned MHz, just above the a hand-held mean™~~">~κ~~--™-™» der coil-based in top of the AM band, •— " ' strument, similar to with peak power of the metal detector wands now in use, that 1 kW. This level is somewhat lower than can be passed over the subject to locate the magnitude used in medical NMR im explosives or contraband on the subject's agers. The pulse sequences used in the body. Given that NQR is affected by the prototype's pulse programmer have been mobility of the analytes, however, sample adapted from modern NMR techniques or instrument movement may be a prob and used to optimize the signal-to-noise lem. Garroway says his group's designs ratio. For small packages, a prototype with are based on the assumption that luggage a smaller rf coil than that of the luggage and package scanners will have a con scanner improves detection because it veyor belt with a stop/start function so scans closer to the packages. that the items are stationary during the As for a practical detection limit for scan time. Achieving perfect subject im explosives in the field, Garroway says the mobility will be more difficult with the prototype detects sub-kilogram samples, passenger scanner. The group is begin but he prefers not to mention specific de ning to examine the effects of slight move tection limits; following the Lockerbie ment on the signal to see whether that incident, such information is considered kind of immobilization is necessary. classified. Kroll says his prototype at Martin Bomb detection experts are increas Marietta has detected a 1-lb sample of an ingly concerned about the sophistication RDX-like simulated explosive with 40% Ν with which bombs are being smuggled
The group is beginning to examine the effects of subject movement on the NQR signal.
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lines will probably be programmed with into airports and other vulnerable sites. just one pulse sequence for its dedicated Given the moldable plastic explosives function. It should be easier to operate available today, detection systems must be able to see bombs in unusual forms. In than the lab version, giving a simple the mid-1980s at least one terrorist almost yes/no signal to the airport operator, and succeeded in slipping through security by should therefore be much cheaper to build. "The number I've been bandying lining a suitcase withflattenedsheets of about is $50,000 for a unit. I'm not sure plastic explosives. The explosives were whether that would be just for the compofound only by a hand search. Because NQR measures the bulk signal from what- nents or for the cost of production in reaever passes through the rf coil, Garroway sonable quantities, but the cost of an insays it doesn't matter what form the explo- strument should be considerably lower than the TNA and sophisticated X-ray desive is in—whether thin sheets or small tectors," says Garroway. pellets. Neither Garroway nor Kroll sees NQR NQR may eventually be used to detectbombs or narcotics with spatial resolution, as a replacement for other airport security devices, and neither is ready to say that it the way X-ray metal detectors do. The will definitely work as well in airports as it meander coil inherently localizes the signal it receives as it moves over the person does in a controlled laboratory setting. But if NQR does work well in thefield,the being scanned, and Garroway says it should be possible to extrapolate the basic relatively low price tag should make it a practical complement to the metal detecluggage system to an array of rf coils. tors and other instruments used in existing airport security systems. Prospects for a new security system
NRL was awarded two world patents in 1993 for the NQR technology, but Garroway says that it has taken two years to sign the initial licensing agreement. "People have been reluctant to invest in NQR because the market isn't well defined," he explains. Funded jointly by the FAA the Advanced Research Projects Agency at the DOD, and a federal interagency counterterrorism program, the NRL technology was recently licensed to Quantum Magnetics of San Diego, CA The company will develop its own NQR spectrometers and will have to solve the engineering problems Garroway says "we've chosen to sweep under the rug while dealing with the analytical method itself." Some of the stumbling blocks to acceptance as an FAA-approved method include price and difficulty of operation. Price causes the airlines to balk, especially in these days of company takeovers and Chapter 11 bankruptcy proceedings. "Many of the components for NQR spectrometers are essentially ham radio electronics, so they're pretty cheap," says Garroway. "The homebuilt version in my lab is actually very expensive—we took apart an NMR system and adapted it for NQR— but we needed a version with flexibility so we could optimize the signal." A commercial instrument for the air-
Analytical Chemistry, Vol. 66, No. 5, March 1, 1994
Is that all it can do?
Now that NQR has been refurbished with modern NMR techniques, Garroway says he hopes someone willfindother practical applications for it in areas such as process control. He says, "NQR is inherently less flexible than NMR, but when it works, it's extremely attractive because of its specificity." He adds that NQR can work with slurries, aggregates, and possibly even emulsions, as long as the molecules are spinning slower than the method's MHz time scale. The ongoing use of NQR as an analytical tool for measuring solid-state phase transitions and order or disorder in materials may also be of interest. Without the appearance of new applications, he says, the fate of NQR may be the same as it was in the mid-70s, in part because only a few funding agencies are used to taking a long-range view of technology development. "Too many times people seem to have the notion that these technologies are just sitting around on the shelf and are available as soon as they're needed," Garroway says. "I testified for the President's Commission on Airport Security and Terrorism after the Lockerbie incident, and I was distressed by the lack of understanding people showed about the actual pace of technology." Deborah Noble
