Over the past 20 years, the Transportation Security Administration (TSA)1—and the Federal Aviation Administration (FAA) before it—invested extensively in the development of systems designed to protect the traveling public from attacks on the commercial aviation system involving explosives. These efforts have resulted in the deployment of two kinds of technologies for screening of baggage and passengers: explosive detection systems (EDSs), which are certified to detect bulk quantities of explosives in checked baggage, and explosive trace detectors (ETDs), which are designed to detect vapor or particles of explosive that would be associated with personal items or carry-on bags as a result of bomb fabrication and transportation. ETDs are also used as one method of resolving alarms from EDSs. An EDS alarm is a more direct indicator of the presence of a potential bomb, since the EDS is designed to detect objects with physical dimensions and densities consistent with threat quantities of explosive materials. The alarm from an ETD, which responds to traces of explosive material, only suggests that a bomb may be present.
In the case of certain explosives, experiments suggest that it is difficult to make a bomb without contaminating persons and things associated with that fabrication. Many of these materials are very sticky, and once a finger has been in contact with the explosive, it is capable of leaving many subsequent fingerprints (on briefcases, clothes, boarding passes, etc.) with detectable amounts of material. The advantages of trace detection are that it can be used on people and baggage without harming them and that it raises minimal privacy issues. This report focuses on opportunities for improving the ability of ETDs to detect terrorist threat materials in transportation—specifically, airport—environments.
Some 7,000 ETDs have been deployed by the TSA in U.S. airports for the interrogation of carry-on baggage for traces of explosives. ETDs are located at passenger checkpoints as well as many other airport venues, and the acquisition of samples involves an operator wiping down surfaces of luggage or carry-on items with a dry pad, which is then most commonly introduced into the sample port of an ion mobility spectrometer (IMS). In the IMS, the target and background molecules in the sample are first ionized and then passed through a drift space, where they are separated based on their mobility. The pattern of separation is compared to a library of known patterns to identify the substance collected. The entire process takes about 1 minute.
These IMS systems have been in development for decades, and the technology is relatively mature. By limiting the detection requirements to certain classes of explosives and by setting the alarm threshold relatively high so as to reduce the number of false positives, instrument complexity and cost are kept low (less than $40,000 per instrument) relative to typical laboratory analytical instruments. However, the currently deployed systems have limitations specific to the physics and chemistry of their operation that make them unsuitable for addressing a variety of emerging threats.
LIMITATIONS OF CURRENT TRACE TECHNOLOGY
Trace detection methods are subject to some inherent limitations that are common to all such methods; currently deployed IMS detectors also have some limitations that are more specific to the IMS technology.
Generic Limitations of Trace Detection
Since trace detection methods are not capable of detecting threat quantities of explosive materials directly (as are bulk detection methods), their efficacy is based on the presumption that in the course of preparing and delivering a bomb, the bomb carrier or his personal items will become contaminated with a residue or vapor that is characteristic of the explosive, and that this residue will be available for sampling at a screening point. Their efficacy also depends on the presumption that the threat residue is present in quantities sufficient to be sampled from the person or thing and detected by the deployed ETDs. If any of these presumptions is incorrect, trace detection is not applicable.
Some of the issues that stem from the inferential nature of trace detection are the following:
Sampling issues. As deployed in airports, trace detection equipment depends on blind sampling, whereby an operator attempts to acquire a sample by wiping areas where threat materials are thought most likely to be present. This method may fail to acquire an adequate sample if the bomb was prepared without leaving sufficient residues, if the external surface was cleaned by the terrorist, or—even when explosive residues are present—if the wiping fails to contact the areas of residue.
Another issue related to sampling is that while passenger screening has been the primary justification for trace detection, currently deployed systems sample neither the passenger’s body nor his or her clothing for residues of threat materials—but rather only selected personal items and carry-on bags that are likely to have been touched by the passenger. Other than metal detectors, no currently deployed technology screens the passengers themselves. One promising approach for detecting explosive residues that may adhere to a passenger’s skin or clothing is the portal sampler.2 Portal prototypes have been tested by TSA, but not one has yet been deployed.
Alarms due to innocently acquired residues. The trace detector may alarm if an individual or bag has had some innocent, incidental contact with a threat material in the past. This might occur, for example, if the individual works in the commercial explosives industry; owns a gun or has contact with someone who does; or is taking nitroglycerin heart medication. In this
case, the detector is functioning as it was designed to, but the alarm does not reflect the individual's one-time proximity to a bomb.
Specific Limitations of Current ETDs
Despite their maturity, IMS-based ETDs also have several specific limitations, discussed below.
Vulnerability to higher false alarm rates at lower alarm thresholds. Current airport IMS systems have an inherently low chemical specificity compared with other analytical instrument systems. In other words, they have a limited ability to distinguish threat substance molecules from interfering molecules that may be in the sample background. As the detection threshold is lowered, this lack of specificity will result in a higher level of false alarms.
Limited number of threat agents concurrently detectable. Current ETDs are designed to detect selected explosives. Because the ionization conditions, dopant gas, and drift time window are optimized for these explosives, IMS systems have limited capability to be reconfigured to concurrently detect new threat materials. As the list of threat materials available to terrorists increases (and assuming the threat scenario is consistent with the expectation of residues), it will be important to develop the capability to concurrently detect a wider range of threat materials.
Improving ETD Performance
The committee offers the following finding and recommendation for improving the performance of currently deployed ETDs:
Finding 1: The trace detection systems currently deployed in airports have limited utility for the following reasons:
The relatively low chemical specificity of IMS means that the instrument alarm threshold must be set high to avoid excessive false alarms; yet, lower alarm levels are desirable to account for inefficient manual and portal sampling techniques and, possibly, “cleaner” perpetrators.
Detection is dependent on the use of blind sampling methods that cover only a small portion of the bag surface for acquisition of adequate residues for analysis.
Current sampling protocols do not allow for the sampling of explosive residues or vapors that may be associated with a passenger’s skin or clothing.
Currently deployed IMS systems are designed to detect only a specific list of explosives and cannot easily be reconfigured to detect an expanded list of explosive, chemical, and biological threat substances.
Recommendation 1: To address these deficiencies in the performance of explosive trace detectors, TSA should do the following:
Place a high priority on the development and deployment of automated trace sampling hardware.
Decrease the threat alarm threshold for ETDs systematically over time to improve the probability of detection of residues while keeping false alarms at current levels.
Deploy passenger screening portals to enable the detection of explosive traces on passengers’ skin and clothing, and assess the acceptability and efficacy of the portals.
Explore new technologies with higher chemical specificity that are capable of detecting a wider range of explosive, chemical, and biological threat materials.
OPPORTUNITIES TO IMPROVE TRACE DETECTION SYSTEMS WITH MASS SPECTROMETRY
To improve upon the IMS trace detection systems currently deployed in airports across the United States, mass spectrometry (MS) is an obvious candidate to consider. It has become the gold standard for resolving high-consequence analyses involving water, air, and ground pollution; pharmaceutical drug development and manufacture; treaty compliance verification relating to the proliferation of nuclear materials; verification of employee drug abuse for prosecution and job termination; detection of performance-enhancing drugs in horses and athletes; and routine analysis in the chemical, drug, and fuel manufacturing industries.
While mass spectrometers have become one of the analytical mainstays of today’s chemistry and biotechnology laboratories, they have historically been large, complex systems that occupied the volume of several file cabinets, were operated by highly trained mass spectrometrists, and sold for around $250,000. More recently, with demand from lab chemists and technicians for instruments that could be used for routine analysis, automated, self-calibrating, auto-tuning, benchtop units of reasonable size and costing $50,000 to $100,000 have become available. These instruments are generally coupled with a gas chromatograph or a liquid chromatograph at the sample inlet to improve chemical selectivity. Indeed, some special-purpose instruments have been miniaturized for mobile applications, though the performance and reliability of these miniaturized systems are still being assessed. In general, however, all of these instruments operate at high vacuum and need professional care and trained operators.
Mass spectrometry is not new to the TSA, which has tested a personnel screener utilizing an MS-based system manufactured by Syagen Technology and a portal developed by Sandia National Laboratories. MS has also been applied by others for the automatic analysis of samples acquired from boarding passes. In the committee’s view, such systems could add significantly to future trace detection capabilities for a variety of threat substances in the transportation context.
Mass spectrometers utilize four steps for analysis: (1) vaporize the sample, (2) place an electric charge on sample molecules to form ions, (3) separate the ions based on their charge-to-mass ratio using an electric or magnetic field, and (4) determine the number of separated ions having a particular charge-to-mass ratio. The uniqueness of mass spectrometry lies in its chemical specificity. It directly measures a fundamental property of the target molecule—its molecular weight—and thus provides a highly specific means of identifying the molecule. By contrast, IMS systems measure a secondary and less specific property of the target molecule—the time it takes for the ionized molecule to drift through a tube filled with a viscous gas under an electric field—and the identity of the molecule is inferred from the time vs. intensity spectrum, which is compared with standard spectra in the instrument’s database. Since different molecules may have similar drift times, IMS inherently has less chemical specificity than MS.
In fact, the committee estimates that a typical tandem mass spectrometer (two mass spectrometer analyzers arranged in series, or a single trapping spectrometer making tandem analyses in time)—an instrument configuration commonly found in analytical laboratories—has a chemical specificity (or informing power) about 10,000 times greater than that of a typical IMS instrument.
Advantages of MS-Based Detection Systems
As a trace detection technology, MS-based systems have the same generic limitations as all the trace detection technologies discussed above. However, MS-based systems enjoy some advantages over current IMS systems:
Lower detection limit while maintaining low false alarm rates. Given the unpredictable efficiency of sample acquisition, discussed above, it is desirable to reduce alarm thresholds below current levels in order to increase the probability of detecting trace residues or vapor while maintaining passenger throughput. IMS systems have alarm thresholds typically set about 100 times the detection limit; however, lowering the alarm threshold will increase the false alarm rate. MS-based systems should be capable of alarm thresholds 1,000 times lower. Given the committee’s estimate that MS-based detectors should have 10,000-fold greater chemical specificity than IMS-based systems, this lower alarm level should be achievable without increasing the rate of false alarms that are due to interfering compounds in the sample background. Note, however, that the lowered detection limit may increase the number of alarms caused by the detection of innocently acquired explosive residues.
Broader range of threat substances concurrently detectable. The flexibility and chemical specificity inherent in MS-based systems make them capable of concurrently detecting a much broader range of threat substances than IMS, including a broader range of explosives, chemical warfare agents, and biological agents. Detection and identification of many of these agents with MS have already been demonstrated under both laboratory and field conditions.
Challenges for MS-Based Detection Systems
MS-based systems face a number of challenges before they can be deployed in airports as trace detectors:
Reducing cost and complexity and increasing ruggedness. The U.S. Army and the Defense Advanced Research Projects Agency (DARPA) have conducted proof-of-principle research and development, testing, and evaluation for both chemical and biological threat analysis using fieldable, rugged, specialized mass spectrometer systems. It is likely that much of the work that these and other entities have done could be used directly or modified for TSA threat scenarios, but TSA needs to focus on its unique needs for a rugged, backbone mass spectrometer that would be useful for many threat detection scenarios. Although extensively used for a variety of laboratory applications, commercially available chemical analysis systems (chromatography followed by two stages of mass analysis, or C/MS/MS) are not designed for an environment as harsh as an airport or other transportation arenas,3 nor are they designed for use by TSA security operators.4
Resolving sampling issues. The configuration of the sample inlet (chromatograph, if needed), the ionization method, the number of mass analysis stages, and the kind of ion detected will depend on the problem to be solved. Even though modern mass spectrometer systems can be automatically reconfigured based on an analysis just performed (e.g., changing the detection from positive ions to negative ions, or changing which ion is selected in the first stage of an MS/MS system), it is not reasonable to expect that one inlet and one ionization method will serve all threat materials or deployment scenarios. Choosing the most appropriate configurations will take time and additional research. An attempt should be made to select a configuration that is extendable to as many threat scenarios and materials as possible.
Improving database robustness. To identify a target molecule using a mass spectrometer, the spectrum obtained would be compared against a library of reference spectra, and a software algorithm would determine if a match occurred. Such algorithms, formats, and spectral libraries already exist and would form the basis for those used in this application. Once the methods of analysis are chosen, corresponding libraries will need to be augmented. All commercial MS data systems allow libraries to be created based on standard samples of interest. Since the chemical specificity of the analysis technique allows for the elimination of most if not all background signals, it is not necessary to run standards in the presence of all known backgrounds, as is always necessary with IMS.
Reported test results on deployed MS-based trace detectors suggest that these instruments are capable of a low limit of detection as well as a low false alarm rate.5 This supports the committee’s view that TSA should initiate a vigorous program to take advantage of the opportunities offered by MS-based detector technologies. Thus, if TSA wishes to invest in the improvement of trace detection technologies, the committee offers the following finding and recommendation:
Finding 2: Owing to their lower limit of detection, higher chemical specificity, and chemical flexibility, MS-based trace detection systems have the capability to address many of the limitations of IMS-based systems.
Recommendation 2: TSA should establish mass spectrometry as a core technology for identifying an expanded list of explosives, as well as chemical and biological agents. Specifically, TSA should
Create a prioritized list of threat materials that are likely to fit a residue scenario and a second list of materials that are not likely to fit the scenario
Determine appropriate MS sampling procedures, inlet configurations, ionization methods, and analysis strategies for relevant materials on this list.
A good way to bootstrap this entire process would be to purchase the best field-deployed instrument to gain experience and to test system applications. One option would be for TSA to purchase an instrument such as the one described in Box 2-1 and evaluate its effectiveness in the airport context.
One example is described in Box 2-1.
A Phased Implementation Plan
Full-scale deployment of MS-based detectors in airports cannot occur immediately; it will require a phased approach involving several generations of instruments, as outlined in the following finding and recommendation.
Finding 3: The many trace detection tasks that can be envisioned in airports will require MS-based detection systems with various levels of cost and performance; in some cases, years of R&D and testing may be required to produce MS instruments with the necessary specifications.
Recommendation 3: If TSA wishes to improve its trace detection capabilities, it should deploy MS-based detectors in a phased fashion, with successive generations of instruments addressing lower quantities of an expanded list of threat materials and more sophisticated security tasks. These tasks range from passenger screening at checkpoints to monitoring of the air handling system.
Here, the committee offers just one plausible scenario for such a phased deployment at a large urban airport. The dollar figures are estimates based on the best judgment of committee members familiar with the development of MS apparatus.
Phase 1 (1 to 3 years). Deploy a limited number of portal sampling systems with both IMS-and first-generation MS-based detectors at airport checkpoints. Compare the performance and reliability of these systems under typical passenger flow conditions.6 This would involve the deployment of perhaps 5 to 10 portals costing $100,000 to $150,000 each for a large urban airport. The first-generation MS-based portals might incur $25,000 to $50,000 more in initial costs compared with comparable IMS portals. Operating costs for an MS-based portal are expected to be $2,000 to $5,000 higher per year.
Phase 2 (3 to 5 years): Develop a second-generation MS instrument with a single geometric configuration that can detect a variety of threat agents, including chemical and biological agents as well as a broader range of explosives. Compare this second-generation instrument with the best available IMS devices at passenger checkpoints and portals to assess suitability and versatility. This would involve deployment of 10 to 20 MS detectors costing perhaps $100,000 to $150,000 each.
Phase 3 (5 to 10 years): Replace current IMS ETDs with fully cost-reduced and automated MS systems that would support both passenger and carry-on screening. This third-generation, automated MS detector would be used in place of IMS in every passenger path and could also be used as an adjunct to EDS x-ray systems in the checked baggage path as well. A major benefit of this device would be to reduce the necessity for hand searches. At a large urban airport, implementation of this phase would mean deployment of perhaps 50 instruments that are assumed to have an initial cost of no more than $75,000 and an operating cost of less than $5,000 per year.
Phase 4 (>10 years): Develop MS-based detectors for use in monitoring for terrorist attacks on the air handling equipment in either a transportation terminal or in the transportation vehicle. This class of instruments is less well-defined than the instruments discussed above, and if biological threats are to be considered, one could expect the cost of this device to exceed the costs of the instruments described above by a factor of between 2 and 3 because of