Opportunities to Improve Airport Passenger Screening with Mass Spectrometry (2004)

The U.S. transportation system is an attractive target for terrorists because it could allow them to cause immediate harm to large numbers of people and create anxiety for many times more, as well as to cause massive economic disruption in the United States and aro+und the world. The system is vulnerable because its mission is to provide service to people with a minimum of intrusion on privacy and disruption of access. The detection and mitigation of attacks on transportation are made more difficult by the transient nature of the passengers and the fact that passengers often carry large amounts of baggage, making it relatively easy to conceal threat materials. The September 11, 2001, attacks on the Pentagon and the World Trade Center, in which commercial airliners were used as weapons, also broadened the concept of what constitutes a threat to U.S. assets in general and the transportation system in particular.

Based on historical terrorist attacks involving the hijacking and bombing of aircraft, current threat detection measures concentrate on detecting weapons or specific explosives. In the future, such attacks could also involve the use of toxic chemicals, chemical and biological weapon agents, or even nuclear materials.^1,2

FAA and TSA efforts over the past 20 years have resulted in the development and deployment of two kinds of technologies for the screening of baggage and passengers: explosive detection systems (EDSs), which are designed to detect bulk quantities of explosives in checked baggage,³ and explosive trace detectors (ETDs), which are designed to detect vapor or particles of explosive that are collected (sampled) from personal items or carry-on bags. From bomb fabrication and transportation, EDSs are systems that are certified by the FAA as being capable of detecting threat quantities of specified

explosives. Currently, the two certified EDS systems are both based on x-ray computed tomography technology. Since this report focuses on trace detection, these systems are not discussed further here.

Some 7,000 ETDs have been deployed by the TSA in U.S. airports.⁴ They are deployed in various venues: at passenger checkpoints, in baggage rooms to resolve alarms from EDS machines, in terminal lobbies together with EDS machines; at curbside check-in kiosks; at remote baggage screening locations, including hotels; at ticket counters where bags are screened; and at lobby drop-and-go points.

This report does not comment on the manner in which ETDs should be deployed—for example, in the checked baggage stream instead of at passenger checkpoints. Rather, it focuses on the potential of mass spectrometry to improve the performance of ETDs, wherever they may be deployed.

The majority of current ETDs are ion mobility spectrometers (IMSs), which utilize an ionizing source, a drift spectrometer, a detector, and an alarm and data presentation processor. Chemical identification is accomplished by tailoring the ion chemistry in the ion source for the material(s) to be detected (along with molecules likely to be present that might interfere with the analysis) and passing the resulting ions through a drift space, where they are separated based on their mobility.

ETDs are deployed at various airport locations, where operators acquire samples by wiping down surfaces of luggage or carry-on items with a pad, which is then introduced into the IMS sample port. For trace detection to be useful for aviation security, these objects or persons would have to be contaminated by residue from the preparation and delivery of the explosive device.

Experiments suggest that it is difficult to fabricate a bomb containing certain explosives 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. Of course, since each subsequent fingerprint will contain less material than the previous one, the actual amount specified as an alarm amount for the trace detection system is a bit arbitrary; however, the lower the limit of detection of the device, the higher the probability that a residue will be detected.

The IMS systems used for trace explosives detection in airports have been in development for decades, and the technology is relatively mature. In the committee’s judgment, only incremental advances in the technology’s performance can be expected in the future. By limiting the detection requirements to a specific set of explosives and by setting the detection limit relatively high, instrument complexity and cost are kept low (~$40,000) relative to typical laboratory analytical instruments.

The advantages of trace detection are that it can be used on people and baggage without harming them and that it raises minimal privacy concerns. In addition, it can be deployed in passenger screening areas because of its relatively small size and low cost. The 7,000 or so units now deployed also have a deterrent value.

An ideal trace system would be capable of inexpensively detecting a specific threat substance and distinguishing it from a complex background on a time scale appropriate for terminating the threat and mitigating the impact on people, property and flight operations. Unfortunately, there is no system that is widely deployable and able to identify all threat substances in real time.

Since trace detection methods do not detect threat quantities of materials directly (as do bulk detection methods), their efficacy presumes that in the course of preparing and delivering a bomb, the terrorist and/or his personal items will become contaminated with a residue or vapor that is uniquely characteristic of the explosive, and that this residue will be available for sampling at a screening point. It also presumes that the threat residue is present in quantities sufficient to be sampled from the person or

thing and detected by deployed ETDs. If these presumptions are incorrect, trace detection is not applicable. The indirect nature of trace detection means that a positive detection does not confirm the presence of a threat amount of explosive, nor does a negative result confirm the absence of an explosive.

Trace threat detection techniques as deployed in airports are subject to a number of generic limitations that stem from the indirect nature of the detection: These include sampling issues and false alarms triggered by innocently acquired residues.

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 if the bomb is prepared without leaving residues, if the external surface has been cleaned, or—even when explosive residues are present—if the wiping fails to contact the areas of residue.

Another limitation is that while passenger screening has been the primary justification for trace detection, in currently deployed systems neither the passenger’s body nor his or her clothing is sampled for residues of threat materials—only selected personal items and carry-on bags that are likely to have been touched by the passenger are sampled. Other than metal detectors, there is no currently deployed technology for screening the passengers themselves. One promising approach for detecting explosive residues that may adhere to a passenger’s skin or clothing is the portal sampler.⁵ Portal prototypes have been tested by TSA and a draft Acceptance Test Plan issued,⁶ but no portal has yet been deployed.

Finally, a 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 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, except that an alarm does not reflect the presence of a bomb—again, a limitation of trace detection.

Despite their maturity, ETDs as currently deployed in airports also have several specific limitations, discussed below.

Current airport IMS systems use a combination of ionization chemistry, dopants, and negative ion sensing to detect specific explosives and have an inherently lower chemical specificity than many other analytical techniques—for example, mass spectrometry (see Chapter 2). In other words, they have a limited ability to distinguish threat substance molecules from interfering molecules that may be in the sample background. Given the uncertainty surrounding the amount of explosive residue that may be

present on a bag and current sampling techniques, which sample only a small fraction of the bag’s surface, the current level of alarm should be reduced to increase the probability of detection. Unfortunately, with a technique such as IMS, which has relatively low chemical specificity, as the alarm threshold is lowered, the number of false alarms will increase when innocent materials are misidentified as threat materials.

False alarms bog down a security system by increasing the time required to screen individuals and requiring use of additional equipment and personnel to meet departure schedules. They also degrade the performance of a screening system by creating a sense among operators that all alarms are false. In extreme cases, responses could include shutting down buildings, clearing airports, and preparing for the isolation and treatment of a significant population. It is important to balance the consequences of an alarm with the certainty of identification: High-impact consequences demand high-certainty identification. As the alarm threshold for trace analyzers is lowered, either to increase the probability of detection or to accommodate situations where there is less threat material available, it will be important to increase the chemical specificity of the device to avoid increasing false alarms.

Technology development has focused on specific classes of explosives almost exclusively, in part because these materials are readily distinguishable from typical background materials and in part because these kinds of explosives were thought to be the most readily available to those who want to harm the aviation system. As a result, current ETDs are only able to detect certain explosives. They have little if any capability for simultaneously detecting other threat substances, including other classes of explosives and chemical and biological threats. Further, IMS systems cannot be reconfigured to concurrently detect a broad palette of these new threat substances. As the list of threat substances available to terrorists increases (and assuming the threat scenario is consistent with the expectation of residues), it will be important to develop the capability for concurrently detecting a wider range of threat substances.

Trace explosives detection as practiced in airports today is a system of limited effectiveness and significant vulnerabilities. Nevertheless, ETDs are the only technology currently available for screening passengers and their carry-on luggage for selected threat materials. With the proliferation of knowledge about how to synthesize a variety of explosives as well as chemical and biological agents, and the known interest in these substances on the part of terrorist groups, new analysis techniques are needed that could reliably detect a wide range of threat substances in lower quantities without increasing the rate of false alarms in high-consequence situations.

If, despite the limitations detailed above, trace detection technology continues to be relied upon for passenger and baggage screening, the committee believes that there are alternative technologies that would greatly increase the probability of detection at acceptably low false alarm rates. In particular, this report focuses on the potential role for mass spectrometry to improve trace detection capabilities now deployed in airports for the growing variety of threat materials. Mass spectrometry was selected because, in the committee’s opinion, it has the greatest potential for addressing the deficiencies in the current trace techniques over the next 5 to 10 years.

Chapter 2 discusses the capabilities of mass spectrometers and the potential opportunities and challenges that they present for the trace detection of threat agents. Chapter 3 discusses R&D priorities

that need to be addressed before such instruments could be deployed in an airport setting and suggests one possible phased strategy for such deployment.

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