Trace Detectors
Much effort has been focused on direct detection of explosive materials in carry-on and checked luggage using X-ray systems, but techniques have also been developed to detect and identify residual traces that may indicate a passenger’s recent contact with prohibited materials.
This trace detection technology is often used in airports in conjunction with X-ray scanners. Trace and residue analysis of explosives is, however, a challenging task due to the small quantities of material available and the presence of other compounds that can interfere with the analysis.
The term “trace detection” refers to both vapour and particulate forms:
Vapour: Gas-phase molecules that are emitted from a solid or liquid explosive or other prohibited substance. The concentration of explosives in the air is related to the vapor pressure of the explosives material and to other factors such as the amount of time the explosives material is present in a location, its packaging, air circulation in the location, etc.
Particulate: Microscopic particles of the solid explosives material that adhere to surfaces (i.e., by direct contact with the explosive, or indirectly, through contact with someone’s hands who has been handling explosives).
(Vapour sampling requires no contact. Particulate sampling requires direct contact to remove explosives material particles from a contaminated surface. All trace detection systems have strengths and weaknesses).
Since trace detection methods are not capable of detecting threat quantities of explosive materials directly (as are bulk detection methods such as X-ray), their efficacy is based on the presumption that in the course of preparing and delivering an IED, the terrorist or his personal items will become contaminated with a particles/vapour (residue) that is characteristic of the explosive, and that this residue will be available for sampling at a screening point.
It is difficult to make an Improvised Explosive Device (IED) without, to a certain extent, contaminating persons and things associated with that fabrication. Once a finger has been in contact with an explosive, it is capable of leaving many subsequent fingerprints (on bag handles, boarding passes, and other items handled) possibly with detectable amounts of material. Other particles may drift about in the air, attaching themselves to fibres of clothing. Trace detection devices can identify these materials. But their efficacy 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 trace detection system. If any of these presumptions is incorrect, trace detection will not be effective. Trace detection technologies differ significantly in sensitivity, specificity, reliability and ease of use.
A common trace detection technology in use at airports is Ion Mobility Spectrometry or IMS. These IMS systems have been in development for decades, and the technology is relatively mature. To use IMS, a security officer wipes a swab across a bag or boarding pass and collects trace materials. Next, the security officer places the swab into a machine the size of a desktop computer. In the machine, 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. However, this method may fail to acquire an adequate sample if the IED was prepared without leaving sufficient residues, if the external surface was cleaned by the terrorist – or when explosive residues are present, if the wiping fails to contact the areas of residue. 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. This lack of specificity often results in a higher level of false alarms. 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 threat substances.
However, when used in conjunction with X-ray and other security technologies, IMS takes the analysis to a higher level which enhances security. Other advantages of IMS are that it can be used on people and baggage without harming them and that it raises minimal privacy issues. New technologies will in future make it possible to detect a wider range of substances with greater accuracy, speed and reliability than ever before.
One of the newest technologies for analysing trace materials is called Mass Spectrometry. The detection method uses a tool common in many chemistry and biology labs called a mass spectrometer. A mass spectrometer uses a magnetic field to identify compounds by the mass of one or more elements in the compound. The devices also are often used by law enforcement to test suspicious-looking residues that could indicate the presence of explosives or drugs inside packages. Most mass spectrometers are unwieldy, cabinet-sized machines that require samples to undergo hours of intensive preparation before testing, which can be a problem if officials need to test a large number of containers quickly. The new portable security screening devices requires about 1,000 times less material than used in regular mass spectrometers. It is hoped that this accurate new technology will reduce nuisance alarms. Mass spectrometers utilise four steps for analysis: (1) vaporise 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 mass spectrometry. The flexibility and chemical specificity inherent in mass spectrometry-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. But again there are challenges to using mass spectrometry: given the range of airport deployment sites (e.g., baggage rooms, kerbside check-in kiosks, passenger checkpoints), an explosive trace detection must ideally be able to operate effectively under a variety of adverse conditions, including extremes of temperature, changes in barometric pressure, high humidity, and high levels of dust or other airborne particles. IMS systems have been known to fail under such conditions, and substantial investment may be required to adapt mass spectrometry-based systems for reliable use in these environments.
There have been many incidents of terrorist attack on civil aircraft by way of a passenger carrying on board an explosive device concealed beneath the clothing. (For example, on 24th August 2004, two female Chechen suicide terrorists exploded IEDs on two separate domestic Russian passenger aircraft that had flown out of Domodedovo International Airport in Moscow. Their devices were suicide belts containing the explosive hexogen. All passengers and crew on both flights died in the attacks). Screening for passengers for trace materials can be achieved by using Walk-through Trace Detection Portals similar to large doorframes. A typical portal system performs a non-intrusive sampling of individuals that takes approximately 10 seconds. An individual enters the portal, where jets of compressed air are pulsed to ruffle clothing and detach particles. The volume of air in the portal is then drawn through a preconcentrator device that strains the particles and condensable vapours onto a mesh. This residue is further concentrated and then sent to an analyser. Some of these portals rely on the passenger’s own body heat to volatilise traces of explosive material for detection as a vapour. Alternatively, a handheld vacuum “wand” may be used to collect a sample. Personnel screening is not usually performed with surface swiping because most individuals would find the method invasive.
Parallel efforts in explosives vapour detection have employed specially trained animals, usually dogs, as detectors. The olfactory ability of dogs is sensitive enough to detect trace amounts of many compounds, but several factors inhibit the general use of canines as explosives trace detectors. Dogs trained in explosives detection have significant upkeep costs, require a dedicated human handler when performing their detection role, and most significantly, can generally only work for brief periods. While very effective, their usefulness becomes degraded as the dog becomes tired or bored.
Because plastic explosives are so deadly and so difficult to detect, a detection taggant is now often added when these explosives are made to make detection easier if someone attempts to use them in terrorist attacks. An example of this is with Semtex, which now is made with ethylene glycol dinitrate added as a detection taggant.
