As terrorists become more sophisticated, there will be a need to detect an increasing variety of threats (new explosives, chemical weapons, etc.). Systems must also be sufficiently automated for non-technical security personnel to operate. Acceptable defenses are now being deployed for baggage scanning, including differential X-ray and X-ray tomography. Swipe methods of sampling are also being used for high-touch areas of baggage and other items. Unfortunately these methods are not appropriate for personnel screening for aviation security and other venues.

Syagen has developed an explosives detecting mass spectrometry (MS) system that has been tested by the Transportation Security Agency (TSA) in combination with a personnel screening portal developed by Sandia National Laboratories. The combined personnel screening system is planned for deployment at airports and other high security venues.

IMS vs. MS

Ion mobility spectrometry (IMS) is the most commonly deployed method for trace detection and about 6000 IMS units are now deployed in US airports for swabbing baggage for evidence of trace explosives contamination that may indicate the presence of an explosives device.

Comparison of ion mobility spectrometry (IMS) and mass spectrometry (MS) methods of detection in an overall high-speed screening system.

The approach being used for personnel screening portals is portrayed in the figure above. A large volume of air is collected and sifted to remove the vast majority of air, while retaining particles, residue, and condensable vapors in a step called preconcentration. Following this enrichment process the collected sample is introduced into the detection system and the compounds further selected using an ionization method that preferentially ionizes the compounds of interest. For example, explosives have properties that make them very reactive to negative ions and electrons. The selectively ionized molecules are then detected by a method that further differentiates different compounds. The two leading detection methods are IMS and MS.

IMS is a method that separates ions in time. Separation is based on the different speeds that ions drift through a viscous gas under the force of an electric field. The drift time depends on molecular size, which does not give an accurate measure of molecular weight. The principal advantages of MS over IMS are higher resolution and molecular identification. As illustrated in the figure above, higher resolution minimizes the problem of interference due to overlapping signals from different compounds. The measure of molecular weight by MS is a very strong indicator of what the compound is, unlike the measure of mobility times by IMS. A recent report by the National Academy entitled “Opportunities to Improve Airport Passenger Screening with Mass Spectrometry,” stated that MS has 10-10,000 times greater resolving power than IMS.

MS Performance

The key to an effective trace explosives detection system is the simultaneous detectability of a broad range of compounds at trace levels in the presence of background compounds at much higher abundance. Achieving this capability is aided by a selective ionization process that maximizes the ion signal of compounds of interest while minimizing the ion signal from the more abundant background compounds. Furthermore, it is essential that each compound have distinct and highly resolved molecular signatures to enable positive identification and differentiation from potential interferents. The figure below shows electron attachment QitTof mass spectra of common explosives. These compounds are observed to have distinct mass spectral signatures. Because the spectral signals are very sharp, the probability of overlap with other compounds, such as background compounds, is very low relative to lower resolution detection methods such as IMS.

Explosives compounds that have been detected by the MS detector with high sensitivity include TNT, ADNT, DNT, MNT, TNB, DNB, DMNB, RDX, HMX, EGDN, NG, PETN, and TATP. Even more importantly, all these compounds can be detected simultaneously with very low false positive rates because of the high-resolution of the MS method. Because of the lower resolution of IMS, increasing the number of target compounds that are detected simultaneously leads to a significant increase in the false positive rate.

MS spectral signatures for selected explosives. Each compound gives a unique spectrum. Nitrate esters all have a common peak at m/z 62.

Detector of the Future

The National Academy of Sciences report strongly recommended a phased plan to upgrade IMS detectors with MS detectors.* The Syagen MS system is a leading explosives detector offering unprecedented levels of performance with regard to speed, sensitivity, and specificity. In combination with the Sandia personnel portal system, it provides a new line of defense against terrorist acts involving bomb-toting individuals. Compared to current technologies the MS detector offers:

• Greater number of detectable threat compounds
  
• Higher detection probability (low false negative rate)
  
• Lower false positive rate
  
• Software-configurable for new threat scenarios

* available from www.nas.edu/nmab

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