Counterterrorist Detection Techniques of Explosives

Counterterrorist Detection Techniques of Explosives, Second Edition covers the most current techniques available for explosive detection. This completely revised volume describes the most updated research findings that will be used in the next generation of explosives detection technologies. New editors Drs. Avi Cagan and Jimmie Oxley have assembled in one volume a series of detection technologies written by an expert group of scientists. The book helps researchers to compare the advantages and disadvantages of all available methods in detecting explosives and, in effect, allows them to choose the correct instrumental screening technology according to the nature of the sample.

• Covers bulk/remote trace/contact or contact-less detection
• Describes techniques applicable to indoor (public transportation, human and freight) and outdoor (vehicle) detection
• Reviews both current techniques and those in advanced stages of development
• Provides detailed descriptions of every technique, including its principles of operation, as well as its applications in the detection of explosives

• Language: English
• Publisher: Elsevier Science
• Release date: Dec 3, 2021
• ISBN: 9780444641052

Book preview

Preface

Detection of packaged items is a challenge. This book focuses on the detection of purposely concealed items—explosives—but the techniques discussed can be applied to many fields. The barriers to explosive detection may be more formidable. The existence of the threat, its location, size, configuration, and chemical makeup are generally unknown. Furthermore, fast, accurate, and reliable detection is required on a growing number of diverse items including humans, luggage, cargo, and mail. Additionally, the operator–detector interaction requires a degree of familiarity with modern technologies.

Initially, explosive detection meant the detection of military and commercial explosives, but in recent years it has meant the detection of improvised or homemade explosives (HMEs). The wide range of materials that fall in this category has increased the level of detection complexity. During the last decade of the 20th century, many technologies were studied, but few have overcome the valley of death and qualification hurdles to become commercially available. Some detection technologies have been updated to include improvised explosives. Such techniques are included in this book since they have the potential to be further updated as new explosives or explosive configurations become a threat.

This book, Counterterrorist Detection Techniques of Explosives, is an update of the first edition (2007) edited by the well-known explosive analyst Jehuda Yinon, who passed away in 2008. This text examines not only emerging technologies but also those that are in present use. This added dimension of matured technologies should benefit decision makers as well as the designers, researchers, and students working in this field. This second edition retains the focus of explosive detection and covers it under three categories:

1.Explosives bulk detection: X-ray, X-ray diffraction, millimeter wave imaging, and nuclear techniques.

2.Stand-off detection: Laser-induced breakdown spectroscopy, polymer sensing, and hyperspectral IR.

3.Trace detection: Colorimetric, ion mobility spectrometry, mass spectrometry, vapor sensors, and canine.

Introduction

The first edition of Counterterrorist Detection Techniques of Explosives was the last of Dr. Jehuda Yinon’s books, all featuring explosive analysis. Dr. Yinon was born in Berlin (1935), educated in Israel (1960–69), and spent most of his research career at the Weizmann Institute of Science. During his distinguished career, he frequently served as a visiting scientist and researcher at various institutions: Soreq Nuclear Research Center (Israel 1960–65), Jet Propulsion Laboratory (California, 1976–77), National Institute of Health (North Carolina, 1980–81), Environmental Protection Agency (Las Vegas, 1988–89), and Central Florida University (2000–04). He died at home in Israel in 2008. Dr. Yinon was educated as an electrical engineer and specialized in mass spectrometry [1]. As a citizen of Israel, he lived through turbulent times: the Six-Day War (June 1967), the Munich massacre (September 1972), and the Yom Kippur War (October 1973).

Although terrorist organizations and events have been a problem for hundreds of years, the 1970s has been termed The Golden Age of Terrorism. Noted terrorist expert Brian Jenkins wrote: "Nearly 9,840 incidents of terrorism were recorded worldwide during that decade, and more than 7,000 people were killed. Although little international terrorism was focused on the United States or its interests (< 10%), domestic terrorism was rampant. In that decade (1970s), 1,470 incidents of terrorism unfolded within the nation’s borders and 184 people were killed," in contrast to only 214 acts of terrorism on US soil between 2002 and 2013 [2]. During the 1970s, more than 150 planes were hijacked in the United States [3]. The explosive threat, when there was one, was generally dynamite or a gun powder formulation [4]. As a result, in 1974, the Federal Aviation Administration (FAA) instituted the Air Transportation Security Act, which sanctioned universal screening, spurring US airports to adopt metal detection screening portals for passengers and X-ray inspection systems for carry-on bags.

While the ability to detect concealed items or chemicals is important in many fields, because so little explosive can kill so many in such a newsworthy manner, aircraft remain one of the first targets to protect. Therefore, it is easiest to follow the development of explosive detectors by tracking the equipment in our airports. Following the extraordinary volume of the hijackings of the 1970s, the US airline industry was hit with deregulation (1978) and the air traffic controllers’ strike (1981). In 1985, world attention focused on the hijacking of TWA flight 847 shortly after takeoff from Athens, the first of several intermediate stops on the Cairo to San Diego route. Lebanese terrorists held the plane and passengers 17 days, taking it from Greek airspace to Beirut to Algiers and back, beating military passengers and killing a US Navy Seabee. That hijacking was followed by a deadlier but shorter-term takeover of Pan Am 73 (Bombay to New York) at the intermediate stop in Karachi (1986). As a result, Federal Air Marshals became an institution on international flights, and permission was given to air carriers operating to and from the United States to add a $5 surcharge to tickets for enhanced passenger security measures. It was the US government’s position at that time that security was the responsibility of the airports and the airlines [5].

The next seminal event was the bombing of Pan Am 103 over Lockerbie, Scotland, in December 1988, killing all 259 on board and 11 on the ground. At the time, US airlines were required to hand search unaccompanied baggage. The bag containing the Semtex (plasticized PETN/RDX) device was loaded in Frankfurt onto the Frankfurt-London-NY-Detroit flight. The Pan Am bombing prompted the International Civil Aviation Organization (ICAO), an agency of the United Nations, to suggest tagging of plastic explosives (1991) to facilitate detection by electron-capture detectors, which coupled with gas chromatography were assumed to be the future of vapor recognition. The convention, in force since 1998, originally specified four chemicals allowable [ethylene glycol dinitrate, ortho- and para-mononitrotoluene, and 2,3,-dimethyl-2,3-dinitrobutane (DMDNB)], but the United States and most other countries now tag plastic-bonded explosives with a small amount of DMDNB. Further changes are found in the 101st Congress Aviation Security Act of 1989 (H.R. 1659) Title 1: Department of Transportation – Amends the Federal Aviation Act of 1958 to direct the Administrator of the Federal Aviation Administration (FAA) to require air carriers to use thermal neutron analysis (or technologically more advanced) explosive detection equipment at certain foreign and domestic airports.

Testifying before the US Senate subcommittee on Appropriations, on May 3, 1990, the Honorable James Busey, the FAA Administrator, revealed that the FAA had purchased six thermal neutron analyzers (TNAs), two actively screening luggage in Kennedy International (TWA facility) and Miami International (Pan Am), a third being installed at Gatwick (UK), a fourth to be installed at Dulles International with United Airlines, and a fifth designated for Frankfurt, Germany. He reported that airline passenger volume had increased 73% since deregulation and that airlines had collected $86.8 million in aggregate (1986–88) and spent $98.7 million on extraordinary security measures according to Air Transport Association (ATA) records. When asked by Senator Lautenberg how US security compared to other countries, Mr. Busey answered: "All developed nations, including the 162 sovereign states who are parties to the Convention of International Civil Aviation (the Chicago Convention), have aviation security programs that are, in varying degrees, similar to those imposed on U.S. air carriers. The nations of Western Europe in particular, have stringent security requirements that are very close, if not a mirror image, of U.S. requirements. The Government of Israel, in contrast, imposes security requirements that are even more ridged."

"The primary difference between U.S. and other nations’ approaches to aviation security is that we require the airport operator to provide a secure operating environment and require the air carrier to ensure that passengers, baggage, and cargo have been screened in accordance with its security program and that access to its aircraft is controlled. In other nations, these latter security services are provided by the government" [5].

Public Law 101-604 embodied the directives of the 101st Congress, but US policy changed with the crash of TWA 800 (July 17, 1996). Though that crash was later shown to have been an accidental fuel-air explosion in the almost empty center fuel tank, initially it was widely assumed to be a terrorist event. The 104th Congress felt the need to do something. The FAA had been using the Office of Technology Assessment (disbanded in 1995) and the National Research Council (NRC) to evaluate various explosive detection instruments. Under President Clinton, the Gore Commission was tasked with reporting on aviation safety and security. Their recommendations led to Public Laws 104-264 and 104-208 and the Federal Aviation Reauthorization Act of 1996, instructing the FAA to deploy certified and non-certified explosive detection instruments and providing the FAA $144.2 million to purchase and assist in the installation of advanced security equipment [6].

Under the rules established by the 101st Congress, FAA was to create a detection standard—what to detect; how much to detect; how fast to detect—and when an instrument met the requirements it could be designated an explosive detection system (EDS). When two vendors made EDS available, the airlines could be required to purchase one. In response to the crash of TWA 800, the 104th Congress directed the FAA to purchase and deploy whatever was available in, what were then, the 19 Category X (largest and busiest) airports [7]. At that time, only one instrument, the InVision CTX-5000, had received the EDS designation and the questions raised were many centering around how to ensure quality control in the factory and at the airport and how to confirm a detection. All explosive trace detectors (ETDs) that are fielded in airport have passed some sort of rigorous testing, details of which are undisclosed for security reasons, but not all are designated EDS. Colorimetric devices, which are still available to airport screeners, receive no designation; but for certain low volatility threats remains an essential part of the screener’s toolbox.

The creation of Transportation Security Administration (TSA) resulted from events of 2001: the 9/11 coordinated attacks in New York and Washington, DC, and the attempt at a concealed shoe bomb (12/22/01). In November that year, President Bush created TSA, which initially was part of the Department of Transportation and later was moved to the Department of Homeland Security when that department was created (March 2003). The federalization of airport screeners was controversial, but a GAO report [8] made it clear that US screeners were far less effective than those in the United Kingdom, Belgium, or the Netherlands and had a turnover rate that (April 1998 to April 1999) "averaged 126 percent at the 19 largest airports; 5 of these airports reported turnover of 200 percent or more, and one reported turnover of 416 percent." This was attributed to low wages (less than at the airport fast-food restaurants); monotony, and poor training.

The shoe bomber’s use of TATP, as well as the use of various peroxides in the London subway bombings of July 7 and July 21, 2005, highlighted a new set of problems in explosive detection. TATP was not a new explosive; it had been first prepared in 1895, studied for chemical reactions in the 1970s, and first reported as a forensic find in the 1980s [9, 10]. However, security was focused on the last problem (the Pan AM 103 bombing); thus, the goal was the detection of concealed Semtex or similar military explosive in checked luggage. Now a non-nitrate explosive must be detected; though it has high vapor pressure, TATP was not easily detected in the negative ion mode by ion mobility spectrometers (IMS), the predominant method at the time. Since that time IMS, deployed in airports, have been modified to detect in both positive and negative mode. To the airport passengers, this change was imperceptible; indeed, the IMS had been used in the positive mode for years for other modalities.

A 1984 Los Alamos report [11] lists as the most promising candidates for vapor detection two techniques, which make use of the electronegativity of (nitro)explosives: ion mobility spectrometry (also called plasma chromatography) and atmospheric pressure ionization mass spectrometry (API-MS). Also considered for vapor detection were novel uses of Raman, fluorescence, various laser techniques, and biological approaches—animals (dogs, gerbils, rats) and enzymes. Interestingly, at this writing, IMS are fielded for detection at airports and elsewhere but are rare in academic laboratories; yet MS is found in most academic and analytical laboratories but only in 2019 has a platform employing MS receive ECAC certification [12]. Raman is used but not for vapors; scans of solids and liquids have become routine for hazmat response. It is likely a matter of time. IMS were being used by the military to detect humans and chemical warfare agents long before it made it into airports [13, 14].

In August 2006, the thwarted liquid threat to aircraft resulted in an immediate ban of all liquids in carryon. Because this was extremely stressful on airlines, the do-something response of 3-1-1 was instituted in the United States. In fact, it was not the first time liquid threats had emerged. In the 1995 Bojinka plot to bomb 12 airlines over the Pacific, at least one of the threats was nitroglycerin. Fortunately, that plot was uncovered after an initial attempt (Philippine Airline 434, Manila to Tokyo) and a subsequent fire in the Manila apartment of the bombmakers, who included Ramzi Ahmed Yousef, known for his involvement in the 1993 World Trade Center bombing [15]. Thus, the liquid threat was not unknown; it was just considered less likely than the military explosives that, themselves, were quite challenging.

The 1984 Los Alamos report [11] grouped the explosives to be detected into six major categories: NG-based dynamites; AN-based dynamites; military explosives (C4, TNT, picric acid); homemade explosive (AN-fuel oil); low-order powders (black and smokeless powder); and special-purpose explosives (cord, caps primers). Liquid explosives were not mentioned as a category, but a pint of gasoline was listed under detection targets in carryon bags. The report only considered two bulk detection techniques, both nuclear-based: thermal neutron activation analysis (TNA) and associated particle time-of-flight. While the latter was cited as a new feasibility study, the former was discussed as having been under development since 1979.

Today (2021) only one of the techniques cited in the 1984 report can be seen in airports. New explosives have not been the problem as much as new ways to present the threat: in shoes (2001), in underwear (December 25, 2009), and in printer cartridges (October 29, 2010). Likewise, the detection tools are not new techniques; the chemistry and physics remain the same; but they are miniaturized, ruggedized, hybridized, and more effective.

References

[1] Sigman M., Zitrin S. Explosive analyses: a tribute to Professor Jehuda Yinon. In: 10th International Symposium on Analysis & Detection of Explosives, Nov. 22-25, 2010, Canberra, Australia. 2010.

[2] Jenkins B.M. The 1970s and the Birth of Contemporary Terrorism. The Rand Blog; 2015 7/30/2015.

[3] Clark N. Why Airline Hijackings Became Relatively Rare. The New York Times; Mar. 29, 2016.

[4] Accessed June https://www.cnn.com/2015/07/28/opinions/bergen-1970s-terrorism/index.html. 2020.

[5] Department of Transportation and Related Agencies Appropriation for Fiscal Year. 1991.45–120.

[6] Assessment of Technologies Deployed to Improve Aviation Security: First Report. National Academies Press; 1999.

[7] Elias B. Airport and Aviation Security: U.S. Policy and Strategy in the Age of Global Terrorism. CRC; Sept 2009.

[8] Dillingham G.L. Aviation Security Terrorist Acts Demonstrate Urgent Need to Improve Security at the Nation’s Airports. GAO-01-1162T Sept 20, 2001.

[9] Wolffenstein R. Ueber die Einwirkung von Wasserstoffsuperoxyd auf Aceton und Mesityloxyd. Ber. Dtsch. Chem. Ges. 1895;28(2):2265–2269.

[10] Evans H.K., Tulleners F.A.J., Sanchez B.L., Rasmussen C.A. An unusual explosive, triacetonetriperoxide (TATP). J. Forensic Sci. 1986;31:1119–1125.

[11] Danen W.C., Morgado R.E., Quick Jr. C.R., Saunders G.C., Taylor D.J. Novel techniques applicable to the detection of concealed high explosives. In: Report LANL to FAA Technical Center under DTFAO3-83-A00321. 1984.

[12] 1st Detect Tracer 1000 received European Civil Aviation Conference (ECAC) certification on Feb. 21, 2019.https://1stdetect.com/about-us/ accessed July 2020.

[13] U.S. Patent 4195513 filed 6/18/69, assignee Franklin GNO Corp. M.J. Cohen

[14] Preston J.M., Karasek F.W., Kim S.H. Plasma chromatography of phosphorous esters. Anal. Chem. 1977;49(12):1746–1750.

[15] Accessed June 2020 https://www.nytimes.com/2006/08/11/world/asia/11iht-web.0811manila.2447764.html.

Chapter 1: Ion mobility spectrometry of explosives, the stability of gas phase ions, and prospectives for future explosive trace detectors

G.A. Eiceman; R. Rajapakse; J.A. Stone    Department of Chemistry & Biochemistry, New Mexico State, Las Cruces, NM, United States

Abstract

The ionization of explosives vapors, principally the formation of chloride adducts and their drift swarms in an electric field, commonly at ambient pressure, is central to the performance of explosive trace detectors (ETDs) based on ion mobility spectrometry (IMS). Although these ions have near thermal energies, decomposition can occur at temperatures in the range 100–180 °C, which brackets the operating temperatures of drift tubes used for commercial ETDs. Consequently, a range of spectral patterns can be observed in IMS-based ETDs operating at a fixed temperature including: a Cl− or NO3− peak formed from decomposition in the ion source; NO3− adducts from secondary reactions in the ion source; baseline distortions caused by ion decomposition during swarm transit through the drift region; hydrogen abstraction; and the unaltered adduct. Studies of the enthalpies of ion decomposition may guide designs and operational parameters of future ETDs having reduced drift region temperatures to promote the lifetimes of chloride adducts and with ions intentionally decomposed in tandem mobility instruments to introduce a new level of selectivity of response over chemical noise.

Keywords

Ion mobility spectrometry; Ion decomposition; Thermal instability; Tandem drift tubes

Acknowledgments

This material is based upon work supported by the U.S. Department of Homeland Security, Science and Technology Directorate, Office of University Programs, under Grant Award 2013-ST-061-ED0001. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Department of Homeland Security. The grant was administered first by Center for Excellence for Explosives Detection, Mitigation, and Response at Univ. Rhode Island and later by the Alert (Awareness and localization of explosives-related threats) Center at the Northeastern University.

1: Introduction

Ion mobility spectrometry (IMS) has been widely accepted for routine screening of hand-held luggage for trace residues of explosives, with 10,000 or more IMS-based explosive trace detectors (ETDs) installed and used routinely in airports worldwide. This high level of interest in ETDs can be associated with the destruction of Air India Flight 182 (June 21, 1985, 329 dead) [1] and of Pan Am Flight 103 (December 21, 1988, 270 dead onboard, 11 killed on ground) [2]. While bombings of aircraft (Fig. 1) have existed for more than 80 years [3–5], an increase in highjackings and bomb-related incidents by 1969 had elevated concerns, nationally and internationally, over commercial aviation security. Increasingly, security check points were placed in airport terminals, equipped initially with metal detectors (against hijackings) and, then in the decades following the bombings of these two high-profile Boeing 747 jumbo jets, with ETDs.

Fig. 1

Fig. 1 Chart of bombings of commercial aircraft by numbers of incidents (A) and numbers of casualties (B).

The deployment of ETDs had been preceded by more than 20 years of developments of ionization detectors for analytical sciences [6, 7] and with special urgency for the US combat involvement in Vietnam [8–10] and the UK conflict in Northern Ireland [11, 12]. Government agencies supported discovery and development within technology companies, yielding sufficient foundations in ionization chemistry for the introduction of vapor detectors such as the PD series in the mid-1970s from Pye Dynamics, Ltd. (Watford, England) [13]. A PD4 reportedly alarmed during inspection and transfer of baggage for Air India Flight 182 in Montreal but, regrettably, the response was considered to be a false alarm and luggage containing mining-grade dynamite was transferred onward to the next aircraft [14].

The PD family of instruments was initially designed for vapor explosives detection using flow-through electron capture by Robert Bradshaw at Pye Dynamics, Ltd. [11, 12]. Later versions incorporated filtering by ion mobility in an electric field [15]. Another simple, mobility filter-based detector was the Ultratek from Ion Track Instruments (Wilmington, MA, with Anthony Jenkins) [15]. While these vapor detectors exhibited superb limits of detection, they were necessarily replaced in time due to the growing threat from bombs containing explosives of lesser volatility and due to their poor discrimination between target and interfering substances. Improved selectivity was provided by a completely different instrument, a flash gas chromatograph with a chemiluminescence detector (the EGIS, from Thermedics Detection), with capabilities to detect both vapors and particulate matter from military-grade explosives [16]. Commercialization of the EGIS was supported by field trials at Logan Airport in 1988; however, IMS-based ETDs were almost contemporaneously developed and commercialized by Ion Track Instruments and Barringer Research Ltd. (Toronto, Ontario Canada, with John Barringer). This new generation of IMS-based ETDs, with thermal desorption of samples, became available in the early 1990s and several hundred units were on-site by the late 1990s [17]. Additional generations with design improvements were needed until IMS-based ETDs were suitable for routine 24/7 screening of hand-held luggage, eventually becoming the predominant checkpoint technology.

Although technologies and methods for detecting explosives using IMS-based ETDs at airports were being refined for improved performance, distribution globally was uneven and attempts to bomb aircraft continued. In an unsuccessful use of TATP (triacetone triperoxide) on American Airlines Flight 63 (Paris to Miami, Dec. 22, 2001) [18], the use of this improvised explosive illustrated still exploitable gaps in ETD performance. In an operational lapse, two flights leaving Domodedovo International Airport in Moscow on August 24, 2004 (Siberia Airlines Flight 1047 and Volga-Avia Express Flight 1353 with combined 89 dead) were simultaneous attacked by female suicide-bombers who were not searched during the boarding process [19]. There have been other attempts, not cited here, to bomb jetliners with two successful in the past decade

Since the discovery of the favorable response to explosives with IMS [9, 20], and the first roll-out of early commercial IMS-based ETDs, trends in technology refinement have arisen from experiences in operating and maintaining sensitive instruments under demanding, highly-visible venues in airport terminals and from the evolving threats of improvised explosives. These include:

a.radioactive ion sources in commercial ETDs exchanged for non-radioactive alternatives, including dielectric barrier discharge, X-ray, and photo discharge. Efforts have been made to retain in each new method the favorable properties of chemical ionization as found with beta emitters, such as 63Ni, the long-established ion source for military- and security-based embodiments of IMS;

b.characterization of the process of collecting particles using wipes, swabs, or traps and of desorbing explosives using heated anvils;

c.methods or technology to extend IMS response to the emerging threats of peroxide-based explosives and inorganic salts, through the use of negative and positive polarity, switched for each measurement or with dual drift tubes, one in each polarity;

d.design and optimization of conditions for parameters and hardware to sustain regular, full-time operation with complex samples or matrices. This robust and sustained operation, nearly 24/7 with trace limits of detection, is an engineering marvel for security applications in airports.

Unlike the early developments and refinements of IMS-based ETDs, which are described in proceedings of explosives-themed conferences [21–25], not all developments after 1980 are published in open literature or have a readily accessible patent record.

A consequence of these experiences has been the regular introduction of new and refined models of ETDs from well-established (and a few relatively new) companies in the United States, Europe, and Asia. In the larger community of IMS researchers, advances have occurred in drift tube technology with improved resolving power [26, 27], in the methods for ion injection [28–30], and in the development of field-dependent mobility methods [31–33]. In all of this activity, change has been gradual nonetheless in the core concepts and in the practice of IMS as ETDs. Possible changes in a next generation of ETDs may be influenced by recent understandings of the stability of gas-phase ions derived from explosives and their mobilities. These include:

a.the temperature dependence for ion decomposition at ambient pressure with reaction energies determined both experimentally and computationally;

b.the decomposition of ions in strong electric fields at ambient pressure;

c.the development of multi-stage drift tubes where ions can be mobility isolated and studied for thermal instability;

d.a combination of points (b) and (c) with the incorporation of reactive stages in tandem drift tubes in ion mobility spectrometry, differential mobility spectrometry, and differential mobility analyzers (see last section below).

In this chapter, recent advances in these four topics will be described with reference to the operation of current IMS-based ETDs and to possible design elements for future analyzers. These topics are fairly specialized and readers wishing general references on ion mobility methods may find helpful several recent reviews, including a few with emphasis on explosives [34–39].

1.1: Mobility spectra of explosives

Explosives have positive electron affinities (EA) and can capture thermalized electrons; however, signal is increased if a dopant is present to provide a reactant ion of higher electron affinity that forms adducts with explosives in the gas phase. The chloride ion (EA of Cl = 3.61 eV) forms suitable adducts and is readily obtained in compact ETDs from electron capture by chlorocarbons of high volatility such as CCl4, CH2Cl2, and C2Cl6. The mobility spectrum of TNT in Fig. 2, was obtained through ionization with a 63Ni source and demonstrates a simple, clean, spectral pattern in response to an explosive.

Fig. 2

Fig. 2 Ion mobility spectrum of TNT with Cl − reactant ion from CCl 4 dopant.

The chloride ion appears as the reactant ion peak (RIP) in the spectrum at ~9 ms and is produced by dissociative capture by CCl4 of thermalized electrons in the ion source region (Eq. 1).

si1_e

   (1)

The baseline is essentially flat between the RIP and the peak at 17.6 ms, due to [TNT − H]−. This product is formed by the dissociation of the hyperenergetic collision complex [TNT + Cl]−* formed between TNT and Cl − that has too short a lifetime to be collisionally stabilized. The methyl hydrogens of TNT (gas phase acidity or GPA = 1320 kJ mol− 1) are acidic and TNT reacts in the gas phase with Cl− by exothermic proton transfer to give the weaker acid HCl (GPA = 1395 kJ mol− 1). The exothermicity of the reaction is the negative of the difference in GPAs (Eq. 1)

si2_e

   (2)

The collisionally stabilized complex [TNT ∙ Cl]− has been reported, in low yield, in an IMS-MS experiment [40]; however, the highest yield of this product was observed at the highest drift field, which is at variance with expected behavior, if dissociation is occurring in the drift region.

Commercial ETDs are equipped with chlorocarbons, such as hexachloroethane [38], that have relatively low vapor pressures and are suitable for long-term unattended operation. At temperatures commonly used for drift tubes in IMS-based ETDs, hydrogen abstraction of TNT to [TNT − H]− is the sole product. In contrast to TNT, a significant number of explosives (M) form stable chloride adducts [M · Cl]−, yet the appearence of mobility spectra are of the same form as Fig. 2, that is two peaks, one for the reactant ion and one for the adduct ion. In other instances, explosives can produce complex mobility spectra with contributions from multiple ion types formed by ion-molecule chemistry in the ion source, by thermal decomposition of neutral explosives, and by decomposition of adduct ions [39]. In the past decade, descriptions of ion decompositions as ions traversed drift tubes have provided clarity both on baseline distortions in spectra and on the appearance of ions such as NO3− for some, but not all, explosives.

2: Thermal decomposition of gaseous ions at ambient pressure

2.1: Methods and technology for the determination of decomposition enthalpies

The stability of an explosive-adduct ion is temperature dependent and the activation energy for its reactive disappearance is an important parameter for response in IMS-based ETDs. The ions gated into the drift region of a low field IMS analyzer are essentially at thermal energy, determined by the ambient gas temperature; negligible energy is derived from the drift field. The constant drift velocity of an ion swarm in the uniform electrostatic field means that its average translational kinetic energy is in equilibrium with the its internal energy. For example, the average translational kinetic energy 3/2RT of any ion at 150 °C is 8.8 × 10− 21 J and for a typical ion of mass 100 g mol− 1 traveling with a drift velocity of 10 m s− 1, the translational energy gained from the field is 8.3 × 10− 24 J. The effective temperature of the ion is 423.4 K, negligibly different from thermal energy. Thus, ions may be considered thermalized and little discussion has been given over the past 5 decades, unsurprizingly, to thermal instability, or to the lifetimes of ions, on the timescale of a mobility measurement. Perhaps the thought of thermalized ions was misinterpreted as an equivalency, namely, low energies = stable ions.

Prior to the early 2000s, only a few reports of ion decomposition in IMS drift regions at ambient pressure can be found, including an early report on the thermal decomposition of protonated monomers (MH+) and proton bound dimers (M2H+) from butyl acetate isomers [41]. Changes in the relative peak intensities of MH+ and M2H+ in mobility spectra obtained at temperatures between 100 and 150 °C suggested the proton bound dimer underwent dissociation to protonated monomer and then protonated monomer was fragmented to protonated acetic acid plus neutral butene (Eq. 3).

si3_e

   (3)

Mass analysis by IMS/MS confirmed ion identities and demonstrated that fragmentation increased with temperatures from 100 to 150 °C. While these findings were supported by mobility-resolved mass analysis, models for reaction pathways were hindered by the drift tube design that allowed sample vapor to diffuse past the ion shutter into the drift region, distorting spectra by clustering between product ions and sample neutrals. Improved drift tube technology or methodology would be required for quantitative understandings and both were achieved in the steps described below.

The first step toward improved experimental control is found in a doctoral dissertation by Ewing in 1996 who studied the thermal dissociation (Eq. 4) of proton bound dimers of dimethylpyridine and of dimethylmethylphosponate (DMMP) by IMS-MS [42].

si4_e    (4)

The spectra were sufficiently simple that the rate constant (k) for the dissociation reaction could be extracted from the slope of the baseline between the peaks for MH+ and M2H+ (Fig. 3). A dual-ion shutter mobility spectrometer allowed the separation of the M2H+ and MH+ ions in a first drift region with opening of the second ion shutter synchronized with the first so that only M2H+ ions entered the second drift region [43]. The mobility spectrum in Fig. 3 shows typical baseline distortion obtained when isolated dimer ions decompose as they traverse the second drift region with a Faraday plate detector.

Fig. 3

Fig. 3 An ion mobility spectrum obtained with dual ion shutters to isolate a proton bound dimer (M 2 H + ) to determine the thermal decomposition to protonated monomer using a distorted baseline as described in Eq. 4 to 6 [42 , 43] .

The Faraday plate detector monitors charge, whether protonated monomer or proton bound dimer, while the arrival time gives information about the ions arriving at a given drift time. If a proton bound dimer decomposes to protonated monomer immediately the shutter is open then it travels at all times as the monomer and arrives at time tm whereas, if it does not decompose, it arrives at time tM2H+. If the ion decomposes between shutter and detector it arrives at time t, intermediate between tMH+ and tM2H+ having spent the first part of its time as proton bound dimer and the rest as protonated monomer. The proton bound dimers that do not decompose provide the peak at tM2H+. The proton bound dimer remaining at time t is proportional to the shaded area shown in the figure and the actual time tx at which dissociation occurred is given by Eq. (5).

si5_e    (5)

The proton bound dimer decomposes in a first order reaction whose integrated rate equation is Eq. (6).

si6_e    (6)

A plot of the natural logarithm of the integrated shaded area at time t as a function of tx yields the Arrhenius graph in Fig. 4, the slope of which is the negative value (ms− 1) of the rate constant at the temperature of gas inside the drift tube.

Fig. 4

Fig. 4 The integrated area, proportional to M 2 H + remaining, as a function of t x , the time at which dissociation occurred. The slope is the rate constant for dissociation of proton bound dimer at a given temperature. Measurements were made in air at ambient pressure (760 torr for organophosphorus compounds ( Table 1) and ~660 torr for ketones ( Table 1).

Rate constants obtained over a range of temperatures allow the calculation of the activation energy, Ea, and the pre-exponential term A from Arrhenius plots (ln k vs 1/T). Values of Ea and A determined for the decomposition of the proton-bound dimers of DMP and DMMP [43] and of several ketones are listed in Table 1. Measurements could be obtained over only a narrow temperature range of ~ 20 °C because of the combination of the limited dynamic range of the mobility spectrometer, the exponential decay of the decompositions, and the absolute values of the activation energies. This limited range of temperatures, from stable to unstable ions (M2H+ was completely stable at lower temperatures and completely absent at higher temperatures), would become a defining feature for other studies on instability of explosive adduct ions by this method, kinetic ion mobility spectrometry.

Table 1

a Ref. [43].

b Ref. [44].

The activation energy may be related to the standard enthalpy change for a reaction by Eq. (7).

si7_e    (7)

The enthalpy changes for the reactions in the temperature ranges of the experiments are then 3–4 kJ mol− 1 higher than the Ea values, consistent with the averages of 127 and 97 kJ mol− 1 for most symmetrical oxygen and nitrogen based dimers [45]. A study of the thermal decomposition of the proton bound dimer of cyclohexanone employing a drift tube and corona discharge ionization found Ea = 69.5 kJ mol− 1 and log A = 10.1 s− 1, both surprisingly low values [46].

3: Ion mobility spectrometry of explosives

3.1: Background

The studies on the dissociation of proton bound dimers described above were comparatively simple without complications from sample impurities, multiple pathways for decomposition, or secondary reactions of ions. All three complications (and more), are encountered when the concepts, methods, and technology of kinetic IMS are extended to the study of gaseous ions derived from explosives. Several questions define the scope of exploration including:

A.Can we recognize the decomposition of chloride adducts of explosives and define timescales and temperatures best suitable for their measurement by ion mobility spectrometry methods?

B.What values for k, Ea, and ΔHo (reaction enthalpy) are measurable or estimatable for a selection of explosives?

C.Are there common principles that govern ion stability?

D.Do the experimental results match values from computational models?

E.What can be learned about current designs of ETDs, i.e., those in use today, and what are possible improvements for future ETD-based IMS methods?

3.2: Chemistry of ion formation and mobility

The study of explosives using ion mobility spectrometry is now somewhat mature and experience has shown that explosives present two challenges for ion formation through reaction at ambient pressure in IMS drift tubes:

a.explosives often contain impurities from manufacture and some exhibit spontaneous decomposition producing materials at levels that interfere with ionization chemistry;

b.the ionization chemistry of explosives, as described in literature available since the early 2000s, is known to produce multiple product ions [39] from the main presumptive ion (M·Cl−) and these can include Cl−, NOx−, [M − H]− and adduct ions such as M·NOx−.

Illustrative of this complexity is the mobility spectrum in Fig. 5 for nitroglycerin (NG) with Clˉ as reactant ion. The spectrum contains not only the anticipated adduct ion, NG˙Clˉ, but several other ion peaks and also an elevated baseline denoting ion decomposition in the drift region. Critically, the baseline distortion is not smooth, suggesting the presence of other, unresolved, peaks. This is an additional complexity when explosives are investigated to determine the kinetic parameters for their further reaction.

Fig. 5

Fig. 5 Ion mobility spectrum for nitroglycerin as might be seen in a commercial ETD without mobility-isolation of an adduct ion.

3.3: Impurities or decomposition products

A first improvement in methodology for the IMS technology for kinetic studies with substances of complex composition was the addition of sample pre-fractionation by gas chromatography (GC), allowing the isolation of the authentic explosive from sample impurities or products of thermal decomposition (Fig. 6). The drift tube was housed in a fan-free oven without ventilation to minimize thermal gradients where the maximum variation in temperature was ± 1 °C in the drift tube (from inlet flange to detector flange).

Fig. 6

Fig. 6 A gas chromatograph and ion mobility spectrometer with tandem drift tube designed to determine the enthalpies of decomposition of ions derived from explosives.

Capabilities to prefractionate a sample are shown in Fig. 7 as a contour plot of ion intensity as a function of gas chromatographic retention time and drift time for four explosives, EGDN, NG, DNT and TNT with Clˉ as reactant ion. These ions were identified by their mass spectra. It is to be noted that although the peaks for the four explosives are prominent in mobility and mass spectra of independent IMS and MS measurements, there are significantly intense peaks that appear in the mobility spectrum that are absent in the mass spectra. This can be attributed to reactions in the mobility spectrometer ion source.

Fig. 7

Fig. 7 Plot of ion intensity, drift time, and chromatographic retention time from analysis of a dilute solution of explosives mixture, (obtained at highest purity) using gas chromatography (GC) prefractionation with the kinetic IMS analyzer.

Signals for nitrate, drift time 13.8 ms, are present at several retention times coincident with the named chloride adducts. There is also one broad, strongly asymmetric peak at chromatographic elution times between 1 min and 2.5 min with the same drift time as NO3ˉ. It is attributable to HNO3 present as an impurity or from prior decomposition in the GC injector port. HNO3 is poorly eluted from the semi-polar chromatographic column and will engage in an exothermic reaction with Clˉ to release NO3ˉ (Eq. 8). Moreover, the distorted shape in the elution profile is likely due to the mismatch of semi-polar chromatographic phase in the column and highly polar