Infrared and Raman Spectroscopy in Forensic Science

John M. Chalmers, Howell G. M. Edwards, Michael D. Hargreaves, John M. Chalmers, Howell G. M. Edwards, Michael D. Hargreaves

• Publisher: Wiley
• Year: 2012
• Print ISBN: 9780470749067
• eBook ISBN: 9781119962984
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Spettroscopia e analisi dello spettro

This book will provide a survey of the major areas in which information derived from vibrational spectroscopy investigations and studies have contributed to the benefit of forensic science, either in a complementary or a unique way. This is highlighted by examples taken from real case studies and analyses of forensic relevance, which provide a focus for current and future applications and developments.

Section III

Counter Terrorism and Homeland Security

Chapter 5

Counter Terrorism and Homeland Security

Vincent Otieno-Alego and Naomi Speers

Forensic and Data Centres, Australian Federal Police, Australia

5.1 Introduction

The detection and prevention of terrorism has never been more relevant than in today's present world climate. Since the September 2001 attack on the World Trade Centre in New York, the public has become sensitised to the terrorist threats. These concerns have become more intense due to the ease by which an improvised explosive device (IED) can be made from legitimate and readily available ingredients. The threat of the use of such “easy to make” IEDs has been highlighted through terrorist bombing incidents such as those in Bali, Indonesia (2002 and 2005), Casablanca, Morocco (2003), the public transport systems in Madrid, Spain (2004) and London, United Kingdom (2005), and the bombing of the World Trade Center in the USA in 1993, followed by its eventual destruction in 2001 [1, 2]. The media also continues to report both successful bombing campaigns as well as foiled bombing attempts in different parts of the world, including the highly publicised failed bombing attempt of the Northwest Airlines Flight 253 on 25 December 2009 [3].

The frequent bombing incidents have ensured that the detection and identification of explosives remains a high priority in homeland security. This, in turn, has generated a great demand for sensitive, reliable, rapid, portable and minimally invasive detection and identification instruments for use by law enforcement and security agencies to enhance the level of protection against terrorist threats that includes explosives, chemical and biological warfare agents, toxic industrial chemicals (TICs) and toxic industrial materials (TIMs). The list of potential threats contains a plethora of individual chemical compounds, and combinations thereof, making it almost impossible for a single analytical technique to be able to meet all of the above requirements. One area that has seen significant innovation since the so-called 9/11 (September 2001) attacks on the United States is the development of vibrational spectroscopic instrumentation, especially field-deployable mid-infrared (IR) and Raman based instruments.

This chapter provides an overview of the use of vibrational spectroscopic techniques in homeland security, including mid-IR, Raman and terahertz (THz). The discussion includes some case studies and is restricted to the analysis and identification of intact (unconsumed) explosives or tiny fragments of explosives that are visible using an optical microscope. The uses of these techniques for trace or vapour explosive detection, as well as their application in the standoff detection of hidden explosives, are not covered in this chapter. The emerging Raman technique known as spatially offset Raman spectroscopy (SORS; see also Chapters 5.4 and 6.2) is of special importance in the current aviation security environment and receives attention in Chapter 5.4.

5.2 Infrared and Raman Spectroscopy for Explosives Identification

Infrared spectroscopy and Raman spectroscopy are both recognised as powerful analytical techniques for the unique identification of a wide range of explosives, explosive components and explosive precursors [4–8]. The two techniques parallel each other in that both spectroscopies provide information on molecular vibrations. However, whilst in infrared spectroscopy the sample absorbs the incident radiation, the Raman phenomenon involves the sample scattering the radiation (refer to Chapter 2). As a consequence of these two differing processes, mid-IR and Raman spectroscopy are complementary rather than identical techniques; vibrations which are usually weak in a mid-infrared spectrum are usually strong in a Raman spectrum and vice versa [9, 10].

In a 2007 survey conducted by the Technical Working Group for Fire and Explosives (TWGFEX) [11], primarily on American forensic laboratories, respondents were asked to rank their usage of the different explosive analysis techniques from 0 (never use) to 7 (use very often). Mid-IR spectroscopy ranked as the most used technique with a mean rank of 5.1, while Raman spectroscopy ranked low with a mean rank of 0.2. For comparison, other common explosive techniques such as gas chromatography-mass spectrometry (GC-MS) and scanning electron microscopy with energy dispersive X-ray (SEM-EDX) spectroscopy had mean ranks of 3.6 and 4.4, respectively [11]. Thus Raman spectroscopy is often described as an emerging technique in forensic explosive examination and forensic science in general. This is because the development and commercialisation of Raman spectroscopy for the analysis of a microscopic specimen was somewhat slower than that for mid-IR spectroscopy. As a consequence, many forensic laboratories are well equipped with infrared spectroscopy, but few are equipped with both mid-IR and Raman spectroscopy, and even fewer equipped with Raman spectroscopy but not mid-IR spectroscopy. This demography is quickly changing due to improvements in Raman instrumental design, which have led to the development of compact computer-controlled systems, incorporating extensive automated protocols that check the instrument's performance, a prerequisite for forensic laboratory accreditation.

In general, an analytical scheme for explosives that features both mid-IR spectroscopy and Raman spectroscopy is likely to be more powerful and flexible than an examination that involves only one or the other as certain compounds are more favourable to one of the two methods of analysis. For example, the mid-IR spectroscopic technique is more suitable for the analysis of coloured formulations of polymeric energetic materials, whereas Raman spectroscopy is often handicapped because of the problem of laser-induced background fluorescence. Raman spectroscopy is generally more suitable for inorganic materials and can identify IR-inactive substances such as sulfur and carbon, which are often encountered in inorganic explosive mixtures [12].

In several respects, mid-IR and Raman spectroscopic techniques are well-suited for the analysis of explosives as well as analysing other types of physical evidence such as paint [12, 13]; see, for example, Chapter 4.2#2. Some of the important advantages of these techniques for explosive identification are discussed below. It is noteworthy that the term Fourier transform or its abbreviation FT is often encountered in conjunction with IR microspectroscopy or IR spectroscopy. This simply refers to the manner in which spectral data are collected and processed (see Chapter 2), and all contemporary mid-IR spectroscopy as described in this chapter refer to using FT instruments.

5.2.1 Level of Chemical Identification

Broadly speaking, all explosives (be they military, commercial, or improvised) require the same chemical building blocks, which consist of a fuel and an oxidiser. Some explosives have the fuel and oxidiser as part of the same molecule (e.g., trinitrotoluene, TNT, and triacetone triperoxide, TATP; see Figure 5.1), and some explosives are comprised of mixtures of separate fuel and oxidiser (e.g., ANFO, which consists of a mixture of ammonium nitrate and fuel oil). Historically, the oxidiser employed by the vast majority of explosives has been the nitro (–NO₂) group. Thus, the nitro group vibrational bands, both symmetric (between about 1260 and 1375 cm⁻¹) and antisymmetric (between 1450 and 1600 cm⁻¹), can be used for explosive detection since they act as vibrational signatures of several classes of explosives including nitroaromatic, nitroaliphatic, nitroamines and nitrate esters [14]. For example, the mid-IR and Raman spectra of TNT shown in Figure 5.1a contain several diagnostic –NO₂ peaks. The two dominant IR absorption bands at 1534 and 1354 cm⁻¹ are attributed to the –NO₂ antisymmetric and symmetric stretching modes, respectively [15]. The Raman data also contains the –NO₂ diagnostic peaks at approximately 1532 cm⁻¹ (antisymmetric stretch) and 1357 cm⁻¹ (symmetric stretch) [16]. Beal and Brill [17] have provided a comprehensive treatise on the behaviour of the vibrational spectra of the –NO₂ group in more than 50 energetic compounds. The authors also noted the invariance of the –NO₂ scissor motion frequency (occurring between 842–846 cm⁻¹) despite the differences in the local chemical environment, thus suggesting the use of this mode as an additional detection tag in –NO₂ containing explosives [17].

Figure 5.1 Transmission mid-IR and Raman spectra of: (a) TNT and (b) TATP. The variation in relative peak intensities highlights the complementary nature of the two techniques. (Raman spectra were recorded using a 785 nm laser.)

Presently, the newest group of explosives to emerge in the arsenals of most terrorist groups are peroxide-based explosives. Notable examples of this class of explosives include the much favoured TATP as well as hexamethylenetriperoxide diamine (HMTD) and methyl ethyl ketone peroxide (MEKP). As shown in Figure 5.1b, for example, TATP does not contain the nitrogen-based chemical markers of familiar explosives, and therefore could easily escape detection by most traditional techniques [18]. In the peroxide family of explosives, diagnostic peaks can be associated with the O–O and C–O bonds, with the details of the other vibrational modes determining the identity of the particular explosive. In fact, the dominant features in both the mid-IR and Raman spectra of TATP correspond to the combination of the ring O–O and C–O stretching modes (see, e.g., Reference [19] and references therein). It is noteworthy that the peroxide O–O stretch is not IR active since it does not induce a change in dipole moment; however, this mode is active in Raman spectroscopy as indi...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• About the Editors
• List of Contributors
• Preface
• Section I: Introduction
• Section II: Criminal Scene
• Section III: Counter Terrorism and Homeland Security
• Section IV: Drugs and Drugs of Abuse
• Section V: Art
• Section VI: Archaeology and Mineralogy
• Section VII: Counterfeit Consumer Products
• Index
• Colour Plates