**1. Introduction**

160 Multivariate Analysis in Management, Engineering and the Sciences

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The detection of harmful materials in bulk and trace levels present in different matrices: gases/vapors, liquids, and solids is an important consideration for the development of sensors and standoff detection systems for use in National Defense and Security applications. Hazardous chemicals such as highly energetic materials (HEM), homemade explosives (HME), chemical and biological agents are classified as imminent threats, providing terrorists with ways to cause damage to civilians or troops. Chemical warfare agents (CWA) are usually classified as skin-damaging, nerve agents and toxins [1]. Examples of exposures have occurred since World War I with the development of chlorine, phosgene, cyanide and sulfur mustard which were also used in the Iran-Iraq war. In recent times, terrorist attempts involving chemical warfare agents have occurred all over the World as in the case of Sarin (Japan, 1994) and Ricin (London and Paris, 2003) [2]. Threat perception of chemical warfare agents has increased since September 11, 2001 [3,4]. Exposure to low levels of these chemical agents can cause respiratory problems, eye irritation, choking and blisters. LD50 (mg/kg) values for Nerve agents include Tabun (0.08), Sarin (0.01), Soman (0.025) and VX (0.007) [5].

Most studies that have been published about detection of these compounds are based on spectroscopic and chromatographic (GC and HPLC) methodologies [6-11]. Vibrational spectroscopy has demonstrated to be valuable for the detection of HEM, HME, CWA and Simulants (CWAS) and Toxic Industrial Compounds (TIC). In particular, infrared spectroscopy (IRS) and Raman spectroscopy (RS) in various modalities have played unique roles in threat compounds detection [6,12-19]. IRS and RS can be employed for detection of

© 2012 Hernández-Rivera et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Hernández-Rivera et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Explosives and Chemical Warfare Agents, as well as other chemical and biological threats in airports, in military environments, in government buildings and other public safety places. Raman Spectroscopy employs a non-invasive approach that provides high resolution and specificity. Some applications of IRS and RS includes lab based characterization studies as well as forensic field studies of organic and inorganic substances through their vibration signatures.

Multivariate Analysis in Vibrational

Spectroscopy of Highly Energetic Materials and Chemical Warfare Agents Simulants 163

measurements were carried out at 10 m by employing a frequency-doubled 532 nm Nd:YAG pulsed laser as excitation line, the quantification study was performed by using partial least squares regression analysis (PLS), Interval-PLS (iPLS) and Synergy-PLS (siPLS) as chemometrics tools to achieve the best correlation between the remote Raman signal and the

In the second case study discussed, Optical Fiber Coupled Raman Spectroscopy (OFP-RS) was employed at 488 nm excitation wavelength for detection of a Chemical Warfare Agent Simulant (CWAS): triethyl phosphate (TEP) inside different commercial bottles: greenplastic, green-glass, clear-plastic, clear-glass, amber-glass and white plastic. Aqueous solutions were also used to discriminate on various bottle materials in commercial beverage

In a third case study a Fourier Transform infrared interferometer with MCT detector was used for recording vibrational infrared signals from nitroaromatic and peroxide explosives in the gas phase. Furthermore, a dispersive IR HEM detection system using a quantum cascade laser was used to record MIR spectral signals of 2,4,6-trinitrotoluene, pentaerythritol tetranitrate and cyclotrimethylenetrinitramine on travel baggage surfaces. Several models

Explosives compounds employed in the studies illustrated were pentaerythritol tetranitrate (PETN) cyclotrimethylenetrinitramine (RDX), triacetone triperoxide (TATP), 2,4 dinitrotoluene (2,4-DNT), 2,4,6-trinitrotoluene (TNT) were synthesized in the laboratory according to methods described by Urbanski [27] and pharmaceutical active compound: acetaminophen (APAP) was purchased from Aldrich-Sigma Chemical Co. (Milwaukee, WI). Powder mixtures were prepared employing the both compounds (PETN and APAP) mentioned above and the compositions of PETN in the mixtures was varied from 1 to 34% w/w. Components of the mixtures were carefully weighed and mixed using an agate mortar

were generated with and without preprocessing throughout MIR spectrum.

ensuring homogeneity throughout the sample with a total weight of 200 mg.

**Figure 1.** Commercial bottles (glass and plastic) used to TEP detection using Optical Fiber Coupled

**2. Description of methodologies** 

concentration (%) of PETN explosive in a mixture with pharmaceutical compound.

products.

Raman spectroscopy.

Optical Fiber Probes (OFP) have been employed in biomedical applications, in communications, in coupling instrumentation to sensing probes and other important modern applications. Moreover, their uses have been extended to excitation and detection of Raman and infrared signals [16, 20]. Fiber optics applications to Raman Spectroscopy can take advantage of a favorable excitation radiation distribution within the sample; allowing the use of higher laser power levels which, in turn, can yield an elevated signal-to-noise ratio (SNR) for a given experiment without increasing the risk of photo-damaging analytes [21, 22]. In 2011 Ramírez-Cedeño et al. utilized Optical Fiber Coupled Raman Spectroscopy (OFC-RS) to detect hazardous liquids concealed in commercial products [6]. They proved that an optical fiber coupled Raman probe was able to discriminate hazardous liquids inside consumer products from common drinks. Elliason et al. (2007) have also reported drug and liquid explosives detection in concealed in colored plastic containers [23].

Recently infrared spectroscopy has shown progress in the use of more powerful IR sources, such as Quantum Cascade Lasers (QCL) by incorporating these devices in IR reflectance, IR transmission and even in IR microscopy applications [24]. QCL-based setups are being developed for in the field applications such as breath analysis, environmental research, airborne measurements, security applications, laser-based isotope ratio measurements, and many others. In particular, for security applications, optical methods are advantageous because of their capability for remote and standoff detection [14, 25, 26]. Due to improvements in QCL development, mid infrared lasers operating at room temperature with high output powers in the CW regime are commercially available and make it possible to set up a ruggedized system that allows sensing of explosives and others materials outside the laboratory and the ability to enter real world scenarios. With laser based standoff spectroscopy, the detection distance can be a few meters to tens of meters. Because of the inverse square dependence of light intensity, larger distances require high power, collimated light sources such as lasers. For homeland security applications such as detection of suicide bombers or improvised explosive devices, a distance of 50-100 m is generally sufficient.

In this chapter we illustrate the usefulness of incorporating powerful statistical routines to all traditional chemistry disciplines: Chemometrics is the application of statistical tools to plan, execute and analyze experiments in chemistry. To illustrate the power of Chemometrics techniques to analyze experiments in chemistry we have chosen three case studies, all involving identification, quantification, discrimination and classification of chemical threats in different matrices from vibrational spectroscopy multivariate data.

In the first case study a remote Raman detection study was performed for quantification of HEM such as pentaerythritol tetranitrate (PETN) present in different mixtures. The remote measurements were carried out at 10 m by employing a frequency-doubled 532 nm Nd:YAG pulsed laser as excitation line, the quantification study was performed by using partial least squares regression analysis (PLS), Interval-PLS (iPLS) and Synergy-PLS (siPLS) as chemometrics tools to achieve the best correlation between the remote Raman signal and the concentration (%) of PETN explosive in a mixture with pharmaceutical compound.

In the second case study discussed, Optical Fiber Coupled Raman Spectroscopy (OFP-RS) was employed at 488 nm excitation wavelength for detection of a Chemical Warfare Agent Simulant (CWAS): triethyl phosphate (TEP) inside different commercial bottles: greenplastic, green-glass, clear-plastic, clear-glass, amber-glass and white plastic. Aqueous solutions were also used to discriminate on various bottle materials in commercial beverage products.

In a third case study a Fourier Transform infrared interferometer with MCT detector was used for recording vibrational infrared signals from nitroaromatic and peroxide explosives in the gas phase. Furthermore, a dispersive IR HEM detection system using a quantum cascade laser was used to record MIR spectral signals of 2,4,6-trinitrotoluene, pentaerythritol tetranitrate and cyclotrimethylenetrinitramine on travel baggage surfaces. Several models were generated with and without preprocessing throughout MIR spectrum.
