DETECTION OF HAZARDOUS MATERIALS USING RAMAN SPECTROSCOPY

Abstract

Hazardous materials have always been one of the most challenging threats to humanity and increasing with time. Hazardous materials cover a broad range of materials like industrial wastage, pollutant gases, poisonous compounds, biological agents causing diseases, narcotics, chemical & biological warfare agents, explosive materials etc. The management of hazardous materials requires monitoring of such materials. So the detection of hazardous threats always remains the top priority of law enforcing agencies. Along with the conventional threats of smuggled weapons & poisons and new threats like explosives, chemical and biological agents has surfaced and posed a serious challenge for society and particularly the law enforcement agencies. The explosive threats i.e. IED blasts, landmine blasts etc. are increasing day by day throughout the world and cause the loss of life of innocent people. Such incidents may be surely minimized by the development of reliable, portable and cost effective explosive detectors; which is the motivational force for carrying the present study and contributing towards safeguarding innocent people and society as a whole against explosive threats. The detection of explosive threats focuses on the detection of bombs, bomb makers, and bomb placers. The constituents of a bomb or IED include ignition system, detonator, booster and main charge. The electronic components like battery, wires etc. may not be an indicator of bombs/IEDs exclusively as these components are used in ordinary items also. The detection of the presence of explosives material is the ultimate indicator of a bomb and may be helpful in preempting the explosion. Further detection of explosive materials plays a vital role in the field of post-blast investigations particularly in case of partial detonation which may take place, results in unburned residual quantities on the blast sites. Such unburned residuals of explosive materials may be found deposited on nearby surfaces especially metallic surfaces or mixed in the soil. The factors involved in the human machine interface depend on the real scenario and should be incorporated at the design level in the process of the development of a particular explosive detector. Laser spectroscopy techniques particularly Raman spectroscopy offers high potential in the identification of explosive materials with high specificity i.e. with a low false viii rate. Raman spectroscopy involving laser as excitation source also promises with the detection of explosive material in the stand-off mode which is required to ensure the safety to the operator or human behind the machine. In the thesis, the potential of different types of Raman scattering based techniques has been investigated for the different real-field scenarios. Chapter 1 provides a basic idea of hazardous materials and deals with the insight of the field of explosive detection in detail like the necessity of detection of hazardous materials especially explosives, different real scenarios etc. Further, the potential techniques for the detection of explosives both non-optical techniques like ion mobility spectrometry, mass spectrometry, gas chromatography, surface-acoustic wave & nuclear-quadrupole resonance; and optical techniques like laser-induced breakdown spectroscopy, photo-dissociated laser-induced fluorescence, cavity-ring down spectroscopy, laser-based photoacoustic spectroscopy, THz spectroscopy and Raman spectroscopy have been discussed. Furthermore, different types of Raman scattering techniques like normal Raman spectroscopy, resonant Raman spectroscopy, time-gated Raman spectroscopy, surface-enhanced Raman spectroscopy, surface enhanced resonant Raman spectroscopy and spatial-offset Raman spectroscopy have been covered. Chapter 2 provides the theoretical and instrumental aspects of the field of Raman scattering. Classical theory and quantum theory have been described with their importance in the explanation of the nature of Raman scattering. The selection rules of Raman spectroscopy and IR absorption spectroscopy and their differences have been explained. Instruments used in the field of Raman spectroscopy have been discussed in detail. Different types of light sources both conventional light sources like mercury lamp, xenon lamp etc. and lasers have been discussed. Different optical schemes like 90-degree configuration and 180-degree configuration have been explained and correlated to the real scenarios. Further optical components like lenses, filters, mirrors, fibers etc. are discussed with specifying their roles in the field of Raman spectroscopy. Different types of wavelength selectors or spectrometers like monochromators, spectrographs and interferometers are discussed in terms of ix application and corresponding detector. Finally, the frequency calibration of spectrometers is discussed. Chapters 3 deals with the studies of explosive materials like ammonium nitrate and p-nitrobenzoic acid in soil samples and through the different types of plastic bottles. Raman spectra of soil samples of different concentrations of ammonium nitrate have been recorded and analysed in terms of the signal-to-noise ratio. Further the effect of integration time on the Raman spectra of these soil samples studied. The potential of the processing of the Raman spectra in terms of the background has been evaluated by recording Raman spectra of lower concentration soil samples with an improved signal-to-noise ratio. Further Raman spectra of p-nitrobenzoic acid through different plastic bottles have been recorded. The importance of the processing of Raman spectra in terms of background and fluorescence of plastic material have been investigated using plastic bottles of different materials. Chapter 4 deals with the development of a customized algorithm for the identification of materials in real time. The real-time detection or identification of the materials in the field itself is one of the main required features of explosive detectors. The continuous wavelet transform (CWT) has been explained in detail. An algorithm has been developed based on CWT for the real-time identification of explosive materials. The Maxican Hat wavelet has been selected for its matching with the nature of the peaks of Raman spectra. Further, the methodology has been evolved and based on which algorithm has been developed represented in form of flow chart. The operational parameters have been sorted out and the graphic user interface is developed accordingly. Further, the developed algorithm and graphic user interface are evaluated by detecting and identifying very similar materials like 2,4,6- dinitrotolune, 1,3,5-trinitrobenze, 2,4,6-trinitrotoluene, ammonium nitrate, potassium nitrate, sodium nitrate, urea nitrate, barium nitrate etc. Chapter 5 presents the trace detection of explosive materials and their derivatives using time-gated Raman spectroscopy. The instrumentation of the time-gated Raman spectroscopy techniques has been explained. The effect of different experimental parameters on the signal-to-noise ratio of peaks of Raman spectra has been x investigated. First, the potential of time-gated Raman spectroscopy over normal Raman spectroscopy has been demonstrated by recording Raman spectra of low concentration sample of 1,3,5-trinitrobenzene using continuous mode and gated mode of intensified charged coupled device. Further, the effect of gain, pulse accumulation and pulse energy has been studied and found that the intensity i.e. sensitivity increases with an increase in any of these parameters. Finally, Raman spectra of low concentration explosive samples like 100 ppm have been recorded successfully. Chapter 6 deals with the detection of explosive materials through translucent plastic bottles using spatially offset Raman spectroscopy. The potential of the technique has been investigated with the excitation wavelength in the visible region. The capability of spatially offset Raman spectroscopy over the normal Raman spectroscopy is evaluated by recording Raman spectra of urea and sodium nitrate through their commercial translucent plastic bottles using both techniques. The effect of spatial offset inserted between incident laser spot and collection spot and integration time is investigated. The Spatially offset Raman spectroscopy ratio has been plotted with respect to spatial offset. Finally, the capability of spatially offset Raman spectroscopy technique has been studied for reduced collection of fluorescence from the container material. Chapter 7 deals with the theoretical study of MDMA molecule for its structural, spectral and thermal characteristics. Geometry optimization of MDMA is performed using DFT and HF methods with different basis sets. Mulliken charge and MEP are studied for negative and positive sites of the molecule. Molecular orbital characteristics like HOMO-LUMO energies, energy gap, ionization potential, electron affinity, global hardness, chemical potential have been investigated. Thermal properties like SCF, zero-point energy, rotational constants, dipole moments are estimated. Enthalpy, specific heat and entropy are investigated in terms of temperature effect in the range 50 K – 700 K. Vibrational analysis are performed on optimized geometries of MDMA with different levels of theory. The vibrational frequencies are scaled and found in an excellent match with experimental values. xi Chapter 8 represents the conclusion of the research work reported in the thesis. The main outcomes of the research work reported have been brought out clearly. Further, the future prospects of the work has been discussed. The important findings and conclusions of the thesis are mentioned below:  Simulated samples of post-blast scenarios have been successfully prepared and studied by recording their Raman spectra at different concentrations of explosive materials.  The capability of detection through transparent plastic bottles is evaluated by recording Raman spectra of explosive materials successfully through transparent plastic bottles of different materials, thickness and transmission.  Customised algorithm for real-time acquisition of Raman signal, its processing for background and fluorescence corrections and identification by peak matching with database has been developed and evaluated.  The capability of time-gated Raman spectroscopy was evaluated by recording of Raman spectra of explosive samples of low concentrations i.e. 1000 ppm, 500 ppm and finally 100 ppm.  The capability of SORS technique is successfully evaluated by recording Raman spectra of explosive materials through translucent-containers and effect of spatial offset on sensitivity was studied.  Theoretical calculations on MDMA molecule have been performed and its structural, thermal and spectral characteristics were estimated and found in good agreement with experimental values. The future prospective of the reported work has been summarized below:  To carry out experimental work of SORS technique with SORS probes at 532nm and 785 nm.  To enhance range and sensitivity of detection of explosive materials.  To study the spectral signatures of hazardous molecules, both theoretically and experimentally, as per requirement of law-enforcement agencies.