<title>Signal processing considerations in NMR detection of liquid explosives</title>

Abstract

High-resolution nuclear magnetic resonance (NMR) provides detailed spectral information on the molecular level. As such, it has been a principal tool of chemists since the early 1950s and used to probe the molecular structure, configuration and composition of liquids. However, high-resolution NMR techniques require very homogeneous DC and RF magnetic fields and are thus inappropriate as the basis for a practical liquid explosives screening system. These field requirements are relaxed for low-resolution NMR, but the unique spectral features of the high-resolution NMR signal are obscured. In spite of this, low-resolution NMR does provide substantial chemical information regarding liquids. Specific parameters available from low- resolution NMR include the signal amplitude (A<SUB>0</SUB>), the spin-lattice relaxation time (T<SUB>1</SUB>), the spin-spin relaxation time (T<SUB>2</SUB>), the diffusion constant (D), and the spin-spin coupling constant (J). Sequences of RF pulses can be designed to respond to one or more of these parameters and, therefore, unique NMR signatures for various liquids can be defined. General considerations of the relative importance of each of these parameters, along with practical considerations regarding allowable scanning and data processing times, will in large part determine the nature of signal processing methods to be used in the NMR Liquid Explosives Screening System.