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Multiparameter NMR Identification of Liquid Substances
Abstract
In this paper we study the method of distinguishing the substances by measuring their nuclear paramagnetic longitudinal and transverse relaxation times and the diffusion coefficient of molecules. Experiments performed using a commercial high magnetic field NMR spectrometer show the possibility to use this method for reliable identification of liquids. Observables in these experiments rather often cannot be described by a single exponential function. In the article we discuss how to utilize the non-single exponential experimental dependences for a quantitative processing of the NMR experimental results.
... In many studies, the conclusion has been made that the TD-NMR is an effective non-invasive, non-contact method to detect content of concealed liquids. Analysis of literature, as well as our recent studies (see Refs. [15,16]), however, shows that for reliable discrimination between a large number of liquids, the scanning device has to involve some additional parameters. They could be NMR ones (not only T2 and T1 relaxation times probed by standard TD-NMR, but also the proton density, diffusion constant [15][16][17], etc.). ...
... Analysis of literature, as well as our recent studies (see Refs. [15,16]), however, shows that for reliable discrimination between a large number of liquids, the scanning device has to involve some additional parameters. They could be NMR ones (not only T2 and T1 relaxation times probed by standard TD-NMR, but also the proton density, diffusion constant [15][16][17], etc.). Another approach is to use techniques, which are totally different from NMR, e.g. ...
The detection of explosives and illicit substances is a very important problem nowadays. Especially, improvised explosive devices (IEDs) manufactured from homemade explosive materials has become an issue of growing importance. In this work, recent developments in the field of time domain (TD) NMR and microwave/sub-THz detection of liquid substances are reviewed. It has been shown that both TD NMR and MW dielectric spectroscopy are potentially very effective techniques to detect content of concealed liquids.
... An alternative, simple approach of TD NMR measurements in non-uniform static magnetic field may be used to obtain information on self-diffusion constant as well [5]. Unfortunately, there is no obvious law for T 1 /T 2 /D value distribution in different groups of liquids [3,4,6,7,8]. Thus, 1 H NMR studies on the detection of dangerous liquids showed that the NMR T 2 and T 1 relaxation times (and in some cases D) can be used as discriminators of substances in a bottle scanner only for the case of limited number of liquids probed or additional pre-known information used. ...
The T1 and T2 relaxation times of some nitrogen-containing liquids were measured by ¹⁴N pulse nuclear magnetic resonance (NMR) in the magnetic field of 0.575 T. The liquids measured in this paper were energetic and toxic substances as well as some electrolytes. A method of the fast discrimination of dangerous liquids by the low field ¹⁴N NMR relaxation parameters was proposed. It is shown that flammable and toxic nitrogen-based liquids have small values of T1 and T2, therefore these liquids may be discriminated from the group of less dangerous liquids.
Fast and reliable detection of liquids will be required for future checkpoint screening techniques. Recently, a new electromagnetic-wave concept based on our high-T c Josephson detectors and Hilbert spectroscopy has been suggested to distinguish between liquids. This technology covers a spectral range of main dispersions of liquids, from a few GHz to a few THz, and thus significantly enhances reliability of identification. The high-T c detectors, due to a power dynamic range of more than five orders, might guarantee short identification times. Several demonstration set-ups of liquid identifiers, consisting of high-T c Josephson detectors, integrated in Stirling coolers, and polychromatic radiation sources, have been developed and characterized. Reflection polychromatic spectra of various liquids in plastic containers have been measured at the spectral range of 15-500 GHz with total scanning time down to 0.2 second. Reliable identification of liquids, both benign and threat, within an accuracy of 0.3% was demonstrated using water as a reflectance reference. The reflectance values for 30%H 2 O 2 /H 2 O solution at frequencies of 30 and 100 GHz were practically undistinguishable from that of for pure water, but an increase of the relative reflectance from 1.017 at 282 GHz to 1.033 at 434 GHz has been found. Last circumstance will be used for optimization of the identifiers.
Liquid explosives pose a threat to security on airplanes and other public places, since they can easily be concealed as benign
liquids. A detector, able to quickly identify liquids, would increase the possibility to detect such threats and speed up
security checks. As a step towards a long-term goal to develop a liquid explosive detector, we have constructed an experimental
setup based on a low-cost 1.1T permanent magnet with huge static magnetic field gradient of 4.8T/m, which allows us to measure
proton relaxation times T
1 and T
2 and the self-diffusion coefficient D in liquid samples in a thin slice excited by radio-frequency pulses. We have developed a simple model in order to explain
diffusion-enhanced non-exponential magnetization recovery in inversion recovery T
1 experiment in this setup. Measuring a wide variety of liquid samples, we have demonstrated that it is possible to discriminate
between the liquids based solely on these parameters. We discuss further improvements to the detection method, among those
the choice of magnetic field, based on the fast field-cycling measurements.
In this work, we present a novel method for non-invasive identification of liquids, for instance to allow for the detection of liquid explosives at airports or border controls. The approach is based on a nuclear magnetic resonance technique with an inhomogeneous magnetic field, forming estimates of the liquid's spin-spin relaxation time, T2, and diffusion constant, D, thereby allowing for a unique classification of the liquid. The proposed detectors are evaluated using both simulated and measured data sets.
1. Elements of Resonance.- 2 Basic Theory.- 3. Magnetic Dipolar Broadening of Rigid Lattices.- 4. Magnetic Interactions of Nuclei with Electrons.- 5. Spin-Lattice Relaxation and Motional Narrowing of Resonance Lines.- 6. Spin Temperature in Magnetism and in Magnetic Resonance.- 7. Double Resonance.- 8. Advanced Concepts in Pulsed Magnetic Resonance.- 9. Multiple Quantum Coherence.- 10. Electric Quadrupole Effects.- 11. Electron Spin Resonance.- 12. Summary.- Problems.- Appendixes.- A. A Theorem About Exponential Operators.- B. Some Further Expressions for the Susceptibility.- D. A Theorem from Perturbation Theory.- E. The High Temperature Approximation.- F. The Effects of Changing the Precession Frequency - Using NMR to Study Rate Phenomena.- G. Diffusion in an Inhomogeneous Magnetic Field.- H. The Equivalence of Three Quantum Mechanics Problems.- I. Powder Patterns.- J. Time-Dependent Hamiltonians.- K. Correction Terms in Average Hamiltonian Theory - The Magnus Expansion.- Selected Bibliography.- References.- Author Index.
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 (A0), the spin-lattice relaxation time (T1), the spin-spin relaxation time (T2), 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.
A particularly disturbing development affecting transportation safety and security is the increasing use of terrorist devices which avoid detection by conventional means through the use of liquid explosives and flammables. The hazardous materials are generally hidden in wine or liquor bottles that cannot be opened routinely for inspection. This problem was highlighted by the liquid explosives threat which disrupted air traffic between the US an the Far East for an extended period in 1995. Quantum Magnetics has developed a Liquid Explosives Screening systems capable of scanning unopened bottles for liquid explosives. The system can be operated to detect specific explosives directly or to verify the labeled or bar-coded contents of the container. In this system, magnetic resonance (MR) is used to interrogate the liquid. MR produces an extremely rich data set and many characteristics of the MR response can be determined simultaneously. As a result, multiple MR signatures can be defined for any given set of liquids, and the signature complexity then selected according to the level of threat. The Quantum Magnetics Liquid Explosives Screening System is currently operational. Following extensive laboratory testing, a field trial of the system was carried out at the Los Angeles International Airport.
A Liquid Explosives Screening System capable of scanning unopened bottles for liquid explosives has been developed. The system can be operated to detect specific explosives directly, or to verify the labeled or bar-coded contents of the container. In this system nuclear magnetic resonance (NMR) is used to interrogate the liquid. NMR produces an extremely rich data set and many parameters of the NMR response can be determined simultaneously. As a result, multiple NMR signatures may be defined for any given set of liquids, and the signature complexity then selected according to the level of threat.
A particularly disturbing development affecting transportation safety and security is the increasing use of terrorist devices which avoid detection by conventional means through the use of liquid explosives and flammables. The hazardous materials are generally hidden in wine or liquor bottles that cannot be opened routinely for inspection. This problem was highlighted by the liquid explosives threat which disrupted air traffic between the US an the Far East for an extended period in 1995. Quantum Magnetics has developed a Liquid Explosives Screening systems capable of scanning unopened bottles for liquid explosives. The system can be operated to detect specific explosives directly or to verify the labeled or bar-coded contents of the container. In this system, magnetic resonance (MR) is used to interrogate the liquid. MR produces an extremely rich data set and many characteristics of the MR response can be determined simultaneously. As a result, multiple MR signatures can be defined for any given set of liquids, and the signature complexity then selected according to the level of threat. The Quantum Magnetics Liquid Explosives Screening System is currently operational. Following extensive laboratory testing, a field trial of the system was carried out at the Los Angeles International Airport.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
The pulsed‐gradient, spin‐echo technique has been used to study self‐diffusion of protons in several colloidal systems in order to examine the usefulness of that technique in determining the extent to which the free movement of molecules in these systems is restricted by the colloidal structures present. The pulsed‐gradient experiment is preferred to the steady‐gradient experiment because it affords better definition and control over the time during which diffusion is observed. Diffusion times between 1 sec and 10−3 sec have been used. One artificial system of thin liquid layers, three different kinds of plant cells, and one emulsion have been studied. Clear indications of restricted diffusion are found in all the systems. When fitted to theoretical expressions derived for such behavior, the data yielded a description of each system, as seen by the diffusing molecules, adequately in agreement with the known structure and properties. Critiera for recognizing and analyzing restricted diffusion are discussed. Necessary conditions for the successful study of restricted diffusion are also discussed.
We apply a novel mobile nuclear magnetic resonance (NMR) scanning system, the NMR-MOUSE® (NMR-MOUSE (Nuclear Magnetic Resonance Mobile Universal Surface Explorer) is a registered trademark of RWTH Aachen University), for measuring porosity of geological drill core sections. The NMR-MOUSE® is used for transverse relaxation measurements on water-saturated core sections using a CPMG sequence with a short echo time. A regularized Laplace-transform analysis by the UPEN program yields the distribution of transverse relaxation times. The signal amplitudes and the distribution integrals correlate directly with the porosity of the cores, in spite of the influence of diffusion in the strong field gradient of the NMR-MOUSE®, which is discussed. The method is particularly attractive because it neither requires a volume calibration nor the samples to be machined to fit the coil, and because the device is mobile and particularly attractive for field use such as on logging platforms and research vessels.
A derivation is given of the effect of a time-dependent magnetic field gradient on the spin-echo experiment, particularly in the presence of spin diffusion. There are several reasons for preferring certain kinds of time-dependent magnetic field gradients to the more usual steady gradient. If the gradient is reduced during the rf pulses, H1 need not be particularly large; if the gradient is small at the time of the echo, the echo will be broad and its amplitude easy to measure. Both of these relaxations of restrictions on the measurement of diffusion coefficients by the spin-echo technique serve to extend its range of applicability. Furthermore, a pulsed gradient can be recommended when it is critical to define the precise time period over which diffusion is being measured.
The theoretical expression derived has been verified experimentally for several choices of time dependent magnetic field gradient. An apparatus is described suitable for the production of pulsed gradients with amplitudes as large as 100 G cm−1. The diffusion coefficient of dry glycerol at 26°±1°C has been found to be (2.5±0.2)×10−8 cm2 sec−1, a value smaller than can ordinarily be measured by the steady gradient method.
An increasingly important need today is to guard against terrorist attacks at key locations such as airports and public buildings.
Liquid explosives can avoid detection at security checkpoints by being concealed as beverages or other benign liquids. Magnetic
resonance (MR) offers a safe, noninvasive technology for probing and classifying the liquid contents inside sealed nonmetallic
containers or packages. MR parameters such as relaxation times and signal amplitudes can be used to verify if the container
actually has the liquid that is claimed. This verification, which can be done quickly, will allow for the detection of containers
whose contents have been tampered with and possibly replaced or mixed with liquid explosives. The development of MR technology
for the detection of liquid explosives has been funded by the Federal Aviation Administration. MR can also be potentially
used for the detection of hazards such as chemical warfare agents and contraband such as dissolved narcotics.
Mobile NMR devices are suited for application at airports. They are low in maintenance, easy to use and offer safe operation.
Recently, great progress has been achieved in developing permanent magnets with high field homogeneity. Achieving high resolution
proton spectroscopy would provide a fast and safe method to identify liquid explosives, without the need of cross checks with
other techniques.
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NMR relaxation data and those from many other physical measurements are sums of exponentially decaying components, combined with some unavoidable measurement noise. When decay data are inverted in order to give quasi-continuous distributions of relaxation times, some smoothing of the distributions is normally implemented to avoid excess variation. When the same distribution has a sharp peak and a much broader peak or a "tail," as for many porous media saturated with liquids, an inversion program using a fixed smoothing coefficient may broaden the sharp peak and/or break the wide peak or tail into several separate peaks, even if the coefficient is adaptively chosen in accord with the noise level of the data. We deal with this problem by using variable smoothing, determined by iterative feedback in such a way that the smoothing penalty is roughly constant. This uniform-penalty (UP) smoothing can give sharp lines, not broadened more than is consistent with the noise, and in the same distribution it can show a tail decades long without breaking it up into several peaks. The noise level must be known approximately, but it can be determined more than adequately by a preliminary inversion. The same iterative procedure is used to implement constraints such as non-negative (NN) or monotonic-from-peak (MT). The significance of an additional resolved peak may be tested by finding the cost of using MT to force a unimodal solution. A bimodal constraint can be applied. Decay data representing sharp lines in contact with broad features can require substantial computing time and some controls to stabilize the iterative sequence. However, UP can be made to function smoothly for a very wide variety of decay curves, which can be processed without adjustment of parameters, including the dimensionless smoothing parameters. Extensive testing has been done with artificial data. Examples are shown for artificial data, biological tissues, ceramic technology, and sandstones. Expressions are given relating noise level to line width and for significance of increase or decrease in error of fit.
Multidimensional NMR techniques used in the measurement of molecular displacements, whether by diffusion or advection, and in the measurement of nuclear spin relaxation times are categorised. Fourier-Fourier, Fourier-Laplace and Laplace-Laplace methods are identified, and recent developments discussed in terms of the separation, correlation and exchange perspective of multidimensional NMR spectroscopy.
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