Potential of Nuclear Quadrupole Resonance in Pharmaceutical Analysis

Chemistry Department, King's College, Strand, London WC2R 2LS, United Kingdom.
Analytical Chemistry (Impact Factor: 5.64). 08/2005; 77(13):3925-30. DOI: 10.1021/ac0503658
Source: PubMed


Nuclear quadrupole resonance is a radio frequency (rf) spectroscopic technique, closely related to NMR, which can be used to detect signals from solids containing nuclei with spin quantum number >1/2. It is nondestructive, highly specific and noninvasive, requires no static magnetic field, and as such is currently used in the detection of explosives and narcotics. Recent technological advances in pulsed NQR methods have shortened detection times, eliminated spurious signals, and enhanced the sensitivity of detection of 14N frequencies, which lie in the low rf range of 0.4-6 MHz, encouraging a wider range of "real world" applications. This Perspective highlights some of the advantages of NQR, the applications in which it could be used, such as the quantification of pharmaceuticals and the identification of polymorphs. Other roles could include detection, analysis, and quality control of pharmaceuticals at all stages of manufacture. Finally, recent advances which enhance even further the sensitivity of detection will be discussed.

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    • "Variation in the chemical shift or NQR frequency from polymorph to polymorph allows identification of differences in the atomic environment. In addition, NMR and NQR are quantitative analytical techniques, i.e. the intensity of their respective signals is directly proportional to the number of nuclei in the sample [158] [160] [161]. Thus it is possible to estimate the quantitative polymorphous composition or a relative proportion of polymorphs in a sample and the result is reliable. "
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    • "The technique is related to both nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). Unlike NMR and MRI, NQR does not require a large static magnetic field to split the energy levels of the nucleus, making it attractive as a noninvasive technique for detecting explosives in landmines and unexploded ordnance [1]–[5] or screening baggage for narcotics and explosives at airports [6], as well as a relatively inexpensive analytical technique for the structural characterization of various compounds [7], [8]. NQR signals are acquired by applying pulsed RF radiation to the sample, which drives transitions between the quadrupolar energy levels, and then measuring the responses. "
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    ABSTRACT: Nuclear quadrupole resonance (NQR) is a radio-frequency technique that can be used to detect the presence of quadrupolar nuclei, such as the <sup>14</sup>N nucleus prevalent in many explosives and narcotics. In a typical application, one observes trains of decaying NQR echoes, in which the decay is governed by the spin echo decay time(s) of the resonant line(s). In most detection algorithms, these echoes are simply summed to produce a single echo with a higher signal-to-noise ratio, ignoring the decaying echo structure of the signal. In this paper, after reviewing current NQR signal models, we propose a novel NQR data model of the full echo train and detail why and how these echo trains are produced. Furthermore, we refine two recently proposed approximative maximum-likelihood detectors that enable the algorithms to optimally exploit the proposed echo train model. Extensive numerical evaluations based on both simulated and measured NQR data indicate that the proposed detectors offer a significant improvement as compared to current state-of-the-art detectors
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    ABSTRACT: Nuclear quadrupole resonance (NQR) is a radio frequency (RF) technique that detects compounds in the solid state and is able to distinguish between different polymorphic forms of certain compounds. For example, a typical sample of trini- trotoluene (TNT) will contain at least two polymorphic forms with rather different NQR properties. In this paper, we pro- pose a frequency selective hybrid detector that exploits th e presence of such polymorphic forms. The presented detector offers both improved probability of detection, as compared to recently proposed detectors, and allows for an estimation of the relative proportions of the multiple polymorphic forms.
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