The Quadrupole Resonance (QR) technique is capable of...

The Quadrupole Resonance (QR) technique is capable of unambiguously identifying a wide range of chemical compounds including specific materials such as explosives and narcotics. Worldwide, in the region of 10,000 different chemical substances have been detected by this technique. QR is one of modern research methods in Physics for the analytical detection of chemical substances in solid form. QR is a radio-frequency (RF) spectroscopy, and it is defined as a phenomenon of resonance RF absorption or emission of electromagnetic energy. It is due to interactions between asymmetrically distributed charges of the atomic nucleus or quadrupole moment and the atomic shell electrons as well as those charges that are outside the atomic radius. Thus, all changes in the quadrupole coupling constants and QR frequencies are meant to be due to their electric origin.

The QR method, is closely related to Magnetic Resonance Imaging ("MRI"), which is widely employed by both industry and the medical profession. In fact, QR is often referred to as zero field MRI; so called because the magnetic field normally used to align the magnetic moments of the nuclei in MRI is absent in QR. A magnetic field is not required in QR because the electric quadrupole moments of the nuclei are already aligned by the electric field gradients of the surrounding molecules. The process of aligning the electric quadrupole moments of the nuclei also aligns their magnetic moments. The requirements in MRI for a large uniform magnetic field make the apparatus bulky and expensive. The QR technique is, in principal, much simpler and extremely sensitive to the structure of the molecule. The size and frequency of the signals in both techniques are related to the strength of the aligning field; this may be increased in MRI by using a bigger magnet, it is of course fixed in QR being a physical characteristic of the material under examination.

In general, the signal strength and the operating frequency are lower in QR. Operating at lower frequencies has the benefits of deeper penetration, but does, however, require a detection system of greater sensitivity. It is towards this goal that the QRSciences QR research has been focused and already considerable progress has been made, to the extent that commercial applications are now a reality.

This technique is known as "pure QR", or direct QR detection, and has a lot of advantages for some applications, such as identification of specific compounds. In turn direct QR detection techniques can be subdivided into two main areas: oscillator-detector methods (continuous-wave (CW) etc) and pulsed methods. At present the pulsed method is the most widely used direct QR method. The use of various multi-pulse sequences permits a considerable increase in the sensitivity, and decreases the time of the experiment.