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The mystery of nuclear quadrupole resonance: How does it work in zero magnetic field?
Nuclear quadrupole resonance spectroscopy (NQR) is a chemical analysis technique related to nuclear magnetic resonance (NMR). Unlike NMR, the NQR technique can detect nuclear transitions in the absence of an external magnetic field, so it is often called "zero-field NMR". The occurrence of nuclear quadrupole resonance depends on the interaction between the electric field gradient (EFG) and the quadrupole moment of the nuclear charge distribution. This interaction makes NQR effective for analyzing solid materials, but not for liquids, where the electric field gradient near the nucleus averages to zero.
Principle
Any nucleus with more than one unpaired nuclear particle (such as a proton or neutron) will have an electric quadrupole moment, which makes its energy levels uneven due to the interaction between the nuclear charge and the electron impurity density. Offset. When radio frequency electromagnetic radiation is applied to the nucleus in an environment without an external magnetic field, the disturbance of the nuclear energy will cause absorption of the quadrupole energy level. This enables NQR to characterize different phase transitions in solid materials."Nuclear quadrupole resonance spectroscopy is extremely sensitive for analyzing chemical structural changes and phase transitions in matter."
Analogies between NQR and NMR
In NMR, nuclei with a rotational spin equal to or greater than 1/2 will produce energy splitting due to the external magnetic field, resulting in resonance absorption. In NQR, nuclei with rotational spin equal to or greater than 1 (such as 14N, 17O, etc.) have an electric quadrupole moment, the existence of which is due to the non-spherical shape of the nuclear charge distribution. Therefore, NQR technology, if performed correctly, can provide a chemical fingerprint of a substance.
“The NQR spectrum is a unique chemical fingerprint that reveals tiny differences between materials.”
Applications and potential
NQR can deeply explore the interaction between the nuclear quadrupole moment and the electric field gradient around the nucleus. Therefore, NQR shows excellent application potential in the study of structural characteristics, chemical bonding and phase transitions of solid-state compounds. For example, in the pharmaceutical field, 14N-NQR has been successfully applied to distinguish the enantiomers in racemic mixtures, such as D-serine and L-serine. D-serine is considered a potential biomarker for Alzheimer's disease, while L-serine has shown potential in treating lateral sclerosis.
Technical Challenges and Developments
However, technical challenges for NQR remain. The main limitations include the large sample volumes required and low signal intensities. Furthermore, the NQR technique requires the nucleus to have a non-zero quadrupole moment, which is limited to nuclei with a spin number greater than or equal to 1. The low intensity of the NQR signal necessitated the use of a large amount of sample in this study.
Future Outlook
Currently, research teams around the world are working to find applications for NQR in explosives detection. The NQR detection system consists of a radio frequency (RF) power source, a coil that generates a magnetic field, and a detection circuit that can detect the NQR reaction of explosive components from the object being tested. Although some counterfeit devices claim to use NQR technology, they are still widely concerned because they cannot actually achieve the functions they claim.
“NQR also shows great potential in oilfield applications, enabling real-time calculation of the remaining capacity of an oil well and the ratio of water, gas and oil during the extraction process.”
With the advancement of technology and the development of various research projects, the application scenarios of NQR will continue to expand. This technology not only demonstrates its value in scientific research, but may also have a profound impact on our environment and health in our daily lives. Can people make better use of this technology to open a new chapter in nuclear quadrupole resonance?
