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Nuclear Magnetic Resonance

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The nuclear magnetic resonance is often heard now a day for imaging, quality control, to determine sample purity of a sample etc. It is an analytical tool used by physicists and chemists to study the structure and dynamics of molecules. In recent years no other technique has grown to such importance as NMR spectroscopy. Lets see what exactly this NMR technique is!.

Definiton

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Nuclear magnetic resonance is well known technique where the nuclei in a magnetic field absorb and even they re-emit the electromagnetic radiation that gives the image which determines the content, sample purity and its molecular structure. So there lies a question how exactly the NMR technique works!
Its well known that many nuclei have spin and every nucleus is charged electrically. Hence if we apply an external magnetic field to the nuclei there will be an energy transfer between lower energy level and higher energy level. The transfer of energy takes place at the wavelength that corresponds to radio frequency and when the spin returns to the lower energy level. It emits energy of same frequency which emits energy yielding an NMR spectrum.
Nuclear Magnetic Resonance


Lets understand it in a more simpler way by discussing about spin!
When a magnetic field is applied the energy gets absorbed in the nuclei that excites from the lower spin state to the upper spin state. Due to this excitation the signal of the NMR spectrum is produced.The external magnetic field strength decides how much is the difference in energy that existing between the two states of the spin. After some time the spin returns to the lower energy level emits the energy at the same frequency. The signal which matches the transfer gives out the NMR spectrum.

Spectroscopy

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Nuclear magnetic resonance is a powerful tool for the structure determination of molecules. In the field of biomedicine two areas of NMR are represented by
  • Magnetic resonance spectroscopy (MRS)
  • Magnetic resonance imaging (MRI)
NMR spectroscopy is a diagnostic technique (also known as Magnetic resonance spectroscopy (MRS)) that provides high quality cross-sectioned two or three - dimensional images of organs without having X-rays or other radiations. The Magnetic resonance spectroscopic imaging (MRSI) is a combination of both spectroscopic as well as imaging methods (Nuclear magnetic resonance imaging (NMI)) that produces spectrum in the sample or the patient.

Instrumentation

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Generally there are two types of NMR instrument:
  • Continuous wave
  • Fourier transform.
Continuous wave NMR spectrometers use the same principle as in optical spectrometers. A strong magnetic field is applied to the sample and the source frequency is scanned slowly (sometimes the frequency of the source is held constant, and the magnetic field is scanned). In Fourier transform Nuclear magnetic resonance, all frequencies of the spectrum are irradiated simultaneously with the radio frequency pulse. By following the radio frequency pulse, the nuclei gets returned to thermal equilibrium. A time domain emission signal is recorded by the instrument as the nuclei relax. The frequency domain spectrum is obtained by using the Fourier transformation.

Applications

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The NMR has a wide application in physics, biochemistry, pharmacy, physiology, food science, veterinary and medicine. Let's see about them :
  • It acts as a great tool in medicine that helps to obtain the clinical images.The MRI technique has proven indispensable for clinicians in diagnosis and monitoring of pathology
  • It can be used to study tissue chemistry and even in chemical labs this technique is used to study molecular structure of various compounds that decides about the purity of the sample
  • The NMR having features coupled with introduction of fourier transform and super conducting magnets has increased the speed, sensitivity and flexibility of the instrumentation that has led to invention of more better computers what we call modern high speed computers.
  • It plays a great role in manufacturing magnetometers.