Nuclear Magnetic resonance

In summary, the rf B1 field in an NMR spectrometer works by circularly polarizing the magnetic moment and causing it to precess about the main field at a specific frequency. The strength and duration of the rf field can be adjusted to control the angle of precession and ultimately flip the net magnetisation by 90 or 180 degrees. However, this process is not exact due to natural broadening of the rf field's frequency.
  • #1
Wadings
3
0
NMR rf Field

hi all,
could anyone help me explain how the rf B1 field in an NMR spectrometer works? Especially the mechanism it uses to flip the net magnetisation by an angle of 90 or 180 degrees?
Would be very gratefull for a link or and explanation to this.

Thanks
Wadings
 
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  • #2
I found this site very useful when studying NMR a few years ago

http://www.cis.rit.edu/htbooks/nmr/

Not sure if it gives the actual mechanism of changing the phase, but it's worth having a look.

Claude.
 
  • #3
A magnetic moment precesses about the net field. It's precession rate is proportional to the strength of the net field. Now imagine that the rf field is circularly polarized (usually is in modern NMR or MRI) and has frequency omega which happens to be the frequency associated with the main field (the
usually large polarizing field). Then in a frame rotating at omega the rf field appears static and the main field appears to be zero. In this frame the magnetic moment then precesses about the field
due to the rf field. The stronger the rf field strength the greater the precession rate. So knowing that rate one can determine how long to apply rf field to achieve a given tip angle.

Note that this argument is only approximate because it assumes a well defined frequency omega. In a real experiment the frequency of the applied rf-field is not delta function-like in its distribution due to its finite lifetime - natural broadening.
 

What is Nuclear Magnetic Resonance?

Nuclear Magnetic Resonance (NMR) is a technique used in chemistry and physics to study the properties of atoms and molecules. It works by measuring the response of atomic nuclei to a magnetic field, providing information about the structure and dynamics of molecules.

How does NMR work?

NMR works by placing a sample of molecules in a strong magnetic field, causing the nuclei of the atoms to align with the field. A radiofrequency pulse is then applied, causing the nuclei to absorb energy and change their alignment. When the pulse ends, the nuclei return to their original alignment, releasing energy that can be measured and analyzed.

What types of information can be obtained from NMR?

NMR can provide information about the number and types of atoms present in a molecule, as well as their chemical environment and connectivity. It can also reveal information about molecular motion, such as rotation and vibration.

What are the applications of NMR?

NMR is widely used in the fields of chemistry, biochemistry, and medicine for structural and dynamic studies of molecules. It is also used in quality control and drug development in the pharmaceutical industry, and in environmental and forensic analysis.

What are the advantages of NMR over other analytical techniques?

NMR has several advantages over other analytical techniques, including its non-destructive nature, high sensitivity, and ability to provide structural and dynamic information. It is also a relatively simple and versatile technique, and can be used to study a wide range of samples, from small molecules to large biomolecules.

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