How Does the RF B1 Field in NMR Spectrometry Flip Net Magnetization?

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SUMMARY

The RF B1 field in NMR spectrometry is crucial for flipping net magnetization by 90 or 180 degrees. This mechanism involves a circularly polarized RF field that matches the precession frequency of the magnetic moment in a rotating frame, effectively making the main magnetic field appear zero. The strength of the RF field directly influences the precession rate, allowing precise control over the tip angle by adjusting the duration of the RF application. The discussion highlights the importance of understanding frequency distribution and natural broadening in real experiments.

PREREQUISITES
  • Understanding of NMR (Nuclear Magnetic Resonance) principles
  • Familiarity with magnetic moments and precession
  • Knowledge of RF (Radio Frequency) field characteristics
  • Basic concepts of phase manipulation in spectroscopy
NEXT STEPS
  • Research the principles of circularly polarized RF fields in NMR
  • Learn about the mathematical modeling of precession rates in magnetic fields
  • Explore the effects of natural broadening on RF field applications
  • Study the impact of RF field strength on magnetization flipping angles
USEFUL FOR

Researchers, physicists, and chemists working with NMR spectrometry, as well as students seeking to deepen their understanding of magnetic resonance techniques and their applications in various scientific fields.

Wadings
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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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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.
 
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.
 

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