SETTING UP YOUR EXPERIMENT
   
Designing an NMR experiment:
Pulse width.
  The presence of a net magnetization vector in a magnetic field does not, by itself, generate an NMR spectrum. In order to create an observeable signal, the magnetization must be altered; this is accomplished by exposing the sample to radiofrequency (rf) radiation. When the radiofrequency matches that of a precessing magnetic moment, a resonance occurs. This will ultimately be seen as a peak in the NMR spectrum, but in the vector representation of the process, a resonance is indicated by a tipping of the net magnetization away from the +z axis down toward the +x axis, as illustrated below. The angle through which it is tipped depends on the length of time the rf burst is left on. Position your mouse over the image below to see the effect of a short pulse along the -y axis (in blue), which moves the magnetization through an angle of only 30o.
 
 

After the pulse, the magnetization vector returns slowly back to its original position along the +z axis. This alignment is called the equilibrium magnetization, and the return trip lasts much longer than the pulse . The rate at which the magnetization returns to equilibrium can reveal important information about the dynamics (motions) of the molecules in the sample, but this is a specialized kind of NMR study.

This action generates a small, but detectable, signal, along the +x axis, as shown below. The magnitude of this signal is equal to the projection of the tipped magnetization vector onto the observation axis. You can think of a projection as the shadow that would be cast by the magnetization if a light were positioned directly over the +z axis. The green arrow in the figure below illustrates this idea.