SETTING UP YOUR EXPERIMENT
   
Designing an NMR experiment:
Pulse width (continued).
  The maximum signal is obtained when the magnetization is tipped through 90o, so that it lies along the +x axis, as shown below (move your mouse over the image to re-run):
 
  This is accomplished by simply using a sufficiently long pulse. In fact, the relationship between tip angle and pulse width (or pulse length) is simple - to double the tip angle, double the pulse width. Thus, if a 90o pulse is 6 msec long, a 45o pulse will be 3 msec, and a 30o pulse, 2 msec. In this way, the magnetization can be tipped through any angle desired. A 12 msec pulse is equivalent to 180o, after which the magnetization lies along the -z axis:
 
 
However, note that the projection of the resulting magnetization onto the +x axis is zero; a 180o pulse generates no observeable signal. A pulse of 18 msec tips the magnetization through 270o, placing it on the -x axis, producing a signal of maximum magnitude but negative sign (which would appear inverted, or upside-down, in the final spectrum). A (24 msec) places the magnetization back on the +z axis, also giving no signal. Note that a 360o pulse is not the same as the equilibrium value. The system must return to equilibrium by tracing its way backwards through all 360 degrees.
 
 

NOTE: Pulse angles are often given in radians, so that : p/4 = 45o; 2p/3 = 60o; p/2 = 90o; p = 180o; 3p/2 = 270o; 2p = 360o, and so on.

ANOTHER NOTE: The pulse angle to time conversions given above (such as 90o = 6msec) are examples only. The exact conversion factor varies from instrument to instrument, and can, in fact, change with time on a given spectrometer, depending on equipment conditions.