Components: Pulse Width [pw]

If the average equlibrium magnetization of all the spins in a sample placed in a magnetic field is represented as oriented along the Z axis in a cartesian coordinate system as shown in Figure II-2, then the length of time the radiofrequency pulse (rf energy) is applied along the X axis (for

example) will have a specific effect on the magnetization. The length of time the radiofrequency pulse is applied to the sample is called the pulse width (pw). The pulse width is normally defined in terms of a flip angle in degrees. As shown in Figure II-2, different pulse widths have different effects on the equilibrium magnetization. The 90 degree pulse width is the length of time the

pulse of rf energy is applied to a particular sample to flip (rotate) all the spins from the Z axis into the XY plane, as shown in Figure II-2A. The 90 degree pulse width, specified as pw90 on the Unity+300 and the VXR-S 400 NMR spectrometers depends upon the nucleus, solvent, and the instrument (probe, etc.,). On the Unity+300, pw90 is 13.3 microseconds for proton and 9.7 microseconds for carbon. On the VXR-S 400, pw90 is 35.0 microseconds for proton and 13.0 microseconds for carbon. Routine experiments on these spectrometers are performed using a pulse width much less than the 90 degree pulse width. The pulse width is entered in microseconds by setting pw to the proper value on both the NMR spectrometers.