90� pulse width calibrations: How to read a pulse width array

One of the most important parts of setting up a Fourier transform–nuclear magnetic resonance experiment is the calibration of the 90° pulse width. There are many ways to calibrate this parameter, but running the experiment as a single array, with the pulse widths ranging from 0° to 450° or more (a nutation experiment), offers a distinct advantage. From this one dataset, a trained eye can evaluate—by simple inspection—as many as nine different aspects of the sample and the spectrometer. These not only include the pw90, but also probe radiofrequency homogeneity, probe arcing, T1 “abuse” (insufficient relaxation delays), radiation damping, off-resonance effects, relaxation during the pulse, nonlinear pulse behavior, and lead pickup.