Tuesday, November 20, 2007
The range of frequency over which NMR resonances are excited depends on the duration of the monochromatic radio frequency pulse applied to the sample. Very long pulses will excite a very narrow range of frequencies whereas very short pulses will excite very wide frequency ranges. For example, presaturation pulses used for solvent suppression, are typically several seconds long while non-selective pulses are typically micro seconds in duration. It is essential to use very short pulses to provide even excitation over large spectral widths. The shape of the excitation profile is related to the Fourier transform of the pulse.