EXPERIMENTAL ASPECTS OF CHIRP NMR-SPECTROSCOPY

Chirp pulses have been used to excite transverse magnetization over bandwidths that are about 50 times wider than the amplitude of the RF field. Experimental profiles are shown of the amplitudes and phases as a function of offset. The efficiency of refocusing by chirp pulses over large bandwidths is proven by experiment. It is shown that RF inhomogeneity leads to an attenuation of the echo intensity but does not perturb its phase. The excitation of multiple-quantum coherences and their reconversion into single-quantum magnetization by chirp pulses is demonstrated experimentally for pairs of carbon-13 nuclei with large chemical-shift differences. The performance of chirp double-quantum excitation methods is compared with that of techniques using monochromatic composite pulses. Various improvements are discussed, such as the smoothing of the RF amplitude profile at the edges of the sweep and the use of phase cycles for the selection of coherence-transfer pathways. The requirements of chirp pulses are compared with those of hard pulses in terms of peak RF power. © 1993 by Academic Press, Inc.