(Very!) Broadband DOSY

Resonance Offset EffectsNuclei such as 13C, 19F and 31P have large chemical shift ranges, e.g. for 19F NMR the range can be up to ± 500 ppm. Limited radiofrequency power available for pulsed excitation makes it very challenging to excite such a wide range of frequencies, especially for multiple pulse methods. Even the best conventional composite pulse methods fall far short of such bandwidths[1], so swept-frequency “chirp” pulses are needed. Broadband DOSY sequences were built in stages by developing broadband excitation and refocusing elements that could be combined to generate broadband spin echo and stimulated echo sequences. Broadband Excitation and Refocusing In principle, a pair of chirp pulses of appropriate relative amplitude can be used to excite very wide bandwidths[2]. Unfortunately, gross phase errors build up towards the edges of the frequency range, reducing the bandwidth of the method. A further, insidious, problem is that the signal phase is extremely sensitive to B1 amplitude, so that B1 inhomogeneity causes large (> 30%) losses in signal even with good modern instrumentation. The double chirp pulse sequence element was therefore adapted and extended (a) to compensate for B1¬ sensitivity and (b) to correct for the phase errors. A uniform, constant-phase broadband excitation was achieved, allowing a 300 kHz bandwidth with maximum 15 kHz RF amplitude, with no undue B1 sensitivity and no loss in sensitivity. Broadband refocusing can be achieved, using an adiabatic composite 180°180°180° chirp pulse[3], which is self-refocusing without phase distortion and is therefore B1 insensitive. Broadband DOSYA broadband DOSY Oneshot[4] sequence, built using the elements described, produces uniform performance over almost a full 300 kHz bandwidth, with no loss in sensitivity, allowing DOSY to be performed over very wide spectral widths (e.g. 600 ppm for 19F at 500 MHz). In comparison, simulation shows that with the same RF amplitude at half height excitation bandwidth, the standard uncompensated DOSY sequence will excite only 30 kHz.References Odedra, S., M. J. Thrippleton, S. Wimperis, J. Magn. Reson. 2012, 225, 81-92. Bohlen, J. M., M. Rey and G. Bodenhausen, J. Magn. Reson. 1989, 84, 191-197. Hwang, T. L., P. C. M. van Zijl and M. Garwood, J. Magn. Reson. 1997, 124, 250-254. Pelta, M. D., G. A. Morris, M. J. Stchedroff and S. J. Hammond, Magn. Reson. Chem. 2002, 40, S147-S152.