Accelerated Relaxation of Sensitive Nuclei for Enhancement of Signal-to-Noise with Time

A pulse sequence is proposed that accelerates the relaxation of sensitive nuclei through inverse polarization transfer from insensitive nuclei that have been subject to NOE during their detection: the sequence is designed to replace normal intersequence relaxation delays. Typically, for the observation of {1H}-13C, the 13C relaxes during data acquisition and is enhanced by NOE from decoupled 1H's. After data acquisition the 13C polarization can be transferred to 1H in order to step-jump accelerate the relaxation of 1H and hence provide 1H polarization more rapidly for transfer to 13C during the next repetition of polarization-transfer sequences. It is suggested that the sequence (SNARE) can advantageously replace the relaxation delays in many common sequences. The benefits of so doing are illustrated using PENDANT and DEPT for 13C investigations. Conditions for the implementation of SNARE are proposed, and it is demonstrated that there is a range of spectral-acquisition parameters of practical interest for which the method can be beneficial. Experimental time savings in achieving a chosen S/N typically exceed 30%.