Pure absorption-mode spectra using a modulated RF mixing period in MQMAS experiments

Different approaches to obtain pure absorption-mode lineshapes in MQMAS exp eriments employing a train of 180 degrees phase-alternating pulses for the multiple-quantum to single-quantum mixing period are investigated. Four pul se sequences, which achieve this by using either the shifted-echo approach or the hypercomplex approach with symmetric coherence transfer pathways, ar e presented and their improved lineshape- and sensitivity-performance is ex perimentally demonstrated by Rb-87 MQMAS of RbNO3. Compared to the original modulated-rf mixing sequence, sensitivity enhancements by factors up to 1. 3 are obtained with the sequences described here. (C) 2000 Academic Press.