Flip-back, an old trick to face highly contrasted relaxation times: Application in the characterization of pharmaceutical mixtures by CPMAS NMR

► To measure the large class of pharmaceutical solid mixtures by 13C-1H CPMAS NMR, the flip-back pulse NMR experiment is revisited.► Kinetics of relaxation and CP transfer with a flip-back pulse has been expressed. ► Gain in 13C signal from the flip-back pulse (in seconds) has been evaluated. ► When highly contrasted protons spin-lattice relaxation times T1 are present, CPMAS with flip-back can lead to routine 13C NMR analysis of complex mixtures.