Signal enhancement in NMR spectra of half-integer quadrupolar nuclei via DFS–QCPMG and RAPT–QCPMG pulse sequences

Two signal enhancement schemes capable of providing large signal-to-noise gains in static and magic-angle spinning NMR spectra of the central transition of half-integer quadrupolar nuclei are presented. Amplitude-modulated double-frequency sweeps (AM-DFS) and rotor-assisted population transfer (RAPT) pulse sequences are used as preparatory sequences in combination with the quadrupolar Carr–Purcell Meiboom–Gill (QCPMG) pulse sequence to obtain large signal enhancements ranging from one to two orders of magnitude. Specific applications to two spin-3/2 nuclei (87Rb and 39K) and one spin-5/2 nucleus (85Rb) are presented.