Enhancing half-integer quadrupolar solid-state NMR signals via steady states: A double frequency sweep-based approach

We introduce a novel signal enhancement technique, termed steadyDFS, for quadrupolar solid-state nuclear magnetic resonance spectroscopy. It can substantially increase the performance of double frequency sweeps (DFSs) for all half-integer quadrupolar spins (I = 3/2, 5/2, 7/2, and 9/2). In steadyDFS, the DFS and readout pulse are repeated multiple times with a repetition time of TR,DFS to generate a steady state that provides substantial sensitivity enhancement. Using a series of simulations, we show that steadyDFS can outperform conventional DFS methods, and enhancements per unit time of ∼5 to 21 can be achieved depending on the value of I. The sensitivity of steadyDFS is robust toward changes in repetition times and quadrupolar relaxation rates of the system. Moreover, steadyDFS is highly modular and can be combined with quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) detection. Using 39K (I = 3/2), 17O (I = 5/2), and 49Ti (I = 7/2) as representative challenging nuclei, we show that enhancements up to 46× can be realized experimentally via steadyDFS-QCPMG, translating to a 20× enhancement per unit time. We applied steadyDFS-QCPMG to protonated and deprotonated samples, as well as to samples with a diverse range of transverse relaxation times (i.e., T2/T2*), where steadyDFS-QCPMG can provide an enhancement per unit time of at least 7. For samples that are not amendable to QCPMG, we explored the use of steady-state free precession (SSFP). Although SSFP is fundamentally incompatible with steadyDFS, we show that beneficial results can be obtained when DFSs are combined with SSFP in an interruptive manner.