Hediger, Sabine, Daniel Lee, Frédéric Mentink-Vigier, and Gaël De Paëpe. “MAS-DNP Enhancements: Hyperpolarization, Depolarization, and Absolute Sensitivity” 7 (2018): 12.
Dynamic nuclear polarization at high magnetic fields has made significant progress over the last decades, and this hyperpolarizing technique is currently revolutionizing the impact of solid-state NMR for the study of complex systems in chemistry, material science, and biology. In this article, we emphasize the importance and difficulty in quantifying sensitivity from DNP under magic-angle spinning. To this end, we provide insight into the cross effect, the current main MAS-DNP mechanism. This includes a description of the microwave-induced hyperpolarization phenomenon but also of the reduction of the NMR signal prior to microwave irradiation for samples doped with polarizing agents (bleaching and depolarization effects). We highlight the importance of the nuclear hyperpolarization buildup time in the evaluation of MAS-DNP efficiency. Finally, we discuss other experimental parameters affecting sensitivity in DNP-enhanced spectra and propose a guideline for its proper characterization depending on the type of investigation.