To my knowledge, this is one of the few methods that allows to measure the absolute polarization of a spin ensemble. Often the on/off signal ratio is used to characterize the efficiency of the DNP – a method that does not take depolarization effects into account. The method is related to the SPY-MR method to measure absolute spin polarization in dissolution DNP experiments.
Sugishita, Tomoaki, Yoh Matsuki, and Toshimichi Fujiwara. “Absolute 1H Polarization Measurement with a Spin-Correlated Component of Magnetization by Hyperpolarized MAS-DNP Solid-State NMR.” Solid State Nuclear Magnetic Resonance 99 (July 2019): 20–26. https://doi.org/10.1016/j.ssnmr.2019.02.001.
Sensitivity of magic-angle spinning (MAS) NMR spectroscopy has been dramatically improved by the advent of high-field dynamic nuclear polarization (DNP) technique and its rapid advances over the past decades. In this course, discussions on ways to improve the DNP enhancement factor or the overall sensitivity gain have been numerous, and led to a number of methodological and instrumental breakthroughs. Beyond the sensitivity gain, however, discussions on accurate quantification of the 1H polarization amplitude achievable in a sample with DNP have been relatively rare. Here, we propose a new method for quantifying the local 1H hyperpolarization amplitude, which is applicable to un-oriented/powdered solid samples under MAS NMR conditions. The method is based on the ability to observe the high-order spin-correlated term (2IzSz) intrinsic to a hyperpolarized I-S two-spin state, separately from the lowest-order Zeeman term (Sz) in quasi-equilibrium magnetization. The quantification procedure does not require evaluation of signal amplitudes for a “microwave-off” condition and for an un-doped reference sample, and thus enables quick and accurate quantification unaffected by the effects of the paramagnetic quenching and the MAS-induced depolarization. The method is also shown to elucidate spatial polarization distribution through the 2IzSz term prepared domain selectively. As a potential application, we also demonstrate 2D DQ-SQ spectroscopy utilizing the 2IzSz term that is generated in a spatially selective manner without using I-S dipolar or J coupling. These salient features may be evolved into a way for characterizing mesoscopic molecular assemblies of medical/biological importance.