Frozen water NMR lineshape analysis enables absolute polarization quantification.

Typical magnetic resonance experiments are routinely limited by weak signal responses. In some cases, the low intrinsic sensitivity can be alleviated by the implementation of hyperpolarization technologies. Dissolution-dynamic nuclear polarization offers a means of hyperpolarizing small molecules. Hyperpolarized water is employed in several dynamic nuclear polarization studies, and hence accurate and rapid quantification of the 1 H polarization level is of utmost importance. The solid-state nuclear magnetic resonance spectrum of water acquired under dissolution-dynamic nuclear polarization conditions has revealed lineshapes which become asymmetric at high levels of 1 H polarization, which is an interesting fundamental problem in itself, but also complicates data interpretation and can prevent correct estimations of polarization levels achieved. In previous studies, attempts to simulate the 1 H spectral lineshape of water as a function of the 1 H polarization led to significant disagreement with the experimental results. Here we propose and demonstrate that such simulations, and therefore polarization quantification, can be implemented accurately, in particular by taking into account the detector dead time during 1 H signal acquisition that can lead to severe spectral distortions. Based on these findings, we employed an echo-based radiofrequency pulse sequence to achieve distortion-free 1 H spectra of hyperpolarized water, and adequate simulations of these echo-based spectra were implemented to extract the absolute 1 H polarization level from the hyperpolarized water signal only, thus alleviating the need for lengthy and insensitive measurements of thermal equilibrium signals.