Sample Ripening affects Dynamic Nuclear Polarization

Phantom IRM
Phantom IRM
(Top) Difference between the steady-state proton polarization and the thermal equilibrium polarization at 4.2 K as a function of the interval between preparation and vitrification of water–glycerol mixtures containing 50 mm TEMPOL. (Bottom) Similar difference with 20 mm AMUPol at 4.2 K.
Mixtures of water and glycerol provide popular matrices for low-temperature spectroscopy of vitrified samples. However, they involve counterintuitive physicochemical properties, such as spontaneous nanoscopic phase separations (NPS) in solutions that appear macroscopically homogeneous. We demonstrate that such phenomena can substantially influence the efficiency of dynamic nuclear polarization (DNP) by factors up to 20% by causing fluctuations in local concentrations of polarization agents (radicals). Thus, a spontaneous NPS of water/glycerol mixtures that takes place on time scales on the order of 30–60 min results in a confinement of polarization agents in nanoscopic water-rich vesicles, which in return affects the DNP. Such effects were found for three common polarization agents, TEMPOL, AMUPol and Trityl.

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