title: Magnetic resonance imaging of DNP enhancements in a rotor spinning at the magic angle.
abstract: Simulations performed on model, static, samples have shown that the microwave power is non-uniformly distributed in the magic angle spinning (MAS) rotor when using conventional dynamic nuclear polarization (DNP) instrumentation. Here, we applied the stray-field magic angle spinning imaging (STRAFI-MAS) experiment to generate a spatial map of the DNP enhancements in a full rotor, which is spun at a low rate in a commercial DNP-MAS NMR system. Notably, we observed that the enhancement factors produced in the center of the rotor can be twice as large as those produced at the top of the rotor. Surprisingly, we observed that the largest enhancement factors are observed along the axis of the rotor as opposed to against its walls, which are most directly irradiated by the microwave beam. We lastly observed that the distribution of enhancement factors can be moderately improved by degassing the sample and increasing the microwave power. The inclusion of dielectric particles greatly amplifies the enhancement factors throughout the rotor. The STRAFI-MAS approach can provide useful guidance for optimizing the access of microwave power to the sample, and thereby lead to further increases in sensitivity of DNP-MAS NMR.
authors.affiliation: U.S. DOE Ames Laboratory, Ames, IA 50011-3020, USA.
authors.forename: Frédéric A
authors.initials: FA
authors.lastname: Perras
authors.affiliation: U.S. DOE Ames Laboratory, Ames, IA 50011-3020, USA.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Kobayashi
authors.affiliation: U.S. DOE Ames Laboratory, Ames, IA 50011-3020, USA; Department of Chemistry, Iowa State University, Ames, IA 50011-3020, USA. Electronic address: mpruski@iastate.edu.
authors.forename: Marek
authors.initials: M
authors.lastname: Pruski
keywords: DNP enhancement; DNP solid-state NMR; STRAFI–MAS MRI