title: Low-temperature dynamic nuclear polarization at 9.4 T with a 30 mW microwave source.
abstract: Dynamic nuclear polarization (DNP) can provide large signal enhancements in nuclear magnetic resonance (NMR) by transfer of polarization from electron spins to nuclear spins. We discuss several aspects of DNP experiments at 9.4 T (400 MHz resonant frequency for (1)H, 264 GHz for electron spins in organic radicals) in the 7-80K temperature range, using a 30 mW, frequency-tunable microwave source and a quasi-optical microwave bridge for polarization control and low-loss microwave transmission. In experiments on frozen glycerol/water doped with nitroxide radicals, DNP signal enhancements up to a factor of 80 are observed (relative to (1)H NMR signals with thermal equilibrium spin polarization). The largest sensitivity enhancements are observed with a new triradical dopant, DOTOPA-TEMPO. Field modulation with a 10 G root-mean-squared amplitude during DNP increases the nuclear spin polarizations by up to 135%. Dependencies of (1)H NMR signal amplitudes, nuclear spin relaxation times, and DNP build-up times on the dopant and its concentration, temperature, microwave power, and modulation frequency are reported and discussed. The benefits of low-temperature DNP can be dramatic: the (1)H spin polarization is increased approximately 1000-fold at 7 K with DNP, relative to thermal polarization at 80K.
authors.affiliation: Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0520, USA.
authors.forename: Kent R
authors.initials: KR
authors.lastname: Thurber
authors.forename: Wai-Ming
authors.initials: WM
authors.lastname: Yau
authors.forename: Robert
authors.initials: R
authors.lastname: Tycko
mesh_terms: D003080:Cold Temperature; D005990:Glycerol / Q000737:chemistry* / Q000528:radiation effects*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008872:Microwaves; D013696:Temperature; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D005990:Glycerol
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