title: Dynamic nuclear polarization at 700 MHz/460 GHz.
abstract: We describe the design and implementation of the instrumentation required to perform DNP-NMR at higher field strengths than previously demonstrated, and report the first magic-angle spinning (MAS) DNP-NMR experiments performed at (1)H/e(-) frequencies of 700 MHz/460 GHz. The extension of DNP-NMR to 16.4 T has required the development of probe technology, cryogenics, gyrotrons, and microwave transmission lines. The probe contains a 460 GHz microwave channel, with corrugated waveguide, tapers, and miter-bends that couple microwave power to the sample. Experimental efficiency is increased by a cryogenic exchange system for 3.2 mm rotors within the 89 mm bore. Sample temperatures ≤85 K, resulting in improved DNP enhancements, are achieved by a novel heat exchanger design, stainless steel and brass vacuum jacketed transfer lines, and a bronze probe dewar. In addition, the heat exchanger is preceded with a nitrogen drying and generation system in series with a pre-cooling refrigerator. This reduces liquid nitrogen usage from >700 l per day to <200 l per day and allows for continuous (>7 days) cryogenic spinning without detrimental frost or ice formation. Initial enhancements, ε=-40, and a strong microwave power dependence suggests the possibility for considerable improvement. Finally, two-dimensional spectra of a model system demonstrate that the higher field provides excellent resolution, even in a glassy, cryoprotecting matrix.
authors.affiliation: Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Alexander B
authors.initials: AB
authors.lastname: Barnes
authors.forename: Evgeny
authors.initials: E
authors.lastname: Markhasin
authors.forename: Eugenio
authors.initials: E
authors.lastname: Daviso
authors.forename: Vladimir K
authors.initials: VK
authors.lastname: Michaelis
authors.forename: Emilio A
authors.initials: EA
authors.lastname: Nanni
authors.forename: Sudheer K
authors.initials: SK
authors.lastname: Jawla
authors.forename: Elijah L
authors.initials: EL
authors.lastname: Mena
authors.forename: Ronald
authors.initials: R
authors.lastname: DeRocher
authors.forename: Ajay
authors.initials: A
authors.lastname: Thakkar
authors.forename: Paul P
authors.initials: PP
authors.lastname: Woskov
authors.forename: Judith
authors.initials: J
authors.lastname: Herzfeld
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Temkin
authors.forename: Robert G
authors.initials: RG
authors.lastname: Griffin
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D009682:Magnetic Resonance Spectroscopy / Q000295:instrumentation*; D012034:Refrigeration / Q000295:instrumentation*; D013048:Specimen Handling / Q000295:instrumentation*; D014159:Transducers