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 Tesla (400 MHz resonant frequency for 1 H, 264 GHz for electron spins in organic radicals) in the 7-80 K 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 80 K.

Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves

“…6 All samples contained 50 mM 15 N, 13 C 3 -L-alanine in partially protonated glycerol/water. Nitroxidedoped samples contained 30 mM of nitroxide radicals (10 mM of the triradicals DOTOPA-4OH or DOTOPA-Ethanol 2,7,8 or 15 mM of the biradical TOTAPOL 9 ), while samples without nitroxides contained 0.2 mM DyEDTA to reduce the lowtemperature nuclear spin-lattice relaxation times (T 1n ). Sample identities and compositions are given in Table I.…”

“…7 Figure 1 compares CP 13 C NMR spectra for samples with and without nitroxide doping and also with and without MAS. Results are summarized in Table II.…”

Materials Characterization by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy

“…62 Second, several groups are developing helium cooled DNP systems. [63][64][65][66] Reducing the sample temperature to less than 30 K can potentially provide further absolute sensitivity gains of one to two orders of magnitude by increasing both DNP enhancements and thermal nuclear polarization. [63][64][65][66] Third, the development of coherent, pulsed microwave sources for time-domain high-field DNP could revolutionize the field in much the same way that pulsed methods revolutionized NMR spectroscopy.…”

Optimizing dissolution dynamic nuclear polarization

“…It has been shown by Thurber et al [53], Cassidy et al [54] that DNP by CE and SE could be greatly improved by using either field modulation or microwave frequency modulation. More recently Hovav et al [55] have shown that frequency modulation could improve DNP at 3.35 T in the temperature range 10-50 K, close to the d-DNP conditions.…”