Dynamic Nuclear Polarization of [1-13C]pyruvic acid at 4.6 tesla.

Dynamic Nuclear Polarization (DNP) of the (13)C nucleus has been investigated for [1-(13)C]pyruvic acid, doped with the trityl radical OX063Me, at 4.64 T and 1.15K. The dependence of the polarization on microwave frequency, radical concentration and electron saturation was studied. For optimized conditions, a (13)C polarization equal to 64+/-5% was obtained, an increase by more than a factor of two compared with earlier results at 3.35 T of the same system. It was furthermore observed that the addition of gadolinium, which resulted in a twofold polarization increase at 3.35 T, only resulted in a minor improvement at 4.64 T. The dependence of the electron saturation on microwave frequency and microwave power was quantified by first moment measurements which were obtained by nucleus-electron double resonance (NEDOR) experiments. Complete electron saturation was observed for a microwave frequency close to the centre frequency of the ESR line, and by using maximum power of the microwave source. The DNP build-up time at 4.64 T (approximately 3000 s) was prolonged by approximately a factor three over the build-up time at 3.35 T (approximately 1200 s). However, after approximately 20 min of microwave irradiation the polarization at 4.64 T exceeded the polarization at 3.35 T.