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

Dynamic Nuclear Polarization (DNP) of the ¹³C nucleus has been investigated for 1-¹³C]pyruvic acid, doped with the trityl radical OX063Me, at 4.64 T and 1.15 K. The dependence of the polarization on microwave frequency, radical concentration and electron saturation was studied. For optimized conditions, a ¹³C 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 (3000 s) was prolonged by approximately a factor three over the build-up time at 3.35 T (1200 s). However, after approximately 20 min of microwave irradiation the polarization at 4.64 T exceeded the polarization at 3.35 T.