Impact of Gd$^{3+}$ doping and glassing solvent deuteration on $^{13}$C DNP at 5 Tesla*

Dynamic nuclear polarization (DNP) is a technique which can be used to amplify signals in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) by several thousand-fold. The most commonly available DNP system typically operates at the W-band field or 3.35 T, at which it has been shown that $^{13}$C NMR signal can be enhanced by deuteration and Gd$^{3+}$ doping. In this work, we have investigated the applicability of these procedures at 5 T. Our results indicate that the deuteration of the glassing matrix still yields an enhancement of $^{13}$C DNP when 4-oxo-TEMPO free radical is used. The effect is attributed to the lower heat load of the deuterons compared to protons. An addition of a trace amount of Gd$^{3+}$ gives a modest enhancement of the signal when trityl OX063 is used, albeit with a less pronounced relative enhancement compared to the results obtained at 3.35 T. The results suggest that the enhancement obtained via Gd$^{3+}$ doping may become saturated at higher field. These results will be discussed using a thermodynamic model of DNP.