van Bentum, P.J.M., et al., Quantitative analysis of high field liquid state dynamic nuclear polarization. Phys. Chem. Chem. Phys., 2011. 13(39): p. 17831-17840.
Dynamic Nuclear Polarization (DNP) in the liquid state has become the focus of attention to improve the NMR sensitivity of mass limited samples. The Overhauser model predicts a fast reduction in DNP enhancement at high magnetic fields where the Electron Larmor frequency exceeds the typical inverse correlation time of the magnetic interaction between a radical spin and proton spins of the water molecules. Recent experiments have shown that an appreciable DNP enhancement in the liquid state is possible also at magnetic fields of 3 to 9 Tesla. At present it is not clear whether the Overhauser model needs to be adapted to explain these results. In the present paper we aim to resolve this question by a combination of in situ temperature dependent NMR relaxation measurements, EPR and DNP experiments. Enhancement factors of up to -165 are obtained with microwave powers below 500 mW. We conclude that at 3.4 Tesla (95 GHz) the various measurements are consistent with each other and in quantitative agreement with Overhauser theory. Microwave heating of the sample does play an important role to reduce the correlation times and allow a substantial Overhauser DNP. The typical enhancement factors may allow new applications in microfluidic NMR.