Inductively coupled NMR probe for versatile dynamic nuclear polarization operation at 7 T: observation of 61 ± 2% ¹H polarization at 4 K.

We have performed dynamic nuclear polarization (DNP) experiments at liquid helium temperatures using a low-power (<70 mW) solid-state diode microwave source at 200 GHz-the electron paramagnetic resonance frequency of stable radicals at 7 T. We employed a home-built Alderman-Grant probe for the detection of ¹H NMR signal at 300 MHz, as such coils are well suited for higher frequency NMR detection. The Alderman-Grant coil is inductively coupled to the rest of the radiofrequency (rf) circuit, whose design allows probe components to be placed away from the sample area, and also enables easy switching of coils with different diameters and resonance frequencies. We have tested our DNP instrument on a frozen nitroxide model system consisting of 4-Amino TEMPO dissolved in a glycerol:water mixture. The largest nuclear spin polarization observed was 61 ± 2% with a sample containing 20 mM 4-Amino TEMPO dissolved in deuterated glycerol (d-glycerol):D₂O:H₂O (50:40:10), amounting to record polarization measured to date at an easily amenable temperature of 4 K.