Simple and cost-effective cross-polarization experiments under dissolution-dynamic nuclear polarization conditions with a 3D-printed 1H-13C background-free radiofrequency coil

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The low sensitivity of conventional nuclear magnetic resonance experiments can be overcome, in suitable cases, by employing hyperpolarization methodologies. One such technique, dissolution-dynamic nuclear polarization, provides a robust means of strongly polarizing a variety of small molecules. A drawback of the dissolution-dynamic nuclear polarization approach, the excessively long polarization timescales for insensitive nuclei, has been circumvented by using cross-polarization radiofrequency pulse sequences, which in general yield quick and substantial 13C polarizations. However, the capacity to effectively perform efficient cross-polarization experiments under dissolution-dynamic nuclear polarization conditions remains challenging, and polarization quantification can be plagued by additional complications including spurious background signals from the probe. Here we propose a background-free1H-13C radiofrequency coil specifically designed for use in cross-polarization experiments on samples of up to 500 μL in volume at liquid helium temperatures. We additionally introduce simple guidelines for the optimization and implementation of cross-polarization radiofrequency pulse sequences. Experimental demonstrations of 13C polarizations reaching ∼60% are presented for the case of [1-13C]sodium acetate.