For completeness, one experiment was performed in which the purification procedure was performed outside the magnet, and as expected the C polarization was lost. The loss of nuclear magnetization when precipitating the sample in a low magnetic field is not yet fully understood. The loss of hyperpolarization could be associated with rapid spin relaxation processes in low magnetic field for the small suspended particles involved in the nucleation process, or efficient contact between the Zeeman and dipolar orders in the solid state at low magnetic field.