The SNR ratio in the ZULF spectra in Fig. is only one order of magnitude higher than a single scan of the thermal reference sample, but the key difference is the hundred-fold dilution of the analytes compared with the thermal polarization experiment. Furthermore, the formate peak of Fig. has SNR > 500, over an order of magnitude higher as compared with previously published work . Nevertheless, the polarization levels in Fig. differ from the optimum performance that could be brought by the d DNP apparatus by an order of magnitude. This could be due to imperfections in the magnetic tunnel leading to zero-field crossings that erode polarization, a mismatch in the synchronization of the apparatuses, or temperature and magnetic field gradients that reduce the magnetometer sensitivity. In Fig. we can observe that the peak arising from glycine at 8 Hz is significantly broader than the other peaks of the hyperpolarized samples. It is well known that quadrupolar relaxation is significant in spin systems where nuclei with spins > 1/2 are involved. The broadening is possibly due to quadrupole relaxation caused by the spin-1 14 N nucleus. This is important because it implies that in the zero-field detection of d DNP-polarized molecules such as amino acids and other nitrogen-containing molecules, the use of 15 N-labelled material may be necessary.