The relaxometry data from Fig. prove that it is possible to observe ZULF spectra of samples even in the presence of mM paramagnetic radical concentration, although at the price of significant paramagnetic broadening and therefore loss in resolution. The dissolution step of our d DNP experiments currently dilutes the sample by a factor of 30, bringing the TEMPOL concentration down to 1-2 mM. The concentration is further reduced by the presence of sodium ascorbate in the beads placed in the DNP sample holder in the dissolution solvent, which acts as a scavenger for TEMPOL radicals . However, the reaction between ascorbate and TEMPOL is not instantaneous. Due to the complex temperature and concentration dynamics of the dissolution step, it is not possible to know the concentration of TEMPOL precisely at the moment of injection. Considering that the TEM-POL concentration after injection is at most on the order of 1 mM, the results presented in Fig. show that the broadening due to paramagnetic relaxation is expected to be within a few hundreds of mHz, acceptable for our hyperpolarized experiment. The spectra of hyperpolarized samples in Fig. have linewidths of approximately 200 mHz: were this due only to TEMPOL, it would mean that 1-2 mM of radical was present at the sample at the moment of detection, consistent with our expectations. This broadening does not decrease the signalto-noise (SNR) ratio (signal amplitude divided by the standard deviation of the nearby noise range) sufficiently to prevent us from observing hyperpolarized peaks but leads to a non-negligible signal loss during the settling time t settle = 2.1 s . The remaining signal after 2.1 s at 100 µT for a TEMPOL concentration of 2 mM should be exp (-a [TEMPOL] t settle ) ≈ 0.28 of the initial value (assuming that TEMPOL has not significantly be quenched by ascorbic acid at this point). Paramagnetic relaxation during the transfer and settling of the solution could therefore explain why the recorded enhancement is lower than expected. This shows that increasing the speed of the transfer, shortening settling time, and suppressing paramagnetic relaxation is of paramount importance to improve the sensitivity of d DNP-ZULF experiments.