To study the influence of the amount of dissolved paramagnetic oxygen on the relaxation process mentioned above, we measured the zero-field NMR spectra of C-formic acid solutions following different numbers of freeze-pump-thaw cycles (Fig. ). The T 2 values extracted (as demonstrated with the example spectrum in Fig. ) for the schemes described above (i.e., with non-adiabatic switching off the field and with the transverse y-pulse) are shown in Fig. . An increase in all relaxation times (to a greater extent in the case of T s ) is observed when a single and two freeze-pump-thaw cycles are performed. This is due to the efficient removal of oxygen from the samples. However, there is no significant effect of the third cycle on relaxation terms as in the case of trimethyl phosphate. It should be noted that the longest relaxation time we measured is shorter than the ones reported in Ref. . The reason is that even though there is no signal associated with the proton of the hydroxyl group, the contribution of this exchangeable proton to the polarization relaxation is non-negligible . In fact, increasing the proton-exchange rate enables elimination of this relaxation mechanism. However, in our study, we used neat C-formic acid (95% in weight in H 2 O), in contrast to the previous study where water and acetonitrile were mixed to accelerate the chemical exchange process. A further increase in the exchange rate leads to the decoupling of the hydroxyl-group proton from the CH 3 group, prolonging the relaxation time and affecting the zero-field spectra, as demonstrated in Refs. . It should be also noted that the experiments with a non-adiabatic transfer result in narrower zero-field peaks compared to the measurements with an additional transverse pulse, which is an indicator of a longer T 2 value. This finding could be explained by the generation of coherences between the |T 0 ⟩ and long-lived |S 0 ⟩ states after non-adiabatic transfer, as mentioned above. On the other hand, the application of transverse pulse increases the intensity of the signal .