Fig. 2.

a, c, d Enhancement ε of ¹H NMR signals of bulk water containing ¹⁵N-labelled Frémy’s salt at 3.4 T as a function of microwave irradiation time t _mw fitted according to Eq. (5). Inset: pulse sequence applied in the DNP experiments. In d we observe a decomposition of the sample at irradiation times longer than 7 s (Frémy’s salt is known to decompose in solution [22]. The rate of decomposition depends on the pH value and the temperature. We observe decomposition only at long irradiation times (up to 10 s), which were only applied for the 5 mM sample). b ¹H NMR inversion recovery experiment on a 25 mM sample at 3.4 T