Fig. 5.

¹³C microwave DNP spectra of ¹³C-enriched substrates doped with a BDPA, b trityl OX063, c galvinoxyl, d DPPH, and e 4-oxo-TEMPO (see Table 1 for details of sample composition) taken at 4.6 T and 1.15 K with a 120 mW microwave source. The up and down arrows correspond to the positive P(+) and negative P(−) polarization peaks, respectively. The corresponding chemical structures of the free radicals are displayed on the right