Fig. 6.

a Representative DNP and thermal ¹³C NMR signals of neat [1-¹³C]pyruvic acid doped with 15 mM trityl OX063 measured at 4.6 T and 1.15 K. The calculated NMR signal enhancement ε of this particular frozen polarized ¹³C sample was 530, which corresponds to 54 % ¹³C polarization under these conditions. b ¹³C polarization buildup curves of ¹³C samples doped with (1) BDPA, (2) trityl OX063, (3) galvinoxyl, (4) DPPH and (5) 4-oxo-TEMPO (see Table 1 for sample composition). The ¹³C samples were irradiated at the positive polarization peak (+) at 4.6 T and 1.15 K with a 120 mW microwave source. The solid lines are fits to a single-exponential buildup equation with a buildup time constant τ_bu. c Calculated temperature dependence of Boltzmann thermal electron and ¹³C nuclear polarization levels at 4.6 T. Experimental values of DNP-enhanced ¹³C polarization levels of ¹³C samples doped with different free radicals using the homebuilt DNP polarizer at 4.6 T and 1.15 K are displayed as symbols (see Table 1 for details of sample composition)