Fig. 3. ¹H Hahn-echo NMR spectra recorded with (red) and without (blue) μw irradiation suitable for CE-DNP on the ¹³C-urea model solution containing 20 mM TOTAPOL (a) or 5 mM AMUPol (b), at a sample temperature of 55 K and a MAS frequency of 10 kHz. The signals recorded in the absence of μw irradiation have been magnified by a factor of 100 for illustrative purposes. Asterisks and daggers denote spinning sideband and ‘plug’ signals, respectively. (c) The measured DNP-enhancement ratio as a function of sample temperature for the sample from (b). (d) The measured DNP-enhancement ratio as a function of the gyrotron beam current, which has been checked to be proportional to μw power, for a ¹³C-urea model solution containing 12 mM AMUPol, recorded at a MAS frequency of 5 kHz and sample temperatures of 105 K (red) and 35 K (blue).