Fig. 4. Transfer efficiency of the ¹H-¹⁵N ramp-CP experiment.

(A) Transfer efficiency calculated for a contact time of 1 ms and a wide range of rf amplitudes. A 70 to 100% linear ramp was applied on the ¹H channel. Calculations assumed an amide ¹H-¹⁵N spin pair, MAS frequency of 16.5 kHz, and a realistic 3D distribution of rf fields generated by a solenoid coil of a typical 3.2-mm MAS probe. Areas of zero-quantum (positive) and double-quantum (negative) conditions are identified. (B) Traces from the 2D map extracted at fixed ¹⁵N rf amplitudes (black dashed lines) and experimental intensities (red lines) measured as a function of ¹H rf amplitude. The calculated map is used to identify suitable rf parameters for the experiment.