Figure 3.

Calibration of the sample position and flip angle. (a) DNP-enhanced, solid-state ¹³C-NMR signal of pyruvate as a function of the position of the sample in probe. To ensure sufficient signal, the sample was first polarized for 40 min. Then the ¹³C signal was acquired at different positions x_s every minute using a low flip angle of 2°. Moving the sample first down, then up, a broad maximum around x_s = 14 mm was found. Straight lines were plotted to guide the eye. (b) DNP-enhanced ¹³C NMR signal acquired by a train of $p_d^{\mathit{RF}}$ = 2 us, $p_a^{\mathit{RF}}$ = 18 dB pulses with TX = 217 µs, TR = 1 s, NS = 1, NX = 49. By fitting a mono-exponential function to the data (red line, $\tau =$ 13.3 s, Eq. 1), the flip angle was determined to α = 3.2° (Eq. 2).