Fig. 3.

(a) Carbon-13 detected NMR noise power spectrum of ¹³C labelled glycerol (300 μl glycerol (99% ¹³C enriched, Aldrich) and 200 μl D₂O (Euriso-top, for field-frequency lock and to allow for adjusting the sample filling height) recorded at 176 MHz ¹³C frequency on the same hardware as in Fig. 2. A total of 1024 noise blocks with 49.6 s acquisition time were recorded at a spectral width of 15 ppm in totally 14 h. Noise power averaging [13] was carried out in Scilab as described in Fig. 2, resulting in the addition of a total of 527,360 power spectra. (b) Proton decoupled (17 mW, 200 μs 90° decoupling pulse length for WALTZ16) ¹³C detected NMR noise spectrum after 2.5 h of acquisition. 10,240 noise data blocks of 0.77 s were acquired in a pseudo-2D mode. Processing was applied as described in Ref. [11]. Note that the noise baselines in these spectra corresponds to the thermal noise levels without spin noise or absorbed circuit noise, the cutoff at the edges being caused by the processing. Carbon-13 chemical shifts are identical to the ones observed in a pulse spectrum referenced to external TSP in D₂O.