Fig. 6 Liquid-phase ¹⁵N nuclear magnetic resonance (NMR) spectra of International Humic Substance Society Pahokee peat humic acid (HA) reacted with Na¹⁵NO₂. Recorded in DMSO-d₆. (a) ACOUSTIC spectrum showing quantitative distribution of all nitrogens, (b) DEPT spectrum showing nitrogens directly bonded to protons, (c) DEPT spectrum, nitrogens bonded to one proton in positive phase, nitrogens bonded to two protons inverted