Condensation degree of burnt peat and plant residues and the reliability of solid-state VACP MAS 13C NMR spectra obtained from pyrogenic humic material

Charred organic matter is assumed to contain heavily condensed polycyclic aromatic domains with a considerable proportion of core carbons. To examine their possible underestimation using variable amplitude (VA) cross polarization (CP) magic angle spinning (MAS) 13C nuclear magnetic resonance (NMR), the condensation degree of a peat subjected to thermal oxidation at 350 °C for up to 180 s was examined by means of elemental analysis and various NMR techniques. For comparison, commercially available barbeque charcoal and charred organic residues collected from a fire-affected soil from Southern Spain were analyzed. The atomic H/C ratios of the aromatic moiety of the chars revealed that on average every second to third C is connected to a proton. Dipolar dephasing NMR data confirm the high extent of and are in agreement with relatively small clusters of six aromatic rings connected by approximately two bridging CS. Alternatively, benzanthracene-like structures with two to five bridging CS are possible. The CP time (TCH) and the 1H and 13C spin lattice relaxation times in the rotating frame (T1ρH; T1ρC) of the chars suggest the presence of crystalline and amorphous domains. Both fractions fulfill the condition TCH < tc < T1ρH which is required for C-detection with the CP-technique. Reliability of the CP-technique was supported by experiments with standards consisting of charred material and untreated peat mixed in defined ratios. The analysis of 13C spin lattice relaxation indicated saturation of some O-alkyl and alkyl C in the Bloch decay (BD) NMR spectrum of the untreated peat. This may explain its higher aromatic C intensity compared to the respective CP MAS NMR spectrum.