Contact-Time Dependence and Cross-Polarization Kinetics of Apparent Signal Distributions in CP/MAS 13C NMR Spectra of Humic Acids.

Humic acids (HAs) represent structurally heterogeneous supramolecular systems whose characterization by CP/MAS 13C NMR spectroscopy strongly depends on experimental conditions. Among these, contact time is one of the most important parameters controlling the efficiency of polarization transfer and the relative contribution of individual carbon groups. The aim of this study was to evaluate the influence of contact time on the apparent CP/MAS 13C NMR signal distribution of HAs isolated from different horizons of a permafrost peat soil (Hemic Folic Cryic Histosol) and to characterize their cross-polarization dynamics using kinetic modeling. A series of CP/MAS 13C NMR spectra was acquired at contact times ranging from 0.01 to 10 ms. Signal intensities of major structural groups were analyzed using both the classical three-parameter model and an extended four-parameter model of cross-polarization dynamics. The results demonstrated pronounced contact-time-dependent variations in the apparent CP/MAS signal distribution among alkyl, aryl, and carboxyl/amide structural domains. Short contact times preferentially enhanced proton-rich aliphatic and carbohydrate structures, whereas longer contact times increased the relative contribution of aromatic and carboxyl/amide fragments. The extended model provided a better fit for selected structurally heterogeneous spectral regions, particularly aromatic and heteroatom-containing domains, whereas the classical model adequately described the relatively homogeneous aliphatic regions. The fitted CP-dynamic parameters provided effective comparative descriptors of the investigated HAs under the experimental conditions employed and demonstrated consistent depth-dependent differences within the investigated peat profile. These depth-dependent differences may reflect changes associated with peat accumulation and long-term cryogenic preservation; however, this interpretation requires confirmation using independently replicated peat profiles.