A Spectroscopic and Chemical Investigation on the De‐ashing of a Humin

The application of 13 C cross‐polarization magic‐angle spinning nuclear magnetic resonance (CPMAS NMR) to humins has been limited because low C and high Fe contents degrade spectral quality. This study investigated the effect of de‐ashing with aqueous HCl/HF (1.16 M HCl and 2.88 M HF) on chemical properties and 13 C CPMAS NMR spectra of a humin. With progressive de‐ashing, the humin became partly soluble in 0.5 M NaOH, and its ash‐free composition approached that of the humic acid fraction. The CPMAS NMR spectra showed increasing improvement in resolution and signal‐to‐noise ratio. The spectrum of the most extensively de‐ashed sample (520 g C kg −1 ) was very similar to that of the humid acid extracted from the same soil. The de‐ashing treatment caused variable changes in the distribution of N determined after acid hydrolysis, but little change in the molar distribution of amino acids. Some monosaccharides were lost, and there was a change in the monosaccharide composition, with glucose predominating (on a molar basis) in the most extensively de‐ashed samples. It is unlikely that major structural changes were caused by the de‐ashing, which provides a useful approach to the NMR investigation of humins.