The quantitative analysis of solution- and CPMAS-C-13 NMR spectra of humic material

Abstract The conditions for obtaining quantitative high resolution C-13 NMR spectra of humic substances in solution and in the solid state are evaluated by determining the relevant relaxation times of the various groups of carbon atoms. Solution spectra at 75 MHz of fulvic and humic acids extracted by standard procedures from 9 German soils were obtained under quantitative conditions in 0.05 M NaOD solution. These are compared to solid state spectra obtained in the cross polarization magic angle spinning (CPMAS) mode at 25 and 75 MHz. In addition to CPMAS spectra of the complete soils and the extracted soils were obtained at the same frequencies. The integration of the spectra obtained for the extracted material shows that the distribution of signal intensity in the various regions of chemical shift is identical for the three types of spectra within experimental error. A different series of measurements comprising various soils with an unusually high content of aromatic carbons showed, however, that for these substances the concentration of aromatic carbon is always found to be too low in the solid state spectra. Comparison of native whole soil solid state spectra with the spectra of the extracts and the solid residues of the extraction procedure casts considerable doubt upon the ability of the standard aqueous sodium hydroxide method to separate defined fractions of humic material.