Quantitation in the solid-state 13C NMR analysis of soil and organic soil fractions

C-13 CP-MAS and DMAS spin-counting experiments have been carried out on an absolute basis for a specific whole soil and its humin, humic acid, and fulvic acid fractions, as well as a sample of the soil that was treated with 2% HF(aq). The results confirm previous conclusions that a substantial fraction of the carbon content indicated by classic elemental analysis is missed in some samples, especially whole soil and humin, by both CP-MAS and DP-MAS C-13 NAIR methods, and that the problem is more serious for CP-MAS than for DP-MAS. This study also confirms the fact that treatment of soil organic matter with 2% HF(aq) dramatically reduces this problem but may generate some structural uncertainties associated with significant structural alterations that accompany the HF(aq) treatment, as indicated by the C-13 NMR data. The relationship between the "missing carbon" problem and the concentration of paramagnetic centers, especially Fe(III) centers, is explored in substantial detail.