The use of spin counting for determining quantitation in solid state 13C NMR spectra of natural organic matter: 2. HF-treated soil fractions

Abstract

Spin counting experiments were used to determine the degree of quantitation achieved in solid state 13C NMR spectra of HF-treated, < 53 μm fractions of eight soils. Observabilities were determined for both cross-polarization (C(obs)-CP) and Bloch decay (C(obs)-BD) techniques. C(obs)-BD ranged between 84% and 106%, indicating that practically all potential signal was observed. Therefore, the BD spectra quantitatively reflected the distribution of 13C environments present in the soils. C(obs)-CP ranged between 54% and 83%, indicating that, although the majority of potential signal was observed, there was a significant fraction of potential NMR signal in each soil that was not detected using the CP technique. Comparison of the BD and CP spectra indicated that the decrease in signal intensity in the CP spectra was more evident for some resonances than for others. The O-alkyl region of the CP NMR spectrum was the least affected and the aromatic region was the most affected for the majority of the soils. This was due, in large part, to the presence of char in the most affected soils, in keeping with the low value of C(obs)-CP reported for charcoal. The carbonyl and alkyl resonances were also under-represented in the CP spectra of some of the soils. (C) 2000 Elsevier Science B.V.