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

Spin counting experiments were used to determine the degree of quantitation achieved in solid state C NMR spectra of HF-treated, < 53 mu m fractions o f eight soils. Observabilities were determined for both cross-polarisation (C-obs-CP) and Bloch decay (C-obs-BD) techniques. C-obs-BD ranged between 8 4% and 106%, indicating that practically all potential signal was observed. Therefore, the ED spectra quantitatively reflected the distribution of C-1 3 environments present in the soils. C-obs-CP ranged between 54% and 83%, i ndicating that, although the majority of potential signal was observed, the re was a significant fraction of potential NMR signal in each soil that was not detected using the CP technique. Comparison of the ED and CP spectra i ndicated that the decrease in signal intensity in the CP spectra was more e vident for some resonances than for others. The O-alkyl region of the CP NM R spectrum was the least affected and the aromatic region was the most affe cted for the majority of the soils. This was due, in large part, to the pre sence 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 Elsevie r Science B.V. All rights reserved.