The use of spin counting for determining quantitation in solid state 13C NMR spectra of natural organic matter 1.: Model systems and the effects of paramagnetic impurities

The degree of quantitation achieved in the solid state C-13 nuclear magnetic resonance (NMR) spectra of a number of organic materials, an HF-treated soil, and a whole soil, was determined for both cross-polarisation (CP) and Bloch decay (BD) techniques using spin-counting experiments. C-13 NMR signal, which was actually observed (C-obs) was in the range The percentage of potential 79-107% for the ED technique and in the range 29-103% for the CP technique. A number of materials, including cellulose, pectin, lignin, and palmitic acid gave quantitative spectra using both CP and ED. A second group, including chitin, collagen, and the HF-treated soil gave quantitative ED spectra, but significantly diminished CP spectra (C-obs-CP = 66-75%). A third group including charcoal, a commercial humic acid, and the whole soil gave significantly diminished ED spectra (C-obs-BD = 79-87%) and severely diminished CP spectra (C-obs-CP = 29-35%). Signal losses in the CP spectra were attributed to rapid relaxation rates (short T1 rhoH) and/or slow magnetisation build-up rates (long T-CH). The spin dynamics of the CP experiment were studied and a new method for correcting for differences in T1 rhoH between the sample and the reference in CP spin-counting experiments was developed. Signal produced by the Kel-F rotor end-caps was significant for the ED spectra and a correction for the end-cap spectrum was required prior to spin counting. The low LH content of the fluorinated Kel-F polymer ensured that the contribution of end-caps to the CP spectra was insignificant. The effects of paramagnetic cations on quantitation in solid slate NMR spectra was investigated by doping model compounds with paramagnetic impurities. Three mechanisms, which bring about signal loss and operate on three different length scales, were identified. The magnitude of the signal loss brought about by a paramagnetic material was shown to be dependent on both the type of cation involved and on the intimacy of contact with the organic matrix. Chemically bound paramagnetic cations were shown to result in large signal losses in the CP spectrum, whereas paramagnetic salts in a physical mixture with an organic material affected both CP and ED spectra equally. (C) 2000 Elsevier Science B.V. All rights reserved.