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

Abstract

The degree of quantitation achieved in the solid state 13C 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. The percentage of potential 13C NMR signal, which was actually observed (C(obs)) was in the range 79-107% for the BD 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 BD. A second group, including chitin, collagen, and the HF-treated soil gave quantitative BD 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 BD 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 T(1p)H) 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 T(1p)H 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 BD spectra and a correction for the end-cap spectrum was required prior to spin counting. The low 1H 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 state 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 BD spectra equally. (C) 2000 Elsevier Science B.V.