13C NMR studies of organic matter in whole soils: I. Quantitation possibilities |
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Authors: | P. KINCHESH D. S. POWLSON E. W. RANDALL |
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Affiliation: | Department of Chemistry, Queen Mary &Westfield College, London El 4NS, UK;Department of Soil Science, Rothamsted Experimental Station, Harpenden, Herts AL5 2JQ, UK |
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Abstract: | A detailed discussion of the quantitative nature of 13C CPMAS NMR spectra as applied to solid samples, such as soil, is given. In particular, the influence of the cross-polarization (CP) time constant (TCH), the relaxation time constant of protons in the rotating frame (T1pH) and the contact time (tc) in the CPMAS experiment are considered. Three distinct quantitation regimes are numerically identified according to sample parameters tCH and T1PH > and the experimental choice of tc: (i) quantitation obtainable from a single CPMAS spectrum; (ii) quantitation obtainable from a series of CPMAS spectra; and (iii) quantitation not possible using CPMAS. Strategies for the measurement of sample parameters TCH and TipH are reviewed. When quantitation is not possible using CPMAS it is necessary to regress to the direct polarization (DP) of 13C nuclei. The sensitivity problems of DPMAS are discussed, as too are general factors that affect the quantitation of 13C data such as spinning sidebands. More specifically in relation to soil samples, the effects on quantitation arising from the presence of paramagnetics and the actual methods for the measurement of signal intensities are covered. |
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