13C NMR studies of organic matter in whole soils: I. Quantitation possibilities

A detailed discussion of the quantitative nature of C-13 CPMAS NMR spectra as applied to solid samples, such as soil, is given. In particular, the influence of the cross-polarization (CP) time constant (T-CH), the relaxation time constant of protons in the rotating frame (T-1 rho H) and the contact time (t(c)) in the CPMAS experiment are considered. Three distinct quantitation regimes are numerically identified according to sample parameters T-CH and T-1 rho H, and the experimental choice of t(c): (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 T-CH and T-1 rho H are reviewed. When quantitation is not possible using CPMAS it is necessary to regress to the direct polarization (DP) of C-13 nuclei. The sensitivity problems of DPMAS are discussed, as too are general factors that affect the quantitation of C-13 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.