C-13 NMR-STUDIES OF ORGANIC-MATTER IN WHOLE SOILS .1. QUANTITATION POSSIBILITIES

A detailed discussion of the quantitative nature of C-13 CPMAS NMR spe ctra as applied to solid samples, such as soil, is given. In particula r, 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 accor ding to sample parameters T-CH and T-1 rho H, and the experimental cho ice 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 measuremen t of sample parameters T-CH and T-1 rho H are reviewed. When quantitat ion is not possible using CPMAS it is necessary to regress to the dire ct polarization (DP) of C-13 nuclei. The sensitivity problems of DPMAS are discussed, as too are general factors that affect the quantitatio n of C-13 data such as spinning sidebands. More specifically in relati on to soil samples, the effects on quantitation arising from the prese nce of paramagnetics and the actual methods for the measurement of sig nal intensities are covered.