Background signal in solid state C-13 NMR spectra of soil organic matter (SOM) - Quantification and minimization

C-13 background signal, obtained for an empty rotor, was shown by spin coun ting experiments to be equivalent to 1 mg of observable carbon for cross-po larization (CP) spectra and 69 mg of observable C for Bloch decay (BD) spec tra. The BD background was mainly due to Kel-F in the stator, with minimal signal detected from the Kel-F end-caps. The CP background was attributed t o non-Kel-F components of the stator, probe, or probe supports. The BD back ground signal was eliminated by using a modified dipolar dephasing pulse se quence in which the absence of F-19 decoupling (rather than the absence of H-1 decoupling) causes selective elimination of the Kel-F signal. (C) 2001 Academic Press.