Effect of ramp size and sample spinning speed on CPMAS C NMR spectra of soil organic matter

Cross polarization (CP) magic angle spinning (MAS) ¹³C NMR spectroscopy is a solid state NMR technique widely applied to study the chemical composition of natural organic matter. In high magnetic fields (>7 T), fast sample spinning is required in order to reduce the influence of spinning sidebands underlying other chemical shift regions. As the spinning speed increases, the Hartmann-Hahn matching profiles break down into a series of narrow matching bands. In order to account for this instability variable amplitude cross polarization techniques (e.g. VACP, ramp-CP) have been developed. In the present study, we experimentally verified the stability of the Hartmann-Hahn condition under two MAS speeds for different samples with known structure and two different humic acids. For a complete restoration of flat matching profiles, large ramp sizes were needed. The matching profiles of the humic acids showed that both samples needed different ramp sizes to restore flat profiles. A set up based on the matching profiles of the commonly used glycine would have led to an insufficient ramp size for one of the humic acids. For the characterization of natural organic matter, we hence recommend to roughly set the matching conditions with a standard and subsequently optimize the matching conditions on a more complex, preferably representative, sample such as a humic acid. We would suggest to either run an array of different ramp sizes until maximum signal intensity is reached for all chemical shift regions or, in the case of unavailable measurement time, to use a ramp size twice the spinning speed.