Solid-state 13C NMR spectroscopic studies of soil organic matter at two magnetic field strengths

The characterization of soil organic matter by solid-state 13C NMR spectroscopy is increasingly being carried out on NMR spectrometers with higher magnetic fields, despite reservations about the quantitative reliability of spectra acquired at these high magnetic fields. In this study, the performance of 200 and 400 MHz spectrometers is compared for the quantitative analysis of two soil organic matter samples of contrasting composition. The higher spinning rate (10 kHz) required on the 400 MHz spectrometer to keep spinning side-band interference at a manageable level results in a 35-40% decrease in signal intensity for cross-polarization (CP) spectra. However, this signal loss is largely non-selective, with only a slight decrease in carbonyl and aromatic signal apparent at the higher field. There is no improvement in spectral resolution at the higher field. Sensitivity is similar for the two spectrometers, with increases in the amount of signal acquired per scan on the higher field instrument negated by the need for a longer delay between scans to allow for slower rates of relaxation. Introduction of a ramped-amplitude CP pulse improves sensitivity relative to the standard CP experiment, especially at high spinning rates, but does not affect relative quantitation. Sensitivity to the Hartman-Hahn match is much greater at the higher MAS rate required on the 400 MHz spectrometer. The robustness of the Bloch decay technique is confirmed, with near identical spectra produced at the two different magnetic fields, and no dependence of signal intensity on the rate of magic angle spinning. The RESTORE technique is used to investigate the causes of the dependence of CP signal intensity on the spinning rate.