Comparison of solid-state 13C NMR spectra of soil organic matter from an experimental burning site acquired at two field strengths

Solid-state 13C cross polarisation (CP) nuclear magnetic resonance spectra were acquired for 15 soil organic matter samples on 2 different spectrometers (200 MHz and 400 MHz). Distributions of broad functional group classes—carboxyl, aryl, O-alkyl, and alkyl—were determined by integration across broad chemical shift regions. The distributions derived from the 2 spectrometers were closely correlated (r2 = 0.77–0.93). Only slight biases were identified; carboxyl C contents were on average 8% lower and alkyl C contents 5% higher for spectra acquired on the 400 MHz spectrometer. These results indicate that valid 13C CP spectra can be acquired at field strengths up to 400 MHz, and that spectra acquired at different field strengths can be directly comparable.