Effects of chemical treatments on the quality and quantitative reliability of solid-state 13C NMR spectroscopy of mineral soils

Three mineral soils were treated by chemical methods traditionally used to increase cross polarization magic angle spinning technique (CP-MAS) 13C nuclear magnetic resonance (NMR) spectral quality. Treatment with 2% HF/BF3 produced the highest quality spectra followed by 4% sodium dithionite and 0.05 M tin(II) chloride, respectively. The improvement in spectral quality was due to both reduction in the paramagnetic character of the soils as well as an increase in the concentration of organic carbon. Surprisingly, removal of iron did not necessarily correlate with higher quality spectra. Dithionite selectively removed aliphatic material, most likely associated with iron, and 2% HF/BF3 removed O-alkyl and carboxyl carbon. Relaxation rate constants (T1H and T1(rho)H) were shown to be a function of the treatment method employed rather than the paramagnetic content of the soil through use of proton spin relaxation editing (PSRE).