IMPROVEMENT OF C-13 AND N-15 CPMAS NMR-SPECTRA OF BULK SOILS, PARTICLE-SIZE FRACTIONS AND ORGANIC MATERIAL BY TREATMENT WITH 10-PERCENT HYDROFLUORIC-ACID

The small organic matter content of mineral soils makes it difficult t o obtain C-13 and N-15 nuclear magnetic resonance (NMR) spectra with a cceptable signal-to-noise ratios. Subjecting such samples to hydrofluo ric acid removes mineral matter and leads to a relative increase in or ganic material. The effect of treatment with 10% hydrofluoric acid on bulk chemical composition and resolution of solid-state C-13 NMR spect ra was investigated with six soils, some associated particle size frac tions, plant litter and compost. The treatment enhanced the signal-to- noise ratio of the solid-state C-13 NMR spectra. The improvement in sp ectrum quality was greatest in the clay fraction of soil contaminated with coal ash. The removal of paramagnetic compounds associated with t he ash may be the main reason for the improvement. Based on total C, t otal N, C/N ratio and intensity distribution of the solid-state C-13 N MR spectra, no changes in organic matter composition could be detected , except for a possible loss of carbohydrates. After treatment with HF , solid-state N-15 NMR spectra of particle size fractions were obtaine d and indicated that the observable nitrogen is present mostly as pept ides and free amino groups. Extraction with hydrofluoric acid is recom mended as a routine treatment prior to solid-state C-13 and N-15 NMR o n soil containing little C or N and soil samples containing paramagnet ic compounds from natural or anthropogenic sources.