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Supercritical Gas Extraction of a Soil with Solvents of Increasing Polarities¹

ABSTRACT

The surface horizon of the Bainsville soil, an Aquoll, was extracted by supercritical gas (SCG) extraction with solvents of increasing polarities. These included n-pentane, ethanol, and mixtures of ethanol and acetone with water. Each extract was analyzed by chemical and ¹³C nuclear magnetic resonance (NMR) methods. Proportions of alkanes, alkanoic acids, and carbohydrates in the extracts decreased as the polarity of the solvent increased. Conversely, proportions of aromatics increased with increasing polarity of the solvent. The presence of carbohydrates and amino acids in the SCG extracts, as indicated by ¹³C NMR, was confirmed by chemical methods. As the polarity of the solvent increased, concentrations of hexoses increased while those of pentoses, except for arabinose, decreased. Similarly, concentrations of basic amino acids increased under these conditions. Hydrolyzates of SCG extracts contained high concentrations of NH₃. A mixture of acetone and water (40:60 v/v%) was found to be the most efficient supercritical gas extractant under the experimental conditions employed. Carbon-13 NMR spectra of the SCG extracts were exceptionally well-defined with sharp and distinct peaks revealing fine chemical structure. The distortion-less enhancement by polarization transfer (DEPT) NMR technique was used to determine C multiplicity and to further enhance resolution. The combination of supercritical gas extraction with ¹³C NMR provides soil scientists with a powerful tool for generating important information on the chemical composition and structure of soil organic matter.

¹ Contribution no. 86-49 from the Land Resource Research Centre, Research Branch, Agriculture Canada, Ottawa, Ontario K1A 0C6, Canada.

² Program Head, Soil Nitrogen and Organic Matter, and Research Scientist, respectively.

Received for publication August 13, 1986.