Analytical Chemistry and Spectroscopy
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Partial structures of a reference groundwater humic substance were quantified by J-resolved two-dimensional 13C NMR spectroscopy applied in its phase-sensitive variant. The results agree with those obtained by the one-dimensional techniques DEPT (distortionless enhancement by polarization transfer) and SEMUT-90 (subspectral editing using a multiple quantum trap), thus demonstrating the accuracy of both methods. However, the two-dimensional technique is recommended, because quantification is based on only one spectrum. Quantification provides the ratio of different substructures (carbons belonging to ketones, carboxylic acids, different aromatics, carbohydrates, paraffinic C, CH, CH2 and CH3 groups) and, in combination with elemental analysis, the partition of hydrogen between C and O (83% bonded to C and 17% to O). Also, insight is obtained into the biogenesis of the humic acids. Aromatic moieties probably originate mainly from lignin rather than from other aromatic compounds. CH—O moieties mainly belong to carbons in ethers and alcohols and only to a small extent to carbons in carbohydrates, as shown by comparing NMR results with gas chromatographic analysis of the carbohydrate part performed by other workers. The ratio of paraffinic CH:CH2:CH3 is about 1:1:1; the high proportion of paraffinic methyl groups cannot yet be explained.
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