QUANTITATIVE ASPECTS OF SOLID-STATE C-13-NMR SPECTRA OF HUMIC SUBSTANCES FROM SOILS OF VOLCANIC SYSTEMS

Cross-Polarisation Magic Angle Spinning Carbon-13 Nuclear Magnetic Res onance spectroscopy (CPMAS C-13-NMR) represents one of the most powerf ul tools to investigate soil organic matter (SOM) mainly because of it s inherent capacity to provide a semi-quantitative evaluation of carbo n distribution. A critical parameter during acquisition of CPMAS C-13- NMR spectra is the contact time required to obtain the cross-polarisat ion between proton and carbon nuclei. The procedure to evaluate the be st contact time for the acquisition of a quantitative CPMAS C-13-NMR s pectrum is to perform Variable Contact Time (VCT) experiments. In this work the structural features of a number of purified humic substances from Italian and Costarican volcanic soils were investigated by CPMAS C-13-NMR spectroscopy after having performed preliminary VCT experime nts. The VCT experiments showed that the average contact times vary ac cording to the origin and chemical structure of the humic material. Th e optimal contact times (OCT) for nine humic samples were between 250 and 800 mu s. These values were different from the time of 1000 mu s t hat is commonly applied as the best average contact time for humic mat erials. Moreover, by comparing the NMR data to those obtained by eleme ntal analysis (C/H ratio), it appeared that the efficiency of the cros s-polarisation between protons and carbons, and hence the contact time , is affected not only by the number of protons, but also by their dis tribution over the molecules. The evaluation of errors in quantitative estimation of the different carbons revealed that the use of OCT gene rally reduced by half the loss of signals occurring when the average c ontact time of 1000 mu s is used in CPMAS C-13-NMR spectra of humic su bstances. (C) 1997 Elsevier Science B.V.