Combination of X-ray photoelectron and solid-state C-13 nuclear magnetic resonance spectroscopy in the structural characterisation of humic acids

Leonardite humic acid (LHA) and soil humic acid (SHA), purchased from the M SS, as well as purified humic acid from Aldrich (HA) are characterised by c ross-polarisation magic angle spinning (CP-MAS) and Bloch decay C-13 NMR sp ectroscopy and X-ray photoelectron spectroscopy (XPS). For the C-13 CP-MAS NMR experiments, the effects of both the sample spinning sidebands and the influence of the cross-polarisation contact time are investigated. The func tional groups distribution obtained by CP-MAS NMR is compared to the one re sulting from chemical analyses. It is found that CP-MAS NMR overestimates t he concentration of CO2H groups for certain humic compounds, whereas the co ncentration of phenolic groups is in reasonable agreement with the values d etermined by chemical methods. XPS was used as a complementary technique to understand the overestimation of carboxylic groups by NMR. The results fro m XPS, a surface characterisation technique, are in good agreement with tho se measured by elemental analyses of the bulk structure. High resolution C 1s and N 1s spectra show that the overestimation of the CO2H content by NMR is due to the presence of amide groups which give overlapping signals in t he CO2H region of the NMR spectra. (C) 2000 Elsevier Science B.V. All right s reserved.