StructURE and elemental composition of humic acids: comparison of solid-state ~(13)C NMR calculations and chemical analyses

摘要

Humic acids (HAs) are found in soils, sediments, waters and some plants. HAs are complex polymeric mixtures. Structural and functional information is critical in determining their reactivity with heavy metals and organic contaminants. The best way to fingerprint HA structures in to examine HAs without any pretreatment. In this respect, solid state C NMR is a powerful tool. Cross Polarization Mgic Angle Spinning (CPMAS) C NMR has been extensively used in HAs studies. There are, however, several problems with this technique, which can lead to nonquantitative spectra. The first problem is the reduced cross-polarization (CP) efficiency for unprotonated carbons, mobile components, or regions with short T_(1_p)~H (proton rotating-frame spin-lattice relaxation time). CP efficiency relies on the distance between ~13C carbons and protons. Thus, C carbons without directly bonded protons have a reduced CP efficiency. Molecular mobility, which reduces the dipolar interaction, can also significantly decrease the CP efficiency. The shortening of T_(1_p)~H by paramagnetic species in HAs can cause protons to lose magnetization too quickly to transfer to carbon (typical T_(1_p)~H are less than 10 ms in HAs~16). Ramp sequence CPMAS is a relatively new technique which was first developed by the Smith group at Yale University. ~(17-19) Cook et al.~(20,21) applied this technique in the study of humic substances. The applicability of this new technique still requires further examination because the relatively long contact time used (10 ms) may render some carbon species with short T_(1_p)~H invisible.~19