title: Comparison of sorption domains in molecular weight fractions of a soil humic acid using solid-state 19F NMR.
abstract: Humic acid was fractionated into eight different molecular size components using ultrafiltration. Solid-state CPMAS 13C NMR demonstrated that fractions larger than 100,000 Daltons were primarily aliphatic in character, while fractions smaller than 30,000 Daltons were predominantly aromatic in character. Solid-state 19F NMR examination of the sorptive uptake of hexafluorobenzene (HFB) by HA and each of the fractions gave spectroscopic evidence for the existence of at least three sorption sites in the smaller molecular size fractions, while two predominant sorption sites could be established in the larger molecular size fractions. Sorbed HFB displayed higher mobility in the smaller, more aromatic fractions while HFB in the larger, more aliphatic fractions displayed lower mobility. The relative mobilities of HFB in each sorption domain suggest that the rigid domain may be composed of aliphatic carbon rather than aromatic carbon moieties. In larger size fractions, this domain may be the result of rigid, glassy regions composed of aliphatic molecules or side chains.
authors.affiliation: Institute of Chemistry and Dynamics of the Geosphere IV, Agrosphere, Research Center Jülich, D-52428 Jülich, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Khalaf
authors.forename: Scott D
authors.initials: SD
authors.lastname: Kohl
authors.forename: E
authors.initials: E
authors.lastname: Klumpp
authors.forename: James A
authors.initials: JA
authors.lastname: Rice
authors.forename: E
authors.initials: E
authors.lastname: Tombácz
mesh_terms: D004784:Environmental Monitoring / Q000379:methods*; D005374:Filtration; D005462:Fluorine Radioisotopes; D006812:Humic Substances / Q000032:analysis* / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008970:Molecular Weight
chemical_list: D005462:Fluorine Radioisotopes; D006812:Humic Substances