MODIFIED PULSED-FIELD GRADIENT NMR EXPERIMENTS FOR IMPROVED SELECTIVITY IN THE MEASUREMENT OF DIFFUSION-COEFFICIENTS IN COMPLEX-MIXTURES - APPLICATION TO THE ANALYSIS OF THE SUWANNEE RIVER FULVIC-ACID

To simplify the complex H-1 NMR spectrum of a fulvic acid sample and g ain structural and molecular size information, spectral editing techni ques were used in conjunction with a general PFG NMR pulse sequence. T hese editing techniques exploit differences in T-1 and T-2 relaxation times as well as differences in coupling constants. The experiments we re initially performed on a model mixture of glutamic acid and ethyl a cetate in order to validate the method as a technique for measurement of diffusion coefficients. The editing experiments were then applied t o the International Humic Substances Society Suwannee River fulvic aci d standard. These editing techniques allowed more selective measuremen t of diffusion coefficients for broad classes of components within reg ions of the H-1 NMR spectrum of the fulvic acid solution. The average radii of gyration calculated for the Suwannee River fulvic acid sample from the diffusion coefficients are in good agreement with literature values.