Correlation of the capacity factor in vesicular electrokinetic chromatography with the octanol:water partition coefficient for charged and neutral analytes.

PURPOSE: The aim of this study was to develop a method based upon electrokinetic chromatography (EKC) using oppositely charged surfactant vesicles as a buffer modifier to estimate hydrophobicity (log P) for a range of neutral and charged compounds. METHODS: Vesicles were formed from cetyltrimethylammonium bromide (CTAB) and sodium n-octyl sulfate (SOS). The size and polydispersity of the vesicles were characterized by electron microscopy, dynamic light scattering, and pulsed-field gradient NMR (PFG-NMR). PFG-NMR was also used to determine if ion-pairing between cationic analytes and free SOS monomer occurred. The CTAB/SOS vesicles were used as a buffer modifier in capillary electrophoresis (CE). The capacity factor (log k') was calculated by determining the mobility of the analytes both in the presence and absence of vesicles. Log k' was determined for 29 neutral and charged analytes. RESULTS; There was a linear relationship between the log of capacity factor (log k') and octanol/water partition coefficient (log P) for both neutral and basic species at pH 6.0, 7.3, and 10.2. This indicated that interaction between the cation and vesicle was dominated by hydrophobic forces. At pH 4.3, the log k' values for the least hydrophobic basic analytes were higher than expected, indicating that electrostatic attraction as well as hydrophobic forces contributed to the overall interaction between the cation and vesicle. Anionic compounds could not be evaluated using this system. CONCLUSION: Vesicular electrokinetic chromatography (VEKC) using surfactant vesicles as buffer modifiers is a promising method for the estimation of hydrophobicity.

Analysis of diffusion coefficient distributions in humic and fulvic acids by means of diffusion ordered NMR spectroscopy.

The use of the computer program CONTIN to analyze pulsed-field gradient NMR (PFG-NMR) data for several standard humic and fulvic acids is described. An advantage of PFG-NMR analysis is that integration of different spectral regions provides a picture of how the diffusion coefficients vary with functional group composition for a given sample. Using prior knowledge of the sample and the principle of parsimony, CONTIN approximates a solution to the inverse Laplace transform applied to the decay of peak intensity with gradient area in the PFG-NMR experiment. Thus, a continuous distribution of diffusion coefficients is resolved for the polydisperse humic and fulvic acids. The results of the CONTIN analyses are in the form of a distribution function and a two-dimensional DOSY plot. The 2D DOSY spectrum displays chemical shifts along one axis and diffusion coefficients along the other, while a number-average diffusion coefficient, D(N), a weight-average diffusion coefficient, D(W), and a most probable diffusion coefficient, D(P), are realized from the diffusion coefficient distribution. For all spectral regions of each humic sample, D(W) was greater than D(N), which in turn was greater than or equal to the D(P), suggesting that the diffusion coefficient distribution is weighted toward smaller, more rapidly diffusing molecules. Polydispersities, estimated from the ratio D(W)/D(N), were less than the reported M(W)/M(N) values for similar humic substances. Thus, the D(W)/D(N) ratio obtained by CONTIN analysis of PFG-NMR data can be at least a qualitative, and at best a semiquantitative, indication of the polydispersity of the humic sample, but should not be used as a quantitative measure of polydispersity.