Khanh H. Bui

Facile Separation of Enantiomers, Geometrical Isomers, and Routine Compounds on Stable Cyclodextrin LC Bonded Phases

The effectiveness of employing stationary phases composed of chemically bonded cyclodextrin molecules in the high performance liquid chromatographic separation of a variety of different types of compounds is summarized. Over one hundred compounds, including optical, geometrical, and structural isomers, diastereomers, and epimers were successfully separated from each other via use of beta- or gamma-cyclodextrin bonded phases and aqueous methanolic mobile phases. The mechanism of separation is based upon inclusion complex formation between the compounds being separated and the cyclodextrin molecules bonded to the stationary phase. The effects of temperature, mobile phase composition and flow rate upon the chromatographic selectivity and resolution are described. The results indicate that the cyclodextrin columns may be more versatile, flexible, and effective compared to the conventional normal or reversed phase columns.

Mechanistic Considerations on the Reversed Phase Liquid Chromato-Graphic Separation of Proteins

Abstract The effect of column length, stationary phase chemistry, flow rate and gradient variation and direction on the separation of seven proteins was evaluated. In general, resolution increased with shorter columns and higher flow rates. Nontraditional reverse gradients (from acetonitrile to buffer) sometimes produced better protein separations than traditional reversed phase gradients. Bonded phase chemistry has little effect on gradient protein separations, but silica gel properties (e.g., surface area, pore size, etc.) did. Both retention and resolution varied considerably with the nature of the gradient used.

Practice, mechanism and theory of reversed phase tlc polymer fractionation

Abstract The use of reversed phase TLC to determine the molecular weight and molecular weight distribution of poly(methyl methacrylate), poly(ethylene glycol), poly(ethylene oxide), poly(tetrahydrofuran), poly(butadiene), poly(isoprene), poly(α-methylstyrene) and poly(styrene) is demonstrated. The mechanism by which fractionation occurs and a theoretical description of the process are given and discussed.

Evaluation of the theory and experimental consequences of gradient liquid chromatographic polymer separations

Abstract The separation of synthetic polymers of different molecular weight by non-aqueous, gradient liquid chromatography does not follow traditional chromatographic mechanisms. As a result one can produce separations where late eluting solutes have narrower bandwidths than early eluting solutes, where shorter columns result in higher resolution than identical longer columns, and where the selectivity and resolution are independent of the type of stationary phase used. These results are explained for polystyrene, poly-α-methylstyrene and polyisoprene using recently developed theory. The results of this work are not only pertinent to the practice and theory of chromatography but also to the study of polymers.

A Comparison of Polymer Separation Efficiency and Resolution by Gradient LC, GPC and TLC

Abstract The specific resolution of gradient LC and reversed phase TLC methods for the separation of different molecular weight standards of poly(isoprene), poly(ethylene glycol), poly(ethylene oxide), poly(styrene) and poly(α-methylstyrene) were determined. It was found that gradient LC has an order of magnitude greater resolving power (for high polymers) than gel permeation chromatography (GPC) while TLC had from two to five times the resolving power of GPC in the molecular weight range investigated. This is a direct result of the greater selectivity of gradient LC and TLC techniques. The specific resolution is also dependent on the type of gradient used to achieve fractionation for the LC technique.

Use Of Aqueous Micellar Mobile Phases In Reverse Phase Tlc

Abstract Aqueous micellar solutions can be used in reverse phase TLC providing the ionic strength of the solution is sufficiently high to prevent the destruction of the stationary phase. Stability curves have been determined for sodium dodecyl sulfate and cetyltrimethylammonium chloride containing aqueous mobile phases. These “pseudophase” solutions allow the unique separation of hydrophobic from hydrophilic compounds. Indeed one can estimate the relative hydrophobicity of a compound by observing its chromatographic behavior in this system.

Determination of Polymer Molecular Weight and Molecular Weight Distribution by Reverse Phase Thin Layer Chromatography

Abstract Reverse phase thin layer chromatography (RPTLC) and scanning densitometry was used to determine various molecular weight averages and the molecular weight distribution of broad molecular weight range samples of poly(styrene) and poly(methyl metharcylate). A basic program was developed which analyzes the analog signals from the scanner, calculates the desired parameters, prints the parameters, graphs the results and simultaneously displays the results on a CRT. The average molecular weight values obtained by this technique compare well to those obtained by other methods.