Soon M. Han

Use of hydroxypropyl- and hydroxyethyl-derivatized β-cyclodextrins for the thin-layer chromatographic separation of enantiomers and diastereomers

Partially substituted hydroxypropyl-beta-cyclodextrin and hydroxyethyl-beta-cyclodextrin proved to be effective chiral mobile phase additives (CMAs) for the thin-layer chromatographic (TLC) resolution of racemic benzyl-2-oxazolidinone, 5-(4-methyl-phenyl)-5-phenylhydantoin, mephenytoin and several dansyl and beta-naphthylamide amino acids. Several diastereomeric compounds including steroid epimers and alkaloids were separated as well. The derivatized beta-cyclodextrins tended to be much more soluble in water and hydro-organic solvents than native beta-cyclodextrin. Their chromatographic selectivity also was somewhat different. The use of CMAs in TLC is a potentially useful and powerful method that has not been considered adequately. The relative lack of chiral stationary phases available in planar format makes the use of CMAs particularly attractive.

Planar chromatographic separation of enantiomers and diastereomers with cyclodextrin mobile phase additives

A variety of racemic compounds were resolved using reversed-phase thin-layer chromatography (TLC) with mobile phases containing highly concentration solutions of beta-cyclodextrin (beta-CD). These include the drugs labetalol and mephenytoin, metallocenes, crown ethers, methyl-p-toluenesulfinate, nornicotine derivaties and several dansyl and beta-naphthylamide substituted amino acids. It was possible to resolve some racemates that could not be separated on beta-CD bonded phase liquid chromatography (LC) columns with this technique. Likewise there were some compounds that could be resolved with the LC approach that failed to separate with the present TLC method. In cases of racemates that could be resolved by either approach, it was found that the retention order was exactly opposite for the two methods. Enantiomeric resolution is highly dependent on mobile phase composition. In particular, the type and amount of organic modifier as well as the concentration of beta-CD affect the observed resolution. Possible reasons for the chromatographic behavior are discussed. Several diastereoisomeric compounds were separated as well, including steroid epimers and pharmaceutical compounds.

Quantitative analytical aspects of reversed-phase liquid chromatography with slurry-packed capillary columns

Conditions for reproducible packing of fused-silica liquid chromatography capillary columns were demonstrated. The plate height vs. velocity curves were determined for successively prepared columns. In addition, the values of separation impedance were calculated. Several liquid chromatography systems were evaluated in conjunction with the slurry-packed capillaries for the retention and peak area reproducibility.

Enantiomeric Separations in Chromatography

I. INTRODUCTION The resolution of enantiomers (nonsuperimposable, mirror-image isomers) has traditionally been considered one of the more difficult problems in separation science. Enantiomers have identical physical and chemical properties in an isotropic environment except that they rotate the plane of polarized light in opposite directions. A mixture containing equal amounts of enantiomers is referred to as a racemic mixture. Neither racemic mixtures nor solutions of achiral compounds are able to rotate the plane of polarized light.

Liquid chromatographic resolution of enantiomers containing single aromatic rings with β-cyclodextrin-bonded phases

Abstract Racemic compounds containing two or more ring moieties are thought to be of optimal size for complexation and resolution on β-cyclodextrin (β-CD) bonded-phase liquid chromatographic columns. There are few reports on the separation of racemates containing one or no rings. Here, seventeen single-ring-containing racemates are resolved via β-CD complexation: (±)-1-phenyl- ethyl propionate; (±)-α-methylbenzyl acetate, (±)-2-chloro-2-phenylacetic acid, (±)-ethyl- 3-phenylglycidate, d , l -3,4-dihydroxyphenyl-α-propylacetamide; (±)-2-methoxy-α-methylbenzyl alcohol, d , l -2-phenylpropionaldehyde, d , l -α-amino-3-thiopheneacetic acid, (±)-mandelic acid, (±)-mandelic acid methyl ester, (±)-O-acetylmandelic acid, (±)-ephedrine; d , l -phenyl- alanineamide, d , l -tyrosine, d , l -tyrosine methyl ester, 3-hydroxy- d , l -kynurenine and N -(3,5- dinitrobenzoyl)- d , l -leucine. These compounds can be separated on β-CD media, and have certain common structural properties that enhance chiral recognition. The factors that permit the resolution of single-ring-containing racemates on β-CD columns are discussed.

Separation of optical isomers of scopolamine, cocaine, homatropine, and atropine

Different drug stereoisomers can have different physiological and therapeutic effects. Difficulties in separating optical isomers often make it impractical to market stereochemically pure products or to monitor isomeric contamination. This is not thought to be a problem with drugs isolated from biological sources (the alkaloids, for example). However, small amounts of isomeric impurities also exist in many biological systems. More importantly the isolation and purification process can cause partial or complete racimization in some cases. Great care must be taken in the handling of some drugs and an efficient, sensitive means to monitor racimization is important. Liquid chromatographic separation on a chiral beta-cyclodextrin bonded phase can be an effective technique in many cases. Its use in separating optical isomers of dl-scopolamine, dl-hyoscyamine, dl-homatropine, and dl-cocaine is discussed.

Evaluation of dye-micelle binding constants using diffusion sensitive band broadening effects

Abstract The diffusion coefficients of small solutes can be significantly altered by the presence of association colloids such as micelles. A relationship is utilized that relates the diffusion coefficient of a solute to its partitioning or binding behavior to a micellar pseudophase. The Taylor-Aris dispersion method was used to evaluate the diffusion coefficient of several dyes in the presence and absence of sodium dodecyl sulfate micelles. With this approach, all binding constants can be determined easily and reproducibly. The theory, experimental approach, and advantages of this technique are discussed.

Inexpensive, low-dead volume flow cells for microcolumn LC

Abstract The dead volume of the detector flow cell is very important in packed microcolumn liquid chromatography. Most commercially available detectors are inadequate because of excessive extra column band broadening. In this work, the conversion of conventional liquid chromatographic UV detectors to low dead volume capillary detectors is described. Inexpensive flow cell conversion has been completed successfully on four different LC detectors.