Apryll M. Stalcup

Cyclodextrins : a versatile tool in separation science

Cyclodextrins have been used extensively in separation science because they have been shown to discriminate between positional isomers, functional groups, homologues and enantiomers. This property makes them one of the most useful agents for a wide variety of separations. The main goal of this review is a discussion of somewhat more exotic applications of cyclodextrins to separation methods. Techniques examined in detail include gel electrophoresis, isotachophoresis, isoelectric focusing, preparative scale electrophoretic techniques, thin-layer chromatography, electrochemically modulated liquid chromatography, use of monolithic media in liquid chromatography, microdialysis, separation on hollow fibers, foam flotation enrichment, solid- and liquid-phase extractions, countercurrent chromatography, separation through liquid and composite membranes, and cyclodextrin applications in molecularly imprinted polymers. Since a lot of attention has been paid to use of cyclodextrins in capillary electrophoresis, liquid, gas and supercritical fluid chromatography, these techniques will be only briefly discussed. The second goal of this review is a discussion of a scaling-up the analytical separations to semi-preparative or preparative techniques. It was found that despite a need for large scale separations in the industry, development of these techniques has been somewhat lagging behind development of miniaturized analytical separations. It is hoped that the focus on areas outside more traditional separation applications might stimulate further research.

(S)-2-Hydroxyprophyl-β-cyclodextrin, a new chiral stationary phase for reversed-phase liquid chromatography

(S)-2- and (R,S)-2-hydroxypropyl-beta-cyclodextrin have been bonded to silica gel and evaluated as stationary phases for reversed-phase liquid chromatography. Stationary phases also were prepared on two silicas having different pore sizes and surface areas. Dissimilarities were observed in enantiomeric selectivities between these columns and also between these and the native beta-cyclodextrin columns. With the exception of compounds 5 and 10, all other racemates reported here which have been successfully resolved on the new phases are enantiomers which have not been previously reported as separated on the beta-cyclodextrin stationary phase. In some cases, there were also differences in enantioselectivities observed between the (S)- and the (R,S)-hydroxypropyl-beta-cyclodextrin phases on the same silica. The results are discussed in terms of the retention mechanism and compared to results reported earlier for beta-cyclodextrin columns.

Ionic Liquids in Chromatography and Capillary Electrophoresis

Abstract One of the major challenges facing the chemical industry is the development of new, “green” technologies which reduce the environmental impact of manufacturing processes. Ionic liquids are emerging as a promising alternative to traditional organic solvents and are, thus, generating considerable interest in a variety of areas. This review examines some of the unique properties of ionic liquids and their applications in analytical separations.

Effect of the configuration of the substituents of derivatized β-cyclodextrin bonded phases on enantioselectivity in normal-phase liquid chromatography

Abstract (R)-(−)-, (S)-(+)- and racemic 1-(1-naphthyl)ethylcarbamate derivatives of β-cyclodextrin (β-CD) bonded phase were used to successfully resolve a large variety of enantiomers in the normal-phase mode. The selectivity (α) and retention (k′) of these new chiral packings in the normal-phase mode are somewhat analogous to a reciprocal Pirkle-type phase. Also, many of the enantiomers resolved on these new phases were not resolvable on the native β-CD phase in the reversed-phase mode or on the naphthylvaline type π-complex, hydrogen bonding phases. Comparison of the selectivity and retention of the (R)-, (S)- and racemic carbamate phases revealed that both the carbamate substituent and the cyclodextrin moieties contribute to chiral recognition. The selectivity of these phases was found to be a function not only of the configuration of the substituent on the CD but also was dependent on the number of substituents (degree of substitution). More importantly, the results indicate that enantioselectivity on these phases arises from two modes of chiral recognition. In some cases, the chiral selectivity of the one configuration of the substituent and the CD combine synergistically while the chiral selectivity of the opposite configuration of the substituent and the CD combine antagonistically. For some compounds, changing the carbamate substituent configuration resulted in a reversal of the elution order.

Capillary Electrophoretic Chiral Separations Using a Sulfated β-Cyclodextrin-Containing Electrolyte

Abstract Sulfated β-cyclodextrin (SO3-β-CD) was utilized as a chiral additive for capillary zone electrophoresis (CZE). Chiral separations of several uhcharged enantiomers, such as phensuximide, indapamide, etc., which are difficult to separate using neutral CDs, were achieved. The effects of SO3-β-CD concentration and the pH and ionic strength of the supporting electrolyte as well as the presence of an organic modifier, methanol, were discussed.

Capillary gas chromatographic separation of enantiomers with stable dipentyl-α-, β- and γ-cyclodextrin-derivatized stationary phases

Abstract Liquid and amorphous solid pentyl derivatives of α-, β- and γ-cyclodextrin were evaluated as gas chromatographic (GC) chiral stationary phase coatings on fused-silica capillaries. The factors controlling the physical state of the pentyl-substituted cycldodextrin derivatives were considered. Sixty-five racemic solutes were resolved, including alcohols, amines, lactones, epoxides, amino alcohols, esters, ethers, amino acids, haloalkanes, sugars and alkaloids. The pentyl-substituted α-, β- and γ-cyclodextrin stationary phases showed pronounced selectivity differences, apparently based on the size, shape and functionality of the analytes. Inclusion complexation may more readily occur at high temperatures in gas than in liquid chromatography (LC). Enantiomeric separations can be accomplished by this GC method that cannot be done by related cyclodextrin LC techniques.

Quinine as a chiral additive in nonaqueous capillary zone electrophoresis

Quinine was used as a chiral additive in nonaqueous capillary zone electrophoresis. The separation buffer consisted of ammonium acetate and quinine in methanol. Quinine was able to enantioresolve a number of N-3,5-dinitrobenzoylated amino acids as well as (±)-1,1′-binaphthyl-2,2′diyl hydrogen phosphate and N-[1-(1-naphthyl)ethyl]phthalamic acid. Enantioresolution deteriorated as the concentration of ammonium acetate was increased but improved with the addition of acetic acid. Chiral recognition seems to arise through an ion-pairing mechanism.

Tetraethylammonium tetrafluoroborate : a novel electrolyte with a unique role in the capillary electrophoretic separation of polyphenols found in grape seed extracts

A capillary electrophoretic method for resolving phenolic compounds found in grape seed extracts was developed using a quaternary ammonium salt as the main electrolyte solution. Seven polyphenols were separated and identified. The separation mechanism seems to involve association between quaternary ammonium cations and the polyphenols. Migration times roughly correlate with the size of the known phenols. The role of the alkyl substituents on the quaternary ammonium ions was investigated and is discussed. The method of analysis proposed herein exhibits high reproducibility in terms of migration times for a complex mixture using a single component background electrolyte.

Characterization of a novel pyridinium bromide surface confined ionic liquid stationary phase for high-performance liquid chromatography under normal phase conditions via linear solvation energy relationships.

Utilizing linear solvation free energy relationship methodology, a novel pyridinium bromide surface confined ionic liquid (SCIL) stationary phase was characterized under normal phase high-performance liquid chromatographic conditions. A limited set of neutral aromatic probe solutes were utilized to rapidly assess the utility of the LSER model, using mobile phases of hexane modified with 2-propanol. The excellent correlation of the global fit across the mobile phase composition range used in this study for the experimental and calculated retention values (R(2)=0.994) indicates that the LSER model is an appropriate model of characterizing this polar bonded phase under normal phase conditions. For a limited subset of compounds, retention on the pyridinium bromide SCIL stationary phase is more highly correlated with that obtained on a cyano column than on a diol column under NP conditions.

The effect of excipients on the stability of levothyroxine sodium pentahydrate tablets

Levothyroxine tablets, 50 microg, have been marketed for many decades but have had numerous recalls due to degradation and failure to meet potency. These experiments were devised to study the effects of various excipients on the stability of levothyroxine sodium pentahydrate in aqueous slurries and in formulated tablets. The active alone was found to be stable in the solid state for 6 months at 40 degrees C/75% RH whether stored in open or closed containers, and was found to be non-hygroscopic under normal processing conditions (>30% RH). In aqueous slurries with an excipient, the stability of the active improved as the pH of the slurry was increased from pH 3 to 11. Tablets manufactured with lactose anhydrous, starch, or microcrystalline cellulose failed to meet USP assay requirements at 3 months at 40 degrees C/75% RH. Tablets manufactured with dibasic calcium phosphate or mannitol met USP assay requirements at 3, but not 6 months when stored at 40 degrees C/75% RH. Tablets manufactured with dibasic calcium phosphate and a basic pH modifier, such as sodium carbonate, sodium bicarbonate, or magnesium oxide, met the USP assay requirements at both 3 and 6 months. Thus, the use of basic pH modifiers is a potential technique for improving the stability of levothyroxine sodium pentahydrate tablets.