Further study on the use of uncharged beta-cyclodextrin polymer in capillary electrophoresis: enantiomeric separation of some alpha-hydroxy acids

Polymeric and polymer-supported pseudostationary phases in micellar electrokinetic chromatography: performance and selectivity

Several types of synthetic ionic polymers have been employed as pseudostationary phases in electrokinetic chromatography. The polymers have been shown to have some significant advantages and different chemical selectivity relative to conventional surfactant micelles. Polymeric phases are effective for the separation and analysis of hydrophobic and chiral compounds, and may be useful for the application of mass spectrometric detection. Additionally, the polymeric phases often demonstrate unique selectivity relative to micellar phases, and can be designed and synthesized to provide desired selectivity. This review covers efforts to develop and characterize the performance, characteristics, and selectivity of synthetic polymeric pseudostationary phases since their introduction in 1992. Some ideas for the future development of polymeric pseudostationary phases and the role they may play in electrokinetic separations are presented.

Allylamine-beta-cyclodextrin copolymer. A novel chiral selector for capillary electrophoresis

A novel, positively charged, copolymer of allylamine and 2-hydroxy-3-methacryloyl-beta-cyclodextrin was synthesized to be used as a chiral selector in capillary electrophoresis. In the copolymer, cyclodextrin molecules are spaced from the backbone though a spacer arm which prevents sterical hindrance of the CD cavity. The self-mobility of the CD polymer in its charged form, opposite to the analytes, is the cause for the enhanced separation factor provided by this selector. Moreover, the positive charged polymer induces a reversal of electroosmotic flow which is beneficial in enantioseparations of acidic compounds as it reduces analysis time and increases peak efficiency. The ability of this copolymer to act as a CE chiral selector in the separation of 2,4-dinitrophenylamino acid enantiomers was investigated in coated and uncoated capillaries and its performance was much better then that of native beta-cyclodextrin.

Enantioselective determination by capillary electrophoresis with cyclodextrins as chiral selectors

This review surveys the separation of enantiomers by capillary electrophoresis using cyclodextrins as chiral selector. Cyclodextrins or their derivatives have been widely employed for the direct chiral resolution of a wide number of enantiomers, mainly of pharmaceutical interest, selected examples are reported in the tables. For method optimisation, several parameters influencing the enantioresolution, e.g., cyclodextrin type and concentration, buffer pH and composition, presence of organic solvents or complexing additives in the buffer were considered and discussed. Finally, selected applications to real samples such as pharmaceutical formulations, biological and medical samples are also discussed.

Selectivity in capillary electrokinetic separations

This review gives a survey of selectivity modes in capillary electrophoresis separations in pharmaceutical analysis and bioanalysis. Despite the high efficiencies of these separation techniques, good selectivity is required to allow quantitation or identification of a particular analyte. Selectivity in capillary electrophoresis is defined and described for different separation mechanisms, which are divided into two major areas: (i) capillary zone electrophoresis and (ii) electrokinetic chromatography. The first area describes aqueous (with or without organic modifiers) and nonaqueous modes. The second area discusses all capillary electrophoretic separation modes in which interaction with a (pseudo)stationary phase results in a change in migration rate of the analytes. These can be divided in micellar electrokinetic chromatography and capillary electrochromatography. The latter category can range from fully packed capillaries, via open-tubular coated capillaries to the addition of microparticles with multiple or single binding sites. Furthermore, an attempt is made to differentiate between methods in which molecular recognition plays a predominant role and methods in which the selectivity depends on overall differences in physicochemical properties between the analytes. The calculation of the resolution for the different separation modes and the requirements for qualitative and quantitative analysis are discussed. It is anticipated that selectivity tuning is easier in separation modes in which molecular recognition plays a role. However, sufficient attention needs to be paid to the efficiency of the system in that it not only affects resolution but also detectability of the analyte of interest.

Chiral separations in capillary electrophoresis

The marked increase in the number of communications on the utilization of electrophoresis for practical chiral separations within the last three years is the most evident, and the most important fact. It reveals that the basic period of intensive research in the field is finished. The search for chiral selectors discriminating racemates in a reasonably analytical manner and the study of both the mechanism and physicochemical aspects of the chiral discrimination process were the main features of that period. Here, we review the state of the art in the field and state the references of the related literature up to the end of 1998.

Vinylpyrrolidine-beta-cyclodextrin copolymer: a novel chiral selector for capillary electrophoresis

The synthesis and characterization of a novel polymer consisting of an alkyl backbone and pendant beta-cyclodextrin units, obtained by radical copolymerization of vinylpyrrolidone and methacryloyl-beta-cyclodextrin (PVP-beta-CD), was reported. The ability of this copolymer to act as a capillary electrophoresis (CE) chiral selector was investigated in the separation of a mixture of basic drugs. The influence of polymeric cyclodextrin concentration, temperature, and pH on the separation of the test analytes was assessed and the advantage of using the polymeric selector over native beta-cyclodextrin was demonstrated.

Comparison of highly sulfated alpha-, beta-, and gamma-cyclodextrins and 18-crown-6-tetracarboxylic acid for the enantiomeric separation of some amino acids and derivatives by capillary electrophoresis

18-Crown-6-tetracarboxylic acid (18C6H4) and highly sulfated cyclodextrins (HS-alpha-, beta-, gamma-CDs) are highly selective chiral selectors for the enantioseparation of solutes bearing the primary amino function. Excellent resolutions were obtained for all solutes on HS-gamma-CD and on 18C6H4. The former, however, is by far the best chiral selector for the solutes studied in this work because the highest resolution is obtained with the shortest migration times. The reversal of the D- and L-migration order on HS-CDs compared to 18C6H4 is an interesting feature for the determination of enantiomeric excess.

Controlling enantioselectivity in chiral capillary electrophoresis with inclusion-complexation

The separation of chiral compounds is of key importance in different fields of application, e.g., pharmaceutical, industrial, forensic, biological, clinical etc. Capillary electrophoresis (CE) is a powerful analytical method applied in chiral analysis and inclusion-complexation is one of the most frequently used mechanism to improve the selectivity of the enantiomeric separation. Cyclodextrins and their derivatives or modified crown-ethers have been successfully applied in CE for the enantiomeric separation of a wide number of analytes. This review surveys the separation of enantiomers by CE when chiral selectors, forming inclusion-complexation, are used. The control of enantioselectivity can be done carefully by considering several experimental parameters such as chiral selector type and concentration, pH, ionic strength and concentration of the background electrolyte, electroosmotic flow, organic modifier etc. The review presents a list of the latest separation of enantiomers by CE where inclusion-complexation plays a key role in the stereoselective separation mechanism.

Enantiomer separation of acidic racemates by capillary electrophoresis using cationic and amphoteric beta-cyclodextrins as chiral selectors

Enantiomer separations of various acidic racemates were performed by capillary electrophoresis (CE) using a commercial cationic beta-cyclodextrin (beta-CD), quaternary ammonium beta-CD (QA-beta-CD), and a commercial amphoteric beta-CD (AM-beta-CD) as chiral selectors. Eleven acidic racemates were successfully separated using QA-beta-CD by changing the CD concentration and the buffer pH. These enantiomer separations were compared with the results using five neutral CD derivatives. Although most racemates were separated with some of the neutral CDs, relatively high CD concentrations were required to obtain baseline separations. In contrast, when QA-beta-CD was employed, the enantiomer separations were successful at low concentrations below 5 mM. Enantiomers of five acidic racemates and ten dansylated amino acids (Dns-amino acids) were separated using AM-beta-CD. Although the baseline separation of racemic 4-chloromandelic acid was not achieved with either QA-beta-CD or five neutral CDs, AM-beta-CD showed complete resolution. Furthermore, the simultaneous enantiomer separation of eight Dns-amino acids was also achieved with AM-beta-CD. Both QA-beta-CD and AM-beta-CD were analyzed by CE and mass spectrometry (MS) in order to identify their compositions because they consisted of a mixture having different degrees of substitution. QA-beta-CD consisted of six components having from one to six quaternary ammonium groups. The composition of AM-beta-CD, however, was very complicated and could not be identified.