Synthesis, characterization, and application of chiral ionic liquids and their polymers in micellar electrokinetic chromatography

Multivariate approach for the enantioselective analysis in micellar electrokinetic chromatography-mass spectrometry. I. Simultaneous optimization of binaphthyl derivatives in negative ion mode

A mixture of two molecular micelles polysodium N-undecenoxy carbonyl-L-leucinate, (poly-L-SUCL) and polysodium N-undecanoyl leucylvalinate, (poly-L-SULV) was utilized in micellar electrokinetic chromatography-electrospray ionization-mass spectrometry (MEKC-ESI-MS) to simultaneously separate and detect enantiomers of binaphthyl derivatives. Separation parameters such as background buffer composition, voltage, temperature, and nebulizer pressure were optimized using a multivariate central composite design (CCD). Baseline enantioseparation for both analytes was achieved. The CCD was also used in the optimization of sheath liquid and spray chamber parameters to achieve optimum ESI-MS response. The results demonstrate that CCD is a powerful tool for the optimization of MEKC-MS parameters and the response surface model analysis can provide in-depth statistical understandings of the significant factors required to achieve maximum enantioresolution and ESI-MS sensitivity.

Versatile method for chiral recognition by the quartz crystal microbalance: chiral mandelic acid as the detection model

Chiral recognition is considered to be the most important, fundamental basis in the development of separation technology for chiral isomers in the pharmaceutical and biotechnology fields. However, the selective detection of individual enantiomers is still one of the most difficult analytical tasks because of the close similarity of the molecular configurations between chiral isomers. This study presents a versatile vapor-diffused molecular assembly (VDMA) reaction approach for chiral recognition by the quartz crystal microbalance (QCM). Chiral L/D-mandelic acid (MA) was used as the detection model, and L-phenylalanine (L-Phe) was used as the selector. The construction of the L-Phe-modified QCM sensor involved a four-step layer-by-layer assembly procedure. Each modification step was analyzed by cyclic voltammetry, the contact angle, and a resonance frequency measurement. The chiral recognizability of the L-Phe-modified QCM sensor to L-mandelic acid was then examined by resonance frequency measurement using the novel VDMA technique and also investigated by atomic force microscope (AFM) measurements. A chiral discrimination factor of up to approximately 9 between L- and d-MA on the L-Phe-modified QCM sensor was obtained by using this gaseous-phase reaction technique. AFM results also showed obvious selective aggregation of L-MA on the L-Phe-modified surface but no noticeable aggregation of D-MA during the VDMA reaction. Both of the QCM and AFM results confirmed the usefulness of this proposed VDMA technique for the study of chiral recognition. The main advantage of the proposed method is that it offers a universal simple application scheme for the QCM detection of small resonance frequency changes due to chiral molecular recognition by a chiral selector immobilized on the QCM sensor surface.

Capillary separation: micellar electrokinetic chromatography

Micellar electrokinetic chromatography (MEKC), a separation mode of capillary electrophoresis (CE), has enabled the separation of electrically neutral analytes. MEKC can be performed by adding an ionic micelle to the running solution of CE without modifying the instrument. Its separation principle is based on the differential migration of the ionic micelles and the bulk running buffer under electrophoresis conditions and on the interaction between the analyte and the micelle. Hence, MEKC's separation principle is similar to that of chromatography. MEKC is a useful technique particularly for the separation of small molecules, both neutral and charged, and yields high-efficiency separation in a short time with minimum amounts of sample and reagents. To improve the concentration sensitivity of detection, several on-line sample preconcentration techniques such as sweeping have been developed.

Electrokinetic chromatography with polymeric pseudostationary phases

Recent research and development efforts concerning polymeric pseudostationary phases (PSPs) for electrokinetic chromatography are reviewed. The introduction of new materials, characterization of structural effects on performance and selectivity, applications, and the use of polymeric PSPs with mass spectrometric detection are considered. Very interesting results concerning the effects of polymer structure have been reported. Significant developments have also been reported in the development of novel applications of polymeric PSPs, particularly for sample preconcentration using micellar affinity gradient focusing. The use of mass spectrometric detection with electrokinetic chromatography has seen significant development, and recent reports indicate that this is a robust and sensitive approach.

Cationic surfactants for micellar electrokinetic chromatography: 2. Representative applications to acidic, basic, and hydrophobic analytes

Changes in MEKC chemical selectivity that are induced by changes in the headgroup structure of cationic surfactants are examined. Separations of acidic, basic, and hydrophobic solutes are examined. The acidic analytes are comprised of methoxyphenols, which are of interest due to their prevalence in wood smoke. The basic solutes consist of compounds often found in forensic urine analysis, and represent typical basic pharmaceuticals. The hydrophobic solutes are six pharmaceutical corticosteroids used in replacement therapy of adrenocortical insufficiency and nonspecific treatment of inflammatory and allergic conditions. The role of the headgroup was found to be quite significant when analyzing acidic compounds with not all the surfactants being able to resolve all of the analytes. The headgroup also induced migration order switches among the acidic analytes. All of the surfactants examined here in were found to be suitable for the analysis of basic analytes with each surfactant providing unique selectivity. The hydrophobic solutes were separated best with the larger more hydrophobic surfactant headgroups. The steroid separation with these two surfactants was achieved without the use of organic modifiers or a mixed micellar phase.

Unusual aqueous-phase behavior of cationic amphiphiles with hydrogen-bonding headgroups

Two cationic surfactants with hydroxyl and carbamate hydrogen-bonding sites at their headgroups were synthesized. Both surfactants are ionic liquids (one of them at room temperature). Samples are isotropic solutions over the entire 0-100% concentration range, which is highly unusual for ionic surfactants. Surface tension, NMR, and conductivity measurements indicate classical micelle formation in aqueous solutions with CMCs below 10 mM. Pulse-gradient spin-echo (PGSE) NMR confirms micelle formation and provides micellar hydrodynamic radii of about 3.8 nm. Because this value is larger than the length of the extended surfactant molecules, about 2.7 nm, it appears that hydrogen-bonded water of hydration contributes substantially to the effective micelle size. At higher concentrations (above 25 wt %), surfactant solutions become viscous, but line broadening in the NMR is small relative to that found with a conventional cationic surfactant (CTAB). Thus, long rod formation, the source of line broadening in the latter, is absent with the new surfactants. Finally, PGSE NMR data show a 5-fold decrease in the diffusion coefficient between 5 and 20 wt %, above which the diffusion coefficients remain constant. The results are best explained by micelle clustering that is likely aided by intermicellar hydrogen bonding. The possibility of an isotropic liquid crystal (LC) phase with cubic symmetry is discussed and dismissed, demonstrating that LC formation of ionic surfactants at high concentrations, the usual behavior in past work, need not occur. Nor is there a definite connection between ionic liquid behavior and isotropic morphology.

Ionic liquids in separation techniques

The growing interest in ionic liquids (ILs) has resulted in an exponentially increasing production of analytical applications. The potential of ILs in chemistry is related to their unique properties as non-molecular solvents: a negligible vapor pressure associated to a high thermal stability. ILs found uses in different sub-disciplines of analytical chemistry. After drawing a rapid picture of the physicochemical properties of selected ILs, this review focuses on their use in separation techniques: gas chromatography (GC), liquid chromatography (LC) and electrophoretic methods (CE). In LC and CE, ILs are not used as pure solvents, but rather diluted in aqueous solutions. In this situation ILs are just salts. They are dual in nature. Too often the properties of the cations are taken as the properties of the IL itself. The lyotropic theory is recalled and the effects of a chaotropic anion are pointed out. Many results can be explained considering all ions present in the solution. Ion-pairing and ion-exchange mechanisms are always present, associated with hydrophobic interactions, when dealing with IL in diluted solutions. Chromatographic and electrophoretic methods are also mainly employed for the control and monitoring of ILs. These methods are also considered. ILs will soon be produced on an industrial scale and it will be necessary to develop reliable analytical procedures for their analysis and control.

Synthesis and Applications of Novel Bis(ammonium) Chiral Ionic Liquids Derived from Isomannide

Carbohydrate-based novel bis(ammonium) chiral ionic liquids have been synthesized by following a straightforward protocol using isomannide as the substrate. Their applications in chiral discrimination and optical resolution of racemates have been established.

Review of aqueous chiral electrokinetic chromatography (EKC) with an emphasis on chiral microemulsion EKC

The separation of enantiomers using electrokinetic chromatography (EKC) with chiral microemulsions is comprehensively reviewed through December 1, 2006. Aqueous chiral EKC separations based on other pseudostationary phases such as micelles and vesicles or on other chiral selectors such as CDs, crown ethers, glycopeptides, ligand exchange moeities are also reviewed from both mechanistic and applications perspective for the period of January 2005 to December 1, 2006.

Evaluation of chiral ionic liquids as additives to cyclodextrins for enantiomeric separations by capillary electrophoresis

A great interest has been drawn these last years towards ionic liquids in analytical chemistry, especially for separation methods. Recent synthesis of chiral ILs opened the way of the evaluation of new potential selectors for enantiomeric separations. This work focused on the evaluation of two chiral ILs (ethyl- and phenylcholine of bis(trifluoromethylsulfonyl)imide) by CE. Particular selectivities are awaited by exploiting unique ion-ion or ion-dipole interactions and by tailoring the nature of the cation and the anion. To evaluate such phenomena, a study was carried out with anti-inflammatory drugs 2-arylpropionic acids as model compounds. The results show that these chiral ILs did not present direct enantioselectivity with regard to these model analytes. The influence of chiral ILs in the electrolytes was then studied in the presence of classical chiral selectors (di- or trimethyl-beta-cyclodextrin). Although no general trend could be established, an increase in separation selectivity and resolution was observed in some cases, suggesting synergistic effects. The complementary determination of apparent inclusion constant values of these IL cations in the used cyclodextrins by affinity CE provided support to the understanding of the phenomena involved.

Utility of ionic liquids in analytical separations

Ionic liquids (ILs), as separation media, have made significant contributions in the past decades in advancing research in gas chromatography (GC), liquid chromatography (LC), and capillary electrophoresis (CE). This review, covering reports published from the mid 1980s to early 2007, shows how ILs have been used so far in separation science, originally primarily as GC stationary phases and later as mobile phase additives (both millimolar and major percent levels) for LC and CE. Representative GC and LC chromatograms as well as CE electropherograms are shown. In addition, the very recent findings on the development of ionic liquids with surfactant properties and its applications for chiral and achiral analysis are discussed.

A comparison of ionic liquids to molecular organic solvents as additives for chiral separations in micellar electrokinetic chromatography

In this study, we report the effects of adding ionic liquids (ILs), as compared to adding conventional molecular organic solvents (MOSs), to aqueous buffer solutions containing molecular micelles in the separation of chiral analyte mixtures in micellar EKC (MEKC). The molecular micelle used in this study was polysodium oleyl-L-leucylvalinate (poly-L-SOLV). The ILs were 1-alkyl-3-methylimidazolium tetrafluoroborate, where the alkyl group was ethyl, butyl, hexyl, or octyl. These ILs were chosen due to their hydrophobicity, good solvating, and electrolyte properties. Thus, it was expected that these ILs would have favorable interactions with chiral analytes and not adversely affect the background current. Common CE buffers, mixed with a molecular micelle, and an IL or a MOS, were used for these chiral separations. The buffers containing an IL in the concentration range of 0.02-0.1 v/v were found to support a reasonable current when an electric field strength of 500 V/cm was applied across the capillary. However, a current break down was observed for the buffers containing more than 60% v/v MOS on application of the above-mentioned electric field. The chiral resolution and selectivity of the analytes were dependent on the concentration and type of IL or MOS used.