Enantioselectivity of alcohol-modified polymeric surfactants in micellar electrokinetic chromatography

A novel method of modifying sodium undecanoyl-L-leucinate (SUL) micelles employed in chiral separation of analytes in micellar electrokinetic chromatography (MEKC) to enhance selectivity toward specific analytes is discussed. The current study aimed at modifying the SUL micelles by introducing different alcohols into the mono-SUL micelles. The micellar solutions were then polymerized in the presence of alcohols followed by postpolymerization extraction of the alcohols to yield alcohol-free polymeric surfactants (poly-L-SUL). The effects of hexanol (C(6)OH) and undecylenyl alcohol (C(11)OH) on micellar properties of this surfactant were investigated by use of surface tensiometry, fluorescence spectroscopy, pulsed field gradient-nuclear magnetic resonance (PFG-NMR), and MEKC. The surface tension and PFG-NMR studies indicated an increase in the critical micelle concentration (cmc) and micellar size upon increasing the alcohol concentration. Fluorescence measurements suggested that alcohols induce closely packed micellar structures. Coumarinic and benzoin derivatives, as well as (+/-)-1, 1'-binaphthyl-2,2'-dihydrogen phosphate (BNP) were used as test analytes for MEKC experiments. Examination of MEKC data showed remarkable resolutions and capacity factors of coumarinic derivatives obtained with modified poly-L-SUL as compared to the unmodified poly-L-SUL. Evaluation of fluorescence, PFG-NMR, and MEKC data suggest a strong correlation between the polarity and hydrodynamic radii of alcohol-modified micelles and the resolution of the test analytes.     

Influence of the polydispersity of polymeric surfactants on the enantioselectivity of chiral compounds in micellar electrokinetic chromatography

Poly(sodium undecenoyl-L-leucinate) (poly-L-SUL) was fractionated by the use of different molecular weight cutoff (MWCO) filters to narrow the polydispersity of the macromolecular sizes of the polymeric surfactant. The resulting polymeric surfactant fractions were characterized by the use of three techniques: (1) pulsed field gradient nuclear magnetic resonance (PFG-NMR) was used to determine the hydrodynamic radii, (2) analytical ultracentrifugation (AUC) was used to determine the molecular weights, and (3) steady-state fluorescence was used to determine the polarity of the nonfractionated and fractionated polymeric surfactants. From the data acquired from PFG-NMR, AUC, and fluorescence, it was noted that the hydrodynamic radii and molecular weight of the fractionated poly-L-SUL increased, while the polarity decreased with the increase in the size of the MWCO filter. However, a similarity in physical properties was observed between the nonfractionated and 10-30K fractionated poly-L-SUL except for the hydrodynamic radius and diffusion coefficients. The influence of different macromolecular sizes of poly-L-SUL on the chiral separation of phenylthiohydantion (PTH)-amino acids and coumarinic derivatives, as test analytes, was elucidated by the use of micellar electrokinetic chromatography (MEKC). The size of polymeric surfactants as a prerequisite for chiral separation was demonstrated by comparing the separation properties of fractionated versus nonfractionated polymeric surfactants. Fractionated poly-L-SUL resulted in enhanced resolution and separation efficiency of the test analytes as compared to the case of the nonfractionated poly-L-SUL. This observation indicates that minimizing polydispersity of polymeric surfactants may be important for some chiral separation applications.     

Enantiomeric separation of furan derivatives and fused polycycles by cyclodextrin-modified micellar capillary electrophoresis

The enantiomeric separations of highly hydrophobic furan derivatives and polycycles were performed and optimized using CD-modified micellar CE. The most effective chiral selector for the enantiomeric separation of these analytes was hydroxypropyl-gamma-CD. The effects of CD and SDS concentration and organic modifier were examined in order to optimize the separation conditions. The ratio of CD to surfactant concentration affected the enantiomeric separation significantly, with increases in the derivatized CD concentration generally enhancing resolution. Addition of an organic solvent modifier to the run buffer served to increase the analytes' solubility and enhance the separation efficiency. A highly acidic pH was necessary to effectively suppress the EOF when operating in the reverse polarity mode.     

Temperature effect on the fluorescence anisotropy decay dynamics of Coumarin-153 dye in Triton-X-100 and Brij-35 micellar solutions

The fluorescence anisotropy decay dynamics of the fluorescent probe Coumarin-153 (C153) have been investigated in two neutral micelles, Triton-X-100 (TX-100) and Brij-35 (BJ-35), at different temperatures and analyzed on the basis of the well-known two-step model. Because steady-state fluorescence spectra of the above probe do not show any noticeable changes with respect to temperature, for either of the studied micelles, suggests a similar polarity in the microenvironment around the probe at all the temperatures studied. The anisotropy results indicated that, for both the micelles, the fluidity inside the Palisade layer increases with temperature. However, the temperature effect on the anisotropy decay is relatively more pronounced in TX-100 than in BJ-35. It is inferred that the temperature effect on the anisotropy decay in the BJ-35 micelle is mainly due to the thermal effect on the microviscosity in the micellar phase. In the case of TX-100, the results indicate that, along with the above thermal effect, an additional effect is observed due to the increased size and hydration of the micelle with temperature, with the result being that the fluorescence anisotropy decay in TX-100 is more sensitive to temperature than in BJ-35. In the TX-100 micelle, our studies show that with an increase in temperature, even though the micellar size increases substantially, the distance of the probe from the micellar core does not increase that significantly. Thus, with increasing temperature, the probe undergoes a relative migration toward the micellar core to avoid the increased hydration in the micellar Palisade layer.     

Approaching over 10 000‐fold sensitivity increase in chiral capillary electrophoresis: Cation‐selective exhaustive injection and sweeping cyclodextrin‐modified micellar electrokinetic chromatography

A novel and simple method that combines an online concentration technique with an enantioseparation technique for capillary electrophoresis—namely, cation‐selective exhaustive injection and sweeping cyclodextrin‐modified micellar electrokinetic chromatography (CSEI‐sweeping CD‐modified MEKC)—realizes the effective enantioseparation of cationic analytes while keeping a significant increase of detection sensitivity. This technique consists of a slight modification of the basic CSEI‐sweeping MEKC. The main idea is to simply add an anionic CD as a chiral selector into the micellar buffer including sodium dodecyl sulfate, but not to change any other buffers in order to preserve the online concentration mechanism. When applied to analysis of the street drug, methamphetamine, the method achieved not only a baseline enantioseparation but also limits of detection (LODs; S/N = 3) of 70–90 pg/mL (ppt) for each isomer. This translates to a more than 10 000‐fold improvement compared to the LODs by the usual injection method. The present technique, which was made from a slight modification of CSEI‐sweeping MEKC, would give an attractive approach that is applicable to almost any analytes for which CSEI‐sweeping MEKC is applicable; all that is required is the selection of an appropriate anionic CD to be added to the micellar buffer.     

Solvation dynamics in triton-X-100 and triton-X-165 micelles: effect of micellar size and hydration

Dynamic Stokes' shift measurements using coumarin 153 as the fluorescence probe have been carried out to study solvation dynamics in two nonionic micelles, viz., triton-X-100 (TX-100) and triton-X-165 (TX-165). In both the micelles, the solvent relaxation dynamics is biexponential in nature. While the fast solvation time tau(s1) is seen to be almost similar for both the micelles, the slow solvation time tau(s2) is found to be appreciably smaller in TX-165 than in TX-100 micelle. Dynamic light scattering measurements indicate that the TX-165 micelles are substantially smaller in size than that of TX-100. Assuming similar core size for both the micelles, as expected from the similar chemical structures of the nonpolar ends for both the surfactants, the Palisade layer is also indicated to be substantially thinner for TX-165 micelles than that of TX-100. The aggregation number of TX-165 micelles is also found to be substantially smaller than that of TX-100 micelles. Fluorescence spectral studies of C153 dye in the two micelles indicate that the Palisade layer of TX-165 micelles is more polar than that of TX-100 micelles. Fluorescence anisotropy measurements indicate that the microviscosity in the Palisade layer of TX-165 micelles is also lower than that of TX-100 micelles. Based on these results it is inferred that the structure of the Palisade layer of TX-165 micelles is quite loose and have higher degree hydration in comparison to that of TX-100 micelles. Due to these structural differences in the Palisade layers of TX-165 and TX-100 micelles the solvation dynamics is faster in the former micelles than in the latter. It has been further inferred that in the present systems the collective response of the water molecules at somewhat away from the probes is responsible for the faster component of the solvation time, which does not reflect much of the structural changes of the micellar Palisade layer. On the contrary, the slower solvation time component, which is mainly due to the single particle response arising from water molecules adjacent to the probe in the micellar Palisade layer, is largely affected by the structural changes in the micellar Palisade layer.     

Control of separation selectivity in micellar electrokinetic chromatography by modification of the micellar phase with solubilized organic compounds

On the basis of the data on the distribution of various neutral solutes between sodium dodecyl sulfate (SDS) micelles and water, the control of separation selectivity in micellar electrokinetic chromatography (MEKC) by modification of the micellar phase with organic additives has been proposed and applied to the separation of simple model compounds. It was found that the distribution constants between the micelles and water (K_d,mc), which were determined by means of MEKC, of the solutes possessing hydrophilic functional groups are much larger than those between heptane and water (K_d,hep), whereas the K_d,mc values of the solutes possessing no hydropholic groups are comparable to their K_d,hep values. This indicates that the former solutes are preferentially solubilized in the Stern layer of the micelles and that the latter are located in the hydrocarbon core. In MEKC separations of aromatic compounds and metal acetylacetonates, considerable changes in separation selectivity were caused by the addition of compounds possessing both hydrophilic functional groups such as alcohols, phenol and ketones to the SDS micellar solution. The variations of the retention factors of the analytes could be explained in terms of saturation of the solubilization sites in the Stern layer with the modifiers, specific interaction of the modifiers with the analytes via hydrogen bonding in the micelles, and expansion of the core volume with the hydrocarbon parts of the modifiers. Such effects of the micellar modification could improve the resolution as well as the selectivity of MEKC separations.     

Determination of organophosphorus pesticides in honeybees after solid-phase microextraction

The solvation parameter model has been applied to the characterization of micellar electrokinetic chromatographic (MEKC) systems with mixtures of lithium dodecyl sulfate and lithium perfluorooctanesulfonate as surfactant. The variation in MEKC surfactant composition results in changes in the coefficients of the correlation equation, which in turns leads to information on solute-solvent and solute-micelle interactions. Lithium perfluorooctanesulfonate is more dipolar and hydrogen bond acidic but less polarizable and hydrogen bond basic than lithium dodecyl sulfate. Therefore mixtures of lithium dodecyl sulfate and lithium perfluorooctanesulfonate cover a very wide range of polarity and hydrogen bond properties, which in turn results in important selectivity changes for analytes with different solute properties.     

Insight into the mechanism of transient trapping in micellar electrokinetic chromatography

Transient trapping is a new mechanism of on-line sample concentration and separation that has recently been presented. It involves the injection of a short length of micellar solution in front of the sample, making it similar to sweeping in partial-filling MEKC. Here, we examine the mechanism of transient trapping by the use of computer simulations and compare it to sweeping in MEKC for the two analytes, sulforhodamine B and 101. The simulation results confirm the mechanism for concentration and separation originally proposed. The mechanism for concentration is similar to sweeping since the analytes are picked and accumulated by the micelles that penetrate the sample zone. The mechanism for separation is however quite unique since the concentrated analytes are trapped for a few seconds on the sample/micelle boundary before they are released as the concentration of micelle is reduced as it undergoes electromigration dispersion and the analytes separate down a micelle gradient. Simulation results suggested that a significant contribution of band broadening arises from the micelle gradient, with shallower gradients resulting in broader peaks. However, this is offset by an increase in selectivity, such that resolution was enhanced even though the peaks are broader. Transient trapping analysis with similar resolution to those obtained by sweeping MEKC could be achieved in 1/10 of the time and 1/4 of the capillary length, which results in a 2-3 times increase in sensitivity.     

Spectral properties and inclusion of 3-(4'-dimethylaminophenyl)-1-(2-furanyl)prop-2-en-1-one in organized media of micellar solutions, beta-cyclodextrin and viscous medium

On the line of a previous work on the spectral properties of some of heteroaryl chalcone, the absorption and fluorescence emission spectral properties of 3-(4'-dimethylaminophenyl)-1-(2-furanyl)prop-2-en-1-one (DMAFP), have been investigated in organized media of aqueous micellar and beta-cyclodextrin (beta-CD) solutions. While the absorption spectra are less sensitive to the nature of the added surfactant or beta-CD, the characteristics of the intramolecular charge transfer (ICT) fluorescence are highly sensitive to the properties of the medium. The ICT maximum is strongly blue-shifted with a great enhancement in the fluorescence quantum yield on adding micellar or beta-CD. This indicates the solubilization of DMAFP in the micellar core and formation of an inclusion complex with beta-CD. The critical micelle concentrations (CMC) as well as the polarity of the micellar core of SDS, CTAB and TX-100 have been determined. The CMC values are in good agreement with the reported values while the polarity is lower indicating that DMAFP molecules are incorporated in the micellar core not at the micellar interface. The inclusion constants of binding of DMAFP in micellar or beta-CD have been also determined. The thermodynamic parameters of formation of DMAFP:CD inclusion complex have been calculated from the temperature dependence of the fluorescence spectra of the formed complex. The highly negative value of formation entropy (DeltaS=-98.0Jmol(-1)K(-1)) reflects the high restrictions imposed on the movement of both the host and included guest molecules which is consistent with the increase of the fluorescence yield and blue shift of the fluorescence maximum.     

离子液体对胶束电动色谱胶束微结构以及分离效果的影响

考察了离子液体对胶束电动色谱胶柬微结构以及分离效果的影响.研究结果表明,离子液体使胶束的表面电荷密度变小、粒径变大及其内核极性增大.以泼尼松、氢化可的松和泼尼松龙为分析对象,氢化可的松与泼尼松龙在十二烷基硫酸钠(SDS)胶束体系中不能实现分离.而在SDS-离子液体混合介质(20 mmol/L SDS-10 mmol/L...     

Understanding chiral molecular micellar separations using steady-state fluorescence anisotropy, capillary electrophoresis, and NMR

Chiral separations employing four diastereomers of poly sodium N-undecanoyl leucylvalinate (p-SULV) as chiral selectors are probed by use of MEKC, steady-state fluorescence anisotropy, and NMR. By employing diastereomers and thus altering the stereochemistry of a single amino acid in a systematic way, one may control the enantiorecognition ability of the chiral selector. As a result, one can gain a better understanding of the mechanisms of chiral recognition for the two classes of neutral or anionic chiral analytes studied. Evaluation of the chiral interactions leading to chiral separations confirmed our earlier observation of a strong relationship between the selectivity (alpha) observed using a chromatographic separation technique (MEKC) and that determined from the spectroscopic parameter, beta. A linear alpha versus beta relationship was observed for the molecular micelle p-(L)-SULV with all eight analytes included in this study. However, as we earlier predicted, different groups of analytes had different slopes, i.e., values of m, suggesting different chiral separation mechanisms. Evaluation of the data allowed a grouping of the analytes according to the primary site of chiral interaction with the leucine or valine moiety of molecular micelle chiral headgroup.     

Separation of metal ions and metal-containing species by micellar electrokinetic capillary chromatography, including utilisation of metal ions in separations of other species

The use of micellar electrokinetic capillary chromatography (MEKC) for the separation of metal ions and metal-containing species is reviewed, together with the use of metal ions as a means to separate other species. Topics covered include the manipulation of separation selectivity through the use of complexation reactions induced by addition of a metal ion to the background electrolyte, enantiomeric separations facilitated through metal-analyte interactions, separation of organometallic species, separation of stable metal complexes in which the entire complex is the analyte and the separation of metal ions as analytes using pre-capillary or on-capillary complexation reactions with a suitable ligand.     

Cationic vesicles as chiral selector for enantioseparations of nonsteroidal antiinflammatory drugs by micellar electrokinetic chromatography

A vesicle-forming chiral cationic surfactant (1R,2S)-(-)-N-dodecyl-N-methyl-ephedrinium bromide was evaluated as a pseudo-stationary phase in micellar electrokinetic chromatography (MEKC) for enantioseparation of eight non-steroidal anti-inflammatory drugs e.g., carprofen, flurbiprofen, fenoprofen, ibuprofen, indoprofen, ketoprofen, naproxen and suprofen by capillary electrophoresis. The effects of varying experimental conditions such as pH and concentration of surfactant in the running buffer on the enantiomer separation of the drugs are reported. A mixture of five of the above drugs was separated and each enantiomeric pair was also separated simultaneously in a single run by use of the surfactant. The strong electrostatic interactions between the analytes and the vesicles seemed to have a major role in the enantiomeric separation of the profens.     

Nonaqueous capillary electrophoresis with laser-induced fluorescence detection: a case study of comparison with aqueous media

A novel method based on separation by nonaqueous capillary electrophoresis (NACE) combined with laser-induced fluorescence (LIF) detection was developed and compared with classic aqueous modes of electrophoresis in terms of resolution of solutes of interest and sensitivity of the fluorescence detection. Catecholamines derivatized with 4-chloro-7-nitro-2,1,3-benzoxadiazole (NBD-Cl) were chosen as test analytes for their subtle fluorescence properties. In aqueous systems, capillary zone electrophoresis (CZE) was not suitable for the analysis of test analytes due to complete fluorescence quenching of NBD-labeled catecholamines in neat aqueous buffer. The addition of micelles or microemulsion droplets into aqueous running buffer can dramatically improve the fluorescence response, and the enhancement seems to be comparable for micellar electrokinetic chromatography (MEKC) and microemulsion electrokinetic chromatography (MEEKC). As another alternative, NACE separation was advantageous when performing the analysis under the optimum separation condition of 20 mM sodium tetraborate, 20 mM sodium dodecyl sulfate (SDS), 0.1% (v/v) glacial acetic acid, 20% (v/v) acetonitrile (ACN) in methanol medium after derivatization in ACN/dimethyl sulfoxide (DMSO) (3:2, v/v) mixed aprotic solvents containing 20 mM ammonium acetate. Compared with derivatization and separation in aqueous media, NACE-LIF procedure was proved to be superior, providing high sensitivity and short migration time. Under respective optimum conditions, the NACE procedure offered the best fluorescence response with 5-24 folds enhancement for catecholamines compared to aqueous procedures. In addition, the mechanisms of derivatization and separation in nonaqueous media were elucidated in detail.     

Comparison of microemulsion electrokinetic chromatography and solvent modified micellar electrokinetic chromatography on rapid separation of heroin, amphetamine and their basic impurities

Microemulsion electrokinetic chromatography (MEEKC) and solvent modified micellar electrokinetic chromatography (MEKC) were investigated with the goal of the rapid separation of complex heroin and amphetamine samples. The rapid simultaneous separation of 17 species of heroin, amphetamine and their basic impurities and adulterants was performed within about 10 min using MEEKC for the first time, whereas solvent modified MEKCs were unable to resolve all the components. The comparisons between MEEKC and solvent modified MEKC proved internal lipophilic organic phase in microemulsions played an important role in improving the separation performance with respect to efficiency. However, the role of internal lipophilic organic phase in MEEKC was disgusted at high concentrations of cosurfactant, and the separations of MEEKC and 1-butanol modified MEKC became similar at high concentrations of 1-butanol. The evaluation of reproducibility, linearity and detection limit of optimized MEEKC method provided good results for all the analytes investigated, thus allowing its application to real controlled drug preparation analysis.     

Use of chiral zwitterionic surfactants for enantiomeric resolutions by capillary electrophoresis

The enantiomeric resolution of 1,1'-binaphthyl-2,2'-diamine and Tr?ger's base was investigated using the commercially available zwitterionic surfactants 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulphonate (CHAPS) and 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulphonate (CHAPSO). Resolution of the weakly basic chiral probes was achieved using varying concentrations of surfactant, above their critical micellar concentrations, in a phosphate buffer (pH 2.5; 100 mM) to ensure ionisation of the analytes. Both CHAPS and CHAPSO were employed in the absence of additional coselectors or surfactants as sole micellar-forming agents. The addition of organic modifiers, methanol and acetonitrile (ACN), to the background electrolyte (BGE) was found to have a detrimental effect on enantioselectivity presumably by alteration of the phase polarity.     

In-capillary derivatization and analysis of amino acids,amino phosphonic acid-herbicides and biogenic amines by capillary electrophoresis with laser-induced fluorescence detection

This paper describes a general approach for the in-capillary derivatization of amino compounds and the subsequent sensitive determination of the derivatives by micellar electrokinetic chromatography (MEKC) or capillary zone electrophoresis (CZE) with laser-induced fluorescence (LIF) detection. Amino acids, biogenic amines and amino phosphonic acid-herbicides were chosen as model analytes to evaluate the analytical potential of this approach. Fulfilment of the in-capillary reaction of the analytes using LIF detection hinged on the excellent labeling chemistry of 5-(4,6-dichloro-s-triazin-2-ylamino)fluorescein (DTAF) and the good resolution achieved in the separation of derivatized analytes. Careful optimization of the electrophoretic conditions in the mixing step of this protocol allowed the determination of amino acids, biogenic amines and phosphorus-containing amino acid-herbicides with concentration limits of detection at the nug/L level and relative standard deviations from 3.5 to 5.8%. The whole analysis is carried out within 20 min, resulting in a very simple, fast and practical approach for the fully automated analysis of amino acids and related compounds in low-volume and low-concentration samples.     

Simultaneous micellar electrokinetic chromatography and liquid chromatography of adriblastina and tarabine PFS Their application to some biological fluids

The current work presents analytical procedures for simultaneous determination of tarabine PFS and adriblastina by micellar electrokinetic chromatography (MEKC) and liquid chromatography (LC). For MEKC analysis, separations and identifications were accomplished using uncoated fused-silica capillary with hydrodynamic injections in the presence of 50mM borate/phophate pH 8.7 and 100mM SDS. The migration times of tarabine PFS and adriblastina were found to be 2.70 and 6.40min, respectively. Calibration curves were established for 10-300ng/mL (r=0.998) tarabine PFS and for 8-120microg/mL (r=0.999) adriblastina. For LC analysis, separations were performed on teicoplanin stationary phase with reversed mobile phase containing methanol:buffer pH 4.05 (20:80%, v/v) at 285nm. The retention times of tarabine PFS and adriblastina were 5.18 and 7.20min, respectively. Calibration curves were established for 3-90microg/mL (r=0.998) tarabine PFS and for 10-120microg/mL (r=0.999) adriblastina. Both MEKC and LC methods were applied for the simultaneous determination of analytes in urine samples.     

Characterisation of retention in micellar high-performance liquid chromatography, in micellar electrokinetic chromatography and in micellar electrokinetic chromatography with reduced flow

The retention (migration) behaviour of various barbiturates, phenylurea and triazine herbicides in micellar electrokinetic chromatography (MEKC) with uncoated fused-silica capillaries was compared with the behaviour in micellar electrokinetic chromatography with reduced electroosmotic flow (RF-MEKC) using capillaries modified with linear polyacrylamide. The error in the values of the retention factors caused by the neglection of the contribution of the electroosmotic flow in RF-MEKC was investigated and a method for correcting this error was suggested. The retention was characterised using the lipophilic and polar indices to characterise and to predict the retention as a function of the concentration of the surfactant (sodium dodecylsulphate) in the running buffer in MEKC and in RF-MEKC. Homologous series of n-alkylbenzenes and of n-alkan-2-ones were compared as the standard sets for the calibration of the retention (migration) index scale. The values of the lipophilic indices of a given solute measured in reversed-phase HPLC, MEKC and RF-MEKC are close to each other. Under ideal MEKC conditions, the values of the polarity indices are close to one for various sample solutes. However, for partially ionised compounds such as weakly acidic barbiturates, where the contribution of the electrophoretic migration is significant, the values of the polarity indices are significantly lower than one. Optimum conditions for separations of mixtures of triazine and phenylurea herbicides and of barbiturates using various techniques tested were compared.