Micellar electrokinetic chromatography in zero-electroosmotic flow environment

Separation of organic cations using novel background electrolytes by capillary electrophoresis

A background electrolyte for capillary electrophoresis containing tris(-hydroxymethyl) aminomethane (THAM) and ethanesulfonic acid (ESA) gives excellent efficiency for separation of drug cations with actual theoretical plate numbers as high as 300,000. However, the analyte cations often elute too quickly and consequently offer only a narrow window for separation. The best way to correct this is to induce a reverse electroosmotic flow (EOF) that will spread out the peaks by slowing their migration rates, but this has always been difficult to accomplish in a controlled manner. A new method for producing a variable EOF is described in which a low variable concentration of tributylammonium- or triethylammonium ESA is added to the BGE. The additive equilibrates with the capillary wall to give it a positive charge and thereby produce a controlled opposing EOF. Excellent separations of complex drug mixtures were obtained by this method.

Effect of water molecules on the fluorescence enhancement of Aflatoxin B1 mediated by Aflatoxin B1:β-cyclodextrin complexes. A theoretical study

In order to explain the observed fluorescence enhancement of Aflatoxin B1 (AFB1) when forming AFB1:beta-cyclodextrin (AFB1:beta-CD) inclusion complexes, we have performed a theoretical (quantum chemistry calculations) study of AFB1 and AFB1:beta-CD in vacuum and in the presence of aqueous solvent. The AM1 method was used to calculate the absorption and emission wavelengths of these molecules. With the help of density functional theory (DFT) and time-dependent DFT (TDDFT) vibrational frequencies and related excitation energies of AFB1 and AFB1.(H2O)m = 4,5,6,11 were calculated. On the basis of these calculations we propose a plausible mechanism for the fluorescence enhancement of AFB1 in the presence of beta-CD: (1) before photoexcitation of AFB1 to its S1 excited state, there is a vibrational coupling between the vibrational modes involving the AFB1 carbonyl groups and the bending modes of the nearby water molecules (CG + WM); (2) these interactions allow a thermal relaxation of the excited AFB1 molecules that results in fluorescence quenching; (3) when the AFB1 molecules form inclusion complexes with beta-CD the CG + WM interaction decreases; and (4) this gives rise to a fluorescence enhancement.

Application of reverse-flow micellar electrokinetic chromatography for the simultaneous determination of flavonols and terpene trilactones in Ginkgo biloba dosage forms

A reverse-flow micellar electrokinetic chromatographic (RF-MEKC) method was developed for the simultaneous qualitative determination of 10 components consisting of the flavonol glycosides, rutin and quercitrin, the flavonol aglycones, isorhamnetin, kaempferol and quercetin, the terpene trilactones, ginkgolides A, B, C and J and the sesquiterpene, bilobalide. This method was used to fingerprint Ginkgo biloba solid oral dosage forms and validated for the quantitation of the marker compounds, rutin and quercetin in some commercial products. In addition to the usual variables, the influence of some essential background electrolyte (BGE) components such as sodium dodecyl sulphate (SDS) and <beta>-cyclodextrin concentrations were investigated. A polyimide fused-silica square capillary column (75 microm I.D. x 360 microm O.D.) with a total length of 60.0 cm and effective length of 45.0 cm was used for the separation. The final BGE consisted of 20 mM phosphoric acid, 40 mM SDS and 12 mM <beta>-cyclodextrin (pH 2.2) using reverse polarity with a voltage of -17.5 kV. Samples were injected electrokinetically at -5 kV for 3 s for the qualitative analysis and hydrodynamically at 20 mbar for 0.6 s for the quantitative assay. The total run time was 22 min and the limits of detection were 3.13 microg/ml and 1.88 microg/ml for rutin and quercetin, respectively. Fingerprint profiles of the solid oral dosage forms and the results of the quantitative analysis indicated that there were major discrepancies in the marker content between products and illustrates the value of this method for use as a procedure to assess product quality of commercially available Ginkgo biloba products.

Determination and regulation of the migration window in electrokinetic chromatography

The determination of the velocities of the mobile and the pseudostationary phases (the migration (time) window) is mandatory for the determination of physicochemical properties by electrokinetic chromatography (EKC). This review offers a detailed discussion on the definition, the importance, the determination and the regulation of the migration (time) window in EKC. An overview on the theoretical treatment of chromatographic processes in EKC is given defining EKC in comparison to the term capillary electrophoresis. Methods to determine and influence the migration window are discussed with emphasis on measures that have been taken to modify the electroosmotic flow velocity. Pseudostationary phases (or separation carriers) that are taken into consideration are anionic and cationic micelles, mixed micelles, microdroplets (microemulsions), polymeric pseudostationary phases and dendrimers.

Ultrafast sodium dodecyl sulfate micellar electrokinetic chromatography with very acidic running buffers

A new micellar electrokinetic chromatography (MEKC) procedure has been developed that allows the use of the anionic surfactant sodium dodecyl sulfate (SDS) together with separation buffers at pHs as low as 1. The technique is based on the employment of a high molecular weight polyethylenimine (PEI)-coated capillary that provides strong cathodal electroosmotic flows at acidic pHs when SDS is added to the running buffer. The procedure is easy to implement since the capillary coating is done by just flushing a PEI aqueous solution through the capillary and the subsequent steps are the same as those for any MEKC protocol. Moreover, the coating renewal provides reproducible separations between injections (migration time RSD values lower than 1.82 and 3.44% were obtained for the same day and three different days, respectively). The good possibilities of this system are demonstrated by showing the separation of a group of eight polyphenolic compounds within a separation time shorter than 2 min. This procedure allows one to extend the optimization of SDS-MEKC separations to the very acidic pH range too.

Chemometric modeling of neurotransmitter amino acid separation in normal and reversed migration micellar electrokinetic chromatography

A chemometric experimental design has been applied for the optimization of neurotransmitter amino acid separation in capillary electrophoresis. The optimizations were carried out for normal micellar electrokinetic chromatography (N-MEKC) and reversed migration micellar electrokinetic chromatography (RM-MEKC). In order to optimize three separation factors and study the interaction between factors, a response function was optimized via searching its optimum (minimum/maximum). For this purpose a central composite design with multivariate linear regression (MLR) analysis was utilized. Modeling with good regression coefficients from the MLR adequately described the interaction of factors such as background electrolyte and sodium dodecylsulfate concentrations which had a large impact on selectivity and migration behaviors. Similar optimal conditions regarding resolution and number of theoretical plates but different retention behaviors as a function of background electrolyte and micellar concentrations were observed for N-MEKC and RM-MEKC. Improved overall performance from the RM-MEKC separation of five neurotransmitter acids, superior to N-MEKC, is demonstrated in terms of repeatability, peak symmetry, sensitivity, and in particular, impurity determination in an overloaded separation system.

Determination of carbamazepine and carbamazepine-10,11-epoxide in human serum and plasma by micellar electrokinetic capillary chromatography in the absence of electroosmosis

Therapeutic drug monitoring of carbamazepine (CBZ), a widely used antiepileptic drug, is required for optimization of pharmacotherapy with this drug and for assessment of the patient's compliance to therapy. The suitability of employing micellar electrokinetic capillary chromatography (MEKC) in the absence of electroosmosis for the determination of CBZ and its main metabolite carbamazepine-10,11-epoxide (CBZE) in extracts of human serum and plasma is reported. Using micelles formed by dodecyl sulfate, analyses performed in untreated fused-silica capillaries at acidic pH and in commercially available coated capillaries under application of reversed polarity are compared. Uncoated and polyvinyl alcohol coated capillaries proved to be unsuitable for this purpose, whereas capillaries coated with linear polyacrylamide and N-acryloylaminoethoxyethanol and operated at pH 7.6 are shown to provide high-quality and reliable data on a short time scale. Assay performance is discussed via statistical analysis of the data produced from a set of quality control sera that contain up to 14 different drugs and via analysis of patient samples. Intraday and interday imprecision data for concentrations between 4.0 and 84 microM are demonstrated to be < 10%. Run times are shown to be < 50% compared to those observed in conventional MEKC at alkaline pH (i.e., in the presence of electroosmosis).

Proceedings of the 10th Annual Frederick Conference on Capillary Electrophoresis. Frederick, Maryland, USA. October 18-20, 1999

Since the introduction of the first commercial capillary electrophoresis (CE) instrument a decade ago, CE applications have become widespread. Today, CE is a versatile analytical technique which is successfully used for the separation of small ions, neutral molecules, and large biomolecules and for the study of physicochemical parameters. It is being utilized in widely different fields, such as analytical chemistry, forensic chemistry, clinical chemistry, organic chemistry, natural products, pharmaceutical industry, chiral separations, molecular biology, and others. It is not only used as a separation technique but to answer physicochemical questions. In this review, we will discuss different modes of CE such as capillary zone electrophoresis, micellar electrokinetic chromatography, capillary gel electrophoresis, capillary isoelectric focusing, and capillary electrochromatography, and will comment on the future direction of CE, including array capillary electrophoresis and array microchip separations.

Drug analysis by capillary electrophoresis and laser-induced fluorescence

This review briefly presents the different laser-induced fluorescence detectors, outlines the different dyes used to derivatize molecules which are used with capillary electrophoresis/laser-induced fluorescence (CE-LIF), and provides an overview and current status of CE-LIF in drug analysis.

On-line concentration of neutral analytes for micellar electrokinetic chromatography. VI. Stacking using reverse migrating micelles and a water plug

Utility of a second enhanced field zone (water zone) is investigated for the on-line concentration of neutral analytes in micellar electrokinetic chromatography. Micellar solutions of sodium dodecyl sulfate prepared in acidic phosphate buffers are used as separation and sample solutions. Prior to long hydrodynamic injection of samples prepared in a low conductivity matrix, a long water plug is hydrodynamically injected to provide a second enhanced field zone. Practical and some fundamental considerations are presented. The technique is selective towards hydrophobic analytes. Notable detector response improvements (>100-fold) for several analytes are observed experimentally.

Electrokinetic chromatography without electroosmotic flow

This review summarizes the various aspects of conducting electrokinetic chromatography in coated columns with suppressed electroosmotic flow. The specific features of the technique will be presented and the potential applications explored. The equations of migration, resolution and zone spreading for neutral solutes will be presented, compared, and contrasted with those of conventional electrokinetic chromatography in bare-silica columns. The principle of separation is the same in electrokinetic chromatography with or without electroosmotic flow; however, there are many significant differences that will be highlighted.

Critical review of recent developments in fluorescence detection for capillary electrophoresis

Recent developments in capillary electrophoresis with fluorescence detection are reviewed. Instrumental advances have led to increased sensitivity and, therefore, a growing number of applications. Capillary electrophoresis has been coupled with various techniques to achieve multi-dimensional separations. Other advances have focused on temporal resolution when sampling from biological environments, increased sample throughput especially for DNA analysis, and fast separation times. New technologies including chip and channel electrophoretic separations with fluorescence detection are also discussed.

Developments in amino acid analysis using capillary electrophoresis

The current status in the analysis of amino acids using capillary electrophoresis is addressed. This area of biological analysis has received increased attention with more than 200 articles being published in the last five years. This review discusses pre-, post-, and on-column derivatization techniques used to tag amino acids providing a detectable moiety. Several separation methodologies which provided resolution for large sets of amino acids are presented. An overview of advances in the enantiomeric resolution methodologies for amino acids is given. Both direct and indirect enantiomeric separation schemes are summarized. Recent advances in detection strategies for both derivatized and underivatized amino acids are presented. Applications utilizing amino acid analysis by capillary electrophoresis are described. This review covers articles published between 1991 and 1996.

Capillary electrophoresis of natural products

Capillary electrophoresis (CE) and micellar electrokinetic chromatography were used for the separation of widely different compounds from natural materials including antibiotics, humic substances, flavonoids, isoflavonoids, illicit drugs, coumarins, alkaloids, steroids, Chinese herbal preparations, nicotine, caffeine, amphetamines, toxins such as aflatoxins B1, B2, G1, G2, mycotoxins, heptapeptide toxins and others, ephedrine compounds, mineral elements, and natural compounds in biological samples. A discussion of sample extraction and clean-up and the advantages of using CE is also presented.

Electrokinetic chromatography in suppressed electroosmotic flow environment: use of a charged cyclodextrin for the separation of enantiomers and geometric isomers

Electrokinetic chromatography (EKC), with negatively-charged cyclodextrins (NCDs) added to the buffer, was conducted in polyacrylamide-coated columns under suppression of electroosmotic flow. The equations of migration and resolution for neutral solutes in this mode of chromatography, which for brevity we term NCD-EKC, are presented. The chiral sulfated cyclodextrin, beta-CD-SBE (IV), used in this study is anionic over the entire pH range accessible to capillary electrophoresis, and the coated columns are stable and provide reproducible performance in the pH range 2.5-8.8. Optimum separation was obtained in the pH range where the solutes are neutral. The incorporation of an alkyl spacer between the sulfate ion and the rim of the cyclodextrin allows an unhindered approach and inclusion of neutral solutes in the cyclodextrin cavity. Solute migration time is inversely proportional to the concentration of the chiral selector. Separation (relative migration time difference) increases with decreasing chiral selector concentration and approaches a maximum, beyond which further decreases in chiral selector concentration result in broad peaks and loss of resolution. A chiral selector concentration of 1% in a 10 mM phosphate buffer produced excellent separation of amino acids and dipeptide enantiomers. In addition to being chiral selectors, cyclodextrins are also known as shape selectors. NCD-EKC is particularly suited for the separation of positional isomers of hydrophobic solutes. The separation of aflatoxin isomers and chlorophenol congeners is presented. In the separation of chlorophenols the more hydrophobic trichlorophenols eluted first and the least hydrophobic, phenol, eluted last.