Micellar electrokinetic chromatography: A practical overview of current methodological and instrumental advances

Strategies for capillary electrophoresis: Method development and validation for pharmaceutical and biological applications-Updated and completely revised edition

This review is in support of the development of selective, precise, fast, and validated capillary electrophoresis (CE) methods. It follows up a similar article from 1998, Wätzig H, Degenhardt M, Kunkel A. "Strategies for capillary electrophoresis: method development and validation for pharmaceutical and biological applications," pointing out which fundamentals are still valid and at the same time showing the enormous achievements in the last 25 years. The structures of both reviews are widely similar, in order to facilitate their simultaneous use. Focusing on pharmaceutical and biological applications, the successful use of CE is now demonstrated by more than 600 carefully selected references. Many of those are recent reviews; therefore, a significant overview about the field is provided. There are extra sections about sample pretreatment related to CE and microchip CE, and a completely revised section about method development for protein analytes and biomolecules in general. The general strategies for method development are summed up with regard to selectivity, efficiency, precision, analysis time, limit of detection, sample pretreatment requirements, and validation.

Chiral Micellar Electrokinetic Chromatography

The potential of Micellar Electrokinetic Chromatography to achieve enantiomeric separations is reviewed in this article. The separation principles and the most frequently employed separation strategies to achieve chiral separations by Micellar Electrokinetic Chromatography are described. The use of chiral micellar systems alone or combined with other micellar systems or chiral selectors, as well as of mixtures of achiral micellar systems with chiral selectors is discussed together with the effect of different additives present in the separation medium. Indirect methods based on the derivatization of analytes with chiral derivatizing reagents and the use of achiral micelles are also considered. Preconcentration techniques employed to improve sensitivity and the main approaches developed to facilitate the coupling with Mass Spectrometry are included. The most recent and relevant methodologies developed by chiral Micellar Electrokinetic Chromatography and their applications in different fields are presented.

A simple and highly sensitive masking fluorescence detection system for capillary array electrophoresis and its application to food and medicine analysis

A high sampling rate, good stability, high throughput masking fluorescence detection system with easy positioning of each channel for capillary array electrophoresis was prepared and studied. A special mask combined with convex lenses was designed to modulate signals, without using any extra device to position each channel. The signal of each channel was detected by a photomultiplier tube, classified and saved by software. The design was used to evidently reduce the rotational vibration of optical components and to stabilize the system, so a high sampling rate was obtained by increasing the DC motor speed. To improve the optical system, optical fibers instead of conventional bulky optical components were used to transmit optical signal and to collect fluorescences in multiple directions, which greatly raised the sensitivity. Other important parameters including sampling rate, rotating speed and driven voltage laser diode (LDs) have also been investigated. Under optimal conditions, the performance of the detection system was evaluated. This novel system had a well-designed structure, and allowed independent multiple capillary operations and easy microanalysis. Its limit of detection for rhodamine 6G was 2.0 × 10^-2 µg/mL.

Efficient separation of tanshinones by polyvinylpyrrolidone-stabilized graphene-modified micellar electrokinetic chromatography

In this work, a PVP-stabilized graphene was used in MEKC for the separation of tanshinones. Seven structurally similar tanshinones were studied, that is, tanshinone IIB, dihydrotanshinone I, tanshinone I, cryptotanshinone, 1,2-dihydrotanshinone I, miltirone, and tanshinone IIA. To achieve optimal conditions, graphene concentration, sample solvent composition, SDS concentration, 2-propanolconcentration, and buffer pH were investigated. At a separation voltage of 30 kV and a 41.5 cm effective length fused-silica capillary, good resolution within 12 min was performed using 10 mM borate buffer (pH 9.3) containing 30 mM SDS, 10% v/v 2-propanol and 6 μg/mL graphene. The method was validated in terms of linearity (r(2) > 0.9970), intra- and inter-day precision were less than 3.56 and 4.83%, respectively. The proposed method was then successfully applied to Danshentong capsule, an herbal preparation from Salvia miltiorrhiza. Our results indicated the high separation efficiency of PVP-stabilized graphene provided new opportunities for the analysis of complex samples.

Electrophoretic extraction of low molecular weight cationic analytes from sodium dodecyl sulfate containing sample matrices for their direct electrospray ionization mass spectrometry

While the use of sodium dodecyl sulfate (SDS) in separation buffers allows efficient analysis of complex mixtures, its presence in the sample matrix is known to severely interfere with the mass-spectrometric characterization of analyte molecules. In this article, we report a microfluidic device that addresses this analytical challenge by enabling inline electrospray ionization mass spectrometry (ESI-MS) of low molecular weight cationic samples prepared in SDS containing matrices. The functionality of this device relies on the continuous extraction of analyte molecules into an SDS-free solvent stream based on the free-flow zone electrophoresis (FFZE) technique prior to their ESI-MS analysis. The reported extraction was accomplished in our current work in a glass channel with microelectrodes fabricated along its sidewalls to realize the desired electric field. Our experiments show that a key challenge to successfully operating such a device is to suppress the electroosmotically driven fluid circulations generated in its extraction channel that otherwise tend to vigorously mix the liquid streams flowing through this duct. A new coating medium, N-(2-triethoxysilylpropyl) formamide, recently demonstrated by our laboratory to nearly eliminate electroosmotic flow in glass microchannels was employed to address this issue. Applying this surface modifier, we were able to efficiently extract two different peptides, human angiotensin I and MRFA, individually from an SDS containing matrix using the FFZE method and detect them at concentrations down to 3.7 and 6.3 μg/mL, respectively, in samples containing as much as 10 mM SDS. Notice that in addition to greatly reducing the amount of SDS entering the MS instrument, the reported approach allows rapid solvent exchange for facilitating efficient analyte ionization desired in ESI-MS analysis.

Recent advances in CE analysis of antibiotics and its use as chiral selectors

Antibiotics are a class of therapeutic molecules widely employed in both human and veterinary medicine. This article reviews the most recent advances in the analysis of antibiotics by CE in pharmaceutical, environmental, food, and biomedical fields. Emphasis is placed on the strategies to increase sensitivity as diverse off-line, in-line, and on-line preconcentration approaches and the use of different detection systems. The use of CE in the microchip format for the analysis of antibiotics is also reviewed in this article. Moreover, since the use of antibiotics as chiral selectors in CE has grown in the last years, a new section devoted to this aspect has been included. This review constitutes an update of previous published reviews and covers the literature published from June 2011 until June 2013.

Recent advances in capillary electrophoretic migration techniques for pharmaceutical analysis

Since the introduction about 30 years ago, CE techniques have gained a significant impact in pharmaceutical analysis. The present review covers recent advances and applications of CE for the analysis of pharmaceuticals. Both small molecules and biomolecules such as proteins are considered. The applications range from the determination of drug-related substances to the analysis of counterions and the determination of physicochemical parameters. Furthermore, general considerations of CE methods in pharmaceutical analysis are described.

Recent developments in capillary and microchip electroseparations of peptides (2011-2013)

The review presents a comprehensive survey of recent developments and applications of capillary and microchip electroseparation methods (zone electrophoresis, ITP, IEF, affinity electrophoresis, EKC, and electrochromatography) for analysis, isolation, purification, and physicochemical and biochemical characterization of peptides. Advances in the investigation of electromigration properties of peptides, in the methodology of their analysis, including sample preseparation, preconcentration and derivatization, adsorption suppression and EOF control, as well as in detection of peptides, are presented. New developments in particular CE and CEC modes are reported and several types of their applications to peptide analysis are described: conventional qualitative and quantitative analysis, determination in complex (bio)matrices, monitoring of chemical and enzymatical reactions and physical changes, amino acid, sequence and chiral analysis, and peptide mapping of proteins. Some micropreparative peptide separations are shown and capabilities of CE and CEC techniques to provide relevant physicochemical characteristics of peptides are demonstrated.

Assessment of DNA damage by micellar electrokinetic chromatography

A simple and inexpensive MEKC method, which is able to assess base damage within DNA samples, is illustrated. After heat-acid hydrolysis of the DNA samples, both the percentage of each canonical DNA base and the relative amount of uncanonical DNA bases can be measured. This method is useful for an evaluation of the integrity of PCR templates used in several fields of investigation.

Contemporary sample stacking in analytical electrophoresis

Sample stacking is a term denoting a multifarious class of methods and their names that are used daily in CE for online concentration of diluted samples to enhance separation efficiency and sensitivity of analyses. The essence of these methods is that analytes present at low concentrations in a large injected sample zone are concentrated into a short and sharp zone (stack) in the separation capillary. Then the stacked analytes are separated and detected. Regardless of the diversity of the stacking electromigration methods, one can distinguish four main principles that form the bases of nearly all of them: (i) Kohlrausch adjustment of concentrations, (ii) pH step, (iii) micellar methods, and (iv) transient ITP. This contribution is a continuation of our previous reviews on the topic and brings an overview of papers published during 2010-2012 and relevant to the mentioned principles (except the last one which is covered by another review in this issue).

Micellar electrokinetic chromatography: a new simple tool for the analysis of synthetic cannabinoids in herbal blends and for the rapid estimation of their logP values

For the first time a capillary separation based on micellar electrokinetic chromatography (MEKC) with diode array detection (DAD) was developed and validated for the rapid determination of synthetic cannabinoids in herbal blends. Separations were carried out on a 30 μm(ID) × 40 cm uncoated fused silica capillaries. The optimized buffer electrolyte was composed of 25 mM sodium tetraborate pH 8.0, 30 mM SDS and n-propanol 20% (v/v). Separations were performed at 30 kV. Sample injection conditions were 0.5 psi, 10s. Diazepam and JWH-015 were used as internal standards. The determination of the analytes was based on the UV signal recorded at 220 nm, corresponding to the maximum wavelength of absorbance of the molecules, whereas peak identification and purity check were also performed on the basis of the acquisition of UV spectra between 200 and 400 nm wavelengths. Under the described conditions, the separation of the compounds was achieved in 25 min without any significant interference from the matrix. Linearity was assessed within a concentration range from 5 to 100 μg/mL. The intra-day and inter-day imprecision values were below 2.45% for relative migration times and below 10.75% for relative peak areas. The present method was successfully applied to the direct determination of synthetic cannabinoids in 15 different herbal blend samples requiring only sample dilution. In addition, the developed MEKC separation was also applied to estimate the octanol/water partition coefficients (logP) of these new and poorly known molecules.

Carbon nanotubes in capillary electrophoresis, capillary electrochromatography and microchip electrophoresis

Carbon nanotubes are among the plethora of novel nanostructures developed since the 1980s. Nanotubes have attracted considerable interest by the scientific community thanks to their extraordinary physical and chemical properties. Research areas have flourished in recent years and now include the nano-electronic, (bio)sensor and analytical field along with many others. This review covers applications of carbon nanotubes in capillary electrophoresis, capillary electrochromatography and microchip electrophoresis. First, carbon nanotubes and a range of electrophoretic techniques are briefly introduced and key references are mentioned. Next, a comprehensive survey of achievements in the field is presented and critically assessed. The merits and downsides of carbon nanotube addition to the various capillary electrophoretic modes are addressed. The different schemes for fabricating electrochromatographic stationary phases based on carbon nanotubes are discussed. Finally, some future perspectives are offered.

High-sensitivity capillary electrophoresis method for monitoring purine nucleoside phosphorylase and adenosine deaminase reactions by a reversed electrode polarity switching mode

A simple, efficient, and highly sensitive in-line CE method was developed for the characterization and for inhibition studies of the nucleoside-metabolizing enzymes purine nucleoside phosphorylase (PNP) and adenosine deaminase (ADA) present in membrane preparations of human 1539 melanoma cells. After filling the running buffer (50 mM borate buffer, 100 mM SDS, pH 9.10) into a fused-silica capillary (50 cm effective length × 75 μm), a large sample volume was loaded by hydrodynamic injection (5 psi, 36 s), followed by the removal of the large plug of sample matrix from the capillary using polarity switching (-20 kV). The current was monitored and the polarity was reversed when 95% of the current had been recovered. The separation of the neutral analytes (nucleosides and nucleobases) was performed by applying a voltage of 15 kV. An about 10-fold improvement of sensitivity for the five investigated analytes (adenosine, inosine, adenine, hypoxanthine, xanthine) was achieved by large-volume stacking with polarity switching when compared with CE without stacking. For inosine and adenine detection limits as low as 60 nM were achieved. To the best of our knowledge, this represents the highest sensitivity for nucleoside and nucleobase analysis using CE with UV detection reported so far. The Michaelis-Menten constants (K(m)) for PNP and ADA and the inhibition constants (K(i)) for standard inhibitors determined with the new method were consistent with literature data.