Large-volume stacking with polarity switching and sweeping for chlorophenols and chlorophenoxy acids in capillary electrophoresis

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.

On-line concentration of environmental pollutant samples by using capillary electrophoresis

This chapter reviews the theory and methodological developments of on-line concentration techniques for the determination of environmental pollutant samples, such as organic and inorganic compounds in capillary zone electrophoresis (CZE) and also in micellar electrokinetic chromatography (MEKC). Topics covered include a variety of on-line preconcentration strategies, which are now generally referred to as sample stacking and sweeping techniques. For each technique, surveyed methods are tabulated in order to assist in method selection. Innovative applications of sample stacking and sweeping to advanced environmental research are also emphasized. In addition, other comparative on-line concentration methods for environmental samples, namely, isotachophoretic stacking and anion and cation selective exhaustive injection-sweeping are briefly discussed.

Flow manipulation for sweeping with a cationic surfactant in microchip capillary electrophoresis

Flow manipulation in sweeping microchip capillary electrophoresis (CE) is complicated by the free liquid communication between channels at the intersection, especially when the electroosmotic flows are mismatched in the main channel. Sweeping in traditional CE with cationic micelles is an effective way to concentrate anionic analytes. However, it is a challenge to transfer this method onto microchip CE because the dynamic coating process on capillary walls by cationic surfactants is interrupted when the sample solution free of surfactants is introduced into the microchip channels. This situation presents a difficulty in the sample loading, injection and dispensing processes. By adding surfactant at a concentration around the critical micelle concentration and by properly designing the voltage configuration, the flows in a microchip were effectively manipulated and this sweeping method was successfully moved to microchip CE using tetradecyltrimethylammonium bromide (TTAB). The sweeping effect of cationic surfactant in the sample solution was discussed theoretically and studied experimentally in traditional CE. The flows in a microchip were monitored with fluorescence imaging, and the injection and sweeping processes were studied by locating the detection point along the separation channel. A detection enhancement of up to 500-fold was achieved for 5-carboxyfluorescein.

Determination of organic acids by CE and CEC methods

A comprehensive overview of the analysis of low-molecular-mass organic acids employing electromigration methods in the capillary format is given. This review includes papers published since 2003 and can be seen as an update of the review paper published by Galli et al. in 2003. Tables included in this review contain application papers describing the determination of organic acids from a variety of fields like the analysis of food and beverages, environmental samples, samples from clinical origin, and from natural products.

On-line preconcentration of weak electrolytes by electrokinetic accumulation in CE: Experiment and simulation

A new on-line preconcentration technique was developed that makes possible to determine nanomolar concentrations of weak acidic analytes in CE. The method consists of long-running electrokinetic sample injection and stacking (electrokinetic immobilization) of the analytes at a boundary of two electrolytes with different pH values (pH 9.5 and 2.5) and consequent mobilization of the stacked uncharged analytes in a micellar system (containing SDS micelles). Several factors including buffer concentration, pH, applied voltage, time of preconcentration, and SDS concentration were tested to optimize the analysis method. An about 4600-fold increase of the sample concentration (in comparison with the standard CZE) can be achieved during the preconcentration step. Two preservatives applied in food industry -- benzoic acid and sorbic acid were used as model samples. The applicability of the proposed method in food analysis was demonstrated by determination of nanomolar concentrations of benzoic acid in sunflower oil. An extended version of the computer program Simul was used for modeling both the preconcentration and mobilization processes taking place in the capillary.

Direct determination of chlorophenols present in liquid samples by using a supported liquid membrane coupled in-line with capillary electrophoresis equipment

Actually there is a great trend on the development of effective analytical methods for monitoring trace levels of various phenols which can indicate, among others compounds, the water quality. A simple, inexpensive supported liquid membrane (SLM) device was used in combination with commercially available capillary electrophoresis (CE) equipment for the direct determination of chlorophenols in surface water samples. The manifold was used simultaneously to extract and preconcentrate the analytes from liquid samples. In the extraction set-up, the donor phase (4 mL) was placed in the CE vial, where a micro-membrane extraction unit (MMEU) accommodating the acceptor phase (100 microL) in its lumen was immersed. The supported liquid membrane was constructed by impregnating a porous Fluoropore Teflon (PTFE) membrane with a water-immiscible organic solvent (dihexyl ether). The extraction process was optimized with regard to the pH of the donor and acceptor phases, membrane liquid, extraction time and voltage applied to the inlet or outlet vial during extraction. The chlorinated phenols pentachlorophenol (PCP), 2,3,6 trichlorophenol (TCP) and 2,6 dichlorophenol (DCP) were thus efficiently separated by CE, using tris(hydroxymethyl)aminomethane (Tris) and an NaH(2)PO(4) solution containing 1% (v/v) methanol at pH 10.5 as running buffer.

Contemporary sample stacking in CE: A sophisticated tool based on simple principles

Sample stacking is a general term for methods in CE which are used for on-line concentration of diluted analytes. During the stacking process, analytes present at low concentrations in a long injected sample zone are concentrated into a short zone (stack). The stacked analytes are then separated and individual zones are detected. Thus stacking provides better separation efficiency and detection sensitivity. Many papers have been published on stacking till now, various procedures have been described, and, many names have been proposed for stacking procedures utilizing the same principles. This contribution brings an easy and unified view on stacking, describes the basic principles utilized, makes a list of recognized operational principles and brings an overview of principal current procedures. Further, it surveys selected recent practical applications ordered according to their operational principles and includes the terms, nicknames, and acronyms used for these actual stacking procedures. This contribution may help both newcomers and experts in the field of CE to orient themselves in the already quite complex topic of sample stacking.

Improved properties of the non-covalent coating with N,N-didodecyl-N, N-dimethylammonium bromide for the separation of basic proteins by capillary electrophoresis with acidic buffers in 25μm capillaries

Capillaries (25microm I.D.) treated with the double-alkyl-chain cationic surfactant N,N-didodecyl-N, N-dimethylammonium bromide (DDAB) in an improved coating procedure were used for separation of four basic proteins in volatile buffers (ammonium acetate and ammonium hydroxyacetate) as well as in a non-volatile buffer (sodium phosphate) at pH 4. The DDAB coating was stable enough to, without recoating, permit consecutive separations of the proteins up to 9h with good precisions in peak areas (RSD=1.1%) and migration times and with high apparent efficiencies (over 1 million theoretical plates/m) in the presence of a strong anodic electroosmosis. Adsorption of the proteins onto the capillary surface, which in previous studies was found to give a certain contribution to zone broadening, was eliminated with the new modified coating method. Complex formation between the proteins and phosphate buffer was studied and confirmed, and it is proposed that slow protein-buffer component interactions are the main contributions to zone broadening in protein separations by CE.

Recent advances in capillary electrophoresis and capillary electrochromatography of pollutants

Recent advances in the CE and CEC separation, detection, and sample preparation methodologies applied to the determination of a variety of compounds having current or potential environmental relevance have been overviewed. The reviewed literature has illustrated the wide range of CE applications, indicating the continuing interest in CE and CEC in the environmental field.

Determination of mercaptopurine and its four metabolites by large-volume sample stacking with polarity switching in capillary electrophoresis

This study describes approaches for stacking a large volume of sample solutions containing a mixture of mercaptopurine monohydrate, 6-methylmercaptopurine, thioguanine, thioguanosine, and thioxanthine in capillary electrophoresis (CE). After filling the run buffer (60 mM borate buffer, pH 8.5), a large sample volume was loaded by hydrodynamic injection (2.5 psi, 99.9 s), followed by the removal of the large plug of sample matrix from the capillary using polarity switching (-15 kV). Monitoring the current and reversing the polarity when 95% of current recovered, the separation of anionic analytes was performed in a run buffer < 20 kV. Around 44- to 90-fold improvement of sensitivity for five analytes was achieved by large-volume stacking with polarity switching when compared with CE without stacking. This method was feasible for determination of the analytes spiked in plasma. Removing most of electrolytes from plasma is a key step for performing large-volume sample stacking. Solid-phase extraction was used for pretreatment of biological samples. To our knowledge, this study is one of few applications showing the possibilities of this stacking procedure to analyze biological samples by large-volume sample stacking with polarity switching (LVSSPS) in CE.

On-line sample concentration techniques in capillary electrophoresis: velocity gradient techniques and sample concentration techniques for biomolecules

Methods with a high sensitivity and high separation efficiency are goals in analytical separation techniques. On-line sample concentration techniques in capillary electrophoresis (CE) separations have rapidly grown in popularity over the past few years because they achieve this goal. This review describes the methodology and theory associated with a number of different techniques, including electrokinetic and chromatographic methods. For small molecules, several on-line concentration methods based on velocity gradient techniques are described, in which the electrophoretic velocities of the analyte molecules are manipulated by field amplification, sweeping, and isotachophoretic migration, resulting in the on-line concentration of the analyte zones. In addition, the on-line concentration methods for macromolecules are described, since the techniques used for macromolecules (DNAs and proteins), are different from those for small molecules, with respect to either mechanism or methodology. Recent studies relating to this topic are also discussed, including electrophoretic and chromatographic techniques on capillary or microchip.

Recent advances in micellar electrokinetic chromatography

This review contains nearly 200 reference citations, and covers advances in electrokinetic capillary chromatography based on micelles, including stabilized micelle complexes, polymeric and mixed micelles from 2003-2004. Detection strategies, analyte determinations, and applications in micellar electrokinetic capillary chromatography (MEKC) are discussed. Information regarding methods of analyte concentration, analyte specific analyses, and nonstandard micelles has been summarized in tabular form to provide a means of rapid access to information pertinent to the reader.