CE frontal analysis employing contactless conductivity detection for determination of CMCs of non-UV absorbing charged surfactants

Micellar systems composed of surfactants are used extensively in academia and industry for many different applications. In this work a highly versatile CE method for determination of CMCs of charged surfactants has been developed. In the case of positively charged surfactants a coating procedure of the fused-silica capillary was used, whereas negatively charged surfactants were analyzed using uncoated capillaries. The CE method is based on frontal analysis (FA) employing use of contactless conductivity and UV detection. The main advantages of the method are that it can be used for non-UV absorbing surfactants without introducing marker compounds which previously has been found to affect CMCs, requires very limited sample volume and is easily implemented and automated using standard CE equipment. The fact that counterions and different aggregated states are separated allows a detailed characterization of the micelle systems using the developed method. In the case of UV absorbing surfactants similar results were obtained employing contactless conductivity and UV detection. Finally, CMCs obtained using conductometry gave similar results as compared to the developed CE-FA procedure.     

Separation of non-UV-absorbing synthetic polyelectrolytes by CE with contactless conductivity detection

A specific method for the separation and detection of non-UV-absorbing polyelectrolytes has been developed. The analysis of such polyelectrolytes by liquid chromatography is nearly impossible due to strong ionic interactions and charge density effects. CE makes use of these charge density effects and thus enables for proper separation. A capacitively coupled contactless conductivity detector has been applied for the detection in CE. A low molar mass poly(acrylic acid) sodium salt standard (PAA1.3k) was separated in free solution CE and detected with the contactless conductivity detector. Different amphoteric electrolytes have been tested for their applicability as BGE for the separation of polyelectrolytes with conductivity detection. It has been shown that the best detection results are obtained with an arginine-sorbate buffer.     

Fabrication and characterization of semicircular detection electrodes for contactless conductivity detector - CE microchips

This study uses simple and reliable microfabrication techniques to fabricate CE biochips, integrating a novel contactless conductivity detector in a miniaturized detection system in a microfluidic biochip. The off-channel electrodes are deposited around side channels by Au sputtering and patterned using a standard "lift-off" process. A vacuum fusion bonding process is employed to seal the lower substrate containing the microchannels and the electrodes to an upper glass cover plate. The variations in the capacitance between the semicircular detection electrodes in the side channels are measured as different samples and ions pass through the detection region of the CE separation channel. Samples of Rhodamine B, commercial sports drinks, mineral waters, and a red wine, respectively, are mixed in different buffer solutions, separated, and successfully detected using the developed device. The semicircular detection electrodes for the contactless conductivity detector have microscale dimensions and provide a valuable contribution to the realization of the lab-on-a-chip concept.     

Conductivity detection for conventional and miniaturised capillary electrophoresis systems

Since the introduction of capillary electrophoresis (CE), conductivity detection has been an attractive means of detection. No additional chemical properties are required for detection, and no loss in sensitivity is expected when miniaturising the detector to scale with narrow-bore capillaries or even to the microchip format. Integration of conductivity and CE, however, involves a challenging combination of engineering issues. In conductivity detection the resistance of the solution is most frequently measured in an alternating current (AC) circuit. The influence of capacitors both in series and in parallel with the solution resistance should be minimised during conductivity measurements. For contact conductivity measurements, the positioning and alignment of the detection electrodes is crucial. A contact conductivity detector for CE has been commercially available, but was withdrawn from the market. Microfabrication technology enables integration and precise alignment of electrodes, resulting in the popularity of conductivity detection in microfluidic devices. In contactless conductivity detection, the alignment of the electrodes with respect to the capillary is less crucial. Contactless conductivity detection (CCD) was introduced in capillary CE, and similar electronics have been applied for CCD using planar electrodes in microfluidic devices. A contactless conductivity detector for capillaries has been commercialised recently. In this review, different approaches towards conductivity detection in capillaries and chip-based CE are discussed. In contrast to previous reviews, the focus of the present review is on the technological developments and challenges in conductivity detection in CE.     

Recent advances of capillary electrophoresis in pharmaceutical analysis

This review covers recent advances of capillary electrophoresis (CE) in pharmaceutical analysis. The principle, instrumentation, and conventional modes of CE are briefly discussed. Advances in the different CE techniques (non-aqueous CE, microemulsion electrokinetic chromatography, capillary isotachophoresis, capillary electrochromatography, and immunoaffinity CE), detection techniques (mass spectrometry, light-emitting diode, fluorescence, chemiluminescence, and contactless conductivity), on-line sample pretreatment (flow injection) and chiral separation are described. Applications of CE to assay of active pharmaceutical ingredients (APIs), drug impurity testing, chiral drug separation, and determination of APIs in biological fluids published from 2008 to 2009 are tabulated.     

Construction and analytical application of an on-column photo reactor for improved detection of iron-species as plant metabolites in capillary flow injection and capillary electrophoresis

An on-column photo reactor for CE, which is constructed from UV-transparent capillaries and a small Pen-Ray UV lamp, is applied to the analysis of small, non-covalent iron-species. These iron-species, e.g. phytosiderophores (PS) in grasses and the non-protein amino acid nicotianamine (NA), play an important role in plant metabolism. The photo reactor is placed directly in front of the on-column absorbance detector, illuminating only some centimeters of the capillary. The photo reactor is used for capillary electrophoresis (CE) and also for capillary flow injection analysis (CFIA). Photoinduced sensitivity changes for model iron-species and for plant extracts are investigated, using UV detection and capacitively coupled contactless conductivity detection (C(4)D). The detection sensitivity for iron-species is enhanced in CFIA; the enhancement factor depends on the type of iron-species. In CE, the sensitivity of iron-species is kinetically dependent on the type of iron-species, decreasing with both detectors, but a photo reaction product is detectable. The relationship between irradiation window length and sensitivity is investigated quantitatively using EDTA-Fe(III). Pure ligands without iron are little affected by the photo reaction in both CFIA and CE. In CE, the detector signals of iron-species in real plant samples are selectively decreased with the proposed photo reactor, thus enabling a simple screening method for such photoactive iron-species.     

Analyte focusing by micelle collapse for liquid extraction surface analysis coupled with capillary electrophoresis of neutral analytes on a solid surface

Liquid extraction surface analysis (LESA) has an advantage of directly sampling analytes on a surface, thus avoiding unnecessary dilution by homogenization of the bulk sample commonly practiced in solid sample analysis. By combining LESA with CE, the additional advantage of separating analytes before detection can be accomplished. For neutral molecules, MEKC needs to be used. Since the detection sensitivity of CE in general suffers from the small capillary dimension, analyte focusing by micelle collapse was employed for enhanced extraction in LESA and sample preconcentration for MEKC. In addition, using a commercial CE instrument, the LESA process was performed much faster and more reliably compared to our first demonstration of LESA‐CE using a homemade CE setup. Three neutral water‐insoluble pesticides sprayed on an apple skin were directly extracted, preconcentrated, and analyzed by the automated LESA‐analyte focusing by micelle collapse‐MEKC with high sensitivity in 10 min. The relative standard deviations of the migration times and peak heights were 0.8–2.1 and 1.2–3.0%, respectively when ametryn was used as an internal standard. The limits of detection obtained with UV absorbance at 200 nm were 1.8–6.4 ppb.     

A Comparison of the Properties of Contactless Conductivity and Diode‐Array Photometric Detectors in Analyses of Low‐Molecular,Biologically Active Substances by Capillary Electrophoresis in Acetic Acid Solutions

The performance of the contactless conductivity (C⁴D) and diode array photometric (DAD) detectors has been compared for CE separations of creatinine, arginine and 3‐methylhistidine in acetic acid background electrolytes. The contactless conductivity detector response has also been modeled. It has been found that the two detectors provide similar responses and can readily be used for dual CE detection. Changes in the acetic acid concentration affect the C⁴D noise less than the DAD noise, but their effect on the C⁴D response to the analytes is greater than with DAD. In general, C⁴D provides better detection results at higher acetic acid concentrations, while DAD is more sensitive and reliable at very low ones. Capillaries with greater internal diameters are preferable for both detectors, provided that the separation efficiency is not adversely affected. Acetic acid is a suitable background electrolyte for CE separations of small, basic organic molecules.     

Impact of Taylor‐Aris diffusivity on analyte and system zone dispersion in CZE assessed by computer simulation and experimental validation

Application of pressure‐driven laminar flow has an impact on zone and boundary dispersion in open tubular CE. The GENTRANS dynamic simulator for electrophoresis was extended with Taylor‐Aris diffusivity which accounts for dispersion due to the parabolic flow profile associated with pressure‐driven flow. Effective diffusivity of analyte and system zones as functions of the capillary diameter and the amount of flow in comparison to molecular diffusion alone were studied for configurations with concomitant action of imposed hydrodynamic flow and electroosmosis. For selected examples under realistic experimental conditions, simulation data are compared with those monitored experimentally using modular CE setups featuring both capacitively coupled contactless conductivity and UV absorbance detection along a 50 μm id fused‐silica capillary of 90 cm total length. The data presented indicate that inclusion of flow profile based Taylor‐Aris diffusivity provides realistic simulation data for analyte and system peaks, particularly those monitored in CE with conductivity detection.     

Characterization of cationic copolymers by capillary electrophoresis using indirect UV detection and contactless conductivity detection

For many industrial applications, the combination of two different monomers in statistical or diblock copolymers enhances the properties of the corresponding polymer. However, during the polymerization reaction, homopolymers might be formed and can influence the properties for the applications. Consequently, the separation and the quantification of the homopolymers contained in copolymer samples are crucial. In addition, the charge density distribution of the statistical copolymer is an important characteristic for the applications. The purpose of this work was to study the characterization of a statistical copolymer of acrylic acid (AA) and diallyldimethyl ammonium chloride (DADMAC) by capillary electrophoresis (CE) in acidic conditions (cationic copolymers). For that purpose, a free solution electrophoretic separation was carried out according to the charge rate (chemical composition) independently of the molar mass. The second objective was to compare contactless conductivity detection and indirect UV absorbance modes for the quantification of DADMAC homopolymers present in copolymer samples. Different coated capillaries based on neutral or positively charged modification were also compared. The comparison of indirect absorbance UV and contactless conductimetric detection demonstrated that both detection modes can be used for a complete CE characterization of non-UV absorbing PAA-DADMAC copolymers.     

A microchip electrophoresis system with integrated in-plane electrodes for contactless conductivity detection

We present a new approach for contactless conductivity detection for microchip-based capillary electrophoresis (CE). The detector integrates easily with well-known microfabrication techniques for glass-based microfluidic devices. Platinum electrodes are structured in recesses in-plane with the microchannel network after glass etching, which allows precise positioning and batch fabrication of the electrodes. A thin glass wall of 10-15 microm separates the electrodes and the buffer electrolyte in the separation channel to achieve the electrical insulation necessary for contactless operation. The effective separation length is 34 mm, with a channel width of 50 microm and depth of 12 microm. Microchip CE devices with conductivity detection were characterized in terms of sensitivity and linearity of response, and were tested using samples containing up to three small cations. The limit of detection for K+ (18 microM) is good, though an order of magnitude higher than for comparable capillary-based systems and one recently reported example of contactless conductivity on chip. However, an integrated field-amplified stacking step could be employed prior to CE to preconcentrate the sample ions by a factor of four.     

Considerations on contactless conductivity detection in capillary electrophoresis

Nearly all analyses by capillary electrophoresis (CE) are performed using optical detection, utilizing either absorbance or (laser-induced) fluorescence. Though adequate for many analytical problems, in a large number of cases, e.g., involving non-UV-absorbing compounds, these optical detection methods fall short. Indirect optical detection can then still provide an acceptable means of detection, however, with a strongly reduced sensitivity. During the past few years, contactless conductivity detection (CCD) has been presented as a valuable extension to optical detection techniques. It has been demonstrated that with CCD detection limits comparable, or even superior, to (indirect) optical detection can be obtained. Additionally, construction of the CCD around the CE capillary is straightforward and robust operation is easily obtained. Unfortunately, in the literature a large variety of designs and operating conditions for CCD were described. In this contribution, several important parameters of CCD are identified and their influence on, e.g., detectability and peak shape is described. An optimized setup based on a well-defined detection cell with three detection electrodes is presented. Additionally, simple and commercially available read-out electronics are described. The performance of the CCD-CE system was demonstrated for the analysis of peptides. Detection limits at the microM level were obtained in combination with good peak shapes and an overall good performance and stability.     

Fast determination of paracetamol and its hydrolytic degradation product p-aminophenol by capillary and microchip electrophoresis with contactless conductivity detection

Paracetamol (PAC) is one of the most extensively used analgesics and antipyretic drugs to treat mild and moderate pain. P-aminophenol (PAP), the main hydrolytic degradation product of PAC, can be found in environmental water. Recently, CE has been developed for the detection of a wide variety of chemical substances. The purpose of this study is to develop a simple and fast method for the detection and separation of PAC and its main hydrolysis product PAP using CE and microchip electrophoresis with capacitively coupled contactless conductivity detection. The determination of these compounds using microchip electrophoresis with capacitively coupled contactless conductivity detection is being reported for the first time. The separation was run for all analytes using a BGE (20 mM β-alanine, pH 11) containing 14% (v/v) methanol. The RSDs obtained for migration time were less than 4%, and RSDs obtained for peak area were less than 7%. The detection limits (S/N = 3) that were achieved ranged from 0.3 to 0.6 mg/L without sample preconcentration. The presented method showed rapid analysis time (less than 1 min), high efficiency and precision, low cost, and a significant decrease in the consumption of reagents. The microchip system has proved to be an excellent analytical technique for fast and reliable environmental applications.     

Recent developments in the separation of inorganic and small organic ions by capillary electrophoresis

Which method should I use for ion analysis, ion chromatography (IC) or capillary electrophoresis (CE)? In terms of actual theoretical plates CE has a clear-cut advantage. The separation ability of IC is adequate for many sample types, and many separation scientists feel that IC offers greater reliability and confidence than CE. However, IC is a more mature technique and there has been more time to solve problems such as peak tailing and to improve reproducibility. The two techniques should be viewed as complementary. A number of recent developments in ion analysis by CE are discussed. These include some simple ways to control electroosmotic flow and improve reproducibility, separation of isotopes, improved methods of indirect photometric detection, a new contactless conductivity detector, separation of ions at low pH, and in solutions of high salt content. Progress in a new technique called IC-CE will be described in which a soluble ion-exchange polymer is added to the capillary electrolyte to separate anions based on differences in both electrophoretic mobility and ion-exchange interactions.     

CE of inorganic species--a review of methodological advancements over 2009-2010

This article is the seventh in a series examining biannually the methodological developments in the field of CE analysis of inorganic species and covers relevant documents published between January 2009 and December 2010. Following an analysis of the significant accomplishments that have impacted the field in two recent years, a survey of advances in general CE methodology is presented. Subsequently, several notable trends that can be perceived in this well-established field are discussed: the continuing rise of ME and consequent development of suitable detection techniques, most notably contactless conductivity detection, the constant pace of advances in speciation analysis, and an increase in non-analytical CE applications to study complexation and (bio)transformation reactions of metal analytes. A range of recently emerged multi-detection designs, ICP-MS interface devices, and separation systems, for which outpacing work has been conducted, are also brought into focus.     

Evaluation of the detection of biomolecules in capillary electrophoresis by contactless conductivity measurement

The sensitivity of contactless conductivity detection to amino acids, peptides and proteins in CE was studied for BGE solutions of different pH values. The LOD and analytical characteristics were compared for acidic and basic conditions and better results were in most cases found for buffers of low pH values. Linear dynamic ranges varied between two orders of magnitude for amino acids and peptides and three orders of magnitude for larger proteins. The concentration detection limits were found to be between 1.2 and 7.5 microM for the amino acids tested and for the larger molecules they varied between 2.6 microM for leucine enkephalin and 0.2 microM for HSA when using a buffer at pH 2.1.     

Application of Capillary Electrophoresis for Determination of Inorganic Analytes in Waters

Aside from HPLC and GC, capillary electrophoresis (CE) is one of the most important techniques for high-performance separations in modern analytical chemistry. Its main advantages are the possibility of using different detection techniques, the possibility of in-capillary sample processing for preconcentration or derivatization, and ease of instrumental miniaturization down to the microfluidic scale. Those features are utilized in the separation of macromolecules in biochemistry and in genetic investigations, but they can be also used in determinations of inorganic ions in water analysis. This review, based on about 100 original research works, presents applications of CE methods in water analysis reported in recent decade, mostly regarding conductivity detection or indirect UV detection. The developed applications include analysis of high salinity sea waters, as well as analysis of other surface waters and drinking waters.     

A chip-based capillary electrophoresis-contactless conductivity microsystem for fast measurements of low-explosive ionic components

A miniaturized analytical system for separating and detecting inorganic explosive residues, based on the coupling of a micromachined capillary electrophoresis (CE) chip with a contactless conductivity detector is described. The low electroosmotic flow (EOF) of the poly(methylmethacrylate) (PMMA) chip material facilitates the rapid switching between analyses of cations and anions using the same microchannel and run buffer (and without an EOF modifier), and hence offers rapid (< 1 min) measurement of seven explosive-related cations and anions. Experimental parameters relevant to the separation and detection processes have been optimized. Addition of a 18-crown-6 ether modifier has been used for separating the peaks of co-migrating potassium and ammonium ions. The ionic-explosive microchip system combines the distinct advantages of contactless conductivity detection with the attractive features of plastic CE microchips. The new microsystem offers great promise for monitoring explosive-related ions at the sample source, with significant advantages of speed/warning, efficiency, cost, or sample size.     

Electrowetting on dielectric actuation of droplets with capillary electrophoretic zones for MALDI mass spectrometric analysis

An automated fraction collection interface was developed for coupling CE with MALDI-MS. This fraction collection approach is based on the electrowetting on dielectric (EWOD) phenomenon performed on a digital microfluidic (DMF) board; it does not rely on a MALDI spotter. In this study, a four-peptide mixture was used as a sample test, and the separations were conducted in a portable CE instrument with a 150 μm o.d. × 50 μm i.d. capillary and a contactless conductivity detector. The CE instrument was interfaced with a robust DMF board. The CE fractions were directly deposited onto the DMF board at predetermined locations prior to MALDI analysis. The series of experiments determined the lowest concentration that produces a measurable MALDI signal. The concentrations were 0.25, 0.5, 0.05, and 0.05 nmol for bradykinin, angiotensin, ACTH (18-39), and insulin, respectively. The contactless conductivity detector limit of detection for the same analytes was 2.5 μmol.     

Rapid determination of partition coefficients of pharmaceuticals by phase distribution and microchip capillary electrophoresis with contactless conductivity detection

A simple microchip CE method integrated with contactless conductivity detection was developed for the direct determination of partition coefficients of selected pharmaceuticals after phase distribution equilibrium. The equilibrium of distribution between two phases for four pharmaceuticals was performed using a 1‐octanol/water system and 1‐octanol/buffer system. During the concentration determination, several major factors affecting detection were investigated in detail for each pharmaceutical to optimize the detection sensitivity. In the optimal conditions, sufficient electrophoretic separation and sensitive detection for each target analyte can be achieved within 40 s. The two systems showed a pH‐dependent partition behavior. Moreover, the measured values showed excellent agreement with those obtained by the traditional shake‐flask method with HPLC–UV detection and literature reports, respectively. The developed method can be successfully applied to measure partition coefficient values of pharmaceuticals and requires much shorter analytical time compared to traditional methods.