Wall-jet conductivity detector for microchip capillary electrophoresis

A new end-column ‘hybrid’ contactless conductivity detector for microchip capillary electrophoresis (CE) was developed. It is based on a “hybrid” arrangement where the receiving electrode is insulated by a thin layer of insulator and placed in the bulk solution of the detection reservoir of the chip, whereas the emitting electrode is in contact with the solution eluted from the channel outlet in a wall-jet arrangement. The favorable features of the new detector including the high sensitivity and low noise, can be attributed to both the direct contact of the ‘emitting’ electrode with the analyte solution as well as to the insulation of the detection electrode from the high DC currents in the electrophoretic circuit. Such arrangement provides a 10-fold sensitivity enhancement compared to currently used on-column contactless conductivity CE microchip detector as well as low values of noise and easy operation. The new design of the wall-jet conductivity detector was tested for separation of explosive-related methylammonium, ammonium, and sodium cations. The new detector design reconsiders the wall-jet arrangement for microchip conductivity detection in scope of improved peak symmetry, simplified study of inter-electrode distance, isolation of the electrodes, position of the wall-jet electrode to the separation channel, baseline stability and low limits of detection.     

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.     

Concurrent determination and separation of inorganic cations and anions in microchip electrophoresis with precisely controlled high‐voltage

An improved method for the concurrent determination and separation of cations and anions by microchip electrophoresis with capacitively coupled contactless conductivity detection (ME‐C⁴D) is described. Two kinds of microchip structures were designed. The first microchip has a long bent separation channel. And for the defects of the first microchip, the second microchip with a Y‐type separation channel has been proposed. The background electrolyte (BGE) composed of 20 mm His/MES and 0.01 mm CTAB was optimized for inhibiting the electroosmotic flow (EOF). Due to the low electroosmotic flow, the cations and anions migrate in opposite directions and can be separated from each other. With the precisely controlled high‐voltage, cations and anions can be migrated in microchannels according to our requirements and sequentially detected by a C⁴D detector built in‐house. Samples containing K⁺, Na⁺, Li⁺, Cl⁻, F⁻ and PO₄³⁻ were analyzed simultaneously in a single run (within 140 s) by both methods. The reproducibility obtained by both methods remained below 5% for migration time and within 3.5–9.1% for peak areas. The proposed concurrent determination methods are inexpensive, simple, fast, ease of operation, high degree of integration.     

Sequential Determination of Inorganic Cations and Anions in Cerebrospinal Fluid by Microchip Electrophoresis

Analysis of inorganic ions in cerebrospinal fluid (CSF) is used mainly in the diagnostics of central nervous system diseases, such as Alzheimer’s disease or multiple sclerosis. A new analytical method for fast determination of inorganic cations (ammonium, calcium, magnesium, sodium and potassium) and anions (chloride, sulfate, nitrite and nitrate) in CSF on an electrophoretic microchip was developed in this context. Zone electrophoresis (ZE) separations were performed on the microchip with coupled channels (CC) and contact conductivity detection. Two different propionate background electrolytes were used for the sequential determination of cations at pH 3.1 and anions at pH 4.3. ZE was used for the determination of cationic constituents while ZE–ZE approach was employed for the determination of chloride in the first separation channel on the CC microchip and other anionic micro-constituents in the second channel. LOD values were in the range of 0.003–0.012 mg L^?1 and 0.019–0.047 mg L^?1 for cations and anions, respectively. Repeatability of migration time was up to 1.2 % for both cations and anions. Repeatability of peak area ranged from 0.3 to 5.6 % for cations and from 0.6 to 6.0 % for anions. Recovery of both cations and anions was in the range 90–106 %. CSF samples were only diluted appropriately without other sample pretreatment prior to analysis. Developed sequential method is suitable for fast determination of the studied cations and anions in CSF with total analysis time <15 min.     

Sequential Determination of Inorganic Cations and Anions in Cerebrospinal Fluid by Microchip Electrophoresis

Analysis of inorganic ions in cerebrospinal fluid (CSF) is used mainly in the diagnostics of central nervous system diseases, such as Alzheimer’s disease or multiple sclerosis. A new analytical method for fast determination of inorganic cations (ammonium, calcium, magnesium, sodium and potassium) and anions (chloride, sulfate, nitrite and nitrate) in CSF on an electrophoretic microchip was developed in this context. Zone electrophoresis (ZE) separations were performed on the microchip with coupled channels (CC) and contact conductivity detection. Two different propionate background electrolytes were used for the sequential determination of cations at pH 3.1 and anions at pH 4.3. ZE was used for the determination of cationic constituents while ZE–ZE approach was employed for the determination of chloride in the first separation channel on the CC microchip and other anionic micro-constituents in the second channel. LOD values were in the range of 0.003–0.012 mg L⁻¹ and 0.019–0.047 mg L⁻¹ for cations and anions, respectively. Repeatability of migration time was up to 1.2 % for both cations and anions. Repeatability of peak area ranged from 0.3 to 5.6 % for cations and from 0.6 to 6.0 % for anions. Recovery of both cations and anions was in the range 90–106 %. CSF samples were only diluted appropriately without other sample pretreatment prior to analysis. Developed sequential method is suitable for fast determination of the studied cations and anions in CSF with total analysis time <15 min.

     

Application of a contactless conductometric detector for the simultaneous determination of small anions and cations by capillary electrophoresis with dual-opposite end injection

A contactless conductometric detection (CCD) system for capillary electrophoresis (CE) with a flexible detection cell was applied for the simultaneous determination of small anions and/or cations in rain, surface and drainage water samples. The applied frequency, the amplitude of the input signal, the electrolyte conductivity and electrode distance were found to be the most significant factors affecting the detection sensitivity. 2-(N-Morpholino)ethanesulfonic acid/histidine-based (MES/His) electrolytes were used for direct conductivity detection of anions and cations, while ammonium acetate was selected for indirect conductivity determination of alkylammonium salts. For the simultaneous separation procedure, involving dual-opposite end injection, an electrolyte consisting of 20 mM MES/His, 1.5 mM 18-crown-6 and 20 microM cetyltrimethylammonium bromide provided baseline separation of 13 anions and cations in less than 6 min. The detection limits achieved were 7-30 micrograms/l for direct conductometric detection of various common inorganic cations and anions, excluding F- (62 micrograms/l) and H2PO4- (250 micrograms/l), and 35-178 micrograms/l for indirect conductometric detection of alkyl ammonium cations. The developed electrophoretic method with conductometric detection was compared to ion chromatography.     

Rapid, low pressure, and simultaneous ion chromatography of common inorganic anions and cations on short permanently coated monolithic columns

Short permanently coated reversed-phase silica based monolithic columns have been investigated for the rapid separation of inorganic anions and cations. One 2.5 x 0.46 cm column was permanently coated with didodecyldimethylammonium (DDAB), for anion analysis; and a second 5.0 x 0.46 cm column was coated with dioctylsulphosuccinnate (DOSS), for cation analysis. The use of a single combined eluent of 2.5 mM phthalate/1.5 mM ethylenediamine, at flow rates of between 4.0 and 8.0 mL/min, resulted in the rapid separation of 8 anions (in under 100 s) and 5 cations (in under 100 s) on the above columns when used individually, with detection limits for common anions ranging from approximately 0.25 to 5 mg/L, and between 2.5 and 50 mg/L for alkaline earth metals, by direct and indirect conductivity detection, respectively. However, with both columns subsequently connected in parallel, with the eluent delivered using a flow splitter from a single isocratic pump, the simultaneous analysis of anions and cations was also possible, based on a single conductivity detector. The potential of this system for the rapid, complete screening of water samples for multiple common anions and cations is shown.     

Simultaneous separation of inorganic anions and cations by using anion-exchange and cation-exchange columns connected in tandem in ion chromatography

Inorganic anions and cations in environmental waters were determined by ion chromatography. Stationary and mobile phases were examined for the simultaneous separation of both anions and cations. Cations detection by UV detection requires a mobile phase with a UV absorbing additive, which indirectly visualizes cations as negative peaks. Simultaneous separation of anions and cations were achieved when using an eluent that consists of inorganic acid with weak basic amino acid as additives. It was convenient to separate both anions and cations by coupling anion-exchange and cation-exchange columns in tandem. The order of the separation columns connected affected the elution profiles. When the eluent comprises of multiple anions and a single cation, the anion-exchange column should be connected in the upper stream, whereas when the eluent comprises multiple cations and a single anion, the cation-exchange column should be connected in the upper stream. Use of switching valves also allowed simultaneous separation of anions and cations in a single chromatographic run. In the present work, operating conditions were optimized for the simultaneous separation of anions and cations.     

Screen‐Printed Contactless Conductivity Detector for Microchip Capillary Electrophoresis

A new method for mass fabrication of silver ink conductivity detector electrodes for poly(methylmethacrylate) (PMMA) microchip electrophoretic systems has been developed based on screen‐printing technology. Printing of silver conductivity electrodes was performed through a patterned stencil on thin PMMA sheets. Following the electrode fabrication, the PMMA sheets are cut into cover sheets, and are aligned and sealed to the channel plate thus establishing a complete microchip separation device. The effects of the electrode width and spacing on the response and resolution have been investigated and the optimized electrode performance was compared to commonly used aluminum electrodes in the determination of ammonium, methyl ammonium, and sodium. The utility of the screen‐printed contactless conductivity detector (SPCCD) electrodes is further demonstrated for the separation and detection of organic acids with excellent reproducibility (RSD values of 3.7% and 4.1% for oxalate and tartrate, respectively). The thick‐film fabrication of the electrode material demonstrates the ability to mass‐fabricate detection devices with total process of device fabrication requiring less than 4 h (including the fabrication of channel plate, cover sheet with the electrodes, and subsequent bonding). The fabrication method described here is convenient and does not compromise the detector performance, hence offers great promise for producing single use field deployable analytical microsystems.     

Simultaneous detection of monovalent anions and cations using all solid-state contact PVC membrane anion and cation-selective electrodes as detectors in single column ion chromatography

The overall efficiency of ion chromatographic procedures allows the possibility of routine separation and detection of common inorganic and organic anions and cations at low levels in a simultaneous system. A simple and rapid independent separation, and sensitive simultaneous detection of monovalent common anions and cations were achieved using 2 mM copper sulfate, (at pH: 5.40), as eluent with low cell-volume potentiometric detectors. This was established using all-solid state contact, tubular, PVC-matrix membrane anion and cation-selective electrodes in series as detectors with mixed-bed ion-exchange column in ion chromatography. The developed method is reproducible and highly selective to monovalent anions and cations, and takes less than 8 min. Under all operation conditions, the detection limits of the developed method, for potassium, rubidium, cesium, thallium(I), nitrite, nitrate, benzoate and bromide, were of the order of tens of ppb, for sodium, ammonium, chloroacetate, cyanate and chloride ions, values were of the order of hundreds of ppb for an injected volume of 20 mul. The method was flexible since most of anions do not interfere the detection of cations and most of cations do not affect the detection of anions, so that the method can be applied to many sample types e.g. environmental. The application of the method for river, sea and tap water samples were illustrated.     

Floating resistivity detector for microchip electrophoresis

A newly developed conductivity detector, the floating resistivity detector (FRD), for microchip electrophoresis was introduced in this work. The detector design permits decoupling of the detection circuit from the high separation voltage without compromising separation efficiency. This greatly simplifies the integration of microchip electrophoresis systems. Its method of detection relies on platinum electrodes being dipped in two buffer-filled branched detection probe reservoirs on the microchip device. In this way, analytes passing through the detection window will not pass through and subsequently adsorb onto the electrodes, alleviating problems of electrode fouling due to analyte contamination and surface reactions. A customized microchip design was proposed and optimized stepwise for the new FRD system. Each branched detection probe was determined to be 4.50 mm long with a 0.075 mm detection window gap between them. The distance between the detection window and buffer waste reservoir was determined to be 1.50 mm. The optimized microchip design was subsequently used in the analysis of four groups of analytes - inorganic cations, amino acids, aminoglycosides antibiotics, and biomarkers. Based on the preliminary results obtained, the detection limits were in the range of 0.4-0.7 mg/L for the inorganic cations and 1.5-15 mg/L for the amino compounds.     

On-line concentration of s-triazine herbicides in micellar electrokinetic chromatography using a cationic surfactant

A new method for the simultaneous determination of anions (sulfate, nitrate, and chloride) and cations (sodium, ammonium, potassium, magnesium, and calcium) in acid rain waters was investigated using high-performance ion-exclusion/cation-exchange chromatography with conductimetric detection on a separation column packed with a polymethacrylate-based weakly acidic cation-exchange resin in the hydrogen-form and an eluent comprising 1.5 mM sulfosalicylic acid–6 mM 18-crown-6 at pH 2.6, operated at 1.5 ml/min. Effective separation and highly sensitive conductimetric detection for the anions and the cations was achieved in about 14 min. Since the ionic balance (equivalents of anions/equivalents of cations) of acid rain waters of different pH (4.40–4.67) ranged from 0.97 to 0.94, evaluation of the water quality of acid rain was possible. This method was successfully applied to the simultaneous determination of the anions and the cations in acid rain transported from mainland China and North Korea to central Japan monitored by a meteorological satellite data analyzer.     

Derivatization of inorganic ions in capillary electrophoresis

This review gives a short overview of the main approaches to the derivatization of inorganic ions in capillary electrophoresis (CE) with emphasis on the most recent works. Various derivatization procedures and detection methods are discussed. A brief account of their advantages and limitations is given. More specific areas such as microchip CE, simultaneous separation of anions and cations, and speciation analysis are also briefly discussed.     

Carbon paste-based electrochemical detectors for microchip capillary electrophoresis/electrochemistry

The first reported use of a carbon paste electrochemical detector for microchip capillary electrophoresis (CE) is described. Poly(dimethylsiloxane) (PDMS)-based microchip CE devices were constructed by reversibly sealing a PDMS layer containing separation and injection channels to a separate PDMS layer that contained carbon paste working electrodes. End-channel amperometric detection with a single electrode was used to detect amino acids derivatized with naphthalene dicarboxaldehyde. Two electrodes were placed in series for dual electrode detection. This approach was demonstrated for the detection of copper(II) peptide complexes. A major advantage of carbon paste is that catalysts can be easily incorporated into the electrode. Carbon paste that was chemically modified with cobalt phthalocyanine was used for the detection of thiols following a CE separation. These devices illustrate the potential for an easily constructed microchip CE system with a carbon-based detector that exhibits adjustable selectivity.     

Simultaneous ion-exclusion/cation-exchange chromatography of anions and cations in acid rain waters on a weakly acidic cation-exchange resin by elution with sulfosalicylic acid

A simple, selective, and sensitive method for the simultaneous determination of anions (sulfate, nitrate, and chloride) and cations (sodium, ammonium, potassium, magnesium, and calcium) in acid rain waters was developed using ion-exclusion/ cation-exchange chromatography with conductimetric detection. A weakly acidic cation-exchange resin column (Tosho TSKgel OA-PAK-A) and a sulfosalicylic acid-methanol-water eluent was used. With a mobile phase comprising 1.25 mM sulfosalicylic acid in methanol-water (7.5:92.5) at 1.2 ml/min, simultaneous separation and detection of the above anions and cations was achieved in about 30 min. Linear calibration plots of peak area versus concentration were obtained over the concentration ranges 0-1.0 mM for anions (R=0.9991) and 0-0.5 mM for cations (R=0.9994). Detection limits calculated at S/N=3 ranged from 4.2 to 14.8 ppb for the anions and from 2.4 to 12.1 ppb for the cations. The reproducibility of retention times was 0.14-0.15% relative standard deviation (RSD) for anions and 0.18-0.31% for cations, and reproducibility of chromatographic peak areas was 1.22-1.75% RSD for anions and 1.81-2.10% for cations. The method was applied successfully to the simultaneous determination of anions and cations in aerosols transported from mainland China to central Japan, as determined by a meteorological satellite data analyzer.     

Simultaneous separation of inorganic anions and cations by capillary zone electrophoresis

A new capillary electrophoretic approach for simultaneous separation of fast anions and cations is demonstrated. Indirect UV detection at 214 nm in conjunction with electromigration sampling from both ends of the capillary was developed. Two electrolyte systems based on imidazole-nitrate and copper(II)-ethylenediamine-nitrate were investigated for the simultaneous separation of chloride, sulphate, hydrocarbonate, potassium, ammonium, calcium, sodium and magnesium ions. Experimental parameters that were evaluated included a nature of UV chromophore, pH of electrolyte, a nature of complexing agent. The method permits the excellent separation of three anions and five cations in only 4 min using electrolyte system containing 2.5 mmol l⁻¹ Cu(NO₃)₂, 5 mmol l⁻¹ ethylenediamine and 1 mmol l⁻¹ fumaric acid at pH 8.5 adjusted with tetraethylammonium hydroxide.     

A flow injection-capillary electrophoresis system with high-voltage contactless conductivity detection for automated dual opposite end injection

The system comprises two flow injection-capillary electrophoresis interfaces into which the opposite ends of the separation capillary are inserted. The electrolyte solution flows through both interfaces by use of hydrostatic pressure. The injection of the samples into the electrolyte flow is accomplished by a rotary-type chromatographic valve at the grounded side and by a pinch-valve injector at the high-voltage side that provides sufficient isolation from the high electric field. The system allows a fully automated dual-injection sequence of samples from both capillary ends and simultaneous electrophoretic separation of anions and cations in the samples. The analytes are detected by a high-voltage contactless conductometric detector positioned approximately in the middle of the separation capillary. The parameters of the system were evaluated. The repeatability of the flow injection-capillary electrophoresis system for the simultaneous determination of anions and cations was evaluated for ten consecutive injections and relative standard deviation (RSD) values for peak areas were better than 1.0%. The sample throughput for total ionic analysis was estimated to be 25 samples per hour. The system was used for automated simultaneous analysis of anions and cations in various real samples. Using a short separation capillary, rapid total ionic analysis in less then 1 min is demonstrated.     

Simultaneous separation of inorganic anions and cations using capillary electrophoresis with a movable contactless conductivity detector

A number of small inorganic anions and cations were separated after injection of the sample into both ends of a separation capillary. The ions were detected using a capacitively coupled contactless conductivity detector (CCCCD) which could be placed at various positions along the capillary length. Counter-directional migration of anions and cations occurs towards the detector, which is placed at an appropriate position along the capillary so that the migration order is determined by the respective effective separation capillary lengths for both anions and cations. As the CCCCD detector can be easily moved to any position along the capillary, virtually any effective separation length can be attained. Depending on the number of analytes in the sample, one can choose to obtain either electropherograms with inter-migrating zones of cations and anions or separations with distinct regions of anion and cation zones, respectively. A new term 'apparent separation selectivity' is introduced to describe the manner in which the position of the detector can be varied in order to determine the final separation.     

Determination of inorganic selenium species by miniaturised isotachophoresis on a planar polymer chip

The use of miniaturised isotachophoresis to allow the simultaneous determination of two inorganic selenium species has been investigated using a poly(methyl methacrylate) chip with a 44-mm-long, 200-microm-wide, 300-microm-deep separation channel. The miniaturised device included an integrated on-column, dual-electrode conductivity detector and was used in conjunction with a hydrodynamic fluid transport system. A simple electrolyte system has been developed which allowed the separation of selenium(IV) and selenium(VI) species to be made in under 210 s. The limits of detection were calculated to be 0.52 mg L(-1) for selenium(IV) and 0.65 mg L(-1 )for selenium(VI). The method allowed the separation of the selenium species from a range of common anions including fluoride, nitrate, nitrite, phosphate, sulfate and sulfite.     

Fluorinated ethylenepropylene copolymer as a potential capillary material in CE

In this work, a new generation UV-transparent polymer, fluorinated ethylenepropylene copolymer (FEP) exhibiting a low degree of crystallinity, extruded in dimensions similar to the most commonly used CE capillaries of approximately 80 mum id and about 360 mum od was investigated for its use as a CE capillary. FEP is transparent down to the low-UV region, and as fluorinated polymers in general are good electrical insulators and exhibit reasonable heat conductivity, it has considerable potential as a material for electrodriven analysis in capillary or microchip formats. The FEP capillary has been characterised with regard to some important aspects for its use as a CE capillary, including its profile of EOF versus pH, as well as procedures for manipulating EOF by coating the inner capillary wall with various semipermanent and dynamic layers. The FEP capillaries were tested and compared with fused-silica capillary for the separation of inorganic and small organic ions using conditions involving direct and indirect detection in the low-UV region. Finally, advantages of the use of the FEP capillary for simultaneous detection of a mixture containing nine inorganic cations and anions using indirect photometric detection with a movable light-emitting diode (LED) detector and a novel electrolyte are demonstrated.