Effects of the cell geometry and operating parameters on the performance of an external contactless conductivity detector for microchip electrophoresis

Quantitative data on the effect of the electrode geometry on the signal strength and the signal-to-noise ratio is given. The measurements are affected by the unavoidable presence of stray capacitance. Best results are achieved for short and narrow electrodes arranged in an antiparallel configuration and separated by a minimal gap, which determines the dimensions of the actual detection volume. Limits of detection between 150 and 250 microg l(-1) and separation efficiencies from 13,000 to 17,000 theoretical plates were achieved for six inorganic cations (NH(4)(+), K(+), Ca(2+), Na(+), Mg(2+)and Li(+)) with electrodes of 1 mm width and a detection gap of 0.5 mm (separation channel length: 7.5 cm) when operating the detector at 20 V(pp) and 500 kHz. The analyses of all major inorganic cations in tap and rain water samples were demonstrated for the first time in microchip electrophoresis with contactless conductivity detection.     

The Effects of the Electrode System Geometry on the Properties of Contactless Conductivity Detectors for Capillary Electrophoresis

The basic analytical parameters of contactless conductivity detectors with planar, semi‐tubular and tubular electrodes have been compared. It has been found that the differences in the analytical parameters of the detectors are not significant for analytical use. The mean values of baseline peak‐to‐peak noise of 0.27, 0.35 and 0.33 mV, sensitivities of 0.97, 2.08 and 2.27 mV/pg (for K⁺ ion), limits of detection of 0.93, 0.65 and 0.53, and the heights equivalent to a theoretical plate of 2.83, 2.39 and 2.40 μm were obtained for the detectors with planar, semi/tubular and tubular electrodes, respectively. Modifications of the basic detectors, namely a detector with thinned capillary wall and planar electrodes, and a detector with semi‐tubular electrodes placed one against the other on the opposite sides of the capillary were also tested. The configuration with the electrodes placed one against the other permits detector construction with zero gap between the electrodes without increasing the noise; when the electrodes overlap, the detector begins to operate as a permittivity detector.     

Comparison of different conductivity detector geometries on an isotachophoresis PMMA-microchip

Conductivity detection is one of the most often employed means of detection in isotachophoresis. In microanalytical devices, thin-film platinum electrodes can be used for conductivity detection and for other electrochemical methods of detection. The design and the performance of different electrode geometries for on-column contact conductivity detection with thin-film platinum electrodes integrated on an isotachophoresis PMMA-microchip is described. Three different electrode geometries for direct conductivity detection were used for the investigation of isotachophoretic separations. The influence of the width of the electrodes and their positioning relative to the separation channel was investigated. The performance of the different detectors is compared for the analysis of organic carboxylic acid anions.     

Amperometric detector designs for capillary electrophoresis microchips

Electrochemical (EC) detection is a sensitive and miniaturisable detection mode for capillary electrophoresis (CE) microchips. Detection cell design is very important in order to ensure electrical isolation from the high separation voltage. Amperometric detectors with different designs have been developed for coupling EC detection to CE-microchips. Different working electrode alignment: in-channel or end-channel has been tested in conjunction with several materials: gold, platinum or carbon. The end-channel detector was based on a platinum or gold wire manually aligned at the exit of the separation channel. Thick- (screen-printed carbon electrode) and thin-film (sputtered gold film) electrodes have also been employed with this configuration, but with a different design that allowed the rapid replacement of the electrode. The in-channel detector was based on a gold film within the separation channel. A gold-based dual electrode detector, which combined for the first time in- and end-channel detection, has been also tested. These amperometric detectors have been evaluated in combination to poly(methylmethacrylate) (PMMA) and Topas (thermoplastic olefin polymer of amorphous structure) CE-microchips. Topas is a new and promising cyclic olefin copolymer with high chemical resistance. Relevant parameters of the polymer microchip separation such as precision, efficiency or resolution and amperometric detection were studied with the different detector designs using p-aminophenol and L-ascorbic acid as model analytes in Tris-based buffer pH 9.0.     

A simplified poly(dimethylsiloxane) capillary electrophoresis microchip integrated with a low-noise contactless conductivity detector

A contactless conductivity detector integrated into a poly(dimethylsiloxane) microchip for electrophoresis is presented. It adopted the simplest configuration of electrodes commonly used in this detection mode for capillary electrophoresis microchips. Although the chip is based on a simple and effective design, it is able to obtain low detection levels due to the low noise of the detection circuit. A circuit based on a lock-in amplifier was designed on printed circuit boards to read out the signal. The property of the detection cell was studied by applying excitation signals of different frequencies and different amplitudes. It was found that the best detection limit could be achieved with a frequency of 50?kHz and amplitude of 20?V. The performance of the detector was demonstrated by successfully separating and detecting several inorganic ions and also a mixture of heavy metal ions. An average detection limit of 0.4?μM was obtained for inorganic cations. This value is significantly improved compared to similar microchip-based detectors. The presented detector could be promising for mass production due to its properties, such as simple construction, high degree of integration, high performance and low cost.     

Fundamental aspects of contactless conductivity detection for capillary electrophoresis. Part I: Frequency behavior and cell geometry

A better understanding of the characteristics of the axial contactless conductivity cell could be obtained by carefully studying the effect of the cell geometry on its frequency behavior. A good fit between theoretical and experimental results shows that the axial contactless conductometric detector can effectively be described by the simplest possible equivalent circuitry consisting of a capacitor, resistor, and a second capacitor. The cell constant is largely defined by the length of the gap between the electrodes. The effective electrode size is thus not related to the dimensions of the real electrodes but more closely to the cross-sectional area of the internal diameter of the capillary. Typical experimental values of 20 MOmega and 0.1 pF were obtained for the resistance and capacitances, respectively, of a cell formed by a 2 mm gap between two 4 mm long electrodes fitted with a capillary of 50 microm ID. It could be shown that the diameter of the electrode is not critical and tight coupling of the electrodes to the outer wall of the capillary is not needed. The peak overshoot phenomenon, which has frequently been reported, is an artefact that can be minimized by optimizing the frequency for cell excitation. The frequency setting has to be optimized for each cell design, operational amplifier, electrolyte solution and capillary.     

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.     

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 radio-frequency moving-boundary detector

A new type of moving-boundary detector is described in which boundaries are detected at platinum ring electrodes situated on the outside of a moving-boundary tube. These rings are part of a bridge circuit operating at radio frequencies. Impedance changes in the region of the rings are converted by a phase-sensitive detector to a voltage output which, when operating in the differential mode, takes the form of a sharp peak as a boundary passes through the rings. Boundary displacements of less then 20 m can be detected, and a precision of better than 0.02% is obtained in the measurement of boundary velocities in typical transference measurements. The fact that only three electrical leads are required in the detecting circuit gives this detector a decided advantage for high-pressure work. Equally important is the elimination of electrode polarization problems associated with other types of electrical detectors.Taken, in part, from a Ph.D. thesis, Carnegie-Mellon University, 1971.     

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.     

Optimization of the high-frequency contactless conductivity detector for capillary electrophoresis

Two constructions of the high-frequency contactless conductivity detector that are fitted to the specific demands of capillary zone electrophoresis are described. The axial arrangement of the electrodes of the conductivity cell with two cylindrical electrodes placed around the outer wall of the capillary column is used. We propose an equivalent electrical model of the axial contactless conductivity cell, which explains the features of its behavior including overshooting phenomena. We give the computer numerical solution of the model enabling simulation of real experimental runs. The role of many parameters can be evaluated in this way, such as the dimension of the separation channel, dimension of the electrodes, length of the gap between electrodes, influence of the shielding, etc. The conception of model allows its use for the optimization of the construction of the conductivity cell, either in the cylindrical format or in the microchip format. The ability of the high-frequency contactless conductivity detector is demonstrated on separation of inorganic ions.     

Enhanced particle trapping performance of induced charge electroosmosis

By increasing the number of floating electrodes or enlarging the width of single floating electrode, this work provides effective ways to strongly improve the particle trapping performance of induced charge electroosmosis (ICEO). Particle trapping with double or triple separate narrow floating electrodes increases the effective actuating range of ICEO flow and therefore enhance the optimum trapping ability to be 1.63 or 2.34 times of that with single narrow electrode (width of ), and the ideal trapping frequency is independent of the electrode number due to the mutual independence of electrochemical ion relaxation over each electrode. Furthermore, using a single wide floating electrode with the effective width equal to three separate narrow floating electrodes () instead of a single narrow one slightly lowers the ideal trapping frequency due to an increase in the characteristic polarization length, but the trapping performance is only up to 1.59 times of that with original single narrow electrode, implying that vertical channel confinement effect may severely suppresses the effective actuating range of ICEO flow and renders the trapping performance not as expected. Trapping experiments over wide floating electrode with different channel height were carried out, showing that the trapping performance increases by correctly increasing the channel height.     

Improved dual photometric-contactless conductometric detector for capillary electrophoresis

A new design of a dual, UV photometric - contactless conductometric detector is described. The separation capillary with an optical window created is pressed onto two semitubular electrodes, 3 mm wide and 2 mm apart. The electrodes form the detection cell of the contactless conductometric detector. An optical fiber, placed in the gap between the conductometric electrodes, brings radiation from the source. The radiation that passes through the separation capillary is recorded by a large-area photodiode. The optical fiber and the photodiode operate the photometric cell which is between the conductometric electrodes. The detector thus permits simultaneous photometric and conductometric detection in the same place of the capillary, while exchanging of the separation capillary is easy and without effect on the detector geometry and performance.     

An electrochemical detector cell for open tubular liquid chromatography and capillary electrophoresis

Summary An electrochemical detector cell has been developed for micro-flow separation systems (OTLC, CE). The cell contains two electrodes, a disk-shaped working electrode made from a carbon fiber bundle, and a tubular Ag/AgCl quasi-reference electrode. The effective cell volume and the coulometric yield have been determined, for different electrode diameters and at different flow rates, in an OTLC system. An effective cell volume of less than 1 nl was observed. The applicability of the cell was demonstrated with the detection of OPA-derivatized amino acids. For use in CE, the cell was equipped with an additional compartment, housing a semi-permeable joint for the decoupling of the high electric field used for the electrophoretic separation. Results are shown on the determination of catecholamines by CE with electrochemical detection. Detection limits with both OTLC and CE were well below 1 fmole.     

Determination of lead in blood by hydrodynamic voltammetry in a flow injection system with wall-jet detector

Lead is one of the most widely distributed toxic heavy metals in the environment. It is a cumulative poison, affecting the brain and nervous system. The threshold between the normal lead level and the level where physiological effects become manifest is relatively narrow. It is therefore desirable to screen exposed populations in order to identify the danger in time. The lead concentration in the blood is a measure to the total amount of lead in the body. A fast, accurate and cheap method for the determination of lead in blood is therefore needed. The conventional method used to determine lead in blood is atomic absorption. Electrochemical methods like stripping voltammetry combine high analytical sensitivity with relatively low cost for the equipment; however, electrode preparation is critical for the success of an analysis, and highly skilled personnel are needed. We describe an automated electrochemical method, using flow injection analysis with a wall-jet detector. Lead is released from its binding site in the blood by ion exchange and quantified by stripping voltammetry with a mercury film electrode (MFE). The method allows for the detection of 0.05 ppm Pb(2+) with an accuracy of about 10%. Electrode poisoning by proteins from the blood can be effectively suppressed when a MFE modified with a Nafion(R)-membrane coating is used. Such modified electrodes can be activated in the solution without further treatment, and used for more than 100 analyses before they have to be replaced. A solid matrix MFE with a Nafion(R)-membrane and all necessary chemicals for mercury film formation and lead release has been developed. Such electrodes are discussed as disposable electrodes for a portable blood lead detector.     

Ion chromatography detector based on solid-state ion-selective electrode array

A variety of neutral carrier type ionophores for monovalent cations were employed to prepare solid-state cation-selective electrodes (SSEs) for use as a detector in single-column ion chromatography (IC). The polyurethane-based pseudoreference electrode made it possible to assemble an array type SSE detector for IC. An SSE-based detector provides not only the overall chromatogram for the separated ion species (monensin methyl ester-nonactin-based membrane), but also the enhanced chromatogram for specified ions of interest (valinomycin as K+ and nonactin for NH4+). This feature makes it possible to perform highly quantitative analysis with low detection limits even if the separation efficiency of the ion-exchange is not sufficient. Since SSE-based IC detectors are easily miniaturized and replaceable at low cost, they are an ideal component of a portable IC system.     

常规离子色谱系统-电容耦合非接触式电导检测器的构建

通过考察电极长度及电极间距、检测管管径及材质、激励信号频率、电压和波形等参数对信噪比的影响,研制了一种适用于常规型离子色谱系统的电容耦合非接触式电导检测器（C⁴D）。在抑制模式下,该检测器对常见无机阴离子（F^-、Cl^-、NO₂^-、Br^-、NO₃^-和SO₄^2-）的检出限（信噪比=3）为0.02~0.08 μmol/L;峰面积的相对标准偏差<1.8%（n=6）;在0.1~10 μmol/L范围内上述6种无机阴离子线性关系良好,相关系数（R²）>0.999。自制C⁴D的主要性能参数与商品化接触式电导检测器相当。该检测器具有结构简单、成本低廉、无电极污染等优点,有利于拓展离子色谱的应用范围。     

Analyzing the response of a contactless conductivity detector in capillary electrophoresis by a resonant method

We report a resonant method to measure the wall capacitance(C_w) and solution resistance(R_S) in a capacitively coupled contactless conductivity detector(C~4D).Under the typical operating conditions in capillary electrophoresis(I.D.50μm,O.D. 360μm,electrode length of 4 mm,electrode gap of 1 mm,frequency of 200 kHz),the values of C_w measured in 1 and 20 mmol/L NaCl solutions are 2.8 and 32 fF,which are only 1.1%and 12%of prediction by the equation in references,respectively.The value of R_S is less than the prediction in solutions withκ<0.02 S/m.The response current of C~4D is due to the change in C_w because the total impedance of a C~4D is composed mainly by the impedance from C_w.     

Characteristics of NaI detector in positron imaging device HEADTOME employing circular ring array

In positron emission computed tomographs employing circular ring arrays of detectors, the performance of the imaging device has been specified ultimately by the characteristics of the detector. The responses of NaI detector were studied when detecting positron annihilation photon (511 keV). The study was mainly by using the NaI detector used in hybrid emission computed tomography (CT) "HEADTOME" we had developed. A series of measurements were carried out positioning two detectors with 40 cm distance and scanning 22Na point source in water. Both detectors was inclined from 0 degrees through 30 degrees to change incident angle of positron annihilation toward crystal face. Energy window was set from 100 to 700 keV. The results were presented as follows: Shortening the crystal length from 7 to 5 cm made sensitivity decrease about 10% and resolution deteriorate about 1 mm (FWHM). As the results of varying the width of the crystal, 20 mm width was optimal at any incident angle. The lead septum between the detectors was the thickness of 4 mm enough to reject multiple detector interactions (crosstalk). Beam mask which was made of lead in order to improve spatial resolution and placed on crystal face worked effectively for incident angles from 0 degrees to 15 degrees but degraded uniformity of spatial resolution from 0 degrees to through 30 degrees.     

3D nanogap interdigitated electrode array biosensors

Three-dimensional interdigitated electrodes (IDEs) have been investigated as sensing elements for biosensors. Electric field and current density were simulated in the vicinity of these electrodes as a function of the electrode width, gap, and height to determine the optimum geometry. Both the height and the gap between the electrodes were found to have significant effect on the magnitude and distribution of the electric field and current density near the electrode surface, while the width of the electrodes was found to have a smaller effect on field strength and current density. IDEs were fabricated based on these simulations and their performance tested by detecting C-reactive protein (CRP), a stress-related protein and an important biomarker for inflammation, cardiovascular disease risk indicator, and postsurgical recuperation. CRP-specific antibodies were immobilized on the electrode surface and the formation of an immunocomplex (IC) with CRP was monitored. Electrochemical impedance spectroscopy (EIS) was employed as the detection technique. EIS data at various concentrations (1 pg/mL to 10 μg/mL) of CRP spiked in buffer or diluted human serum was collected and fitted into an equivalent electrical circuit model. Change in resistance was found to be the parameter most sensitive to change in CRP concentration. The sensor response was linear from 0.1 ng/mL to 1 μg/mL in both buffer and 5% human serum samples. The CRP samples were validated using a commercially available ELISA for CRP detection. Hence, the viability of IDEs and EIS for the detection of serum biomarkers was established without using labeled or probe molecules.