Article

Comparison of Different Contactless Conductivity Detectors for the Determination of Small Inorganic Ions by Capillary Electrophoresis

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Abstract

The analytical performance parameters of four different contactless conductivity detectors for capillary electrophoresis were determined. These detectors were designed either with a miniature cell for versatility, with high voltage excitation for high sensitivity or with a battery power supply for field application. One of the units is commercially available. The plate numbers and reproducibility of peak areas (typically about 2%) were very similar, indicating that these parameters are generally limited by the separation and injection procedures rather than the detectors. The linear dynamic ranges are better than 2 orders of magnitude for all detectors and the correlation coefficients were also almost identical. Significant differences were found with regard to the detection limits. For the detector with a miniature cell, which lacks an in-cell amplifier, the detection limit was typically 1.5 μM for the inorganic ions tested (K+, Ca2+, Na+, Mg2+, Li+, Cl−, NO3− and SO42−), while with the other 3 detectors this parameter was as low as about 0.1 μM. The use of buffer solutions with relatively high background conductivity was found to lead to detection limits which were up to one order of magnitude higher. The extent of deterioration of this parameter with buffer conductivity was found to be related to the excitation voltage and was most pronounced for the high voltage detector.

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... The LODs and S/Ns obtained are listed in Table 1. The 3DP detector showed comparable LODs (Table S3) to other benchmark oncapillary detectors [49,50] using the same id capillary. The signal outputs by C 4 D and PD were compared (Fig. S3). ...
... Detectors with single detection can also be printed by using the strategy in building C 4 D or PD. The twoin-one detector's LOD values were greater than those of the benchmark detectors [49,50]. The use of high conductance 3DP material(s) should provide more effective shielding and improve C 4 D performance. ...
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Piezo-catalysis was first used to degrade a non-dye pollutant, 4-chlorophenol (4-CP). In this process, hydrothermally synthesized tetragonal BaTiO3 nano/micrometer-sized particles were used as the piezo-catalyst and the ultrasonic irradiation with low frequency was selected as the vibration energy to cause the deformation of tetragonal BaTiO3. It was found that the piezoelectric potential from the deformation could not only successfully degrade 4-chlorophenol, but also effectively make it dechlorination at the same time. And five kinds of dechlorinated intermediates, hydroquinone, benzoquinone, phenol, cyclohexanone, cyclohexanol, were determined. This is the first sample of piezo-dechlorination. Although various active species, including h+,e-,•H,•OH,O2•-,1O2 and H2O2, were generated in the piezoelectric process, it was confirmed by ESR, scavenger studies and LC-MS that the degradation and dechlorination were mainly attributed to •OH radicals. These •OH radicals was chiefly derived from the electron reduction of O2, partly from the hole oxidation of H2O. These results indicated that the piezo-catalysis was an emerging and effective advanced oxidation technology for degradation and dechlorination of organic pollutants.
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The analysis of ionic content of exhaled breath condensate (EBC) from one single breath by CE with C4D is demonstrated for the first time. A miniature sampler made from a 2 ml syringe and an aluminum cooling cylinder for collection of EBC was developed. Various parameters of the sampler that influence its collection efficiency, repeatability and effect of respiratory patterns were studied in detail. Efficient procedures for the cleanup of the miniature sampler were also developed and resulted in significant improvement of sampling repeatability. Analysis of EBC was performed by CE-C4D in a 60 mM MES/L-Histidine BGE with 30 μM CTAB and 2 mM 18-crown-6 at pH 6 and excellent repeatability of migration times (RSD < 1.3% (n = 7)) and peak areas (RSD < 7% (n = 7)) of 12 inorganic anions, cations and organic acids was obtained. It has been shown that the breathing pattern has a significant impact on the concentration of the analytes in the collected EBC. As the ventilatory pattern can be easily controlled during single exhalation, the developed collection system and method provides a highly reproducible and fast way of collecting EBC with applicability in point-of-care diagnostics.This article is protected by copyright. All rights reserved
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A novel approach for diagnosis of cystic fibrosis is presented. A simple and fast procedure to obtain sweat sample was developed. It consists of repeatedly wiping the skin of the forearm with deionized water moisturized cotton swab and extraction in 1mL of deionized water. Double opposite end injection capillary electrophoresis with contactless conductometric detection is used for the analysis of the extract. Chloride, sodium and potassium as the three target ions that participate in the ion transfer across the cellular membranes, and are affected by CF, are simultaneously determined in approximately 3min in a background electrolyte containing 20mM 2-(N-morpholino)ethanesulfonic acid, 20mM l-histidine and 2mM 18-crown-6. By using the target ion ratios rather than the concentrations of each individual ion combined with principal component analysis, the diagnosis of CF can be made more accurately and greatly reduce the number of false positive or negative results as is often the case when single ion (chloride) is analyzed.
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Chapter
This chapter describes the theoretical fundamentals of the capacitively coupled contactless conductivity detection (C4D) applied to capillary electrophoresis (CE) and microchip electrophoresis (MCE). In addition, the instrumental advances and strategies reported in the last 10 years to integrate detection electrodes for C4D measurements on MCE have been presented and discussed. Finally, this chapter covers a wide range of applications in both CE and MCE systems including bioanalytical and analytical assays, on-chip enzymatic reactions, food analysis, explosive and chemical warfare agents.
Article
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Contactless conductivity detection is universal for CE in that all charged species can be quantified, and it is particularly attractive for those inorganic and organic ions that are not directly accessible by optical means. It is not necessary to make precise alignments or to create an optical window into the capillary. Commercial detectors that can be retrofitted to existing instruments are available at a cost that is lower than that of any other electrophoresis detector. The approach is also suited for lab-on-chip devices. It is attempted in this review to summarize some of the expertise accumulated since the introduction of the axial contactless conductivity detector to CE 10 years ago.
Article
Recent advances and key strategies in capillary electrophoresis and microchip CE with electrochemical detection (ECD) and electrochemiluminescence (ECL) detection are reviewed. This article consists of four main parts: CE-ECD; microchip CE-ECD; CE-ECL; and microchip CE-ECL. It is expected that ECD and ECL will become powerful tools for CE microchip systems and will lead to the creation of truly disposable devices. The focus is on papers published in the last two years (from 2005 to 2006).
Article
This review points out some important trends in the development of the detection techniques for small ions in CE. On the basis of selected literature references it briefly discusses some general requirements on detection techniques in CE. Various optical measurements, mass spectrometric approaches and electrochemical detection techniques are dealt with. Some specific features of microchip CE separation and detection are pointed out and possibilities of dual detection are mentioned. The principal parameters of the above detection techniques are then briefly compared.
Article
Capacitively coupled contactless conductivity detection (C(4)D) in the axial electrode configuration was introduced in 1998 as a quantification method for capillary electrophoresis. Its universality allows the detection of small inorganic ions as well as organic and biochemical species. Due to its robustness, minimal maintenance demands and low cost the popularity of this detector has been steadily growing. Applications have recently also been extended to other analytical methods such as ion chromatography, high-performance liquid chromatography and flow-injection analysis. C(4)D has also found use for detection on electrophoresis based lab-on-chip devices. Theoretical aspects of C(4)D in both the capillary and microchip electrophoresis format have been comprehensively investigated. Commercial devices are now available and the method can be considered a mature detection technique. In this article, the achievements in C(4)D for the time period between September 2004 and August 2007 are reviewed.
Article
The separation and detection of small oligopeptides in CE with contactless conductivity detection were demonstrated. A strongly acidic separation buffer (0.5 M acetic acid) was employed in order to render the species cationic. Separation of the stereoisomers was achieved in typically 10-15 min by using either dimethyl-beta-CD (DM-beta-CD), (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18C(6)H(4)), a combination of the two substances, or of histidine, as buffer additives. Calibration curves were determined for isomers of Gly-Asp and H-Pro-Asp-NH(2), in the range of 0.05-0.5 mM and 0.1-1 mM, respectively, and were found to be linear. LODs were determined to be in the order of 1.0 microM. The determination of isomeric impurities down to about 1% was found possible. Species showing good separation could also be successfully determined on an electrophoretic lab-on-chip device, with analysis times of a few minutes.
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Although simple equivalent circuits have been used to explain the basic functioning of a capacitively coupled contactless conductivity detector (C4D), more sophisticated models are required to take into account the effects of the spatial non-homogeneity of the solution conductivity as the electrophoretic zones pass inside the detector. The overshooting phenomenon observed in real electropherograms may be explained by modeling the coupling of the electrodes with the inner capillary with a network of resistors and capacitors and its dependence with the stray capacitance becomes evident. An even more detailed model of the cell based on electrostatics allows one to calculate the stray capacitances. For the typical geometries and materials, this capacitance is on the order of a few to hundreds of femtofarads. It was possible to demonstrate that the ground plane, sometimes used, reduces the capacitance, but does not eliminate it completely. Possible noise sources are also discussed. The electrode tightness minimizes a possible source of mechanical noise due to variation of the coupling capacitances. Thermal control should also be ensured; the calculations showed that a temperature fluctuation as low as 7×10−3 °C induces artifacts as high as the limit of quantification of K+ in a typical electrophoretic condition, for which the technique has one of its highest sensitivities.
Article
Modifications to a high-frequency contactless detector are described. Its properties (reproducibility, stability and dependence of the sensitivity on the concentration or conductivity and relative permittivity of the measured solutions) are demonstrated and its applicability to capillary isotachophoresis is illustrated on the example of a separation of a test mixture.
Article
A new detection system for isotachophoresis, the high-frequency contactless conductivity detector, is described. This detector has a high resolving power and gives good reproducibility.
Article
The new features of the capacitively coupled contactless conductivity detector for capillary electrophoresis described are higher peak-to-peak excitation voltages for the detector cell of up to 250 V, a pick-up amplifier in close proximity to the electrode and synchronous detection. The electrical performance of the cell was characterized and found to follow readily predictable patterns. The alterations led to a higher signal strength, a better signal-to-noise ratio (S/N) and improved stability. The 3 × S/N detection limits obtained for inorganic cations and anions are in the range 0.1–0.2 μM. For the indirect detection of model compounds of organic cations and anions (aliphatic amines and sulfonates), detection limits of typically 1 μM were achieved.
Article
An oscillometric detector for capillary electrophoresis (CE) has been described. Two 2-mm silver rings separated by 1 mm were painted over the polyimide coating of a fused-silica capillary (75-μm i.d. and 360-μm o.d.) and used as electrodes for oscillometric measurements. A function generator was used to apply a sinusoidal signal over one of the electrodes; the other one was connected to a current-to-voltage converter. The rectified signal is proportional to the admittance of the cell, which is a function of the inner solution conductivity in the region of the electrodes. Electropherograms of alkaline and alkaline-earth cations showed good signal-to-noise ratio. For typical electrophoretic conditions, the limit of detection for lithium was 1.5 μM, and there was good linearity (R = 0.998 for eight data points) up to 2 mM. Indirect conductivity detection of quaternary ammonium salts was achieved by using potassium acetate running buffer, showing results similar to those from conventional conductometric detectors. Despite the cell length (5 mm), good resolution was obtained in the electropherograms. Equivalent electrical circuits were proposed for the cell. The most simplified model comprises a resistor−capacitor couple in parallel with another capacitor. The resistor stands for the inner solution resistivity, the series capacitor stands for the fused-silica wall dielectric properties in the region between the electrodes and the solution, and the parallel capacitor stands for the leakage through the wall and edge capacitance effects.
Article
Very simple contactless conductivity cells have been designed, suitable for detection in flow analytical techniques (classical HPLC, FIA, SIA, continuous flow, etc.) in which this detection has so far rarely been used. The cells consist of two wire electrodes covered with a very thin layer (units of μm) of an insulating material (the dielectric) and punctured through the walls of a plastic tubing at a close mutual distance (from tenths to units of a mm) and placed either across the tube or along the tube axis, so that the detection occurs within the space limited by the two electrodes. As the thickness of the dielectric layer is substantially smaller than that in common contactless conductivity cells used in CE, the AC current flowing between the electrodes is higher and the measuring sensitivity is enhanced. The principal operational characteristics of these detectors have been tested using KCl solutions and compared with those calculated from a simple theoretical model. It has been shown that this model permits theoretical description of these cell types and reasonable prediction of their behavior. Practical application has been demonstrated on a FIA determination of inorganic carbonate.
Article
Capacitively coupled contactless conductivity detection (C4D) is presented in a progressively detailed approach. Through different levels of theoretical and practical complexity, several aspects related to this kind of detection are addressed, which should be helpful to understand the results as well as to design a detector or plan experiments. Simulations and experimental results suggest that sensitivity depends on: 1) the electrolyte co-ion and counter-ion; 2) cell geometry and its positioning; 3) operating frequency. Undesirable stray capacitance formed due to the close placement of the electrodes is of great importance to the optimization of the operating frequency and must be minimized.
Article
The popularity of contactless conductivity detection in capillary electrophoresis has been growing steadily over the last few years. Improvements have been made in the design of the detector in order to facilitate its handling, to allow easy incorporation into available instruments or to achieve higher sensitivity. The understanding of its fundamental working principles has been advanced and the detection approach has also been transferred to lab-on-chip devices. The range of applications has been extended greatly from the initial work on small inorganic ions to include organic species and biomolecules. Concurrent determination of cations and anions by dual injection from opposite ends has been demonstrated as well as sample introduction by using flow-injection systems for easy automation of the process.
Article
The prototype of a field-portable battery-powered capillary electrophoresis instrument described here includes a high voltage supply capable of delivering the standard 30 kV at both polarities. The instrument has dimensions of 340 mm×175 mm×175 mm (w×h×d) and a weight of 7.5 kg. Data acquisition is carried out with a portable laptop or palmtop computer. Electrochemical detection was chosen as the straightforward signal transduction methods are relatively easily implemented. For robustness, the amperometric and potentiometric detection modes are carried out in a fixed wall-jet cell without decoupler. Both methods rely on the electrophoretic ground electrode as reference and counter electrode. For conductometric detection the only recently reported contactless version was implemented. The availability of the three complementary electrochemical detection methods allows for great versatility, which includes the ability to determine inorganic cations and anions as well as many organic species of interest.
Article
A contactless capacitively coupled conductivity detector for capillary electrophoresis is introduced. The detector consists of two electrodes which are placed cylindrically around the outer polyimide coating of the fused-silica capillary with a detection gap of 2 mm. The electrodes form a cylindrical capacitor, and the electric conductivity of the solution in the gap between the electrodes is measured. A high audio or low ultrasonic frequency for coupling of the ac voltage is used in order to minimize the influence of reactance of the liquid. For an improved version of the detector, two syringe cannulas are used as the electrodes and the capillary is simply assembled into the tubing. This allows an easy placement of the detector on various positions along the capillary. The limit of detection of inorganic cations and anions is 200 ppb, as determined for sodium and chloride, respectively.
Article
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.
Article
The detection of alkali, alkaline earth and heavy metal ions with a high-voltage capacitively coupled contactless conductivity detector (HV-C(4)D) was investigated. Eight alkali, alkaline earth metal ions and ammonium could be separated in less than 4 min with detection limits in the order of 5 x 10(-8) M. The heavy metals Mn2+, Pb2+, Cd2+ Fe2+, Zn2+, Co2+, Cu2+ and Ni2+ could also be successfully resolved with a 10 mM 2-(N-morpholino)ethanesulfonic acid/DL-histidine (MES/His)-buffer. Zn2+, Co2+, Cu2+ and Ni2+ showed an indirect response. The detection limits for the heavy metals were determined to range from about 1 to 5 microM.
Article
Potentiometric detection is rarely used in separation methods but is promising for certain classes of analytes which can only with difficulty be quantified by more standard methods. Conductimetric detection of ions is very versatile and has recently received renewed interest spurned by the introduction of the capacitively coupled contactless configuration. Both are useful and complementary alternatives to the established optical detection methods, and to the more widely known electrochemical method of amperometry. The simplicity of the electrochemical methods makes them particularly attractive for microfabricated devices, but relatively little work has to date been carried out with regard to potentiometric and conductimetric detection.
Article
A contactless conductivity detector integrated into the capillary cassette of Agilent (3D)CE equipment is described. The detector is user-friendly, compact and easily modified. The UV detector of the (3D)CE equipment is available parallel with the contactless conductivity detector increasing the detection power. Two electrolyte solutions, 2-(N-morpholino)ethanesulfonic acid-histidine solution (20 mM, pH 6.0) and ammonium acetate (10 mM, pH 4.0), were used as the separation media for inorganic cations and organic catecholamines, respectively. The detection limit for all metal cations except barium was under 0.5 mg/l, and that for four catecholamines was ca. 10 mg/l. This last value was the same order of magnitude as achieved with parallel UV detection.
Article
A miniaturized capacitively coupled contactless conductivity detector (mini-C(4)D) cell has been designed which is small enough to allow it to slide along the effective capillary length inside the capillary cassette of an Agilent capiillary electrophoresis system (CE) (or other CE brand of similar construction), including the possibility of positioning it close to the point of optical detection (4 cm), or even putting two such detector cells in one cassette. The cell was tested and the performance characteristics (noise, sensitivity, and peak width) were compared with those obtained with the previously used large C(4)D cell. No significant differences were observed. The mini-C(4)D was used in simultaneous separations of common cations and anions where its advantage over a larger C(4)D cell is the ability to vary the point of detection with the mini-C(4)D cell continuously at any point along the capillary length, so that the optimum apparent selectivity can be chosen. Other applications include providing a convenient second point of detection in addition to photometric detection, such as to measure accurately the linear velocity of a zone, or to allow placement of two mini-C(4)D cells in one capillary cassette simultaneously.
Article
Capacitively coupled contactless conductivity detection (C(4)D) has become an accepted detection method in capillary electrophoresis (CE) for a variety of analytes. Advantages of this technique over optical detection modes and galvanic contact conductivity detection include great flexibility in capillary handling and rather simple mechanical parts and electronics, as it can be performed in an on-capillary mode. Furthermore, the detection principle can be used with capillaries made of other materials than fused silica (PEEK, Teflon), with chip-based separation technologies, or with capillaries having very small inner diameters. This review presents a discussion of the published literature on C(4)D for CE and capillary electrochromatography.
Article
Despite the availability of commercial capillary electrophoresis systems for over ten years, where quantitative analysis is required, capillary zone electrophoresis (CZE) has often failed to replace ion chromatography as the method of choice for a large number of analytes, not least inorganic anions. To investigate the reasons for this apparent failing, a review is presented of work that has been carried out to-date involving the quantitative application of CZE to the determination of inorganic anions in industrial and environmental samples. This review summarizes work both investigating and improving the quantitative aspects of the CZE of inorganic anions. A complete survey of how CZE has been applied to the determination of inorganic anions in real samples is given, including what, if any, analytical performance parameters were investigated and quoted, and if quality assurance data and validation methods were briefly considered, thoroughly investigated or simply ignored.
Article
The signal-to-noise ratio of a contactless conductivity detector for capillary electrophoresis was examined for different cell arrangements and operating parameters. The best signal-to-noise ratios, and hence the best detection limits, are obtained for frequencies which give highest sensitivity. Comparative experiments for three different excitation voltages (20, 100, and 200 V(pp)) showed that the best signal-to-noise ratios were achieved for the highest excitation voltage of 200 V(pp). Low conductivity of the background electrolyte solution is mandatory to obtain lowest noise levels, and also the improvement on applying high excitation voltages was best for the electrolyte solution with lowest conductivity. The diameter of the electrodes was found to have only a negligible effect, so that a tight fitting of the electrodes to the external diameter of the capillary is not necessary. A cell without shielding between the two electrodes showed significant direct coupling (stray capacitance) and lower signal-to-noise ratios for all experimental conditions used. A serious distortion of the peak shapes was also observed for this cell arrangement.
Article
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.
Article
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.
Article
A theoretical model of the contactless conductivity detector (CCD) has been developed consisting of a network of resistors and capacitors. The output of the model is compared to experimental results and to the output of a simpler model. Experimentally, a lock-in amplifier is added to the detection scheme of the contactless conductivity detector to provide a more sensitive method of signal isolation. The detector is assembled on a printed circuit board with the electrodes in a co-axial configuration. The electrodes are chosen to allow for use with fused silica capillaries in capillary electrophoresis. The use of a lock-in amplifier in place of a previous rectification/filtering circuit allows for an approximate 10-fold improvement in S/N. The detector shows a linear response to changes in excitation voltage and to changes in analyte concentration. Mass limits of detection of 60, 63, and 50 fg are determined for the inorganic cations potassium, sodium, and lithium, respectively (for a signal three times the level of the rms noise).
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