Electrochemical detection in the capillary electrophoresis analysis of inorganic compounds

Capillary electrophoresis-based approaches for the study of affinity interactions combined with various sensitive and nontraditional detection techniques

Nearly all processes in living organisms are controlled and regulated by the synergy of many biomolecule interactions involving proteins, peptides, nucleic acids, nucleotides, saccharides, and small molecular weight ligands. There is growing interest in understanding them, not only for the purposes of interactomics as an essential part of system biology, but also in their further elucidation in disease pathology, diagnostics, and treatment. The necessity of detailed investigation of these interactions leads to the requirement of laboratory methods characterized by high efficiency and sensitivity. As a result, many instrumental approaches differing in their fundamental principles have been developed, including those based on capillary electrophoresis. Although capillary electrophoresis offers numerous advantages for such studies, it still has one serious limitation, its poor concentration sensitivity with the most commonly used detection method-ultraviolet-visible spectrometry. However, coupling capillary electrophoresis with a more sensitive detector fulfils the above-mentioned requirement. In this review, capillary electrophoresis combined with fluorescence, mass spectrometry, and several nontraditional detection techniques in affinity interaction studies are summarized and discussed, together with the possibility of conducting these measurements in microchip format.

Analysis of Small Ions with Capillary Electrophoresis

Small inorganic ions are easily separated through capillary electrophoresis because they have a high charge-to-mass ratio and suffer little from some of the undesired phenomenon affecting higher molecular weight species like adsorption to the capillary wall, decomposition, and precipitation. This chapter is focused on the analysis of small ions other than metal ions using capillary electrophoresis. Methods are described for the determination of ions of nitrogen, phosphorus, sulfur, fluorine, chlorine, bromine, and iodine.

Fundamentals of electrochemical detection techniques for CE and MCE

The electroanalytical techniques of amperometry, conductometry and potentiometry match well with the instrumental simplicity of CE. Indeed, all three detection approaches have been reported for electrophoretic separations. However, the characteristics of the three methods are quite distinct and these are not related to the optical methods more commonly employed. A detailed discussion of the underlying principles of each is given. The issue of possible effects of the separation voltage on the electrochemical detection techniques is considered in depth, and approaches to the elimination of such interferences are also discussed for each case.

Analysis of inorganic and small organic ions by CE with amperometric detection

Capillary electrophoresis has become a widely useful analytical technology. Amperometric detection is extensively employed in capillary electrophoresis for its many inherent virtues, such as rapid response, remarkable sensitivity, and low cost of both detectors and instrumentations. Analysis of inorganic and small organic ions by capillary electrophoresis is an important research field. This review focuses on the recent developments of capillary electrophoresis coupled with amperometric detection for analysis of inorganic and small organic ions. Advancements in electrophoresis separation modes, amperometric detection modes, working electrodes, and applications of inorganic ions, amino acids, phenols, and amines are discussed.

Optimization of CZE for analysis of phytochemical bioactive compounds

Advantages of CZE such as high efficiency, low cost, short analysis time, and easy implementation result in its wide applications for analysis of phytochemical bioactive compounds (e.g. flavonoids, alkaloids, terpenoids, phenolic acid, saponins, anthraquinones and coumarins). However, several aspects, including sample preparation, separation, and detection have significant effects on CZE analysis. Therefore, optimization of these procedures is necessary for development of the method. In this review, sample preparation such as extraction method and preconcentration, separation factors including buffer type, concentration and pH, additives, voltage and temperature, as well as detection, e.g. direct and indirect UV detection, LIF and MS were discussed for optimization of CZE analysis on phytochemical bioactive compounds. The optimized strategies were also reviewed.

Coated Wire Potentiometric Detection for Capillary Electrophoresis Studied Using Organic Amines, Drugs, and Biogenic Amines

Capillary electrophoresis was coupled successfully and reliably to potentiometric sensors, which are based on an ionically conductive rubber phase coating, applied on a 250 microm diameter metal substrate. The membrane components included potassium tetrakis(p-chlorophenyl)borate (TCPB), bis(2-ethylhexyl)sebacate (DOS), and high molecular mass poly(vinyl chloride) (PVC). Potentiometry reveals a very sensitive CE detection mode, with sub-micromolar detection limits for amines and the randomly chosen drugs quinine, clozapine, cocaine, heroine, noscapine, papaverine, and ritodrine. The lowest detection limit, 1 x 10(-8) M injected concentration, was obtained for the quaternary ammonium compound tetrahexylammonium chloride. The more polar lower aliphatic amines and the biogenic amines dopamine, adrenaline, and cadaverine have much higher detection limits. The detection limits are log P dependent. Addition of a commercially available calixarene molecule or a synthetic macrocyclic amphiphilic receptor molecule to the electrode coatings enhanced the sensitivity respectively for the lower aliphatic amines and for the biogenic amines. A transpose of the Nikolskii-Eisenman-type function was suggested and used to convert the signal of the detector to a concentration-dependent signal.

Analysis of Metal Ions by Sweeping via Dynamic Complexation and Cation-Selective Exhaustive Injection in Capillary Electrophoresis

To improve the detection sensitivity of metal ions in capillary zone electrophoresis (CZE), a novel method that combines complex formation and on-line sample preconcentration by sweeping was developed. Sweeping is defined as the picking and accumulating of analytes by a carrier in the background solution, with which they have considerable affinity. In this sweeping method, using ethylenediaminetetraacetic acid as carrier, dynamic complexation to form a UV-absorbing chelate and on-line preconcentration occur simultaneously during a run. The technique was validated in terms of the limit of detection, reproducibility, and sensitivity enhancement. Detection responses of some divalent metal ions, in terms of peak heights, were improved from 60- to 180-fold, relative to conventional CZE which employed precapillary complexation. The limits of detection were in the range of (1.8-23.4) x 10(-8) M. This method was applied to the analysis of trace metal ions in factory wastewater. Furthermore, sweeping in conjunction with sample stacking accompanying electrokinetic injection, cation-selective exhaustive injection (CSEI-sweeping), was also examined. Up to 140 000-fold improvement in detector responses for some divalent and trivalent metal ions was realized by CSEI-sweeping. The limits of detection were in the range (2.4-25.2) x 10(-11) M.

Quantitative capillary zone electrophoresis of inorganic anions

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.

Elemental speciation analysis in capillary electrophoresis

The growing awareness of the strong development of the toxicity of heavy metals upon their chemical forms has led to an increasing interest in the qualitative and quantitative determination of specific metal species. Speciation has therefore become an important topic of present-day analytical research. The development in the elemental speciation analysis by capillary electrophoresis (CE) is reviewed. Various CE separation modes and detection techniques applied are discussed. A comprehensive description of reported methods to date in CE speciation analysis including metals, metalloids and nonmetallic elements is demonstrated. Some examples are presented to demonstrate CE's ability to solve real-world speciation analysis with emphasis on the applications in biological and environmental samples. Further, some issues concerning the limitations and the future of CE with regard to speciation studies are also discussed.

Capillary-electrode alignment by an optical-fiber connector for amperometric detection in capillary electrophoresis

We describe here an electrochemical cell ideal for routine analysis of CE-EC experiments (capillary electrophoresis coupled with electrochemical detection). The cell was modified from a fiber-optic connector, named MT, which allowed frequent change and fast alignment between a pair of 4-strand fiber ribbons. The relative standard deviations of the current response and migration time for 100 microM dopamine were, respectively, 3.7 and 0.5% in five repetitive routines of disconnecting, polishing, and assembling the CE cell. The time required for alignment of the separation capillary and the working electrode was < 10 s, once all components were assembled in an MT fiber-optic plug. These features enabled CE-EC users to polish the working electrode and reassemble the EC cell as in HPLC-EC. However, to accommodate the channel dimensions of the commercially available MT, a special order capillary with outer diameter of 125 microm is necessary at this stage.

Conductivity detection cell for capillary zone electrophoresis with a solution mediated contact of the separated constituents with the detection electrodes

A contact conductivity detection cell for capillary zone electrophoresis (CZE) with an electrolyte solution mediated contact of the separated constituents with the detection electrodes (ESMC cell) was developed in this work. This new approach to the conductivity sensing in CZE is intended to eliminate detection disturbances due to electrode reactions and adsorption of the separated constituents when these are coming into direct contact with the detection electrodes. An optimum detection performance of the cell was achieved when the carrier electrolyte solution mediated the electric contact of the detection electrodes with the separated constituents. Different compositions of the mediator and carrier electrolyte solutions led to large drifts of the detection signals. Isotachophoresis experiments performed in this context with the ESMC cell revealed that origins of these drifts are in transport processes (diffusion and electromigration) between the detection compartment and the detection electrodes in the cell. These processes affected, to some extent, other analytically relevant performance parameters of the ESMC cell of the present construction as well [e.g., concentration limits of detection (LODs), a contribution of the cell to the band broadening]. For example, the ESMC cell gave, under optimum operating conditions, 3-4 times higher concentration LODs for the test analytes than a current on-column conductivity cell employed under identical working conditions. On the other hand, these LOD values (25-150 nmol/l) were still 20-25 times lower than those estimated from reference experiments for a contactless conductivity detector. CZE experiments with iodide, carried out under working conditions leading to electrochemical reactions of this anion on the detection electrodes of current conductivity cells, did not occur in the ESMC cell. In addition, this cell, contrary to a reference contact conductivity cell, required no special care (e.g., cleaning of the surfaces of the detection electrodes by chemical or electrochemical means) to maintain its reliable long-term performance. Anionic CZE analyses of tap and mineral water samples monitored by the conductivity detector provided with the ESMC cell demonstrated a practical applicability and certain limitations of this detection approach in the analysis of ionic constituents present in high ionic strength sample matrices.

Anion-exchange capillary electrochromatography with indirect UV and direct contactless conductivity detection

Conductivity detection is applied to ion-exchange capillary electrochromatography (IE-CEC) with a packed stationary phase, using a capacitively coupled contactless conductivity detector with detection occurring through the packed bed. Columns were packed with a polymeric latex-agglomerate anion-exchanger (Dionex AS9-SC). A systematic approach was used to determine suitable eluants for IE-CEC separations using simultaneous indirect UV and direct conductivity detection. Salicylate and p-toluenesulfonate were identified as potential eluant competing anions having sufficient eluotropic strength to induce changes in separation selectivity, but salicylate was found to be unsuitable with regard to baseline stability. It was also found for both indirect UV and direct conductivity detection that homogenous column packing was imperative, and monitoring of the baseline could be used to assess the homogeneity of the packed bed. Using a p-toluenesulfonate eluant, the separation of eight common anions was achieved in 2.5 min. Direct conductivity detection was found to be superior to indirect UV detection with regard to both baseline stability and detection sensitivity with detection limits of 4-25 microg/L being obtained. However, the calibration for each anion was not linear over more than one order of magnitude. When using conductivity detection, the concentration of the eluant could be varied over a wider range (2.5-50 mM p-toluenesulfonate) than was the case with indirect UV detection (2.5-10 mM), thereby allowing greater changes in separation selectivity to be achieved. By varying the concentration of p-toluenesulfonate in the eluant, the separation selectivity could be manipulated from being predominantly ion-exchange in nature (2.5 mM) to predominantly electrophoretic in nature (50 mM).

Recent progress in capillary electrophoresis of metal ions

Advances in the fundamental studies and methodology of capillary electrophoresis (CE) as applied to metal ion analysis over the last two years are reviewed, with the objective of providing the interested reader with a state-of-the-art picture of technique's potentialities in the area. In particular, novel strategies for separation selectivity control and CE system innovations designed to enhance the detection sensitivity are described. In addition, a brief overview of the primary metal analytes and samples for which the technique appears to be best suited is given. The current limitations of the technique regarding most of all the implementation for routine use are considered along with the approaches on how they could be addressed. Finally, some pointers as to the likely trends in the future research are discussed.

Surface-enhanced Raman scattering: a structure-specific detection method for capillary electrophoresis

A new approach to detecting capillary electrophoresis (CE) eluent components by interfacing CE with a surface-enhanced Raman scattering (SERS) system is described. In this approach, CE-based separation of a mixture of trans-1,2-bis(4-pyridyl)ethylene and N,N-dimethyl-4-nitrosoaniline has been detected by SERS in a postcolumn geometry. The retention time obtained from SERS corresponds well with that from conventional UV-visible detection. Meanwhile, CE eluants are identified by their characteristic vibrational spectra, demonstrating the validity of SERS as a structure-specific detection method for CE. In addition, the ability to monitor SERS intensity changes at molecule-specific frequencies makes selective detection of individual analytes possible, even when separation is incomplete. Finally, CE-SERS is evaluated for separation of amino acids (tyrosine and tryptophan) and environmental pollutants (chlorophenol mixtures).

Conductivity detection of aliphatic alcohols in micellar electrokinetic chromatography using an oscillometric detector

Although conductivity is usually applied to detect ionic species in capillary electrophoresis (CE), nonionic species can also be detected by their indirect effects on the conductivity of the running electrolyte. This approach was used for detection of aliphatic alcohols in micellar electrokinetic chromatography (MEKC) with an oscillometric detector. Although the detector operates at 600 kHz, for the range of electrolyte concentration used in CE, the response is mainly due to variations of conductivity. A 50 mM phosphate and 50 mM SDS solution was used as running electrolyte and as the solvent for mixtures of some isomers of propanol, butanol, and pentanol. A set of negative peaks was obtained and assigned to the components by spiking the samples. The limits of detection (LOD) ranged from 2.1 mM for 2-methyl-2-propanol to 5.3 mM for 1-pentanol. Due to the high affinity for the interior of the micelles, 1-hexanol could not be easily-detected, but by the addition of 10% methanol to the running electrolyte it was possible. For this electrolyte, the LOD was improved, ranging from 0.8 mM for 2-methyl-2-propanol to 1.5 mM for 1-pentanol. Calibration plots were linear up to 40 mM at least. These results indicate that conductivity may be useful for detection of nonionic species in CE, especially when optical methods can not be conveniently applied.

Capillary electrophoresis with end-capillary potentiometric detection using a copper electrode

Potentiometric end-capillary detection in capillary electrophoresis has the advantage of relatively easy miniaturisation without having to compromise the concentration sensitivity. Potentiometric end-capillary detection using a copper electrode is also attractive because of the sensitive detection of many inorganic and organic UV-transparent ions and the ability to work in both direct and indirect mode. In this work, detection of a number of common anions in a tartrate electrolyte at pH 3 was studied. The influence of the end-capillary detection geometry on the detection performance was investigated. An end-capillary detection cell allowing the separation capillary to be changed without the need to realign the detection electrode was constructed and fitted into a commercial CE apparatus. Under the optimal configuration, which was a 25 microm diameter copper electrode aligned coaxially with a 25 microm capillary and positioned at a distance of about 25 microm from the capillary end, excellent peak shapes were achieved and comparison with simultaneous on-capillary photometric detection showed no additional peak broadening. Good sensitivity was obtained, resulting in concentration limits of detection (LODs) in the low microM range and mass LODs in the low amol range. Examples of separations of inorganic and organic anions are presented and the analytical potential of the detection method is assessed.