Determination of salbutamol in human plasma and urine by high-performance liquid chromatography with a coulometric electrode array system

A method is developed to determine salbutamol in human plasma and urine using high-performance liquid chromatography (HPLC) with a coulometric electrode array system, based on the electrochemical behavior of salbutamol at graphite electrode. The mobile phase component A is 30 mM sodium dihydroxy phosphate-30 mM triethylamine and is adjusted to pH 6.0 with 20% phosphate acid. The mobile phase component B is methanol. The optimized mobile phase composition was A and B in the proportion of 90:10 (v/v). Paracetamol is selected as the external standard. The human plasma and urine samples are pretreated using solid-phase extraction cartridges (Sep-Pak Silica), and the eluting solution is monitored by the coulometric electrode array system. The electrode potentials are set at 300, 400, 550, and 650 mV, respectively. Calibration curves show good linearity, and the recovery of salbutamol proves to be constant and unaffected by the concentration of the drug. This method, developed using HPLC-electrochemical detection, is reproducible and sensitive enough for the determination of salbutamol in human plasma and urine.     

Preparing the key metabolite of Z‐ligustilide in vivo by a specific electrochemical reaction

The key in vivo metabolites of a drug play an important role in its efficacy and toxicity. However, due to the low content and instability of these metabolites, they are hard to obtain through in vivo methods. Electrochemical reactions can be an efficient alternative to biotransformation in vivo for the preparation of metabolites. Accordingly, in this study, the metabolism of Z‐ligustilide was investigated in vitro by electrochemistry coupled online to mass spectrometry. This work showed that five oxidation products of the electrochemical reaction were detected and that two of the oxidation products (senkyunolide I and senkyunolide H) were identified from liver microsomal incubation as well. Furthermore, after intragastric administration of Z‐ligustilide in rats, senkyunolide I and senkyunolide H were detected in the rat plasma and liver, while 6,7‐epoxyligustilide, a key intermediate metabolite of Z‐ligustilide, was difficult to detect in vivo. By contrast, 6,7‐epoxyligustilide was obtained from the electrochemical reaction. In addition, for the first time, 6 mg of 6,7‐epoxyligustilide was prepared from 120 mg of Z‐ligustilide. Therefore, electrochemical reactions represent an efficient laboratory method for preparing key drug metabolites.     

Qualitative and quantitative high performance liquid chromatographic analysis of monoamine neurotransmitters and metabolites in cerebrospinal fluid and brain tissue using reductive electrochemical detection

The application of reductive coulometric electrochemical detection for analysis of the monoamine neurotransmitters norepinephrine, dopamine, and serotonin and their common metabolites in brain and cerebrospinal fluid following separation by isocratic high performance liquid chromatography is described. The high sensitivity and screening capabilities of coulometric electrodes permits the accurate quantitation of as little as 3-5 pg of these compounds in tissue following a simple single step purification procedure. Moreover, comparison of peak height ratios obtained from analysis of authentic reference standards and tissue samples at selected multiple electrode potentials provides a straightforward means for qualitative evaluation of peak identification and purity during analysis of biological samples. The method is comparatively inexpensive and precise within and between day coefficients of variation for most compounds range from 2-5%. Thirty samples can be run in duplicate in a 24 h period.     

Graphite/Nanocrystalline Zeolite Platform for Selective Electrochemical Determination of Hepatitis C Inhibitor Ledipasvir

A simple and efficient procedure is described for the electrochemical determination of ledipasvir (LDP) in presence of co‐formulated drug sofosbuvir (SOF). Herein, a highly sensitive, low‐cost electrochemical protocol was utilized to fabricate a zeolite modified carbon paste electrode (ZY/CPE) through mixing of zeolite nanostructures with graphite powder. The fabricated sensor displayed high sensitivity, allowing optimal charge transfer/electrode kinetics. Different experimental and chemical factors, including electrode composition, effect of pH, scan rate and amount of ZY were evaluated carefully to obtain the highest electrochemical response. Under optimized conditions and using square wave voltammetry (SWV), the current response of the ZY/CPE electrochemical sensing platform exhibited a detection limit of 7.5×10^?9 M and a large linear range from 5.0×10^?8 to 1.0×10^?4 M. The practical applicability of the suggested electrochemical platform was verified in the assessment of LDP in pharmaceutical formulations with excellent recoveries in the range of 99.50–98.87 %. Moreover, a biological relevance of the developed sensor was established by the analysis of LDP in human urine and plasma samples with satisfactory recoveries of 99.00 and 99.68 %, respectively. Due to the simplicity and ease of preparation of the proposed sensor, it can be used in quality control laboratories and for clinical analysis.     

Fast procedure for no-reference inversion-voltammetric determination of lead in aqueous solutions

Possibility of no-reference inversion-voltammetric determination of lead in aqueous solutions with the use of a preliminarily determined coulometric constant of the electrochemical cell, which enables substantially faster analysis as compared with the known analogs, was studied. The coulometric constants of electrochemical cells with a mercury film electrode were found for different solution volumes.     

Determination of pipecuronium bromide and its impurities in pharmaceutical preparation by high-performance liquid chromatography with coulometric electrode array detection

A sensitive and selective HPLC method with coulometric electrode array detection for the determination of pipecuronium bromide and its four impurities has been developed. The coulometric electrode array detection at increasing potentials from +300 to +900mV of the porous graphite electrode versus the palladium reference electrode was used. The limit of detection and quantitation for pipecuronium bromide was 8 and 25ngml(-1), respectively. This elaborate method for the simultaneous analysis of pipecuronium bromide and its impurities proved to be fast, precise, accurate, sensitive, and could be applied to analysis in substances and in pharmaceutical preparations.     

The behaviour of an electrochemical detector used in liquid chromatography and continuous flow voltammetry: Part III. Potential dependent current

The potential dependence of the steady-state current at a channel electrode has been calculated approximately. Six regions of behavior can be described from a consideration of the characteristic times of diffusion, fluid flow and electrochemical reaction. Wave shapes and half-wave potential shifts are as expected for amperometric cells under all values of the heterogeneous reaction rate constant, and for reversible electrochemical reactions in a coulometric cell. The wave adopts a peculiar shape for irreversible reactions at a coulometric electrode. This is caused by the inapplicability of the concept of mass transfer control in these systems.     

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.     

Impedimetric Detection of DNA Damage with the Sensor Based on Silver Nanoparticles and Neutral Red

Novel electrochemical DNA‐sensor based on glassy carbon electrode (GCE) modified with Ag nanoparticles, Neutral red covalently attached to its surface and native DNA adsorbed on modifier coating was developed for the estimation of DNA damage on example of model system based on Fenton reagent. As was shown, the oxidation process resulted in synchronous increase of electron transfer resistance and capacitance measured by electrochemical impedance spectroscopy (EIS). The contribution of each sensor component on the signal was specified and sensitivity estimated against similar surface coatings. The shift of EIS parameters was found to be higher than that of similar biosensors reported. The DNA sensor was tested on the estimation of antioxidant capacity of green tea infusions again the results of coulometric titration with electrogenerated bromine.     

Electrochemical generation of manganese (III) at glassy carbon electrode in acetic acid

Conditions were established for the electrochemical generation of manganese(III) at glassy carbon in acetic acid. In the oxidation of manganese-(II) in potassium acetate supporting electrolyte high current efficiencies were achieved in a wide range of current densities at the working electrode, whereas in the presence of sodium perchlorate a successful generation of manganese(III) could be performed only at low current densities. With increasing content of water in the anolyte the amount of generated manganese (III) was abruptly decreased and its stability in the solution was diminished. Procedures are given for a successful coulometric titration of reducing substances with anodically generated manganese (III); biamperometric, potentiometric and bipotentiometric methods for the location of the end-point were employed. The error of the determinations did not exceed ± 2%.     

Coulometric detector based on porous carbon felt working electrode for flow injection analysis

A coulometric detector based on carbon felt as working electrode has been designed. Ascorbic acid, hydroquinone, gallic acid and sulfur dioxide were used as electroactive compounds to determine the electrochemical characteristics of this detector. The coulometric conversion efficiency, selectivity, linear response range, detection limit and mass transfer coefficient were investigated in order to use the detector as a cleanup device in a flow injection system with dual-detector. This system has been used for determination of sulfur dioxide in wine, and the results are compared to those from the aspiration-oxidation method.     

Coulometric Anodic Stripping Voltammetry of Lead at Copper Column Electrode

A flow coulometric electroanalytical system using a copper column electrode with a copper wire inserted into a Nafion tube was developed to determine Pb(II) content based on anodic stripping voltammetry. The electrolysis efficiency of 5 μM Pb(II) was evaluated to be 100.4±4.5 % (n=5) when the length of the copper wire and flow rate of the Pb(II) solution were 50 cm and 0.1 mL min⁻¹, respectively. The amount of electricity due to the re-oxidation of Pb electrodeposited at the copper column electrode was proportional to the concentration of Pb(II) in the range between 0.1 to 100 μM, and the limit of detection for Pb(II) was 0.8 μM for a deposition time of 15 min. Interference from the presence of Cd(II) could be avoided and the selective determination of Pb(II) was successfully achieved by adjustment of the electrodeposition potential.     

Vapor-transportation preparation and reversible lithium intercalation/deintercalation of alpha-MoO3 microrods

We report on the preparation and electrochemical application of rechargeable lithium-ion batteries of alpha-MoO3 microrods. A simple and efficient vapor-transportation approach was developed to yield large-scale alpha-MoO3 microrods. The as-prepared products were present in long, uniform, rodlike structures with a diameter of approximately 2 approximately 6 microm, and the proportion of the rod morphology was about 95% according to the analysis of scanning electron microscopy (SEM). The electrochemical lithium intercalation/deintercalation characteristic of the as-prepared microrods was investigated by cyclic voltammetry and a galvanostatic charge-discharge method. The results showed that the alpha-MoO3 microrods exhibited high capacity (225 mAh g(-1)) and excellent cycling reversibility, and are thus promising cathode candidates in advanced rechargeable lithium-ion batteries. The correlation between the specific structural features of the microrods and their superior electrode performance is discussed in detail, revealing that the unique rodlike structure plays an important role in optimizing the electrochemical performance of the electrode.     

基于单纯型的非线性拟合求解多孔碳毡电极库仑检测器的质量转移系数

基于单纯型的非线性拟合求解多孔碳毡电极库仑检测器的质量转移系数陈国南（福州大学化学系,福州,３５０００２）关键词多孔碳毡电极;质量转移系数;非线性拟合在流动注射电化学检测中库仑检测器已经得到广泛的应用［１,２］。以多孔材料作为库仑检测器的工作电极,具...     

Determination of clenbuterol in pig liver by high-performance liquid chromatography with a coulometric electrode array system

A method has been developed to determine clenbuterol in pig liver using HPLC with coulometric electrode array system, for this compound can be irreversibly oxidized at high potentials by ordinary methods. Investigation into the effect of the pH of mobile phase on the retention factor and peak height of clenbuterol was made. The electrochemical behavior of clenbuterol at graphite electrodes was taken into account. Optimization of different extract conditions was also performed. The samples were pretreated using liquid-liquid extraction based on diethyl ether and the organic layer was evaporated to dryness. The residue was dissolved in mobile phase and monitored by an ESA electrochemical detector. Four electrodes in series were used for quantitation and the potentials of electrodes were set at 450, 600, 650 and 680 mV, respectively. Calibration curve showed good linearity and the detection limit of clenbuterol was 1.2 ng/g. This method developed using HPLC-ECD is reproducible, and sensitive enough for the determination of clenbuterol in pig liver. It is easy to perform.     

An algorithm for thorough background subtraction from high-resolution LC/MS data: application to the detection of troglitazone metabolites in rat plasma, bile, and urine

Interferences from biological matrices remain a major challenge to the in vivo detection of drug metabolites. For the last few decades, predicted metabolite masses and fragmentation patterns have been employed to aid in the detection of drug metabolites in liquid chromatography/mass spectrometry (LC/MS) data. Here we report the application of an accurate mass-based background-subtraction approach for comprehensive detection of metabolites formed in vivo using troglitazone as an example. A novel algorithm was applied to check all ions in the spectra of control scans within a specified time window around an analyte scan for potential background subtraction from that analyte spectrum. In this way, chromatographic fluctuations between control and analyte samples were dealt with, and background and matrix-related signals could be effectively subtracted from the data of the analyte sample. Using this algorithm with a +/- 1.0 min control scan time window, a +/- 10 ppm mass error tolerance, and respective predose samples as controls, troglitazone metabolites were reliably identified in rat plasma and bile samples. Identified metabolites included those reported in the literature as well as some that had not previously been reported, including a novel sulfate conjugate in bile. In combination with mass defect filtering, this algorithm also allowed for identification of troglitazone metabolites in rat urine samples. With a generic data acquisition method and a simple algorithm that requires no presumptions of metabolite masses or fragmentation patterns, this high-resolution LC/MS-based background-subtraction approach provides an efficient alternative for comprehensive metabolite identification in complex biological matrices. Copyright (c) 2008 John Wiley & Sons, Ltd.     

Comparison of μm and mm sized disk electrodes for end-column electrochemical detection in capillary electrophoresis

End-column electrochemical detection based on either the use of a 25 μm microdisk electrode or a 0.5 mm macrodisk electrode has been compared with respect to performance and influence on non-aqueous capillary electrophoretic separations. Despite the much higher coulometric efficiency obtained with the larger disk electrode, the microdisk electrode configuration offers comparable limits of detection (LOD) for the neutral and positively charged ferrocene compounds employed in conjunction with a non-aqueous acetonitrile-based buffer. The LODs for ferrocene were found to be 4.0 × 10^–8 M and 6.7 × 10^–8 M for the microdisk and macrodisk detector, respectively. In addition, both detector arrangements showed different relative responses for neutral and positively charged analytes. The macroelectrode-based detector introduced additional zone broadening while this was not found to be the case with the microelectrode arrangement. Using the microelectrode detector, the band broadening in an electro-osmotically driven flow system was compared to that in a gravity flow-based system. It was demonstrated that the zone broadening under gravity flow conditions was approximately twice as large as under electro-osmotic flow conditions for a typical set of experimental parameters.     

The preparation of a fast response Ag/Ag2SO4 reference electrode

A procedure for the preparation of a fast response Ag/Ag(2)SO(4) reference electrode to be used for the detection of the endpoint of a coulometric titration is described.     

Determination of some phytoestrogens in soybeans and their processed products with HPLC and coulometric electrode array detection

A reliable and sensitive method for quantification of daidzein and genistein by HPLC with coulometric electrode array detection is presented using bisphenol A as internal standard. Acid hydrolysis during extraction of foods allows the quantitative determination of total phytoestrogens as aglycones. The substances are separated on a reversed phase column (Hypersil^® Elite C-18), eluted with methanol/acetonitrile/50 mM sodium acetate pH 4.8 (40/ 20/40, v/v/v) and detected in a coulometric electrode array detector using an oxidative detection mode (+390 to +810 mV in increments of 60 mV, vs palladium reference electrode). Phytoestrogen levels from several food items were determined. High levels of daidzein (819 mg/kg) and genistein (960 mg/kg) were found in soy products whereas biochanin A could not be detected in any of these food samples. The recovery depends on the kind of food and was found to be between 72 and 94% for daidzein and genistein.     

Direct electrochemistry of heme proteins: effect of electrode surface modification by neutral surfactants

Direct electrochemical studies on horse heart myoglobin and horseradish peroxidase (HRP) have been carried out using tin-doped indium oxide (ITO) and surfactant modified glassy carbon working electrodes. These proteins show very slow electron transfer kinetics at metal or untreated electrodes. Moreover, small amounts of surface-active impurity were drastically affects the electrode reaction of these proteins. The results showed that modification of the electrode surface with neutral surfactants significantly improves the electrochemical response of myoglobin as well as of HRP. The electrode response was found to depend on the structure of the surfactants. The amount of surfactant required per unit area of the electrode surface to promote the maximum electron transfer rate constants was found to be constant. This indicated that the surfactant molecules interacted with the electrode surface in a specific manner and anchored the protein molecules to align in the suitable orientation. The hydrophobicity of the surfactants rather than their charge was found to be crucial in promoting the electrode response of these proteins at the glassy carbon electrode.