Electrochemical Determination of 4-Nitrophenol Using a Single-Wall Carbon Nanotube Film-Coated Glassy Carbon Electrode

A single-wall carbon nanotubes (SWNT) film coated glassy carbon electrode (GCE) was fabricated for the direct determination of 4-nitrophenol (4-NP). The electrochemical behaviors of 4-NP at the SWNT-film coated GCE were examined. In 0.1M phosphate buffer with a pH of 5.0, 4-NP yields a very sensitive and well-defined reduction peak at the SWNT-modified GCE. It is found that the SWNT film exhibits obvious electrocatalytic activity towards the reduction of 4-NP since it not only increases the reduction peak current but also lowers the reduction overpotential. Based on this, an electrochemical method was proposed for the direct determination of 4-NP. The reduction peak current varies linearly with the concentration of 4-NP ranging from 1×10^–8 to 5×10^–6M, and the detection limit is 2.5×10^–9M after 3min of open-circuit accumulation. The relative standard deviation at 2×10^–7M 4-NP was about 6% (n=10), suggesting excellent reproducibility. This new method was successfully employed to determine 4-NP in several lake water samples.     

Electrochemical Study and Selective Determination of Dopamine at a Multi-Wall Carbon Nanotube-Nafion Film Coated Glassy Carbon Electrode

A homogeneous and stable suspension of multi-wall carbon nanotubes (MWNT) was achieved by dispersing MWNT into 0.1% Nafion ethanol solution. A uniform MWNT-Nafion cast film was obtained over a glassy carbon electrode (GCE) via solvent evaporation. The electrochemical behavior of dopamine (DA) was examined, and a reversible two-electron redox reaction was observed. In comparison with the bare GCE and the Nafion-modified GCE, the MWNT-Nafion modified GCE shows obvious electrocatalytic activity towards DA. Moreover, the MWNT-Nafion film coated electrode exhibits excellent selectivity towards DA even in the presence of high concentrations of ascorbic acid (AA) and uric acid (UA). The oxidation peak current was proportional to the concentration of DA over the range of 1×10^–8 to 1×10^–5molL^–1, and a detection limit of 2.5×10^–9molL^–1 was obtained after 2min. of open-circuit accumulation. The dispersion and morphology of MWNT-Nafion film were investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Fourier transfer (FT) IR spectra.     

Voltammetric Behavior of Dopamine at ct-DNA Modified Carbon Fiber Microelectrode

A sub-micrometer thin-layer DNA modified carbon fiber microcylinder electrode was prepared by electrodeposition of ct-DNA at 1.5V (vs. Ag/AgCl). The voltammetric behavior of dopamine (3-hydroxytyramine) was investigated at the modified electrode. It was found that the modified electrode exhibits a highly electrocatalytic activity toward dopamine oxidation. Differential pulse voltammetry was used for determination of dopamine in pH 7.4 phosphate buffer solution. A linear response of the peak current versus the concentration was found in the range of 4×10^–6 to 10^–4molL^–1 at 10^–4molL^–1 AA (ascorbic acid) coexistence (R=0.9959) and the range of 6×10^–5 to 10^–3molL^–1 at 10^–3molL^–1 AA (R=0.9960). The presence of a high concentration of ascorbic acid did not interfere with the determination. The proposed method exhibited good recovery and reproducibility. This method can be applied to the detection of DA in real samples.     

Adsorptive Catalytic Voltammetry of Physcion in the Presence of Dissolved Oxygen at a Carbon Paste Electrode

A novel electroanalytical method for the determination of physcion is described for the first time. Physcion yields an adsorption catalytic voltammetric peak at –0.74V (vs. SCE) in 0.4molL^–1 NH₄Cl–NH₃·H₂O buffer solution (pH 10.5) at a carbon paste electrode (CPE). The experimental results indicated that physcion is efficiently accumulated at a CPE by adsorption. In the subsequent potential scan, physcion was reduced to a homologous anthrahydroquinone compound. The compound was then immediately oxidized to physcion by the dissolved oxygen in the solution, and then physcion was again reduced at the CPE. As a result, a cyclic catalytic reaction was established. The second-order derivative peak current is proportional to the physcion concentration in the ranges of 2.0×10^–104.0×10^–9molL^–1 (accumulation 90s) and 4.0×10^–92.0×10^–8molL^–1 (accumulation 60s). The limit of detection is 8×10^–11molL^–1 (S/N=3) for a 120s accumulation time. The method was applied to the direct determination of physcion in the medicinal plant polygonum multiflorum Thumb with satisfactory results.     

Ultra-Sensitive Anodic Stripping Voltammetry for the Determination of Xanthine at a Glassy Carbon Electrode

An electrochemical anodic stripping procedure for ultra-trace assay of xanthine in Cu²⁺ solution at a glassy carbon electrode (GCE) is described. Cyclic voltammetry was used to characterize the nature of the process taking place at the GCE. The anodic stripping response in the presence of Cu²⁺, at 150mV (peak I) and 600mV (peak II), is evaluated with respect to various experimental and instrumental conditions. Voltammetric studies show that the mechanism of the overall reaction is similar to that of the oxidation of purine derivatives at a pyrolytic graphite electrode. It is found that the copper metal deposited onto the GCE was oxidized to Cu⁺ at around –180mV vs. Ag/AgCl and the generated Cu⁺ reacted with xanthine to accumulate on the GCE as an insoluble compound. The Cu⁺-xanthine compound accumulated on the GCE was redissolved by the oxidation of Cu⁺ to Cu²⁺ at ca. 150mV, and the concentration of xanthine in the vicinity of the GCE increased. The results enabled us to use the measurement of the oxidation peak current as the basis of a simple, accurate and rapid method of determining xanthine within a concentration range of 19.9 to 166nM for peak (I) and 0.24 to 17.2µM for peak (II). Promising results were obtained for xanthine determination by using an external mixing step prior to stripping measurements, which yielded a detection limit of 0.138µgL^–1 (9.1×10^–10M) xanthine. The effect of some interferences (e.g. purine compounds, amino acids and some metal ions) was considered.     

Electrocatalytic Reduction of Nitrite Using a Carbon Nanotube Electrode in the Presence of Cupric Ions

A simple, inexpensive method for determining nitrite is presented. With a carbon nanotube modified glass carbon electrode (GC), the overpotential for reduction of nitrite decreased, and direct reduction could be achieved in acid solution. Sensitivity, however, was not very high. When cupric ions were added to the solution, the reduction peak current increased significantly, and in particular the presence of multiple nitrate did not interfere. Experimental conditions were optimized, and preliminary studies were performed on the electrochemical mechanism of nitrite reduction in the presence of cupric ions on the carbon nanotube modified electrode. Under optimized conditions, the peak current of reduction achieved with the differential pulse voltammetric method was proportional to the concentration of nitrite in the ranges of 2.0×10^–6–1.0×thinsp;10^–5molL^–1 and 2.0×10^–5–1.0×10^–3molL^–1. The detection limit reached 5.0×10^–7molL^–1, and most of the inorganic ions did not interfere. The determination of nitrite in samples of rain water and river water was satisfactory.     

Solid-Phase Extraction with Slurry Injection of the Resin Into ETAAS for Trace Determination of Thallium in Natural Water

Thallium in natural water samples was determined by electrothermal atomic absorption spectrometry after 1000-fold enrichment by mini solid-phase extraction from a 100-mL sample solution. A Tl-pyrrolidine-1-carbodithioate complex formed in a sample solution of pH 1.6 was extracted on fine particles of a cellulose nitrate resin dispersed in the sample solution. The cellulose nitrate resin was then collected on a membrane filter (25mm^ø) by filtration under suction using a glass funnel with an effective filtration area of 0.64cm². As a result, a circular thin layer of the resin phase with a diameter of 9mm was obtained. Then the resin phase was carved out by an acrylate resin puncher with a 10-mm^ø hole to put it into a sample cup containing 100µL of 10mM HNO₃ containing 0.5mM NaCl. The resin phase was suspended in the solution by ultrasonication. 1000-fold enrichment was thus attained within 15min, and the suspension was delivered to electrothermal atomic absorption spectrometry. The linear calibration graph was obtained in the range of 0–4ng of Tl in 100mL of a sample solution. The detection limit obtained by 3 method was 0.19ng. The proposed method was applied to the determination of Tl in natural water samples. The results showed the concentration of Tl in seawater was 12.1±1.8pgmL^–1 for the calibration graph method and 12.6±1.4pgmL^–1 for the standard addition method. A snowmelt sample contained 20.7±1.0pgmL^–1 of Tl.     

Voltammetric Determination of Bismuth in Water and Nickel Metal Samples with a Sodium Montmorillonite (SWy-2) Modified Carbon Paste Electrode

A simple and reliable electrochemical method for the determination of bismuth in water and nickel metal samples using a sodium montmorillonite (SWy-2) modified carbon paste electrode was described. Due to its strong cation-exchange ability and adsorptive characteristics, SWy-2 significantly enhances the sensitivity of determination for Bi³⁺. Bi³⁺ is firstly preconcentrated and then reduced on the modified electrode surface at –0.50V. After that, reduced bismuth is stripped from the electrode surface during the positive potential sweep of –0.50V to 0.20V, and a well-defined stripping peak at –0.12V appears. The stripping peak current is proportional to the concentration of Bi³⁺ from 4×10^–9molL^–1 to 1×10^–6molL^–1. The detection limit (signal-to-noise=3) is 1×10^–10molL^–1 after 5min. accumulation. The proposed method was successfully applied to the determination of bismuth in water and nickel metal samples.     

Flame Atomic Absorption Spectrometric Determination of Gold and Palladium Using Microcolumn On-line Preconcentration and Separation

A microcolumn on-line preconcentration and separation system was developed for the flame atomic absorption spectrometric (FAAS) determination of trace levels of gold and palladium. The analytes were selectively adsorbed onto the microcolumn packed with 2-mercaptothiazole immobilized silica gel (MBTSG) in an acidity range of 0.1 to 6.0M HCl at a sampling flow rate of 4.0mLmin^–1. The analytes adsorbed could be desorbed by a thiourea solution with a flow rate of 2.0mLmin^–1. Most of the common coexisting metal ions at a concentration of 25.0mgmL^–1 and anions at a concentration of 50.0mgmL^–1 did not interfere with the preconcentration and determination of Au and Pd. The limits of detection (LOD), defined as three times the standard deviation of the blank (3), of Au and Pd are 10ngmL^–1 and 26ngmL^–1, respectively, with a preconcentration time of 60s. The relative standard deviation (RSD) is about 2.0% for 0.20µgmL^–1 Au and 0.30µgmL^–1 Pd. With a sample loading time of 30min, 6.7ngmL^–1 Au and 10ngmL^–1 Pd can be preconcentrated quantitatively. A geological sample, an anode slime and a secondary nickel alloy were successfully determined with the proposed method, and the results obtained showed good agreement with the certified values.Received December 23, 2002; accepted May 14, 2003 Published online August 8, 2003     

Determination of Heavy Metal Ions in Chinese Herbal Medicine by Microwave Digestion and RP-HPLC with UV-Vis Detection

A new method for the simultaneous determination of heavy metal ions in Chinese herbal medicine by microwave digestion and reversed-phase high-performance liquid chromatography (RP-HPLC) has been developed. The Chinese herbal medicine samples were digested by microwave digestion. Lead, cadmium, mercury, nickel, copper, zinc, and tin ions in the digested samples were pre-column derivatized with tetra-(4-chlorophenyl)-porphyrin (T₄-CPP) to form the colored chelates which were then enriched by solid phase extraction with C₁₈ cartridge and eluted from the cartridge with tetrahydrofuran (THF). The chelates were separated on a Waters Xterra RP₁₈ column by gradient elution with methanol (containing 0.05molL^–1 pyrrolidine-acetic acid buffer salt, pH=10.0) and THF (containing 0.05molL^–1 pyrrolidine-acetic acid buffer salt, pH=10.0) as mobile phase at a flow rate of 0.5mLmin^–1 and detected with a photodiode array detector in the range of 350–600nm. In the original samples the detection limits of lead, cadmium, mercury, nickel, copper, zinc and tin are 4ngL^–1, 3ngL^–1, 6ngL^–1, 5ngL^–1, 2ngL^–1, 6ngL^–1, and 4ngL^–1, respectively. This method was applied to the determination of lead, cadmium, mercury, nickel, copper, zinc and tin in Chinese herbal medicine samples with good results.     

Kinetics of formation,dissociation and equilibrium constants of pentacyanoaquoruthenate(II) with heterocycles

The kinetics of formation and dissociation reactions of [Ru(CN)₅L]^3– with a series of heterocyclic ligands were studied in aqueous media. In this presence of an excess of heterocycle, the observed second order rate constants were calculated from the k_obs versus [ligand] plot at =0.100m NaClO₄. Activation parameters for the formation reactions (H=28±7kJmol^–1 and S=140±35JK^–1mol^–1) are comparable for all systems, indicating a common mechanism. The kinetics of exchange of coordinated heterocycles for 1,3,5-triazine yielded a rate saturation typical of a limiting dissociative mechanism. Activation parameters of the limiting first order specific rate of dissociation reactions were H=85±7kJmol^–1 and S=18±4JK^–1mol^–1. Equilibrium constants were calculated from the second order rates of formation and pseudo-first order rates of dissociation reaction.     

Electrocatalytic Properties and Voltammetric Determination of Melatonin at a Nanostructured Film Electrode

A multi-wall carbon nanotube (MWNT) film coated glassy carbon electrode (GCE) was fabricated, and the electrochemical behavior of melatonin on the MWNT film coated GCE was investigated. The oxidation peak current of melatonin increases significantly and the oxidation peak position shifts positively at the MWNT film modified GCE compared to that at a bare GCE. This indicates that MWNTs feature highly effective catalysis to the electrochemical oxidation of melatonin. A simple and sensitive electroanalytical method was developed for the determination of melatonin. The oxidation peak current is proportional to the concentration of melatonin from 8×10^–8 to 1×10^–5molL^–1. The detection limit is about 2×10^–8molL^–1 for 3min accumulation. The proposed method was demonstrated to work satisfactorily with commercial capsules.     

The Electrochemical Behavior of p-Aminophenol at a ω-Mercaptopropionic Acid Self-Assembled Gold Electrode

A -mercaptopropionic acid (MPA) self-assembled monolayer modified electrode (MPA/SAM/Au) on a gold electrode has been fabricated. The characterization of the MPA/SAM/Au was investigated using attenuated total reflection-fourier transform infrared (ATR-FTIR) and A.C. impedance. The electrochemical behaviors of p-aminophenol (p-AP) were studied at the MPA/SAM/Au by cyclic voltammetry and semi-derivative voltammetry (SDV) in BR buffer solution. The modified electrode shows excellent electrocatalytic activity for the redox of p-AP and accelerates the electron transfer rate. The diffusion coefficient (D) is 4.55×10^–6cm²s^–1. The oxidative peak current increases linearly with the concentration of p-AP in the range of 4.0×10^–88×10^–6molL^–1 and 1.0×10^–52×10^–4molL^–1 by square wave voltammetry response, respectively. The detection limit (three times the signal blank/slope) is up to 1.2×10^–8molL^–1. The modified electrode is able to eliminate the interference of p-benzenediol, o-benzenediol and o-AP at a 40-, 90- or 70-fold concentration of p-AP, and it has been satisfactorily used for the determination of the real sample.     

Non-Aqueous Capillary Electrophoresis with Indirect Photometric Detection for Direct Analysis of Pb2+, Zn2+ and Cd2+ Ions

A new approach, based on non-aqueous capillary electrophoresis separation and indirect photometric detection, was established for the determination of the transition metal ions Pb²⁺, Zn²⁺ and Cd²⁺. Under optimized conditions, the method produced baseline separation of these three metal ions. The linear range and detection limits were 1050µM, 1.9µM for Cd²⁺; 1050µM, 2.1µM for Zn²⁺; and 20100µM, 3.8µM for Pb²⁺, respectively.     

Determination of Na,K, Rb and Cs Distribution in Human Brain Using Neutron Activation Analysis

This study aims to investigate the distribution of Na, K, Rb and Cs in human brains (5 individuals, 12 brain parts, mean age: 75 years). Distribution of the trace metals between lipid fraction and brain tissue was investigated in solvent extraction experiments. Determinations were carried out by instrumental neutron activation analysis. The present results show a rather non-homogeneous distribution for Na and a relatively uniform distribution for K, Rb and Cs. The mean concentrations found are 7440µgNag^–1 dry weight, 12800µgKg^–1, 14µgRbg^–1 and 50ngCsg^–1. A highly significant positive correlation was found between Rb and Cs. Solvent extraction experiments showed that 19% of Rb and 26% of Cs of the total content is located in lipid fraction.     

Determination of Attogram Quantities of Silver via Quenching of the Solid Substrate Room-Temperature Phosphorescence of Rhodamine B Based on the Catalysis of its Oxidation by Potassium Persulfate

A new method of SS-RTP for the determination of trace silver has been established. This method is based on the fact that Ag⁺, when activated by ,-bipyridyl (bipy) in a media of HAc–NaAc (pH=4.9), can catalyze the reaction of Rhodamine B (RhoB) oxidized by K₂S₂O₈, thus causing the Solid Substrate Room-Temperature Phosphorescence (SS-RTP) of RhoB to be quenched. The activating efficiency of bipy is 6.7 times higher than that of o-phenanthroline (phen). The reduction of the phosphorescence intensity (I_p) of RhoB is directly proportional to the concentration of Ag⁺ ions in the range of 1.6016.0agspot^–1 (0.40µLspot^–1). The regression equation of the working curve can be expressed as I_p=18.78+5.100m_Ag+ (agspot^–1) (r=0.9994, n=6), the detection limit is 0.28agspot^–1. This rapid, accurate and sensitive method has been successfully applied to the determination of trace silver in tea and human hair samples, and the results agree well with the Atomic Absorption Spectroscopy (AAS) method. The mechanism of the catalyzing reaction is also discussed.     

Determination of Caffeine Using a Simple Graphite Pencil Electrode with Square-Wave Anodic Stripping Voltammetry

A simple commercial graphite pencil electrode (GPE) was utilized for monitoring caffeine using the square-wave anodic stripping voltammetry (SWASV) method. This method was applied to determine the caffeine levels in several tea samples, which yielded a relative error of 1% in the concentrations. Caffeine was deposited at 0.0V (vs. Ag/AgCl), then reduced at +1.40V to strip it on the GPE. Optimal experimental conditions for the analysis were found to be as follows: pH value of 9 for the medium; deposition potential of 0.0V; deposition time of 120s; SW frequency of 25Hz; SW amplitude of 45mV, and step potential of 6mV. Given these optimum conditions, a linear range was observed within the concentration of 0500mgL^–1. At caffeine concentrations of 50.0, 250.0, and 500.0mgL^–1, the relative standard deviations in measured concentrations (n=12) were 0.19, 0.09, and 0.11%, respectively. The detection limit was found to be 9.2mgL^–1, which is comparable with the result obtained using a carbon paste electrode, equivalent to 8.2mgL^–1.     

Energy Transfer Electrogenerated Chemiluminescence for the Determination of Sulfite

A simple and novel electrogenerated chemiluminescence (ECL) method for the determination of sulfite has been developed based on the energy transfer ECL process. It was found that a weak ECL signal of sulfite was electrochemically generated on a platinum electrode in neutral aqueous solution. The signal was strongly enhanced by rhodamine B as an energy receptor and further enhanced by the neutral surfactant Tween 80. In 0.10M phosphate buffer solution (pH=7.5) containing 2.0×10^–6gmL^–1 rhodamine B and 0.4% (v/v) Tween 80, the ECL response to the concentration of sulfite at a potential of 0.82V was linear over a range of 1.0×10^–7gmL^–1 to 8.0×10^–6gmL^–1, and the detection limit was 5×10^–8gmL^–1. The relative standard deviation (n=11, 1.0×10^–6gmL^–1) was 3.8%. The proposed method has been successfully applied to the determination of sulfite in pharmaceutical injections and white sugar samples.     

Electrochemical Determination of Cinnamtannin B1 with a Pyrolytic Graphite Electrode

Cinnamtannin B1 (trimeric proanthocyanidin), which is identified and isolated from the effective fraction of the root of Lindera aggregata (Sims) Kosterm, is one kind of condensed tannin used as an effective antipyrotic and antitumor agent. Its electrochemical response can be obtained at a pyrolytic graphite electrode. Consequently, an easily performed and sensitive method for the determination of cinnamtannin B1 is developed. The detection limit is estimated to be 1.0×10^–7M with the linear determination range of 2.0×10^–7M to 1.8×10^–6M. Five replicate analyses of 1.0×10^–6M cinnamtannin B1 yields an RSD value of 2.1%. Since the working electrode does not need to be modified with any other species, it is very stable, repeatable and easily treated, and this method therefore potentially useful in real sample analysis.     

Flow Injection Determination of Lomefloxacin Based on Photochemically Sensitized Chemiluminescence

A photochemical chemiluminescence (CL) method for the determination of lomefloxacin (LFX) is proposed. LFX undergoes a photochemical reaction when irradiated with ultraviolet light, and a complex is formed when the photoproduct reacts with terbium(III), which can greatly enhance the CL of the Ce⁴⁺–Na₂SO₃ system. Under optimum experimental conditions, the linear range is between 9.0×10^–10 and 1.0×10^–5M, and the detection limit is 2.2×10^–10M. The relative standard deviation for the determination of 5.0×10^–8M LFX was 3.0% (n=11). The method has been successfully applied to the determination of LFX in dosage form, serum samples and urine samples. The recoveries were 97.9–102.3% for serum and urine samples. The possible mechanism is presented.