Determination of dopamine in synthetic cerebrospinal fluid by SWV with a graphite–polyurethane composite electrode

This work describes an electroanalytical investigation of dopamine using cyclic voltammetry (CV) and the graphite–polyurethane composite electrode (GPU). In CV studies, well-defined redox peaks characterize the oxidation process at the GPU electrode, which is indicative of electrocatalytic effects associated with active sites on the GPU electrode surface. A new analytical methodology was developed using the GPU electrode and square wave voltammetry (SWV) in BR buffer solution (0.1 mol L^–1; pH 7.4). Analytical curves were constructed under optimized conditions (f=60s^–1, E_a=50 mV, E_I=2 mV) and detection and quantification limits of 6.4×10^–8 mol L^–1 (12.1 g L^–1) and 5.2×10^–6 mol L^–1 (0.9 mg L^–1), respectively, were achieved. The precision of the method was checked by performing ten successive measurements for a 9.9×10^–6 mol L^–1 dopamine solution. For intra-assay and inter-assay precisions, the relative standard deviations were 1.9 and 2.3%, respectively. In order to evaluate the developed methodology, the determination of dopamine was performed with good sensitivity and selectivity, without the interference of ascorbic acid in synthetic cerebrospinal fluid, which indicates that the new methodology enables reliable analysis of dopamine.     

Kinetics and mechanism in the decomposition of CCl4 in a radio-frequency pulse discharge

In the decomposition of CCl₄ in an r.f. pulse discharge the observation of time-resolved absorption spectra of the CCl radical allowed concentration measurements and a kinetic and mechanistic investigation of the system. Kinetic spectroscopy and end product analysis, with computer simulation of proposed mechanisms, indicates that the major decomposition reaction is CCl₄»CCl+3Cl (Cl₂+Cl), with a minor contribution from CCl₄»CCl₂+2Cl. Radical concentrations were of the order of 10^–7 mol dm^–3 (10¹⁴ molec. cm^–3). CCl removal was kinetically second order with a rate coefficient value of (3.7±8)×10¹⁰ mol^–1 dm^–3 s^–1 at 295±3 K at gas pressure 0.1 torr.     

Determination of subnanomolar concentrations of cobalt by adsorptive stripping voltammetry at a bismuth film electrode

Procedures for trace cobalt determinations by adsorptive stripping voltammetry at in situ and ex situ plated bismuth film electrodes are presented. These exploit the enhancement of the cobalt peak obtained by using the Co(II)–dimethylglyoxime–cetyltrimethylammonium bromide–piperazine-N,N-bis(2-ethanesulfonic acid) system. The calibration graph for an accumulation time of 120 s was linear from 2 × 10^–10 to 2 × 10^–8 mol L^–1. The relative standard deviation from five determinations of cobalt at a concentration of 5 × 10^–9 mol L^–1 was 5.2%. The detection limit for an accumulation time of 300 s was 1.8 × 10^–11 mol L^–1. The proposed procedure was applied to cobalt determination in certified reference materials and in tap and river water samples.     

Perchlorate-selective membrane electrode based on a new complex of uranil

A potentiometric ion-selective electrode based on new compound, as a carrier, has been successfully developed for detection of perchlorate anion in aqueous solution. Within the perchlorate ion concentration range 1.0×10^–6 to 1.0 mol L^–1 the electrode had a linear response with a Nernstian slope of 60.6±1.0 mV per decade . The limit of detection as determined from the intersection of the extrapolated linear segments of the calibration plot was 8.0×10^–7 mol L^–1. The proposed electrode has fairly a good discriminating ability towards ClO₄^– ion in comparison to other anions. The sensor has a response time of 10 s and can be used for at least 2 months without substantial divergence in potential. It was successfully applied to direct determination of perchlorate in urine and water.     

Determination of berberine by measuring the enhanced total internal reflected fluorescence at water/tetrachloromethane interface in the presence of sodium dodecyl benzene sulfonate

A highly sensitive method for determination of berberine is proposed based on the measurements of total internal reflected fluorescence (TIRF) at water/ tetrachloromethane (H₂O/CCl₄) interface. In the pH range of 2.6–5.7, the co-adsorption of the berberine with the anionic surfactants such as sodium dodecyl benzene sulfonate (SDBS), sodium dodecylsulfonate (SDS), and sodium lauryl sulfate (SLS) occurs at the H₂O/CCl₄ interface, resulting in greatly enhanced TIRF signal characterized by the emission at 526 nm when excited with a 351 nm light beam. The enhanced TIRF intensity is in proportion to the berberine concentration in the range 0.2–10.0×10^-7 mol L^-1. The limit of detection is 1.7×10^-9 mol L^-1 (3). It was found that ions such as Ca(II), Cu(II), Fe(III), Cd(II), Mg(II), Zn(II), Pb(II), and Al(III) can be allowed larger than 1.0×10^-4 mol L^-1. Meanwhile, the organic compounds such as vitamin B, saccharine, and amino acid do not display any effect for the present TIRF method even if they are larger than 1.0×10^-2 mol L^-1in high concentration levels (larger than 1.0×10^-5 mol L^-1). The results of determination for synthetic samples were agreement with the desired values, and the ones for tablets were identical with those obtained according to the method of Chinese Pharmacopoeia.     

Electrochemical behavior of a covalently modified glassy carbon electrode with aspartic acid and its use for voltammetric differentiation of dopamine and ascorbic acid

Aspartic acid was covalently grafted on to a glassy carbon electrode (GCE) by amine cation radical formation in the electrooxidation of the amino-containing compound. X-ray photoelectron spectroscopic (XPS) measurement and cyclic voltammetric experiments proved the aspartic acid was immobilized as a monolayer on the GCE. Electron transfer to Fe(CN)₆^4– in solution of different pH was studied by cyclic voltammetry. Changes in solution pH resulted in the variation of the charge state of the terminal group; surface pK_a values were estimated on the basis of these results. Because of electrostatic interactions between the negatively charged groups on the electrode surface and dopamine (DA) and ascorbic acid (AA), the modified electrode was used for electrochemical differentiation between DA and AA. The peak current for DA at the modified electrode was greatly enhanced and that for AA was significantly reduced, which enabled determination of DA in the presence of AA. The differential pulse voltammetric (DPV) peak current was linearly dependent on DA concentration over the range 1.8×10^–6–4.6×10^–4 mol L^–1 with slope (nA mol^–1 L) and intercept (nA) of 47.6 and 49.2, respectively. The detection limit (3) was 1.2×10^–6 mol L^–1. The high selectivity and sensitivity for dopamine was attributed to charge discrimination and analyte accumulation. The modified electrode has been used for determination of DA in samples, in the presence of AA, with satisfactory results.     

Cholinesterase sensors based on screen-printed electrodes for detection of organophosphorus and carbamic pesticides

Cholinesterase sensors based on screen-printed electrodes modified with polyaniline, 7,7,8,8-tetracyanoquinodimethane (TCNQ), and Prussian blue have been developed and tested for detection of anticholinesterase pesticides in aqueous solution and in spiked grape juice. The influence of enzyme source and detection mode on biosensor performance was explored. It was shown that modification of the electrodes results in significant improvement of their analytical characteristics for pesticide determination. Thus, the slopes of the calibration curves obtained with modified electrodes were increased twofold and the detection limits of the pesticides were reduced by factors of 1.6 to 1.8 in comparison with the use of unmodified transducers. The biosensors developed make it possible to detect down to 2×10^–8 mol L^–1 chloropyrifos-methyl, 5×10^–8 mol L^–1 coumaphos, and 8×10^–9 mol L^–1 carbofuran in aqueous solution and grape juice. The optimal conditions for grape juice pretreatment were determined to diminish interference from the sample matrix.Abbreviations ChE Cholinesterase - TCNQ 7,7,8,8-Tetracyanoquinodimethane - ChO Choline oxidase - AChE Acetylcholinesterase - BChE Butyrylcholinesterase - BSA Bovine serum albumin - 2-PAM 2-Pyridine aldoxime methiodide     

Rapid determination of telmisartan in pharmaceuticals and serum by the parallel catalytic hydrogen wave method

The polarographic characteristics of telmisartan have been investigated in 0.8 mol L^–1 NH₃.H₂O–NH₄Cl (pH 8.9)–0.01 mol L^–1 H₂O₂ as supporting electrolyte. The results demonstrate that the polarographic reduction wave at ca. –1.30 V in the absence of H₂O₂ is a catalytic hydrogen wave, and the reduction wave enhanced by H₂O₂ is a so-called parallel catalytic hydrogen wave. The analytical sensitivity of the parallel catalytic hydrogen wave is ca. 60 times higher than that of the corresponding catalytic hydrogen wave. Based on the parallel catalytic hydrogen wave a novel method has been developed for determination of telmisartan by linear sweep polarography. The calibration curve is linear in the range 2.0×10^–8–2.0×10^–6 mol L^–1 and the detection limit is 1.0×10^–8 mol L^–1. The precision is excellent with relative standard deviations of 2.6% at a concentration of 1.0×10^–7 mol L^–1 telmisartan. The proposed method has been applied to the direct determination of the telmisartan in capsule forms and biological samples. The proposed method has been proved to be advantageous over existing CZE and MEKC methods in simplicity, rapidity, and reproducibility.     

Adsorptive stripping voltammetric determination of cobalt in the presence of dimethylglyoxime and cetyltrimethylammonium bromide

A sensitive procedure for determination of micro-traces of Co(II) by adsorptive stripping voltammetry is proposed. The procedure exploits the enhancement of the cobalt peak obtained by use of the system Co(II)–dimethylglyoxime–piperazine-1,4-bis(2-ethanesulfonic acid)–cetyltrimethylammonium bromide. Using the optimized conditions, a detection limit (based on the 3 criterion) for Co(II) of 1.2×10^–11 mol L^–1 (0.7 ng L^–1) was achieved. The calibration plot for an accumulation time of 30 s was linear from 5×10^–11 to 4×10^–9 mol L^–1. The procedure was validated by analysis of certified reference materials and natural water samples.     

Determination of Iron in Tap and Waste Water Using Liquid–Liquid Extraction in a Flow-Injection System

A flow-injection procedure for the determination of iron(III) in water is described. The procedure is based on the formation of an ion pair between the tetraphenylarsonium (Ph₄As⁺) (TPA) or tetrabutylammonium (But₄N⁺) (TBA) cations and the tetrathiocyanatoferrate(III) complex (TTF). This ion pair is extracted with chloroform, and the absorbance of the organic phase is measured at 503nm (for Ph₄As⁺) or 475nm (for But₄N⁺). Iron concentrations higher than 0.9×10^–6molL^–1 (50µgL^–1) can be detected in the first case, with a relative standard deviation of 1.9% (n=12), a linear application rangeof between 1.34 and 54.0×10^–6molL^–1 (75–3015µgL^–1), and a sampling frequency of 30h^–1. For the ion pair with But₄N⁺, the detection limit is 0.52×10^–6molL^–1 (29µgL^–1), with a relative standard deviation of 1.6% and a linear application range between 0.73 and 54.0×10^–6molL^–1. Under the proposed working conditions, only Pd(IV), Cu(II) and Bi(III) interfere. With the application of the merging zones technique, considerable amounts of organic reagent can be saved. The TBA method was applied to the analysis of iron(III) in tap and industrial waste waters.     

Determination of traces of iron with the catalytic maximum wave in differential pulse polarography

Summary A differential pulse polarographic method for the determination of iron employing the catalytic maximum wave has been studied. A well-defined differential pulse polarographic peak for iron(III) in Britton-Robinson buffer solution containing 50 mol/l N-(2-hydroxyethyl) ethylenediamine N,N,N-triacetic acid (HEDTA) and 5 mmol/l KBrO₃ is observed in the potential range from +0.2 to –0.3 V vs. SCE. The peak current is very large compared to that of the Fe(III)/EDTA complex, being proportional to the concentration of iron(III) between 1.00×10^–8 and 3.58×10^–6 mol/l under optimum conditions. The relative standard deviations for 3.58×10^–7 mol/l and 1.79×10^–6 mol/l iron(III) were 1.38 and 0.54%, respectively (n=5), and the calculated detection limit was 5.2×10^–9 mol/l iron(III). The method has been applied to the determination of iron in fresh snow and rain waters.

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Spurenbestimmung von Eisen mit Hilfe der katalytischen Maximumsstufe in der Differential-Puls-Polarographie

Zusammenfassung Das Verfahren beruht auf der Tatsache, daß in Britton-Robinson-Puffer (mit 50 mol/l HEDTA und 5 mmol/l KBrO₃) im Potentialbereich von +0,2 bis –0,3 V gegen SKE ein gut definierter puls-polarographischer Peak für Eisen(III) auftritt. Der Peakstrom ist im Vergleich zu dem des Fe(III)/EDTA-Komplexes sehr groß und ist unter optimalen Bedingungen im Konzentrationsbereich von 1,00·10^–8 bis 3,58·10^–6 mol/l der Eisen(III)-Konzentration proportional. Die relative Standardabweichung beträgt 1,38% bzw. 0,54% (n=5) für 3,58·10^–7 mol/l bzw. 1,79·10^–6 mol/l Fe(III). Die berechnete Nachweisgrenze liegt bei 5,2·10^–9 mol/l Fe(III). Das Verfahren wurde zur Eisenbestimmung in Schnee- und Regenwasser eingesetzt.

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This work was supported in part by a Grant-in-Aid for Scientific Research from Hokkaido-prefecture, 1982.     

Molecular Imprinting-Chemiluminescence Determination of Norfloxacin Using a Norfloxacin-Imprinted Polymer as the Recognition Material

A selective molecular imprinting-chemiluminescence method was developed for the determination of norfloxacin by using a norfloxacin-imprinted polymer as recognition material and the Ce(IV)-sodium sulfite-norfloxacin chemiluminescence reaction as the detection system. The chemiluminescence intensity responded linearly to the concentration of norfloxacin within 1.0×10^–71.0×10^–5molL^–1 with a detection limit of 3×10^–8molL^–1. The relative standard deviation for 5.0×10^–7molL^–1 norfloxacin solution was 2.4% (n = 7). This method was applied to the determination of norfloxacin in urine samples, and the results obtained were satisfactory.     

Integrated procedure for determination of endocrine-disrupting activity in surface waters and sediments by use of the biological technique recombinant yeast assay and chemical analysis by LC–ESI-MS

An integrated procedure using mass spectrometry and molecular biology for determination of estrogenicity in natural waters and sediments is reported. Solid-phase extraction (SPE) and pressurized-liquid extraction (PLE), respectively, were used for isolation of endocrine-disrupting compounds (EDC) from surface waters and sediments, followed by liquid chromatography–mass spectrometry using an electrospray interface (LC–ESI-MS). Twenty seven EDC were determined: non-ionic surfactants (nonylphenol ethoxylate), alkylphenols (e.g. nonylphenol and octylphenol), bisphenol A, phthalates, and natural and synthetic steroid sex hormones. Limits of detection varied from 0.02 to 0.22 g L^–1 and from 1 to 10 g kg^–1 in water and sediments, respectively. Recoveries ranged from 65 to 125% and 73 to 97% for waters and sediments, respectively. In addition to LC–ESI-MS determination, extracts obtained by SPE and PLE were analyzed by the recombinant yeast assay (RYA) to assess total estrogenic activity. This bioassay detects natural estrogens and xenoestrogens, producing a quantitative measurement of EDC irrespective of the identity of the chemical responsible for the activity. As a novelty, a relative estrogenicity factor was determined for 19 analytes with EC ₅₀ values ranging from 10^–10 to 10^–9 mol L^–1 for synthetic estrogens, from 10^–7 to 10^–5 mol L^–1 for alkylphenol derivatives, and from 10^–5 to 10^–4 mol L^–1 for phthalates and benzothiazoles. By use of this integrated chemical–ecotoxicological approach good correlation was usually established between chemical composition and estrogenic effects for surface water and sediment samples from Portugal. Estrogenic activity observed was mainly attributed to the presence of nonylphenolic compounds (with concentrations of NP ranging from 0.1 up to 44 g L^–1 in waters and up to 1172 g kg^–1 in sediments), and to the sporadic presence of estrogens, detected at ng L^–1 levels.     

Polarographic Determination of Lovastatin in the Presence of H2O2 in Pharmaceuticals, Human Urine and Serum

The polarographic reduction and catalytic behavior of lovastatin are studied by polarography and cyclic voltammetry. The reduction wave of lovastatin appears at ca. –1.49V (vs. SCE) in 0.16molL^–1 Na₂B₄O₇–KH₂PO₄ (pH=7.4) supporting electrolyte containing 20% ethanol. It is ascribed to a 2e^–, 2H⁺ addition to the carbonyl group on lactone ring. If H₂O₂ is present, the reduction wave is catalyzed to produce a polarographic catalytic wave. Based on the catalytic wave, a novel method for the determination of lovastatin is proposed. A rectilinear calibration curve of the catalytic wave was obtained for lovastatin concentration in the range 1.5×10^–8 to 1.0×10^–6molL^–1. The peak current of the catalytic wave is ca. 12 times higher than that of the corresponding reduction wave. The detection limit is 8.0×10^–9molL^–1. The proposed method is simpler, faster and more sensitive than the known methods for lovastatin analysis, and can be applied to the direct determination of lovastatin in pharmaceuticals, urine and serum without preliminary separation.     

Determination of levodopa methyl ester and its metabolites in rat serum by CZE with amperometric detection

A reliable and reproducible method, capillary zone electrophoresis with amperometric detection (CZE–AD), has been developed for separation and quantification of levodopa methyl ester (LDME) and its biotransformation products levodopa (L-DOPA) and dopamine (DA) in rat serum. A carbon-disk electrode was used as working electrode. The optimum conditions for CZE detection were 50 mmol L^–1 phosphate solution at pH 7.0 as running buffer, 17 kV as separation voltage, 1.0 V (vs Ag/AgCl, 3.0 mol L^–1) as detection potential, and sample injection for 8 s at 17 kV. The linear ranges were from 2.4×10^–2 to 2.2 g mL^–1 for LDME, 2.9×10^–1 to 49.5 g mL^–1 for L-DOPA, and 1.4×10^–2 to 1.5 g mL^–1 for DA with correlation coefficients of 0.9997, 0.9994, and 0.9999, respectively. The detection limits for LDME, L-DOPA, and DA were 14.6, 98.0, and 9.7 ng mL^–1, respectively. Recoveries were 80.3% for LDME, 93.5% for L-DOPA, and 86.5% for DA. This method was applied to serum samples after intravenous injection of LDME and L-DOPA to rats.     

Simple and rapid catalytic spectrophotometric determination of superoxide anion radical and superoxide dismutase activity in natural medical vegetables using phenol as the substrate for horseradish peroxidase

The coupling reaction of 4-aminoantipyrine (4-AAP) with phenol using the superoxide anion radical ( ) as oxidizing agent under the catalysis of horseradish peroxidase (HRP) was studied. Based on the reaction, produced by irradiating vitamin B₂ (V_B2) was spectrophotometrically determined at 510 nm. Under the optimum experimental conditions, the relationship between A ₅₁₀ and concentration was linear in the range 9.14×10^–6–1.2×10^–4 mol L^–1. The detection limit was determined to be 1.37×10^–6 mol L^–1. A possible reaction mechanism was discussed. The effect of interferences and surfactants on the determination of was also investigated. The proposed method was applied to determine superoxide dismutase activity in garlic, scallion, and onion with satisfactory results.     

Study of a bis-furaldehyde Schiff base copper(II) complex as carrier for preparation of highly selective thiocyanate electrodes

A new highly selective thiocyanate electrode based on N,N-bis-(furaldehyde)-1,2-phenylenediamine-dipicolyl copper(II) complex [Cu(II)-BFPD] as neutral carrier is described. The electrode has an anti-Hofmeister selectivity sequence: SCN^–>I^–>Sal^–>ClO₄ ^–>Br^–>NO₂ ^–>Cl^–>NO₃ ^–>SO₄ ^2–>SO₃ ^2–>H₂PO₄ ^– and a near-Nernstian potential linear range for thiocyanate from 1.0×10^–1 to 5.0×10^–6 mol L^–1 with a detection limit 2.0×10^–6 mol L^–1 and a slope of 57.5 mV decade^–1 in pH 5.0 of phosphate buffer solution at 20 °C. The response mechanism is discussed on the basis of results from A.C. impedance measurement and UV spectroscopy. The anti-Hofmeister behavior of the electrode results from a direct interaction between the central metal and the analyte ion and a steric effect associated with the structure of the carrier. The electrode has the advantages of simplicity, fast response, fair stability and reproducibility, and low detection limit. The selectivity of electrodes based on [Cu(II)-BFPD] exceeds that of classical anion-sensitive membrane electrodes based on ion exchangers such as lipophilic quaternary ammonium or phosphonium salts. Application of the electrode for determination of thiocyanate in waste water samples from a laboratory and a gas factory, and in human urine samples, is reported. The results obtained were fair agreement with the results obtained by HPLC.     

Electrochemical Properties and Analytical Applications of Keggin-type Phosphomolybdic Anions in Electrostatically Linked 2-Aminoethanethiol Self-Assembled Monolayers

A composite film containing a heteropolyanion was prepared on a 2-aminoethanethiol (AT) self-assembled monolayer film-modified gold electrode by attaching the Keggin-type phosphomolybdic anion. The surface structures and electrochemical properties of the composite films were characterized by using ATR-FTIR, AC impedance, cyclic voltammetry and chronocoulometry. FTIR studies indicated that there was some electrostatic interaction between Pmo₁₂O₄₂^7– and surface NH₃⁺. Three reversible redox couples were observed in 1.0molL^–1 H₂SO₄ in the potential range of 0–0.7V for the composite film modified electrode, which were attributed to two-electron and two-proton electrochemical processes of PMo₁₂O₄₂^7–. The diffusion coefficient is determined to be 2.04×10^–7cm²s^–1. The composite film shows good catalytic activities for the reduction of nitrite (NO₂^–) in acidic solution and the catalytic mechanisms are described. The modified electrode provides a linear response for NO₂^– in the concentration range of 1.0×10^–4 to 1.0×10^–7molL^–1 by differential pulse adsorptive stripping voltammetry with a correlation coefficient of 0.9965. The detection limit (three times the signal blank/slope) was 2.0×10^–8molL^–1. The modified electrode was used for the determination of NO₂^– in wastewater.     

Simultaneous Determination of Aromatic Amines by Liquid Chromatography Coupled with Carbon Nanotubes/Poly (3-methylthiophene) Modified Dual-Electrode

Liquid chromatography with amperometric detection (LC-AD) is developed and applied to simultaneously determine five aromatic amines. In the LC-AD, a new carbon nanotubes/poly(3-methylthiophene) modified dual-electrode is fabricated and then used as the working electrode. It is found that this chemically modified electrode (CME) exhibits efficiently electrocatalytic oxidation for aromatic amines with relatively high sensitivity, stability and long-life. Thus, lower detection in LC-AD can be achieved, which are 4.0 × 10^–8 mol L^–1 for aniline, 1.6 ×10^–7 mol L^–1 for 4-nitroaniline, 1.0 × 10^–7 mol L^–1 for 4-chloroaniline, 1.5 × 10^–7 mol L^–1 for 1-naphthylamine, 1.7 × 10^–7 mol L^–1 for 2-bromoaniline. The recoveries of the five analytes are also determined, which range between 0.95 and 1.05 for drinking water, 0.86 and 1.10 for the LiWa River water.     

Oxidation of ascorbic acid in the presence of phthalocyanine metal complexes. Chemical aspects of catalytic therapy of cancer 2. Catalysis by cobalt octacarboxyphthalocyanine. Reaction products

The products of ascorbic acid oxidation in the presence of cobalt octa-4,5-carboxy-phthalocyanine sodium salt (TPH) were identified. These include the ascorbate radical (A^·), hydroxyl radical (OH^·), and hydrogen peroxide (H₂O₂). The kinetics of accumulation and consumption of the reaction products was studied. For the concentration ranges of ascorbic acid = 0–2.5 ⋅ 10⁻³ mol L⁻¹ and the catalyst C _TPH = 0–3.5 ⋅ 10⁻⁵ mol L⁻¹, the the highest possible concentration of the ascorbate radical is ∼10⁻⁷ mol L⁻¹, the concentration of H₂O₂ is 7 ⋅ 10⁻⁴ (30% of the starting concentration of ascorbic acid) and the concentration of the hydroxyl radical is at most 10⁻⁶ mol L⁻¹.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2224–2228, October, 2004.