Voltammetric determination of isoprenaline in pharmaceutical preparations using a copper(II) hexacyanoferrate(III) modified carbon paste electrode

The electroanalytical determination of isoprenaline in pharmaceutical preparations of a homemade carbon paste electrode modified with copper(II) hexacyanoferrate(III) (CuHCF) was studied by cyclic voltammetry. Several parameters were studied for the optimization of the sensor such as electrode composition, electrolytic solution, pH effect, potential scan rate and interferences in potential. The optimum conditions were found in an electrode composition (in mass) of 15% CuHCF, 60% graphite and 25% mineral oil in 0.5 mol l⁻¹ acetate buffer solution at pH 6.0. The analytical curve for isoprenaline was linear in the concentration range from 1.96×10⁻⁴ to 1.07×10⁻³ mol l⁻¹ with a detection limit of 8.0×10⁻⁵ mol l⁻¹. The relative standard deviation was 1.2% for 1.96×10⁻⁴ mol l⁻¹ isoprenaline solution (n=5). The procedure was successfully applied to the determination of isoprenaline in pharmaceutical preparations; the CuHCF modified carbon paste electrode gave comparable results to those results obtained using a UV spectrophotometric method.     

Evaluation of a carbon paste electrode modified with organofunctionalized amorphous silica in the cadmium determination in a differential pulse anodic stripping voltammetric procedure

The determination of cadmium using a carbon paste electrode modified with organofunctionalized amorphous silica with 2-benzothiazolethiol was investigated. The Cd(II) oxidation peak was observed around −0.80 V (vs. SCE) in phosphate buffer (pH 4.0) in differential pulse anodic stripping voltammetry. The best results were obtained under the following optimized conditions: 1 min accumulation time, 50 mV pulse amplitude, 20 mV s⁻¹ scan rate in phosphate buffer pH 4.0. Using such parameters a linear dynamic range from 5.6×10⁻⁷ to 3.5×10⁻⁵ mol l⁻¹ Cd(II) was observed with a sensitivity of 2.83 μA mol⁻¹ l, limit of detection 1.0×10⁻⁷ mol l⁻¹. Cd(II) spiked in a natural water sample was determined with 99% mean recovery at 10⁻⁷ mol l⁻¹ level. Interference were also evaluated.     

Simultaneous UV and electrochemical determination of the herbicide asulam in tap water samples by micellar electrokinetic capillary chromatography

A simple end-column electrochemical detector was designed and attached to an available commercial capillary electrophoresis instrument with UV detection to detect different kind of herbicides and to determinate methyl-4-aminophenyl-sulfonylcarbamate (asulam) in water samples. The designed cell is very easy to assemble and disassemble in a short period of time; the working electrode positioning is also quickly achieved without micropositioners. The alignment between working electrode and capillary outlet was very reproducible for the all checked electrodes; the R.S.D. obtained was lower than 6.0% for 100 μm gap distance. In this mode, the non-electroactive and electroactive compounds could be detected by UV and electrochemical detection, respectively at the same time. The electrochemical determination of asulam using micellar electrokinetic capillary chromatography (MEKC) is the first time that is reported. In both detection systems, a linear range was obtained for asulam concentrations lower than 25.0 mg l⁻¹, in boric acid 0.020 mol l⁻¹ at pH 8.20 and containing 0.025 mol l⁻¹ of sodium dodecyl sulfate, to obtain selectivity additional separation by the micellar distribution process. Under these conditions, an experimental detection limit of 0.4 mg l⁻¹ was achieved. A new experimental scheme is also described for asulam determination in tap waters with a previous preconcentration step. Using both, UV and electrochemical detection, with a previous extraction procedure, the detection limits of asulam in tap water samples were of 1.0 and 0.8 μg l⁻¹, respectively.     

Immobilization of horseradish peroxidase to a nano-Au monolayer modified chitosan-entrapped carbon paste electrode for the detection of hydrogen peroxide

A procedure for fabricating an enzyme electrode has been described based on the effective immobilization of horseradish peroxidase (HRP) to a nano-scaled particulate gold (nano-Au) monolayer modified chitosan-entrapped carbon paste electrode (CCPE). The high affinity of chitosan entrapped in CCPE for nano-Au associated with its amino groups has been utilized to realize the use of nano-Au as an intermediator to retain high bioactivity of the enzyme. Hydrogen peroxide (H₂O₂) was determined in the presence of hydroquinone as a mediator to transfer electrons between the electrode and HRP. The HRP immobilized on nano-Au displayed excellent electrocatalytical activity to the reduction of H₂O₂. The effects of experimental variables such as the operating potential of the working electrode, mediator concentration and pH of measuring solution were investigated for optimum analytical performance by using an amperometric method. The enzyme electrode provided a linear response to hydrogen peroxide over a concentration range of 1.22×10⁻⁵-2.43×10⁻³ mol l⁻¹ with a sensitivity of 0.013 A l mol⁻¹ cm⁻² and a detection limit of 6.3 μmol l⁻¹ based on signal per noise =3. The apparent Michaelis-Menten constant (K_m^app) for the sensor was found to be 0.36 mmol l⁻¹. The lifetime, fabrication reproducibility and measurement repeatability were evaluated with satisfactory results. The analysis results of real sample by this sensor were in satisfactory agreement with those of the potassium permanganate titration method.     

A flow injection method for biamperometric determination of dipyrone in pharmaceuticals

A flow-injection method for determination of dipyrone (novalgin, metamizol) in pharmaceutical tablets with biamperometric detection is described. Flow-parameters such as flow rate, coil length and sample injection volume were optimized. The calibration graph was linear (r=0.9997) within the range of 10-50 mg l⁻¹ in a 1-mmol l⁻¹ HCl carrier solution for an applied potential of 100 mV between the two platinum wire electrodes. Two indicating redox systems were studied, namely Fe(III)/Fe(II) and I₂/I⁻. The former proved to be well suited as a selective and sensitive biamperometric indicating system for dipyrone in the presence of ingredients commonly accompanying the drug. The developed method was successfully applied to the determination of dipyrone in several commercial pharmaceutical preparations. The sampling rate was 71 samples per hour with a rsd of 1.6% (eight injections of a 14-mg l⁻¹ dipyrone solution). Recoveries close to 100% demonstrated the reliability of the proposed method.     

Electrochemical determination of maltol in beverages with glassy carbon electrode and its silica sol-gel modified electrode

The electrochemical behavior of maltol on a glassy carbon (GC) electrode was investigated. The results were applied to differential pulse voltammetric determination of maltol in beverages pretreated by ultrafiltration. Under the optimum experimental conditions, the linear range is 1×10⁻⁵ to 6×10⁻⁴ mol l⁻¹ maltol and the relative standard deviation for 0.4 mmol l⁻¹ maltol is 0.6% (n=9). The detection limit was 5 μmol l⁻¹. Furthermore, silica sol-gel film on GC electrode could be used as suitable selective membrane, which integrated selective membrane on the electrode and substituted for the pretreatment of ultrafiltration. Under the above conditions, maltol was determined by semi-differential linear sweep voltammetry at a silica sol-gel modified GC electrode in the concentration range of 5×10⁻⁶ to 5×10⁻⁴ mol l⁻¹. The detection limit was 2 μmol l⁻¹ and the relative standard deviation for 0.1 mmol l⁻¹ maltol was 0.7% (n=7). The proposed method is of sensitivity, simplicity, rapidness and no contamination. It had been applied to the direct determination of maltol in beverages such as grape wines, drinks and beers without any pretreatment. The results obtained with the present method were satisfactory with those obtained by spectrophotometry. It could be used as a simple and practical method for the determination of the flavor enhancer maltol in beverages.     

Voltammetric determination of trace mercury at a sonogel-carbon electrode modified with poly-3-methylthiophene

The sonogel-carbon electrode is a new class of sol-gel electrode that exhibit favourable mechanics and electrics properties to be used as electrochemical sensor. In this paper, a modified sonogel-carbon electrode is proposed to determine mercury at trace levels. The modified electrode is obtained by electropolymerization of 3-methylthiophene on the surface of a bare sonogel-carbon electrode. This electrode shows high selectivity and sensitivity and linear response towards Hg(II), with a detection limit of 1.4 × 10⁻³ mg l⁻¹. The electrode is reusable by a simple chemical cleaning procedure. No deterioration was observed in the electrode response during at least 1 week of successive measurements.     

Simultaneous determination of lead(II) and cadmium(II) at a diacetyldioxime modified carbon paste electrode by differential pulse stripping voltammetry

A simple and effective chemically modified carbon paste electrode (CMCPE) for the simultaneous determination of lead(II) and cadmium(II) was developed in this work. The electrode was prepared by the addition of diacetyldioxime into a carbon paste mixture. Pb²⁺ and Cd²⁺ were preconcentrated on the surface of the modified electrode by complexing with diacetyldioxime and reduced at a negative potential (−1.10 V). Then the reduced products were oxidized by differential pulse stripping. The fact that two stripping peaks appeared on the voltammograms at the potentials of −0.65 V (Cd²⁺) and −0.91 V (Pb²⁺) demonstrates the possibility of simultaneous determination of Pb²⁺ and Cd²⁺. Under the optimized working conditions, calibration graphs were linear in the concentration ranges of 1.0×10⁻⁷-1.5×10⁻⁵ mol l⁻¹ (Pb²⁺) and 2.5×10⁻⁷-2.5×10⁻⁵ mol l⁻¹ (Cd²⁺), respectively. For 5 min preconcentration, detection limits of 1×10⁻⁸ mol l⁻¹ (Pb²⁺) and 4×10⁻⁸ mol l⁻¹ (Cd²⁺) were obtained at the signal noise ratio (SNR) of 3. To evaluate the reproducibility of the newly developed electrode, the measurements of 5×10⁻⁷ mol l⁻¹ Pb²⁺ and Cd²⁺ were parallel carried out for six times at different electrodes and the relative standard deviations were 2.9% (Pb²⁺) and 3.2% (Cd²⁺), respectively. Interferences by some metals were investigated. Only Ni²⁺ and Hg²⁺ apparently affected the peak currents of Pb²⁺ and Cd²⁺. The diacetyldioxime modified carbon paste electrode was applied to the determination of Pb²⁺ and Cd²⁺ in water samples. The results indicate that this electrode is sensitive and effective for the simultaneous determination of Pb²⁺ and Cd²⁺.     

Determination of ethamsylate in pharmaceutical preparations by irreversible biamperometry

A novel flow-injection irreversible biamperometric method is described for the determination of ethamsylate. The proposed method is based on the oxidation of ethamsylate at one platinum electrode and the reduction of permanganate at another to form an irreversible biamperometric detection system. Ethamsylate can be determined over the range 1.0×10⁻⁶-1.0×10⁻⁴ mol l⁻¹ with a sample measurement frequency of 180 samples h⁻¹. The detection limit for ethamsylate is 4.0×10⁻⁷ mol l⁻¹. The stability of the proposed method is shown by a RSD of 0.52% for 11 replicate determinations of 2.0×10⁻⁵ mol l⁻¹ ethamsylate. The proposed method was applied to the determination of ethamsylate in pharmaceutical preparations.     

A multicommuted flow system for sequential spectrophotometric determination of hydrosoluble vitamins in pharmaceutical preparations

A multicommuted flow system is proposed for spectrophotometric determination of hydrosoluble vitamins (ascorbic acid, thiamine, riboflavine and pyridoxine) in pharmaceutical preparations. The flow manifold was designed with computer-controlled three-way solenoid valves for independent handling of sample and reagent solutions and a multi-channel spectrophotometer was employed for signal measurements. Periodic re-calibration as well as the standard addition method was implemented by using a single reference solution. Linear responses (r=0.999) were obtained for 0.500-10.0 mg l⁻¹ ascorbic acid, 2.00-50.0 mg l⁻¹ thiamine, 5.00-50.0 mg l⁻¹ riboflavine and 0.500-8.00 mg l⁻¹ pyridoxine. Detection limits were estimated as 0.08 mg l⁻¹ (0.5 μmol l⁻¹) ascorbic acid, 0.8 mg l⁻¹ (2 μmol l⁻¹) thiamine, 0.2 mg l⁻¹ (0.5 μmol l⁻¹) riboflavine and 0.1 mg l⁻¹ (0.9 μmol l⁻¹) pyridoxine at 99.7% confidence level. A mean sampling rate of 60 determinations per hour was achieved and coefficients of variation of 1% (n=20) were estimated for all species. The mean reagent consumption was 25-fold lower in relation to flow-based procedures with continuous reagent addition. Average recoveries between 95.6 and 100% were obtained for commercial pharmaceutical preparations. Results agreed with those obtained by reference methods at 95% confidence level. The flow system is suitable for application in quality control processes and in dissolution studies of vitamin tablets.     

The electrochemical properties of dopamine,epinephrine and their simultaneous determination at a poly(L-methionine) modified electrode

A poly(L-methionine) modified electrode, fabricated by electrochemical immobilization of the L-methionine on a glassy carbon electrode, was used for simultaneous determination of dopamine and epinephrine through cyclic voltammetry. The electrochemical properties of dopamine and epinephrine have been investigated. This sensor gave two separated cathodic peaks at −0.282 and 0.112 V for EP and DA, respectively. A linear response was obtained in the range of 5.0 × 10⁻⁷ to 1.0 × 10⁻⁴ mol l⁻¹ for epinephrine, and 1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ mol l⁻¹ for dopamine. The detection limits were 3.6 × 10⁻⁷ mol l⁻¹ and 4.2 × 10⁻⁷ mol l⁻¹ for epinephrine and dopamine, respectively. This method was successfully applied for simultaneous determination of dopamine and epinephrine in human urines. The text was submitted by the authors in English.     

Voltammetry determination of trace manganese with pretreatment glassy carbon electrode by linear sweep voltammetry

This paper described the determination of trace manganese using linear sweep voltammetry at a pretreatment glassy carbon electrode. The glassy carbon electrode pretreated by electrochemical method in the 0.1 mol l⁻¹ NaOH solution greatly improved the electrode responsibility in the determination of manganese(II). The barrier to the detection of low manganese concentration was overcome by means of autocatalytic effect of manganese oxide deposited on the electrode in advance. Under the optimum experiments condition (0.04 mol l⁻¹ NH₃-NH₄Cl buffer solution, pH 9.0), the linear range was 4×10⁻⁸ to 1×l0⁻⁶ mol l⁻¹ Mn(II) for linear sweep voltammetry and 1×10⁻⁹ to 4×10⁻⁸ mol l⁻¹ Mn(II) for convolution voltammetry. The relative standard deviation for 2×10⁻⁸ mol l⁻¹ Mn(II) is 3.4%. The proposed method is simple, rapid, sensitive and selective. It had been applied to the determination of trace manganese in samples with satisfactory results.     

On-line electrochemical preconcentration and flame atomic absorption spectrometric determination of manganese in urine samples

A sensitive and selective method was developed for the determination of traces of manganese in urine using on-line electrochemical preconcentration followed by flame atomic absorption spectrometry detection. A home made flow-through polypropylene cell (4.5 cm long × 0.8 cm diameter filled with glass marbles) with an effective inner volume of 0.5 ml containing a working and a counter electrode, both of glassy carbon and a Pt pseudo reference electrode was located in a flow injection manifold specially designed for the purpose of this work. The manganese was deposited from buffer solution of NH₃/NH₄Cl at pH 9.00 through an oxidizing process at a current of 400 mA during 7 min. A flow of HCl 0.1 mol l⁻¹ at 4 ml min⁻¹ through the cell, chemically dissolved the deposit. A small portion (15 μl) of the concentrate was introduced in a continuously flowing system by means of a timing device and was then carried to the detector for the manganese quantification. All electrochemical and spectroscopic variables as well as possible interferences in both systems were systematically studied. The relative standard deviations for ten consecutive measurements of manganese solutions of 2.0 and 20 μg l⁻¹ were of 2.3 and 1.5%, respectively, while for a sample processed five times was less then 5%. The accuracy of the developed procedure was evaluated by adding known amounts of manganese standard to urine samples and following the whole procedure. Recoveries within the range 97.2-102.8% were obtained. To further prove the accuracy, a Seronorm Trace Elements in Urine, Batch 403125 sample with a reported concentration of 13 μg Mn l⁻¹ was also analyzed. The experimental value obtained was of 12.7 ± 0.1 μg l⁻¹, which does not differ significantly from the reported amount (p < 0.05). A preconcentration factor of 40, a linear range between 0.015 and 60 μg l⁻¹ and a limit of detection of 15 ng l⁻¹ permitted the determination of manganese in real urine samples from non-exposed subjects in the range 0.5-2.8 μg l⁻¹.     

Determination of Cd by electrothermal atomic absorption spectrometry after electrodeposition on a graphite probe modified with palladium

Traces of Cd were determined by electrothermal atomic absorption spectrometry after electrochemical preconcentration on a commercial graphite ridge probe modified with Pd. The Pd electrochemically deposited on the probe surface served not only as the modifier but it also protected the graphite surface. Cd was electrodeposited at a controlled potential − 1.2 V (vs. saturated calomel electrode) using the Pd-modified graphite probe as a working electrode. The sensitivity of Cd determination remained unchanged for 300 electrodeposition and atomization cycles. The detection limit (3σ_blank) was improved with increasing time of electrolysis and was 1.2 ng l^− 1 for a 10 min electrolysis time in the presence of 0.1 mol l^− 1 NaNO₃. The procedure was applied for the determination of Cd in (1 + 1) diluted seawater and in (1 + 1) diluted urine samples using the standard addition method.     

Semi-online preconcentration of Cd, Mn and Pb on activated carbon for GFASS

This paper presents a method whereby trace elements in NH₄Cl-NH₃ medium are adsorbed on activated carbon in a micro-flow-injection (FI) semi-online sorbent extraction preconcentration system and then determined by graphite furnace atomic absorption spectrometry (GFAAS). The analytical performance of the proposed method for determining Cd, Mn and Pb was studied. A microcolumn packed with activated carbon was used as a preconcentration column (PCC). The metals to be determined were preconcentrated onto the column for 60 s and then rinsed with 0.02% (v/v) HNO₃ and eluted with 30 μl of 2 mol l⁻¹ HNO₃. Compared with the direct injection of 30 μl of aqueous sample solution, enrichment factor of 32, 26, and 21 and detection limits (3σ) of 0.4, 4.7, and 7.5 ng l⁻¹ for Cd, Mn and Pb, respectively, were obtained with 60 s sample loading at 3.0 ml min⁻¹ for sorbent extraction, 30 μl of eluate injection, and peak area measurement. The precisions (RSD, n=6) were 2.8% at the 0.05 μg l⁻¹ level for Cd, 3.0% at the 0.3 μg l⁻¹ level for Mn, and 3.1% at the 0.5 μg l⁻¹ level for Pb. The experimental results indicate that the procedure can eliminate the fundamental interferences caused by alkali and alkaline earth metals and the application of it to the determination of Cd, Mn and Pb in some water samples is successful.     

Flow-injection biamperometry of pyrogallol compounds

A biamperometric method for the direct determination of pyrogallol compounds has been designed for flow-injection analysis. The method is based on the electrocatalytic oxidation of pyrogallol compounds at one pretreated platinum electrode and the reduction of platinum oxide at the other pretreated platinum electrode to form a biamperometric detection system with the applied potential difference of 10 mV. Three important compounds, pyrogallol, gallic acid and tannic acid, have been detected by the method. The linear relationships between currents and the concentrations of pyrogallol, gallic acid and tannic acid are obtained over the range 1.0×10⁻⁶-1.0×10⁻⁴, 1.0×10⁻⁶-1.0×10⁻⁴ and 1.0×10⁻⁶-2.0×10⁻⁴ mol l⁻¹ with the detection limit of 6.0×10⁻⁷, 6.0×10⁻⁷ and 8.0×10⁻⁷ mol l⁻¹ (S/N=2), respectively. The R.S.D. observed for 30 successive determinations of 5.0×10⁻⁵ mol l⁻¹ pyrogallol, gallic acid and tannic acid are 1.9, 2.5 and 2.0%, respectively. Most ions and organic compounds tested are found not to cause significant interference in the determinations. The method is simple, selective and efficient (180 h⁻¹), performing well as a routine assay, and has been validated by the determination of pyrogallol compounds in tea and Chinese gall.     

Electrochemical detection of phenolic estrogenic compounds at carbon nanotube-modified electrodes

The use of a carbon nanotube-modified glassy carbon electrode (CNT-GCE) for the LC-EC detection of phenolic compounds with estrogenic activity is reported. Cyclic voltammograms for phenolic endocrine disruptors and estrogenic hormones showed, in general, an enhancement of their electrochemical oxidation responses at CNT-GCE attributable to the electrocatalytic effect caused by CNTs. Hydrodynamic voltammograms obtained under flow injection conditions lead to the selection of +700 mV as the potential value to be applied for the amperometric detection of the phenolic estrogenic compounds, this value being remarkably less positive than those reported in the literature using other electrode materials. Successive injections of these compounds demonstrated that no electrode surface fouling occurred. A mobile phase consisting of a 50:50 (v/v) acetonitrile:0.05 mol l⁻¹ phosphate buffer of pH 7.0 was selected for the chromatographic separation of mixtures of these compounds, with detection limits ranging between 98 and 340 nmol l⁻¹. Good recoveries were obtained in the analysis of underground well water and tap water samples spiked with some phenolic estrogenic compounds at a 14 nmol l⁻¹ concentration level.     

Preconcentration of palladium in a flow-through electrochemical cell for determination by graphite furnace atomic absorption spectrometry

An electrochemical preconcentration at a controlled potential on the electrode in a flow-through mode followed by graphite furnace atomic absorption spectrometric (GFAAS) detection is proposed for determination of trace amounts of palladium. After electrolysis the polarization of the electrodes was changed and deposited metal was dissolved electrochemically in the presence of an appropriate stripping reagent. Conditions for the electrodeposition, such as pH of the solutions, a deposition potential, dissolution potential and a composition of stripping solution were optimised. The graphite electrode (GE) and glassy carbon electrode (GCE) were tested for the palladium reduction process. The detection limit of 0.05 ng ml⁻¹ Pd (1 pg) was obtained after palladium preconcentration on the GCE and dissolution with 0.2 mol l⁻¹ thiourea in 0.1 mol l⁻¹ HCl followed by GFAAS detection. The method was applied for the determination of palladium in spiked tap water and road dust samples.     

Voltammetric determination of pyridoxine (Vitamin B6) at a carbon paste electrode modified with vanadyl(IV)-Salen complex

The preparation and electrochemical characterization of a carbon paste electrode modified with the N,N-ethylene-bis(salicylideneiminato)oxovanadium(IV) complex [VO(Salen)] as well as its behavior in relation to the oxidation of pyridoxine (Vitamin B₆) are described. The electrochemical behavior of the modified electrode and the electrooxidation of pyridoxine were investigated using cyclic voltammetry. The best voltammetric response was obtained for an electrode composition of 15% (m/m) [VO(Salen)] in the paste, KCl solution of pH 5.5-8.0 and scan rate of 25 mV s⁻¹. A sensitive linear voltammetric response for pyridoxine was obtained in the concentration range of 4.5×10⁻⁴ to 3.3×10⁻³ mol l⁻¹ with a slope of 42.5 μA mmol⁻¹ l, and a detection limit (3σ/slope) of 3.7×10⁻⁵ mol l⁻¹ using linear sweep voltammetry. Among several compounds tested only Vitamin B₁ seems to interfere in the analyte signal. The concentrations of pyridoxine in pharmaceutical formulations using the proposed electrode and an official spectrophotometric method based in the reaction with N,N-diethyl-p-phenylenediamine are in agreement at the 95% confidence level and within an acceptable range of error.     

Chromium(III) ion selective electrode based on glyoxal bis(2-hydroxyanil)

A poly(vinyl chloride) membrane based on glyoxal bis(2-hydroxyanil) as membrane carrier was prepared and investigated as a Cr(III)-selective electrode. The electrode has a linear dynamic range of 3.0×10⁻⁶-1.0×10⁻² mol l⁻¹, with a Nernstian slope of 19.8±0.5 mV per decade and a detection limit of 6.3×10⁻⁷ mol l⁻¹. It has a fast response time of <20 s and can be used for at least 3 months without any considerable divergence in potential. The proposed electrode revealed good sensitivities for Cr(III) over a wide variety of metal ions and could be used in a pH range of 2.7-6.5. Above all, the membrane sensor has been used very successfully for the analysis of some food materials and alloys for the determination of Cr(III) ion.