Spectrophotometric determination of methyldopa and dopamine in pharmaceutical formulations using a crude extract of sweet potato root (Ipomoea batatas (L.) Lam.) as enzymatic source

A rapid, precise and low cost spectrophotometric method is proposed for the determination of methyldopa and dopamine in pharmaceutical formulations. The crude extract of sweet potato root (Ipomoea batatas (L.) Lam.) was used as an enzymatic source of polyphenol oxidase (PPO; EC.1.14.18.1). This enzyme catalyses the oxidation of catecholamines to the corresponding methyldopaquinone and dopaminequinone. Those compounds are converted by a rapid spontaneous auto-oxidation to methyldopachrome and dopaminechrome which have a strong absorption at 480 or 470 nm, respectively. The calibration graphs are linear from 2.0x10(-4) to 6.0x10(-3) M. The results obtained by the proposed enzymatic method are in close agreement with those obtained using a Pharmacopoeia procedure and also with the label values. The detection limit (three times the signal blank/slope) was 3.4x10(-5) and 3.0x10(-5) M for methyldopa and dopamine, respectively, the recovery of methyldopa and dopamine from three samples ranged from 97.5 to 102.9% of the added amount.     

Simultaneous square-wave voltammetric determination of aspartame and cyclamate using a boron-doped diamond electrode

A simple and highly selective electrochemical method was developed for the simultaneous determination of aspartame and cyclamate in dietary products at a boron-doped diamond (BDD) electrode. In square-wave voltammetric (SWV) measurements, the BDD electrode was able to separate the oxidation peak potentials of aspartame and cyclamate present in binary mixtures by about 400 mV. The detection limit for aspartame in the presence of 3.0x10(-4) mol L(-1) cyclamate was 4.7x10(-7) mol L(-1), and the detection limit for cyclamate in the presence of 1.0x10(-4) mol L(-1) aspartame was 4.2x10(-6) mol L(-1). When simultaneously changing the concentration of both aspartame and cyclamate in a 0.5 mol L(-1) sulfuric acid solution, the corresponding detection limits were 3.5x10(-7) and 4.5x10(-6) mol L(-1), respectively. The relative standard deviation (R.S.D.) obtained was 1.3% for the 1.0x10(-4) mol L(-1) aspartame solution (n=5) and 1.1% for the 3.0x10(-3) mol L(-1) cyclamate solution. The proposed method was successfully applied in the determination of aspartame in several dietary products with results similar to those obtained using an HPLC method at 95% confidence level.     

Promising electrochemical performance of high-surface-area boron-doped diamond/carbon nanotube electroanalytical sensors

Porous boron-doped diamond (p-BDD) electrodes of high-surface-area have been prepared on vertically aligned carbon nanotube substrates, and their electrochemical performance has demonstrated promising results for application in electroanalysis. The electrochemical features of the p-BDD electrodes were investigated and compared with those of a conventional flat BDD electrode (f-BDD). From cyclic voltammetry studies performed for the electrochemical probes [Fe(CN)₆]^3? and N,N,N′,N′-tetramethyl-para-phenylenediamine (TMPD), a fast charge transfer was observed at the p-BDD/electrolyte interface. For the [Fe(CN)₆]^3? redox probe, the heterogeneous electron-transfer rate constant (k ⁰) value obtained for p-BDD was 10.9 times higher than that obtained using a f-BDD electrode. Moreover, the p-BDD electrodes also gave a smaller peak potential separation, ΔE _p, and larger analytical signal magnitude for different biomolecules, such as dopamine (DA), acetaminophen (AC), and epinephrine (EP). These set of results demonstrated that the p-BDD electrode is a suitable candidate for applications in electroanalytical chemistry.     

Sensitive and Selective Voltammetric Determination of Ciprofloxacin Using Screen-printed Electrodes Modified with Carbon Black and Magnetic-molecularly Imprinted Polymer

This work reports the construction of screen-printed electrode (SPE) modified with carbon black (CB) and magnetic Fe₃O₄ nanoparticles coated with molecularly imprinted polymer (mag@MIP) based on an alternative low-cost approach. The proposed device was used for the sensitive and selective voltammetric determination of the antibiotic ciprofloxacin. The results obtained from cyclic voltammetry pointed to the improvement of the SPE response toward the irreversible oxidation of ciprofloxacin after the modification of the electrode surface with CB-based film and the magnetic preconcentration of mag@MIP containing adsorbed ciprofloxacin molecules. Compared to the non-molecularly imprinted sensor (mag@NIP), the presence of cavities selective for ciprofloxacin recognition in the mag@MIP nanoparticles accounted for the improvement in analytical signal of the molecularly imprinted sensor. The application of differential pulse voltammetry for ciprofloxacin determination yielded a linear response in the concentration range of 0.5 to 7.0 μmol L⁻¹. The repeatability and interference tests results showed that the proposed electrochemical sensor has good measurement accuracy and selectivity. The proposed device was applied for the voltammetric determination of ciprofloxacin in river water and synthetic urine samples, where recovery rates close to 100 % were recorded for all the samples. The quantification data obtained from the application of the proposed voltammetric method were quite consistent with those of the HPLC reference method.     

Potentiometric determination of acids and bases using a silica gel based carbon-epoxy indicator electrode

The construction and the application of a silica gel based carbon-epoxy indicator electrode for the potentiometric determination of acids and bases are described. The effect of composition of silica gel and carbon-epoxy, slope (mV/pH), linear response (pH range) and the use for acid-base titrations were investigated. The data obtained for the acid-base titrations were compared with those obtained using a glass electrode in the same conditions. The electrode showed a linear response in the pH 2 to 13 range with a slope of -40.5 +/- 0.4 mV/pH (at 25 degrees C) and a response time of less than 15 s. The lifetime of the electrode was higher than one year (over 6000 determinations) with a decrease of only 5% of the initial potentiometric response. The silica gel based carbon-epoxy electrode showed excellent results in the end-point indication potentiometric titrations in determination of acids and bases. The miniaturization of the proposed electrode for flow injection analysis was investigated.     

PbO2-based graphite-epoxy electrode for potentiometric determination of acids and bases in aqueous and aqueous-ethanolic media

The construction and analytical evaluation of a PbO2-based graphite-epoxy electrode sensitive to H3O+, based on incorporation of lead(IV) oxide in a graphite-epoxy matrix, are described. The data obtained from a variety of acid-base titrations in aqueous and aqueous-ethanolic media were compared with those obtained by use of a glass electrode under the same conditions. The proposed electrode provides a linear response in the pH range from 1 to 11 with a slope of -58.7+/-0.3 mV pH(-1) and -60.8+/-0.2 mV pH(-1) in aqueous and ethanolic media, respectively. The response time was less than 15 s and the lifetime of the electrode was at least eight months (ca. 5000 determinations) and its performance is good in pH determination and end-point detection in potentiometric acid-base titrations in both aqueous and aqueous-ethanolic media.     

Flow-injection turbidimetric determination of homatropine methylbromide in pharmaceutical formulations using silicotungstic acid as precipitant reagent

A flow-injection turbidimetric procedure exploiting merging zones is proposed for determining homatropine methylbromide (HMB) in pharmaceutical preparations. The determination is based on the precipitation reaction of homatropine methylbromide with silicotungstic acid in acidic medium to form a precipitate, which was measured at 410 nm. The analytical curve was linear in the HMB concentration range from 8.1 × 10⁻⁵ to 2.2 × 10⁻⁴ mol l⁻¹, with a detection limit of 5.0 × 10⁻⁶ mol l⁻¹. The recoveries ranged from 96 to 103%, the sampling frequency was 70 determinations per hour and relative standard deviations were less than 1.5% (n = 10). The results obtained for commercial formulations using the FIA procedure were in good agreement with those obtained by using a comparative method.     

Flow injection spectrophotometric determination of reducing sugars using a focalized coiled reactor in a domestic microwave oven

A flow injection (FI) spectrophotometric procedure for determining reducing sugars content in sugar cane juices using a focalized PTFE coiled reactor positioned at the output antenna of a domestic microwave oven at 700 W is proposed. In this system, sample solution converge to 1.0 mol l(-1) NaOH and 5.2 mmol l(-1) K(3)Fe(CN)(6) solutions previously mixed and the decrease of hexacyanoferrate(III) concentration was monitored at 420 nm. Under best analytical conditions, there was a direct relationship between absorbance decrease and reducing sugar content (fructose plus glucose concentrations) in the concentration range from 50 to 1200 micromol l(-1) with a detection limit of 15 micromol l(-1). The relative standard deviations (rds) were less than 1.4% for ten injection of 400 and 800 micromol l(-1) fructose solution and the analytical frequency was 70 h(-1). A paired t-test showed that all results obtained for sugar cane juices using this FI procedure and the Somogyi-Nelson batch procedure agree at the 95% confidence level.     

Simultaneous voltammetric determination of paracetamol and caffeine in pharmaceutical formulations using a boron-doped diamond electrode

A simple and highly selective electrochemical method was developed for the single or simultaneous determination of paracetamol (N-acetyl-p-aminophenol, acetaminophen) and caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) in aqueous media (acetate buffer, pH 4.5) on a boron-doped diamond (BDD) electrode using square wave voltammetry (SWV) or differential pulse voltammetry (DPV). Using DPV with the cathodically pre-treated BDD electrode, a separation of about 550 mV between the peak oxidation potentials of paracetamol and caffeine present in binary mixtures was obtained. The calibration curves for the simultaneous determination of paracetamol and caffeine showed an excellent linear response, ranging from 5.0 × 10⁻⁷ mol L⁻¹ to 8.3 × 10⁻⁵ mol L⁻¹ for both compounds. The detection limits for the simultaneous determination of paracetamol and caffeine were 4.9 × 10⁻⁷ mol L⁻¹ and 3.5 × 10⁻⁸ mol L⁻¹, respectively. The proposed method was successfully applied in the simultaneous determination of paracetamol and caffeine in several pharmaceutical formulations (tablets), with results similar to those obtained using a high-performance liquid chromatography method (at 95% confidence level).     

Synergic effect studies of the bi-enzymatic system laccase-peroxidase in a voltammetric biosensor for catecholamines

Several bi-enzymatic carbon paste biosensors modified with enzymes laccase from Pleurotus ostreatus fungi and peroxidase from zucchini (Cucurbita pepo) were constructed for evaluating the synergic effect of the two enzymes on the voltammetric biosensor response for various catecholamines. Initially was investigated the effect of pH from 5.0 to 7.5, temperature from 25 to 50 °C, initial stirring time from 30 to 150 s, scan rate from 10 to 60 mV s⁻¹ and potential pulse amplitude from 10 to 60 mV on the biosensor response for several catecholamines such as dopamine, adrenaline, isoprenaline and l-dopa. It was observed a biosensor signal increase employing both enzymes, indicating thus there is a synergic effect between laccase and peroxidase, verified also in spectrophotometric studies, in the determination of these catecholamines.