Valdir Souza Ferreira

Direct amperometric determination of tert-butylhydroquinone in biodiesel

The direct amperometric determination of tert-butylhydroquinone (TBHQ) in biodiesel at an unmodified glassy carbon electrode is reported. A biodiesel aliquot was added into an electrochemical cell containing a 75% (v/v) ethanol-water solution under stirring (with final concentration of 50 mmol L(-1) HClO(4)). The amperometric method involved the continuous application of three sequential potential pulses to the working electrode (700 mV for 300 ms, 0 mV for 100 ms and -50 mV for 1s). TBHQ was continuously monitored at the first (direct oxidation) and optionally at the second (reduction) potential pulse while the third potential pulse was applied for cleaning of the electrode surface. For comparison, the samples were also analyzed by high-performance liquid-chromatography and a good agreement between the results was verified. Recovery values for spiked samples were between 90% and 95% and the reproducibility of the proposed method was around 5% (n=5). The proposed method can be easily adapted for on-site analysis.

Differential pulse polarographic determination of ceftazidime in urine samples with and without prior extraction

Abstract Ceftazidime shows two main polarographic reduction peaks at pH 4.0, that at −0.45 V owing to reduction of the CN bond in the methylethoxyimino group and that at −1.00 V owing to the reductive elimination of pyridine: the first peak is particularly suitable for the determination of ceftazidime. Ceftazidime can also be determined indirectly using the tensammetric peak at −0.60 V (in Britton-Robinson buffer pH 9.5) of pyridine liberated on hydrolysis. Ceftazidime can be determined in urine using the direct method only after C 18 solid phase extraction, but it can be determined directly in the urine by hydrolysing it and using the pyridine peak.

Determination of thiodicarb using a biosensor based on alfalfa sprout peroxidase immobilized in self-assembled monolayers

A biosensor based on alfalfa sprout (Medicago sativa) homogenate as a source of peroxidase is proposed for the determination of thiodicarb by square-wave voltammetry. This enzyme was immobilized in self-assembled monolayers of l-cysteine on a gold electrode. Several parameters were investigated to evaluate the optimum conditions for operation of the biosensor. The analytical curve was linear for thiodicarb concentrations of 2.27 x 10(-6) to 4.40 x 10(-5) mol L(-1) with a detection limit of 5.75 x 10(-7) mol L(-1). The lifetime of the Au-alfalfa sprout-SAMs was 20 days (at least 220 determinations). The average recovery of thiodicarb from samples of vegetable extracts ranged from 99.02 to 101.04%. The results obtained for thiodicarb in vegetable extracts using the proposed method are in close agreement with those using a high performance liquid chromatography procedure at the 95% confidence level.

Voltammetric determination of tert-butylhydroquinone in biodiesel using a carbon paste electrode in the presence of surfactant

This paper reports an electroanalytical method developed for determining the antioxidant tert-butylhydroquinone (TBHQ) in biodiesel, based on the enhancement effect of cetyltrimethylammonium bromide (CTAB). In pH 6.5 Britton-Robinson buffer, a poorly defined oxidation peak was observed for TBHQ at a carbon paste electrode (CPE). In the presence of low concentrations of CTAB, however, the oxidation peak current was markedly increased. Several parameters were studied and optimized for the development of this methodology, and under optimal conditions the oxidation peak current was proportional to TBHQ concentration in the range of (1.05-10.15) x 10(-6) mol L(-1), with limits of detection and quantification of 7.11 x 10(-8) mol L(-1) and 2.37 x 10(-7) mol L(-1), respectively, by linear sweep voltammetry (LSV). The method was applied to TBHQ determination in soybean biodiesel samples. The results were satisfactory in comparison with those obtained using high-performance liquid chromatography (HPLC).

Interaction study of moxifloxacin with Cu(II) ion using square-wave voltammetry and its application in the determination in tablets

Este trabalho apresenta um metodo eletroanalitico para a determinacao de moxifloxacina em uma formulacao farmaceutica a partir da interacao com o ion Cu(II), monitorando a reducao sobre eletrodo de gota de mercurio pendente (HMDE). Um pico de reducao bem definido em -0.21 V vs. Ag/AgCl foi observado apos a interacao da MOXI com o ion Cu(II) por voltametria de onda quadrada (SWV) em tampao fosfato 0.04 mol l-1 pH 8.0. Usando 10 segundos de pre-contracao em -0.40 V um limite de deteccao de 3.60 10-8 mol l-1 foi encontrado. A comparacao dos resultados obtidos com um metodo espectrofometrico nao mostrou diferenca significativa entre as medidas.

Indirect cathodic-stripping voltammetric determination of ceftazidime as a mercury salt

At accumulation potentials close to +0.1 V at a hanging mercury drop electrode, ceftazidime is accumulated at pH 9.5, probably in a hydrolysed or otherwise chemically altered form, in an anodic process to give an adsorbed mercury salt. The accumulation of this mercury salt allows the indirect cathodic-stripping voltammetric determination of ceftazidime using the reduction peak of the mercury salt at -0.70 V. The high sensitivity of the method coupled with high sample dilution allows ceftazidime to be determined in milk samples at the 28 μg ml -1 level without prior separation. In order to determine lower levels of ceftazidime in milk (ca. 10 ng ml -1 ) a separation process would be required.

Determination of linuron in water and vegetable samples using stripping voltammetry with a carbon paste electrode.

A carbon paste electrode was used for the electrochemical determination of linuron concentrations in water and vegetable extracts. Optimal conditions were established with respect to electrode activation (electrochemical pretreatment), time accumulation, potential accumulation, scan rate, and pH. The limit of detection achieved with a pre-concentration step was 23.0 μg L(-1). Recovery measurements in vegetable extract and natural water samples were in the range of 98-103%, indicating that the proposed electrochemical method can be employed to analyze linuron in these matrices. The determination results were in good agreement with HPLC results.

Carbon-nanotube-modified screen-printed electrodes, a cationic surfactant, and a peak deconvolution procedure: alternatives to provide satisfactory simultaneous determination of three synthetic antioxidants in complex samples

The development of electroanalytical methods for determining three or more analytes simultaneously in a complex sample is highly desirable. Unfortunately, this is not an easy task in electrochemical studies of organic compounds, and alternative methods to achieve this goal have been increasingly sought in recent years. To this end, the present investigation employed a combination of screen-printed electrodes (SPEs), cetyltrimethylammonium bromide (CTAB), and a peak deconvolution procedure as an alternative approach to improve the sensitivity and selectivity in the simultaneous determination of three antioxidants in samples of biodiesel and cosmetic oils. The use of CTAB also prevents the occurrence of SPE fouling by matrix constituents, being essential in the simultaneous detection of the antioxidants tert-butylhydroquinone (TBHQ), propyl gallate (PG), and butylhydroxyanisole (BHA). Under optimized conditions, the limit of detection proved satisfactory, with relative standard deviations of 0.99% for TBHQ, 2.1% for PG, and 0.61% for BHA, indicating that the method has satisfactory repeatability. The approach was successfully applied to determine the target antioxidants in biodiesel samples at different concentrations after simple, rapid dilution, yielding recovery rates of 98–106% for TBHQ, 98–103% for PG, and 92–101% for BHA. In the analysis of cosmetic oils, a deconvolution procedure was applied to achieve separation of the oxidation peak potentials, with recovery rates of 94% for PG and 101% for BHA using the standard addition method. The results were satisfactory compared with those obtained using high-performance liquid chromatography.

Polarographic determination of sunscreen agents in cosmetic products in micellar media

This paper introduces a simple, fast and reliable electroanalytical method for differential-pulse polarography based on electrochemical reduction at a dropping mercury electrode. The method was validated for the determination of 2-ethylhexyl-4-methoxycinnamate (EHMC) alone and in association with 4-methylbenzylidene camphor (MBC) or 2-hydroxy-4-methoxybenzophenone (BENZ-3) in samples of commercial cosmetic preparations. The supporting electrolyte that provided the best-defined and most intense peak current for EHMC determination was Britton-Robinson buffer (pH 4.0) in the presence of a cationic surfactant. Under optimized conditions, EHMC exhibited one single peak of reduction at -1.49 V versus Ag/AgCl. A limit of detection of 3.76 x 10(-8)mol L(-1) and a limit of quantitation of 1.25 x 10(-7) mol L(-1) were found for the pure EHMC standard. A good average recovery rate was reached for all the samples analyzed.

Voltammetric determination of the phytoestrogen genistein in soy flours and soy based supplements using cationic surfactant cetyltrimetylammonium bromide

This paper proposes a methodology based on electrochemical reduction for a fast, specific and sensitive determination of genistein by square-wave voltammetry (SWV). The highest peak currents and optimal definition of reduction peak of genistein was obtained by using a supporting electrolyte consisted of 0.04 mol L(-1) phosphate buffer-methanol (8:2, v/v) solution at pH 7.5 and cationic surfactant 2.21 x 10(-4) mol L(-1) cetyltrimethylammonium bromide (CTAB). The peak current was proportional to the concentration of genistein in the range 1.14 x 10(-7) to 1.09 x 10(-6) mol L(-1), with a detection limit and quantification of 3.43 x 10(-8) mol L(-1) and 1.14 x 10(-7) mol L(-1), respectively. The extraction procedure was evaluated for a better determination of genistein contained in soy flour and soy based supplements. The efficiency of the proposed methodology was demonstrated by comparing the results obtained by SWV with those obtained by the high-performance liquid chromatography (HPLC) method.