Square wave adsorptive cathodic stripping voltarnmetry automated by sequential injection analysis - Potentialities and limitations exemplified by the determination of methyl parathion in water samples

This paper describes the development and evaluation of a sequential injection method to automate the determination of methyl parathion by square wave adsorptive cathodic stripping voltammetry exploiting the concept of monosegmented flow analysis to perform in-line sample conditioning and standard addition. Accumulation and stripping steps are made in the sample medium conditioned with 40 mmol L-1 Britton-Robinson buffer (pH 10) in 0.25 mol L-1 NaNO3. The homogenized mixture is injected at a flow rate of 10 mu Ls(-1) toward the flow cell, which is adapted to the capillary of a hanging drop mercury electrode. After a suitable deposition time, the flow is stopped and the potential is scanned from -0.3 to -1.0 V versus Ag/AgCl at frequency of 250 Hz and pulse height of 25 mV The linear dynamic range is observed for methyl parathion concentrations between 0.010 and 0.50 mgL(-1), with detection and quantification limits of 2 and 7 mu gL(-1), respectively. The sampling throughput is 25 h(-1) if the in line standard addition and sample conditioning protocols are followed, but this frequency can be increased up to 61 h(-1) if the sample is conditioned off-line and quantified using an external calibration curve. The method was applied for determination of methyl parathion in spiked water samples and the accuracy was evaluated either by comparison to high performance liquid chromatography with UV detection, or by the recovery percentages. Although no evidences of statistically significant differences were observed between the expected and obtained concentrations, because of the susceptibility of the method to interference by other pesticides (e.g., parathion, dichlorvos) and natural organic matter (e.g., fulvic and humic acids), isolation of the analyte may be required when more complex sample matrices are encountered. (C) 2007 Elsevier B.V. All rights reserved.

On the activation and physical degradation of boron-doped diamond surfaces brought on by cathodic pretreatments

The electrochemical activation and physical degradation of boron-doped diamond (BDD) electrodes with different boron doping levels after repeated cathodic pretreatments are reported. Galvanostatic cathodic pretreatment passing up to -14000 C cm(-2) in steps of -600 C cm(-2) using -1 A cm(-2) caused significant physical degradation of the BDD surface, with film detachment in some areas. Because of this degradation, a great increase in the electrochemically active area was observed in Tafel plots for the hydrogen evolution reaction (HER) in acid media. The minimum cathodic pretreatment needed for the electrochemical activation of the BDD electrodes without producing any observable physical degradation on the BDD surfaces was determined using electrochemical impedance spectroscopy (EIS) measurements and cyclic voltammetry: -9 C cm(-2), passed at -1 A cm(-2). This optimized cathodic pretreatment can be safely used when electrochemical experiments are carried out on BDD electrodes with doping levels in the range between 800 and 8000 ppm.

Nitriding in cathodic cage of stainless steel AISI 316: influence of sample position

R.R.M. de Sousa et al. Nitriding in cathodic cage of stainless steel AISI 316: Influence of sample position. Vacuum, [s.l.], n.83, 2009. Disponivel em: . Acesso em: 04 out.2010.

The shielding effect of glycerol against protein ionization in electrospray mass spectrometry

Most commercial recombinant proteins used as molecular biology tools, as well as many academically made preparations, are generally maintained in the presence of high glycerol concentrations after purification to maintain their biological activity. The present study shows that larger proteins containing high concentrations of glycerol are not amenable to analysis using conventional electrospray ionization mass spectrometry (ESI-MS) interfaces. In this investigation the presence of 25% (v/v) glycerol suppressed the signals of Taq DNA polymerase molecules, while 1% (v/v) glycerol suppressed the signal of horse heart myoglobin. The signal suppression was probably caused by the interaction of glycerol molecules with the proteins to create a shielding effect that prevents the ionization of the basic and/or acidic groups in the amino acid side chains. To overcome this difficulty the glycerol concentration was decreased to 5% (v/v) by dialyzing the Taq polymerase solution against water, and the cone voltage in the ESI triple-quadrupole mass spectrometer was set at 80-130 V. This permitted observation of a mass spectrum that contained ions corresponding to protonation of up to 50% of the ionizable basic groups. In the absence of glycerol up to 85% of the basic groups of Taq polymerase became ionized, as observed in the mass spectrum at relatively low cone voltages. An explanation of these and other observations is proposed, based on strong interactions between the protein molecules and glycerol. For purposes of comparison similar experiments were performed on myoglobin, a small protein with 21 basic groups, whose ionization was apparently suppressed in the presence of 1% (v/v) glycerol, since no mass spectrum could be obtained even at high cone voltages. Copyright (C) 2003 John Wiley Sons, Ltd.

Gravity, antigravity and gravitational shielding in (2+1) dimensions

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Voltammetric characteristics of miconazole and its cathodic stripping voltammetric determination

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Electrochemical reduction and cathodic stripping voltammetric determination of clotrimazole

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

The Cathodic Cleavage of the Nitrobenzoyl Group from Protected Aliphatic Amines in N,N-Dimethylformamide

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Cathodic stripping voltammetric determination of ceftazidime in urine at a hanging mercury drop electrode

A method was developed for the differential-pulse cathodic stripping voltammetric determination of ceftazidime with a hanging mercury drop electrode using its reduction peak at -0.43 V in Britton-Robinson buffer pH 4.0. The optimum accumulation potential and time were -0.15 V and up to 60 s, respectively. Linear calibration graphs were obtained from 1 x 10(-8) M and 1.5 x 10(-7) M. The limit of determination was calculated to be 5 x 10(-9) M. The coefficient of variation was 4% (n = 7) at 1 x 10(-7) M ceftazidime. The effect of various components of urine on the voltammetric response was studied, and creatinine, uric acid, urea, and glucose were shown to interfere in the method. Ceftazidime bound to human albumin gives a unique stripping peak at -0.48 V. Recoveries of 87% +/- 2% of the ceftazidime (n = 5) were obtained from urine spiked with 1.27 mu g ml(-1) using C-18 solid phase extraction cartridges. (C) 1997 Academic Press.

Determination of brilliant blue FCF in the presence and absence of erythrosine and quinoline yellow food colours by cathodic stripping voltammetry

A study of the voltammetric behaviour of the food colours brilliant blue FCF (C.I. 42090), erythrosine (C. I. 45430) and quinolin e yellow (C. I. 47005) in the pH range 2-10 have been carried out by cathodic stripping voltammetry. At pH 4.5 (acetate buffer) with an accumulation potential of 0 V and accumulation time of 30 s, the voltammograms presented well-defined reduction peaks at potential - 0.76 V for brilliant blue FCF, - 0.85 V for quinoline yellow and - 0.54 V for erythrosine. Linear calibration graphs were obtained from 8 to 80 mug l(-1) brilliant blue, from 4 to 43 mug l(-1) quinoline yellow and from 10 to 70 mug l(-1) erythrosine. The method has been successfully applied to identify and quantify binary mixtures of these dyes and applied for determining brilliant blue FCF in commercial food products.

Cathodic stripping voltammetric detection and determination at a hanging mercury-drop electrode of dye contaminants in purified biomaterials: study of the human serum albumin and reactive dye 120 system

Procion red HE-3B (RR120) is an example of dye currently used in affinity purification. A method is described for determining trace amounts of RR120 dye contaminant in human serum albumin by cathodic stripping voltammetry. The method is based on a measure of a well-defined peak at -0.58 V, obtained when samples of HSA protein (0.01-2% w/v) containing dye concentrations are submitted to a heating time of 330 min at 80degreesC in NaOH, pH 12.0 and the samples are removed to a solution containing Britton-Robinson buffer, pH 4.0. Using an optimum accumulation potential and tune of 0 V and 240 s, respectively, linear calibration curves were obtained from 1.0 X 10(-9) to 1.0 X 10(-8) mol 1(-1) for RR120 dye. Leakage/hydrolysis of reactive red 120 from an agarose support (e.g. at pH 2 or 12) can also be conveniently determined at very low levels (sub-mug ml(-1)) by means of cathodic stripping voltammetry, which involves adsorptive accumulation of the dye onto the hanging mercury-drop electrode. (C) 2002 Elsevier B.V. B.V. All rights reserved.

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 mu g 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. (C) 1998 Elsevier B.V. B.V. All rights reserved.

Cathodic stripping voltammetric determination of ceftazidime with reactive accumulation at a poly-L-lysine modified hanging mercury drop electrode

Ceftazidime is hydrolysed only slowly at pH 10 at room temperature. This is indicated by a small cathodic stripping voltammetric peak obtained at pH 10 at a hanging mercury drop electrode at about -0.6 V which corresponds to the reduction of the hydrolysis product. This peak is enhanced more than tenfold by the addition of poly-L-lysine (PLL) to the electrolyte solution. The optimum accumulation potential is between 0 and -0.1 V: the size of the peak decreases steadily, however, as the accumulation potential is moved to more negative potentials and is about one-sixth the size for accumulation at -0.4 V. Existing knowledge of the organic chemistry of cephalosporins indicates that the accumulation must involve an aminolysis reaction of the unprotonated PLL with the beta-lactam ring of the ceftazidime. The limit of detection (3 sigma) in standard solutions was calculated to be 1 x 10(-10) mol l(-1). The detection limit in buffer solution containing 1% of urine was calculated to be 5 x 10(-9) mol l(-1), i.e. 5 x 10(-6) mol l(-1) in the urine. (C) 1999 Elsevier B.V. B.V. AU rights reserved.

Molecular modeling and small angle X-ray scattering studies of Hoplosternum littorale cathodic haemoglobin

Considerable interest is currently focused on fish haemoglobins in order to identify the structural basis for their diversity of functional behavior. Hoplosternum littorale is a catfish that presents bimodal gill (water)/gut (air) -breathing, which allows this species to survive in waters with low oxygen content. The hemolysate of this fish showed the presence of two main haemoglobins, cathodic and anodic. This work describes structural features analyzed here by integration of molecular modeling with small angle X-ray scattering. Here is described a molecular model for the cathodic haemoglobin in the unliganded and liganded states. The models were determined by molecular modeling based on the high-resolution crystal structure of fish haemoglobins. The structural models for both forms of H. littorale haemoglobin were compared to human haemoglobin. (C) 2004 Elsevier B.V. All rights reserved.