Stripping Square Research Articles

Spectrophotometric and adsorptive stripping square wave voltammetric determination of iron in olive oils, as complex with 5,5-dimethylcyclohexane-1,2,3-trione 1,2-dioxime 3-thiosemicarbazone (DCDT).

Two methods for the determination of iron in olive oil by spectrophotometry and by adsorptive stripping square wave voltammetry (Ad-SSWV) have been developed. These two methods are based on the formation of a 5,5-dimethylcyclohexane-1,2,3-trione 1,2-dioxime 3-thiosemicarbazone (DCDT)-iron(II) complex in strongly acid media. In both, iron is extracted from the olive oil by using HCl. Spectrophotometric determination of iron with DCDT is based on the feature that the DCDT-Fe complex shows an absorbance maximum at 550 nm. A calibration graph has been constructed from 0 to 4000 ng mL(-)(1), and the detection limit was 115 ng mL(-)(1) (57 ng g(-)(1) in olive oil). On the other hand, the voltammetric determination of the metal is based on the appearance of a peak due to an adsorptive reductive process of the complex that it is observed when the Ad-SSWV technique is used. A calibration graph has been constructed from 0 to 30 ng mL(-)(1), and the detection limit was 0.55 ng mL(-)(1) (13.75 ng g(-)(1) in olive oil according to the proposed procedure).

Application of avidin-biotin technology and adsorptive transfer stripping square-wave voltammetry for detection of DNA hybridization and avidin in transgenic avidin maize.

The proteins streptavidin and avidin were electrochemically detected in solution by adsorptive transfer stripping square wave voltammetry (AdTS SWV) at a carbon paste electrode (CPE). AdTS SWV was used to quantify biotinylated oligonucleotides, DNA hybridizations, and avidin in extracts of transgenic avidin maize. The detection limits of denatured and native streptavidin were 6 pM and 120 nM, respectively. The results demonstrated that streptavidin/avidin AdTS SWV is a sensitive and specific method for quantifying DNA and proteins in biological samples such as foods and tissue extracts, including genetically modified crops (avidin maize) and other plants in neighboring fields.

Electrochemical study of taxol (paclitaxel) by cathodic stripping voltammetry: determination in human urine

An analytical method for the determination of taxol (paclitaxel) via cathodic stripping square wave voltammetry has been developed. The method allows to achieve a detection limit of 5.2 ng ml −1 and a determination limit of 11.0 ng ml −1 working with a mercury drop electrode in 0.05 M boric acid/borate buffer pH 9.0, previously accumulated at −0.1 V (v.s. Ag/AgCl/KCl 3 M) for 360 s. Moreover, information about the mechanisms governing the reduction of taxol has been obtained from the studies of the instrumental parameters of square wave voltammetry technique. After addition of an internal standard, taxol was extracted from spiked human urine and the proposed method was successfully applied to the quantification of the analyte.

Voltammetric detection of copper(II) at a carbon paste electrode containing an organically modified silica

A carbon paste electrode modified with aminopropyl-grafted silica gel (APS) was evaluated as an electrochemical sensor for copper species in aqueous medium. The overall analysis involved a two-step procedure: an accumulation step at open circuit, followed by medium exchange to a pure electrolyte solution for the voltammetric quantification. During the preconcentration step, copper(II) was accumulated on the APS surface by complexation with the amine groups. In the detection step, copper(II) was desorbed in acidic medium, then cathodically electrolyzed at −0.5 V for 60 s, and detected by anodic stripping square wave voltammetry. The influence of various experimental parameters on the sensor response was investigated (i.e. pH of the detection and accumulation media, electrolysis time, carbon paste composition, silica gel porosity, accumulation time). Reproducible results were obtained, giving after optimization a linear calibration in the 5×10 −8–2×10 −7 M concentration range, with a detection limit of 3×10 −9 M. This new sensor was applied to copper analysis in laboratory tap water.

Silica-modified electrode for the selective detection of mercury

Pure silica particles were dispersed within carbon paste and the resulting modified electrode was applied to the selective voltammetric detection of mercury(II) species after their accumulation at open circuit. The remarkable selectivity observed between pH 4 and 7 was attributed to the intrinsic adsorption mechanism which involves a condensation reaction between mercury(II) hydroxide and hydroxyl groups on the silica surface, leading to the formation of an inner-sphere-type surface complex. After optimization with respect to the electrode composition, the detection medium, and the voltammetric scan mode, a linear response was obtained in the concentration range between 2 × 10−7 M to 1 × 10−5 M, by applying anodic stripping square wave voltammetry. Various silica samples were used and their sorption behavior was discussed in relation to their specific surface area and porosity. The effect of chloride and pH on the accumulation of mercury(II) on silica was also investigated.

Voltammetric behavior of sildenafil citrate (Viagra) using square wave and adsorptive stripping square wave techniques: Determination in pharmaceutical products

The behavior of sildenafil citrate (SC) was studied by square wave technique, leading to two methods for its determination in aqueous samples (pH 2.0) and pharmaceutical formulations. The application of the square wave (SW) without the adsorptive accumulation shows the maximum response at –1.03 V. Besides, SC gave two adsorptive stripping voltammetric (AdSV) peaks at –1.03 and −1.15 V using an accumulation potential of −0.80 V. The effect of experimental parameters that affect this determination are discussed. For the stripping technique, SC proved to be more sensitive, yielding signals three or four times larger than those obtained by applying a square wave scan without the previous accumulation. The calibration graph to determine SC was linear in the range 5×10 −9 to 9×10 −7 M by stripping mode with a time of accumulation t acc of 10 s. The relative standard deviations obtained for concentration levels of SC as low as 1.0×10 −7 M with square wave was 2.5% ( n=10) and for 1.0×10 −7 M with stripping square wave was 3.1% ( n=10) in the same day. The two proposed methods (SW and SWAdSV) were applied to the determination of SC in three pharmaceutical products (Viagra 25, Viagra 50 and Viagra 100) with very good recoveries with respect to the labelled values.

Geochemical factors controlling free Cu ion concentrations in river water

Copper speciation was determined monthly at seven sites on four rivers in southern New England to understand which geochemical factors control free metal ion concentrations in river water. Samples were conventionally filtered (<0.45 μm) and then ultrafiltered (3.000 molecular weight cut-off) to determine Cu speciation in the truly dissolved size fraction. Differential pulse anodic stripping voltammetry (DPASV) was used to quantify natural organic complexation and cathodic stripping square wave voltammetry (CSSWV) to measure directly both Cu sulfide complexes and total EDTA concentrations. The results showed both dissolved organic matter (DOM) and sulfide complexation dominate Cu speciation and control the concentrations of free ion. Free Cu2+ was calculated to be in the subnanomolar range for the majority of the year. Only in the winter months, when concentrations of DOM and metal sulfides complexes were at a minimum were free metal ions directly measurable by DPASV at low nanomolar concentrations. The extent of sulfide complexation appears to be dominated by the size of headwater marshes (upstream sampling sites) and by the amount of sewage treatment plant effluent (downstream sites). DOM complexation was related to the organic matter composition and followed model organic ligands. Indirect evidence suggests variations in river water pH and Ca2+ (metal competition) has only a minor role in Cu complexation. Measured concentrations of total EDTA suggest this synthetic ligand can control Cu speciation in some highly developed watersheds; however, competition from Ni (and possibly Fe) limits the extent of this complexation.

Electroanalytical study and square wave voltammetric techniques for the determination of β-blocker timolol at the mercury electrode

An electroanalytical study of timolol in Britton–Robinson buffer is described. The reduction process on the hanging mercury drop electrode gives rise to two adjacent peaks above −1.0 V (versus Ag/AgCl), the latter on the background discharge, within the entire pH range studied (1.8–10.5), the polarographic p K a value obtained was 6.75. The results proved that the reduction of timolol is irreversible and the limiting current is adsorption controlled. The dependence of the peak current on concentration was studied by means of different polarographic and voltammetric techniques. Using adsorptive stripping square wave voltammetry (AdS-SWV), the height for the first peak was proportional to the concentration of timolol in two linear ranges: 1.0×10 −9 to 1.2×10 −8 M and 1.2×10 −8 to 1.1×10 −7 M, the 3 σ detection limit being 6.6×10 −10 mol l −1. For square wave polarography (SWP) the linear range and the detection limit were 4.0×10 −8 to 3.0×10 −6 M with two different slopes and 2.5×10 −8 M, respectively. A mechanism is proposed for the reduction of timolol. Two procedures, based on SWP and AdS-SWV, were developed for the determination of timolol in ophthalmologic drops.

Electrochemical behaviour of Venlafaxine and its determination in pharmaceutical products using square wave voltammetry

An electrochemical method for the determination of Venlafaxine in pharmaceutical formulations recently available on the European market is described. The electrochemical oxidation of Venlafaxine was studied at a HMDE electrode over a wide pH range (1.9–10.0) of buffered aqueous solutions using different potential sweep techniques. The results obtained showed that the best definition of the analytical signals was found in boric acid/potassium tetrahydroxoborate buffer at pH 8.7 using anodic stripping square wave voltammetry. Recovery trials were performed to assess the accuracy of results and these were compared to those provided by a HPLC technique according to the method described in literature and to the features of the pharmaceutical formulations. A relative deviation of <0.2% was obtained. With the developed methodology, the limit of detection was 0.124 mg/l.

Determination of abscisic acid by cathodic stripping square wave voltammetry

In this paper a study is accomplished on behavior in a mercury electrode, of the phytohormone abscisic acid and of the conditions of accumulation in a HMDE. A mechanism is proposed of reduction based on its electrochemical behavior and proving the product of the reduction through mass spectrometry of bulks. A method is proposed for the determination of Abscisic acid (ABA) with a quantification limit of 58 ng ml −1. The procedure is applied wing determination of ABA in pears through the combination of high performance liquid chromatography (HPLC) with electrochemical quantification.

Nickel quantification in mice organs by adsorptive cathodic stripping voltammetry using mercury microelectrodes

AbstractMercury film microelectrodes (MFMs) were used to quantify trace levels of nickel accumulated in mice organs samples after dosing the mice with a physiological solution containing 150 ppm of Ni ions over a period of time, by performing adsorptive cathodic stripping square wave voltammetry (AdSWV). The residues obtained after digestion of the biological organs with nitric acid which affects the electrochemical response of Ni were investigated by XRD and IR spectrometry and found to be mainly composed by p‐nitrobenzoic acid. Electrochemical conditions for the quantification of Niwith MFMs in the biological solutions were optimized. The electrochemical data compared favorably with that obtained by atomic absorption spectrometry.

On the existence of free and metal complexed sulfide in the Arabian Sea and its oxygen minimum zone

Free hydrogen sulfide was not detected in the oxygen minimum zone (OMZ) of the Arabian Sea during legs D1 (September 1992) and D3 (October–November 1992) of the Netherlands Indian Ocean Programme (NIOP). However, sulfide complexed to metals was detected by cathodic stripping square wave voltammetry at 2 nM or less throughout the water column. A slight increase in sulfide was measured in the OMZ relative to the surface waters and may be related to sulfur release from organic matter during decomposition. Sulfide complexes are of two general types at low concentrations of metal and sulfide. First, metals such as Mn, Fe, Co and Ni form complexes with bisulfide ion (HS −) that are kinetically labile to dissociation and are reactive. Second, metals such as Cu and Zn form multinuclear complexes with sulfide (S 2−) that are kinetically inert to dissociation; thus, they are less reactive than free (bi)sulfide and the labile metal bisulfide complexes. Zinc and copper sulfide complexes are important in allowing hydrogen sulfide to persist in seawater which contains measurable oxygen.

Use of 4-chloro-7-nitrobenzofurazane as a derivative reagent for the determination of amino acids by cathodic stripping square wave voltammetry in biological fluids

The rapid and simple reaction of 4-chloro-7-nitrobenzofurazane with amino acids allows the determination of amino acids in urine using cathodic stripping square wave voltammetry. The obtained compounds are adsorbed on a hanging mercury drop electrode for determination by voltammetric methods using phosphate buffer at pH 2.0 as supporting electrolyte. The proposed method allows determination with an error of ⩽ 5.68%. A limit of detection (3 σ) of 3.64 nM (0.766 ng ml −1) and a limit of determination (10 σ) of 12.12 nM (2.55 ng ml −1) are obtained for arginine determination. The proposed method has been applied to the determination of amino acids in urine.

Iodine speciation: a potential indicator to evaluate new production versus regenerated production

Vertical profiles of iodide, iodate and total free iodine were determined monthly for one year (from July 1993 to June 1994) at the DYFAMED permanent station located in the northwestern Mediterranean Sea. Dissolved iodate and iodide were directly determined by differential pulse polarography and cathodic stripping square wave voltammetry, respectively. Iodate is the predominant species, ranging from 390 nM in surface waters to 485 nM in deep waters. Iodide is present in significant concentrations up to 80 nM in surface waters and from undetectable levels to several nanomolar (< 10 nM) in deep waters. Concentrations of total free iodine are slightly lower in surface waters (467–478 nM with an average of 472 nM) than in deep waters (475–486 nM with an average of 481 nM). Considerable variations in iodine speciation were observed. Iodide concentrations in surface waters were relatively low and stable from February to April (about 20 nM), increased up to 80 nM from May until November, and then decreased from December to February. Comparison between iodide abundance and primary production demonstrates that the transformation from iodate to iodide in surface waters is linked to the regenerated production. The stability of regenerated iodide in surface seawater makes iodide a potential indicator to evaluate new production vs regenerated production.

Iodine speciation in the water column of the Rogoznica Lake (Eastern Adriatic Coast)

Specific concentration profiles of iodate, iodide and organo-iodine in the water column of the Rogoznica Lake, where both oxic and anoxic conditions occur, were observed. Iodate and iodide concentrations were determined directly in water samples by differential pulse voltammetry (DPV) and cathodic stripping square wave voltammetry (CSSWV), respectively. Concentrations of ‘labile’ and ‘stabile’ organo-iodine were determined by DPV in the samples pretreated with chlorine water and UV-irradiation (combined with hydrogen peroxide and followed by chlorine water addition), respectively. In this stratified water column (salinity and dissolved oxygen gradients) with pronounced biological productivity, high concentrations of iodide (up to 0.87 μM) and significant percentages of ‘labile’ and ‘stabile’ organo-iodine, up to 37% and 30% respectively, were found. The formation of ‘labile’ organo-iodine is primarily governed by chemical reactions with dissolved sulphur forms and organic compounds, as well as remineralization processes of ‘stabile’ organo-iodine.

Determination of Zeatin in plant extracts by square wave stripping polarography and differential pulse stripping polarography

The electrochemical behaviour of the phytohormone trans-Zeatin at a hanging mercury drop electrode was studied. Three adsorption waves are observed, the first is due to the complexation reaction between Hg(II), produced at the electrode, and Zeatin and the other two by specific adsorption of Zeatin and reduction of the C = N group, respectively. The methodological and instrumental variables were studied in order to develop a determination method based on stripping differential pulse and stripping square wave polarography. The determination limits were between 15.5 and 53.6 ng ml −1 and a linear variation of peak intensity versus Zeatin concentration was observed for the three waves. The procedure was applied to the determination of Zeatin in plant extracts, after separation of cytokinins by means of a column packed with Amberlite XAD-4 ion exchange resin. The proposed procedure was compared with the determination by gas chromatography-mass spectrometry with prior methylation of Zeatin.

Iodine speciation in the northwestern Mediterranean Sea, method and vertical profile

Abstract Iodine speciation analysis was carried out upon seawater samples collected in July 1993 at the Dyfamed station (43 °25′N, 7 °52′E) located in the northwestern Mediterranean Sea. Dissolved iodate and iodide were directly determined by differential pulse polarography and cathodic stripping square wave voltammetry, respectively, and organically bound iodine was estimated by wet-chemical oxidation with sodium hypochlorite. Iodate is the predominant species ranging from 416 nM in surface waters to 480 nM in bottom waters. Iodide is present in significant concentrations up to 60 nM in surface waters, undetectable between 500 and 1000 m depth and present in very low but measurable concentrations (about 6 nM) in deep waters. The vertical profile of total free iodine demonstrates observable removal from surface waters, slight enrichment at about 200 m depth and constant there below. Up to 40 nM of organically bound iodine has been estimated between 20 to 30 m. Factorial analysis of different iodine species with biologically relevant parameters provided strong evidence for iodine biophilic features.

Investigation of metal sulfide complexes in sea water using cathodic stripping square wave voltammetry

Cathodic stripping square wave voltammetry has been used to detect H2S in sea water over a wide range of concentrations (nM to mM). The addition of metal ions to the solutions was found to depress the signal. This depression was attributed to the formation of metal sulfide complexes [MHS +, M(HS)2]. Stability constants for the formation of sulfide complexes with Cd2+, Cu2+, Cu+, Pb2+, Co2+, Fe2+, Mn2+, Ni2+, Hg2+ have been estimated in sea water at pH 8.0 and 25 °C using this method. The stability constants of cadmium sulfide complexes (log βCdHS = 6.3 and log βCd(HS)2 = 12.7) were found to be in reasonable agreement with previous measurements. The values of log βCuHS = Cu(HS)2 = 13.0; log βPb(HS) = 7.1 and log βPb(HS)2 = 13.5; log βZnHs = 6.0 and log βZn(HS)2 = 13.7, log βFeHS = log βCoHS = log βNiHS = 5.3 and log βMnHS = 6.7 were also obtained. Correction for the formation of strong chloro complexes for Hg2+ and Cu+ in sea water gave measured values close to the literature values. The results have been used to determine the speciation of HS- in surface sea water and of a number of metal ions in various anoxic basins.

Chemical removal of oxygen with sulfite for the polarographic or voltammetric determination of iodate or iodide in seawater

In the direct determination of iodate and iodide in seawater by differential pulse polarography and cathodic stripping square wave voltammetry, respectively, sulfite can be used to facilitate the removal of oxygen in the samples before they are analyzed. The reaction between sulfite and dissolved oxygen is almost instantaneous. In comparison with the removal of oxygen by bubbling the sample with an inert gas alone, the time needed for the analysis of a sample for iodate and iodide may be reduced to about 15 min and 5 min, respectively.

Determination of total inorganic iodine in seawater by cathodic stripping square wave voltammetry

A method has been designed for the reduction of iodate to iodide in seawater and subsequent determination of total dissolved iodine as iodide by cathodic stripping square wave voltammetry. The pH of the sample is lowered to about 1–2 and iodate is reduced to iodide with sodium sulfite under this acidic condition. The pH of the sample is then raised to 8–9 before the concentration of iodide is measured.