Dušan Bustin

Voltammetric determination of caffeine in beverage samples on bare boron-doped diamond electrode

A sensitive and selective electrochemical method for the caffeine determination using bare boron-doped diamond electrode was developed. It was found that caffeine provided highly reproducible and well-defined irreversible oxidation peak at very positive potential. The effects of supporting electrolyte, pH and scan rate on the voltammetric response of caffeine oxidation were studied to select the optimum experimental conditions. Linear response of peak current on the concentration in the range from 4×10(-7) to 2.5×10(-5)M, good repeatability (RSD of 2.1%) and detection limit of 1.5×10(-7)M without any chemical modifications and electrochemical surface pretreatment were evaluated. The effect of possible interferents appeared to be negligible which evidently proved very good selectivity. The proposed method was successfully applied for the caffeine determination in commercially available beverage samples, with results in a close statistical agreement to those declared by manufacturer and HPLC used as independent method.

Voltammetric determination of penicillin V in pharmaceutical formulations and human urine using a boron-doped diamond electrode

Simple, sensitive and selective differential pulse voltammetric method for determination of penicillin V on a bare (unmodified) boron-doped diamond electrode has been developed. Penicillin V provided highly reproducible and well-defined irreversible oxidation peak at very positive potential of +1.6V (vs. Ag/AgCl). The optimum experimental conditions for oxidation of penicillin V were achieved in acetate buffer solution (pH 4.0). The modulation amplitude of 0.1V, modulation time of 0.05s and scan rate of 0.05Vs(-1) were selected as optimum instrumental parameters for differential pulse voltammetry. Linear response of peak current on the concentration in the range from 0.5 to 40μM with coefficient of determination of 0.999, good repeatability (RSD of 1.5%) and detection limit of 0.25μM were observed without any chemical modifications and electrochemical surface pretreatment. The effect of possible interferents such as stearic acid, glucose, urea, uric acid and ascorbic acid appeared to be negligible which evidently proved the good selectivity of method. The practical analytical utility of proposed method was demonstrated by determination of penicillin V in pharmaceutical formulations (tablets) and human urine samples with satisfactory recoveries (from 98 to 101% for tablets and 97 to 103% for human urine).

Determination of pharmaceutical dosage forms via diffusion layer titration at an interdigitated microelectrode array

This paper presents a method for the analysis of drugs in dosage form. It is based on galvanostatic generation of oxidation agent from a suitable precursor on one segment of interdigitated microelectrode array (IDA) and its consecutive amperometric detection on second segment. High collection efficiency of this process in comparison to rotational ring disc electrode (RRDE) is a unique feature of IDA system. The transfer of oxidation agent can be influenced by addition of species, which reacts with oxidant. This influence can be used for its determination. Evaluation of generator-collector current dependence (diffusion layer titration curve) reveals the value of generator current I(genE) of the end-point of titration. I(genE) is proportional to the bulk phase concentration of determined species. The method was applied to the analysis of pharmaceuticals Antabus (Disulfiram Alpharma NOR, tetraethylthiuram disulfide (TETD)), a popular drug for alcoholism treatment, and Celaskon (vitamin C, Léciva CZE, ascorbic acid (AA)). From model samples analysis rather low detection limits, 9x10(-7) mol dm(-3), respectively, 4x10(-6) mol dm(-3), were estimated which enables trace content analysis of the drugs. A small size of IDA sensor also makes it suitable for microanalytical improvement.

Titration of As(III) with electrogenerated iodine in the diffusion layer of an interdigitated microelectrode array

Abstract The transfer of electrogenerated iodine from generator to collector microelectrodes has been studied with an interdigitated array system. The high value of the collection efficiency (ca. 85%) compared with that of rotating ring-disk electrodes, measured as the slope of the collector versus generator current plot, was observed. The plot was linear with a positive intercept on the generator current axis. An oxidation wave associated with surface redox processes of materials used for interdigitated array (IDA) fabrication was found to cause the intercept. An analytical technique was developed based upon the evaluation of the influence of As(III) on the iodine collector versus generator current plot. Increasing the As(III) concentration leads to a proportional increase of the intercept on the generator current axis. The collector potential is set at the limiting current value of iodine cathodic reduction and serves as its amperometric detector. The collector current will remain zero until the current flowing through the IDA generator produces an iodine flux larger than the flux of incoming As(III). Titration curves (collector versus generator current plots) measured with a slowly scanning generator current show very good reproducibility. Since no chemical reaction takes place in the bulk of the sample (diffusion layer titration), the experiment can be repeated many times in the same solution. The sensitivity of this method is 1920 μAM −1 and the determination limit estimate is 7 × 10 −6 M As(III).

Application of redox cycling enhanced current at an interdigitated array electrode for iron-trace determination in ultrapure spectral carbon

Abstract Vertically arranged interdigitated array (IDA) electrodes with a continuous Pt-film basis and a 0.5 μm thick insulation layer in the middle separating the upper Pt-IDA electrodes (26 pairs of 400 μm long fingers, 5 μm wide with 5 μm gaps) from the Pt-basis were applied for Fe trace analysis in ultrapure spectral carbon. The testing with spiked samples as well as comparison of results with those obtained by independent graphite-furnace atomic absorption spectrometry (AAS) proved the reliability of the proposed sample preparation procedure and IDA signal generation in the range of Fe content 1 × 10 −5 to 1.5 × 10 −4 wt.%. The method can be considered as a less expensive alternative to the AAS determination of Fe pollutants in this ultrapure material.

Determination of tetramethylthiuram disulfide on an interdigitated microelectrode array

Abstract A novel analytical technique for the determination of tetramethylthiuram disulfide (TMTD) was developed. It is based on total oxidation of TMTD with hypobromite. Hypobromite is generated on one segment of interdigitated array. It reacts rapidly and quantitatively with TMTD and unreacted flux of hypobromite is detected on second segment of the interdigitated microelectrode array (IDA). Detection is amperometric therefore the potential of the second (detector) segment is fixed at the value corresponding to the limiting current of the opposite reaction to that taking place on the generator segment. Rather low detection limit of this method ( 7×10 −7 mol dm −3 ) allows the analysis of samples with trace content of TMTD.

Titrations with electrogenerated halogens in the diffusion layer of an interdigitated microelectrode array

A technique based on diffusion layer titration was developed for the iodimetric determination of low concentrations of thiosulfate and the bromimetric determination of allyl alcohol. The diffusion layer titrations utilize chemical reactions proceeding quantitatively only in the close vicinity of the electrode. One set of segments of an interdigitated array (IDA) microelectrode serves for galvanostatic anodic generation of titrant (iodine or bromine) and the second set, immersed in the diffusion layer of the generator, detects its unreacted flux. The detector (collector) is potentiostated to the potential of the limiting diffusion current of iodine/bromine cathodic reduction. The diffusion layer titration curves (collector current versus generator current plots) measured ‘point by point’ or by slowly scanning the generator current, show very good reproducibility. Since no bulk phase chemical reaction actually proceeds, the experiment can be repeated extensively in the same solution. The sensitivity of this method is 1424 µA l mol–1 and the determination limit is 6 × 10–7 mol l–1 for thiosulfate determination. The substantially lower sensitivity compared with the rotating ring-disc diffusion layer titration is compensated for by the possibility of a many-fold decrease in the sample volume for IDA microelectrode diffusion layer titration. The technique was applied to the trace determination of thiosulfate in analytical-reagent grade potassium iodide. The thiosulfate content found was slightly lower than that specified by the manufacturer. A sensitivity of 486 µA l mol–1 and a determination limit of 2 × 10–5 mol l–1 were found for the bromimetric determination of allyl alcohol. Analytically favourable titration curves with negligible curvature around the end-point were obtained in IDA diffusion layer titrations, in contrast to the rotating ring-disc electrode measurement where considerable curvature was encountered owing to the lower rate of the bromination of allyl alcohol. The absence of almost any curvature in IDA experiments is explained by a significantly slower allyl alcohol flux compared with convective diffusion at a rotating electrode. The slower flux results in more time for the titration reaction to proceed close to equilibrium.

Indirect voltammetric detection of fluoride ions in toothpaste on a comb-shaped interdigitated microelectrode array

A novel technique based on dynamic electrochemistry for the detection of fluoride ions was developed. It is based on its strong complexation with ferric ion. Formed fluoroferric complex is cathodically inactive at the potential of the reduction of free ferric aquo ion. The voltammetric and amperometric response of platinum comb-shaped interdigitated microelectrode array is decreased after fluoride addition. This decrease serves for the quantification of fluoride ions added to the solution. The detection limit of 4.5×10(-5) mol dm(-3) was achieved when one of the segments of interdigitated microelectrode array (IDA) was used as an indicating electrode. The detection limit is about one order of magnitude lower than in the case of conventional platinum macroelectrode. In comparison with ISE electrodes this method is faster and also avoiding large error resulting from the antilogarithmization of ISE Nerstian response. The method was applied to the analysis of toothpaste.

Determination of mercury in high-purity gallium arsenide using a gold-fibre voltammetric microelectrode

A gold-fibre microelectrode is proposed for the determination of mercury in high-purity gallium arsenide by stripping analysis. The precision and accuracy were tested using mercury-spiked gallium arsenide samples and were found to be satisfactory for mercury contents higher than approximately 10 ppb. Both the precision and accuracy exceed those of the method applying a compact gold rotating disc electrode. De-aeration of the sample solution and its stirring during accumulation are unnecessary in the analyses with gold-fibre microelectrodes.

Study of cyanoaquonitrosyl complexes of chromium. IV. Electrochemical reactions and complexation equilibria of the NO(H2O)4CrCN+ adducts with mercury(II)

Abstract The electrochemical reactions of bond isomers cis-NO(H 2 O) 4 Cr-CN + and cis-NO(H 2 O) 4 Cr-NC + and of their complexes with Hg 2+ are described. The reaction schemes proposed are based chiefly on polarographic and coulometric measurements. The measurement of the potentials of the reversible reduction of Hg(II) complexed in {NO(H 2 O) 4 Cr-NC} 2 Hg 4+ was used for the calculation of the formation constant of this complex from NO(H 2 O) 4 Cr-NC + and Hg 2+ , K = 1.43 X 10 21 M −2 at 25°C and an ionic strength of 1 M (ClO − 4 , Na + ), and of the acid-base dissociation constant of NO(H 2 O) 4 Cr-NCH 2+ , K a = 2.75 X 10 −2 M for the same conditions. An analysis of the limiting diffusion current of reduction of the complexed Hg(II) yielded the value of the equilibrium constant for the reaction of Hg 2+ with {NO(H 2 O) 4 Cr-NC} 2 Hg 4+ , Q = 7 ± 4 for 25°C and an ionic strength of 1 M (ClO − 4 , Na + , H + ).