Peter Tomčík

Manganese detection in marine sediments: anodic vs. cathodic stripping voltammetry

Three different electroanalytical techniques for the detection of manganese in marine sediments are evaluated. The anodic stripping voltammetry of manganese at an in situ bismuth-film-modified boron-doped diamond electrode and cathodic stripping voltammetry at a carbon paste electrode are shown to lack the required sensitivity and reproducibility whereas cathodic stripping voltammetry at a bare boron-doped diamond electrode is shown to be reliable and selective with a limit of detection, from applying a 60s accumulation period of 7.4 x 10(-7)M and a sensitivity of 0.24AM(-1). The method was used to evaluate the manganese content of marine sediments taken from Sibenik, Croatia.

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.

Electrochemical determination of superoxide and nitric oxide generated from biological samples

Abstract The biological samples from the cardiovascular system were measured by amperometric enzyme electrode for superoxide determination which was prepared by anodic polymerization of pyrrole and concomitant incorporation of superoxide dismutase (SOD) on a Pt wire in phosphate buffer solution. The amperometric response to superoxide was measured at a potential of 0.7 V to oxidize the hydrogen peroxide generated. The response time of biosensor is less than 5 s and a 15 nM detection limit was achieved. Simultaneous with superoxide production was measured nitric oxide on metalloporphyrine biosensor.

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.

The ammonia-free partial reduction of substituted pyridinium salts

This paper reports a study into the partial reduction of N-alkylpyridinium salts together with subsequent elaboration of the intermediates thus produced. Activation of a pyridinium salt by placing an ester group at C-2, allows the addition of two electrons to give a synthetically versatile enolate intermediate which can be trapped with a variety of electrophiles. Furthermore, the presence of a 4-methoxy substituent on the pyridine nucleus enhances the stability of the enolate reaction products, and hydrolysis in situ gives stable dihydropyridone derivatives in good yields. These versatile compounds are prepared in just three steps from picolinic acid and can be derivatised at any position on the ring, including nitrogen when a p-methoxybenzyl group is used as the N-activating group on the pyridinium salt. This publication describes our exploration of the optimum reducing conditions, the most appropriate N-alkyl protecting group, as well as the best position on the ring for the methoxy group. Electrochemical techniques which mimic the synthetic reducing conditions are utilised and they give clear support for our proposed mechanism of reduction in which there is a stepwise addition of two electrons to the heterocycle, mediated by di-tert-butylbiphenyl (DBB). Moreover, there is a correlation between the viability of reduction of a given heterocycle under synthetic conditions and its electrochemical response; this offers the potential for use of electrochemistry in predicting the outcome of such reactions.

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.