Mauro Bertotti

Amperometric detection of nitrite and nitrate at tetraruthenated porphyrin-modified electrodes in a continuous-flow assembly

The modification of a glassy carbon surface by coating with an electrostatically assembled film of tetraruthenated cobalt porphyrin/(meso-tetra(4-sulphonatephenyl)porphyrinate zinc(II) yields an indicator electrode that allows the determination of nitrite to be performed with a limit of detection of 0.1 μM in a flow injection configuration. The dynamic range extends up to 1000 μM and the repeatability of the measurements was evaluated to be 1.5% with a throughput of 50 samples per hour. The efficiency of the bilayered film to mediate the electron transfer allows the determinations to be performed at a less positive potential (+0.75 V) with enhanced sensitivity. The coating also prevents the surface poisoning and its stability is maintained over several weeks. The same detector was used for determination of nitrate after reduction to nitrite in a reductor column containing copperised cadmium. This method was used for the determination of nitrate and nitrite in mineral water, saliva and cured meats, the results being in agreement with certified values and those obtained by using recommended procedures.

Determination of nitrate in mineral water and sausage samples by using a renewable in situ copper modified electrode

A new approach for in situ electrodeposition of a renewable copper layer onto a copper electrode is reported. The active surface was obtained by anodic dissolution of a copper electrode at an appropriate potential and further redeposition of copper ions still remaining at the diffusion layer. Under optimal experimental conditions the peak current response increases linearly with nitrate concentration over a range of 0.1-2.5mmol L(-1). The repeatability of measurements for nitrate was evaluated as 1.8% (N=15) and the limit of detection of the method was found to be 11micromol L(-1) (S/N=3). Nitrate contents in two different samples (mineral water and sausages) compared well with those obtained from using the standard Griess protocol at a 95% of confidence level measured by the t-student test. The interference from chloride on the nitrate analysis and the possibility of simultaneous determination of nitrite were also examined.

Amperometric determination of nitrite via reaction with iodide using microelectrodes

Abstract The measurement of a steady state current at a Pt microdisc electrode (radii 2.5–10 μm) is used as the basis of a rapid and simple procedure for the determination of trace nitrite in waters. The method is based on the reaction of nitrite ion with iodide in 0.1 M H 2 SO 4 and has a lowest detection limit of 0.1 μM. The method can be used for the determination of the nitrite level in natural waters and human saliva.

A renewable copper electrode as an amperometric flow detector for nitrate determination in mineral water and soft drink samples

A novel approach was developed for nitrate analysis in a FIA configuration with amperometric detection (E=-0.48 V). Sensitive and reproducible current measurements were achieved by using a copper electrode activated with a controlled potential protocol. The response of the FIA amperometric method was linear over the range from 0.1 to 2.5 mmol L(-1) nitrate with a detection limit of 4.2 micromol L(-1) (S/N=3). The repeatability of measurements was determined as 4.7% (n=9) at the best conditions (flow rate: 3.0 mL min(-1), sample volume: 150 microL and nitrate concentration: 0.5 mmol L(-1)) with a sampling rate of 60 samples h(-1). The method was employed for the determination of nitrate in mineral water and soft drink samples and the results were in agreement with those obtained by using a recommended procedure. Studies towards a selective monitoring of nitrite were also performed in samples containing nitrate by carrying out measurements at a less negative potential (-0.20 V).

Determination of Iodate in Salt Samples with Amperometric Detection at a Molybdenum Oxide Modified Electrode

A method for the amperometric determination of iodate in a flow injection configuration is described. The electroreduction of the analyte occurs at a glassy carbon disk electrode modified by a molybdenum oxide layer electrochemically deposited, the film acting as an efficient electrocatalyst for the mass transport controlled reduction of iodate. The sensor is applied to the determination of iodate in a wall-jet cell coupled to a flow injection apparatus after convenient deoxygenation of the carrier solution. The linear response of the sensor is extended from 10×10–6 mol L–1 to 10×10–3 mol L–1 iodate with a limit of detection (signal-to-noise=2) of 6×10–6 mol L–1. The repeatability of the method for injections of a 50×10–6 mol L–1 iodate solution was 6 % (n = 22). Interference from other oxidant anions (nitrate, nitrite and perchlorate) was not noticeable, whereas bromate interferes at concentrations up to 10 times that of iodate. The method was used in the determination of the iodate content in commercial salt samples.

Determination of Parathion and Carbaryl Pesticides in Water and Food Samples Using a Self Assembled Monolayer/Acetylcholinesterase Electrochemical Biosensor

An acetylcholinesterase (AchE) based amperometric biosensor was developed by immobilisation of the enzyme onto a self assembled modified gold electrode. Cyclic voltammetric experiments performed with the SAM-AchE biosensor in phosphate buffer solutions (pH = 7.2) containing acetylthiocholine confirmed the formation of thiocholine and its electrochemical oxidation at Ep = 0.28 V vs Ag/AgCl. An indirect methodology involving the inhibition effect of parathion and carbaryl on the enzymatic reaction was developed and employed to measure both pesticides in spiked natural water and food samples without pre-treatment or pre-concentration steps. Values higher than 91-98.0% in recovery experiments indicated the feasibility of the proposed electroanalytical methodology to quantify both pesticides in water or food samples. HPLC measurements were also performed for comparison and confirmed the values measured amperometrically.

Determination of sulphite in wine by coulometric titration

A method for the determination of both free and bound S(IV) in white wine samples by coulometric titration with electrogenerated iodine is described, the S(IV) speciation being performed by extracting SO2 from samples acidified with hydrochloric acid. Titrations of samples treated with NaOH led to the total S(IV) concentration, the results being in agreement with the ones obtained by the distillation procedure as well as by using the classical Monier-Williams method. An average recovery of 97% was obtained for samples spiked with S(IV) in the range 0.4–1.2 mM, which demonstrated the validity of the proposed procedure. The repeatability was 1.3% and the limit of detection was estimated as 0.01 mM. Results from direct titrations of wine added to the electrochemical cell are reported, which give indications of the stability of adducts towards the reaction with iodine.

Amperometric determination of acetylsalicylic acid in drugs by batch injection analysis at a copper electrode in alkaline solutions.

This paper describes the determination of acetylsalicylic acid (ASA) as salicylate (SA) in pharmaceutical formulations by using amperometric detection with copper electrodes in 0.10 mol l(-1) NaOH solution. Batch injection analysis (BIA) was explored for this application. The system exhibited sharp current response peaks, rapid washout and excellent repeatability. A large linear dynamic range from 1 to 1000 mumol l(-1) was obtained by using an injected volume of 100 mul, with a detection limit of 0.48 mumol l(-1). R.S.D. of 0.37% for 30 repetitive (1x10(-4) mol l(-1)) injections and sampling frequency of 60 h(-1) were achieved. The results obtained using this system for ASA determination in seven different drug samples compared well with those found by spectrophotometry (Trinder test).

An analytical application of the electrocatalysis of the iodate reduction at tungsten oxide films.

Films of non-stoichiometric tungsten oxides have been deposited onto glassy carbon surfaces by electrodeposition from acidic W(VI) solutions and the chemical stability of these oxides was investigated by using the electrochemical quartz crystal microbalance. At these modified surfaces, rotating disc electrode voltammetric experiments indicated that iodate is electrocatalytically reduced in a mass-transport controlled process. The influence of the film thickness on the response to iodate was investigated and the results indicated a linear relationship between catalytic current and film thickness for relatively thin oxide layers. The modified electrode was employed successfully as an amperometric sensor for iodate in a flow injection apparatus. The linear response of the developed method is extended from 5 micromol L(-1) to 5 mmol L(-1) iodate with a limit of detection (signal-to-noise=3) of 1.2 micromol L(-1). The repeatability of the method for 41 injections of a 1 mmol L(-1) iodate solution was 0.8% and the throughput was determined as 123 h(-1). Interference from other oxidant anions such as nitrate and nitrite was not noticeable, whereas bromate and chlorate interfere at slight levels. The method was used in the determination of the iodate content in table salt samples.

Charge transfer at electrostatically assembled tetraruthenated porphyrin modified electrodes

Molecular interfaces constituted by electrostatically assembled tetraruthenated cobaltporphyrin (Co(TRP)) and tetrasulphonated zincporphyrin (Zn(TPPS)) multi-bilayer films, have been recently exploited for the development of sensors and electrochemical devices. In this work, their conducting properties were investigated by means of cyclic and rotating disk voltammetry, and by impedance spectroscopy. The films exhibited an unusually high conductivity, which increased further when the potential approached the redox potential of the Ru(III/II) couple. This behavior is reflected in a very low heterogeneous charge transfer resistance, leading to Nyquist diagrams dominated by or exhibiting solely a wide Warburg region. The kinetics of a previously reported analytical application involving heterogeneous charge transfer from nitrite ions to the porphyrin film interface was carefully examined. According to this study, the limiting step for the nitrite oxidation is the heterogeneous charge transfer to the interfacial Ru(III) complexes, with k=2.7×103 dm3 mol−1 s−1.