Simple flow injection for screening of total antioxidant capacity by amperometric detection of DPPH radical on carbon nanotube modified-glassy carbon electrode.

An amperometric flow injection (FI) method suitable for evaluation of 'total antioxidant capacity' (TAC) is presented. In this method, a carrier stream of a solution of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH(•)) continuously flows through an electrochemical cell, furnished with a carbon nanotube modified-glassy carbon electrode (CNT/GC) as the working electrode. At the applied voltage of 0.05V (vs. Ag/AgCl), DPPH(•) is reduced resulting in a constant electric current. For measurement of the TAC, a sample zone containing antioxidant(s) is injected into the carrier stream therein reduction reaction of DPPH(•) occurring within the sample zone. The decreased amount of the radical in the sample zone leads to a drop of the amperometric signal at the CNT/GC electrode. We have also compared the performance of the CNT/GC electrode to the unmodified GC electrode using cyclic voltammetry. The sensitivity of the CNT/GC electrode was more than twenty five times greater than the bare GC electrode. The study of the sweep rate dependence showed that the cathodic and anodic current of 0.1mM DPPH solution varied linearly (r(2)=0.998) with the square root of the scan rate, from 0.02 to 0.12 Vs(-1). These results demonstrated that the CNT/GC electrode is appropriate for the quantitation of antioxidants via amperometric detection of the residual concentration of non-reacted DPPH(•). We obtained linear calibrations for all the antioxidants tested including gallic acid, catechin, quercetin, caffeic acid and Trolox. The system offers rapid sample throughput (45 samples h(-1)) and good precision of 3.2% R.S.D., for 20 µL-injection of 2.5 µM Trolox (n=30). This method was applied to evaluate the TAC of extracts of some Thai indigenous vegetables.

Determination of iodide by detection of iodine using gas-diffusion flow injection and chemiluminescence.

This work describes development of a flow injection (FI) system for determination of iodide, based on the chemiluminescence (CL) reaction between iodine and luminol. Iodide in the sample zone is oxidized to iodine. Employment of a gas-diffusion (GD) unit allows for selective detection of the generated CL (425nm). Preliminary results showed for concentrations of less than 2mgL(-1), that signals were irreproducible and that the calibration was not linear. In order to solve these problems, a method of 'membrane conditioning' was investigated, in which iodide stream was continuously merged with oxidant to generate I(2) that conditioned the GD membrane and tubing. This minimized surface interaction between the active surface and the I(2) generated from the samples, thus improving both precision and sensitivity. By employing membrane conditioning, it has been possible to reliably detect concentrations down to 0.1mgL(-1). At the optimized condition, an excellent linear calibration (r(2) = 0.999) was obtained from 0.1 to 1.0mgL(-1). The method was successfully applied to determine iodide in some pharmaceutical products such as potassium iodide tablets and a liquid patent medicine. However, for vitamin tablets, ascorbic acid was found to interfere seriously by causing a negative signal.

Flow-injection determination of iodide ion in nuclear emergency tablets, using boron-doped diamond thin film electrode.

The electrochemical determination of iodide was studied at boron-doped diamond thin film electrodes (BDD) using cyclic voltammetry (CV) and flow-injection (FI) analysis, with amperometric detection. Cyclic voltammetry of iodide was conducted in a phosphate buffer pH 5. Experiments were performed using glassy carbon (GC) electrode as a comparison. Well-defined oxidation waves of the quasi-reversible cyclic voltammograms were observed at both electrodes. Voltammetric signal-to-background ratios (S/B) were comparable. However, the GC electrode gives much greater in the background current as usual. The potential sweep rate dependence exhibited that the peak current of iodide oxidation at 1mM varied linearly (r(2) = 0.998) with the square root of the scan rate, from 0.01 to 0.30Vs(-1). This result indicates that the reaction is a diffusion-controlled process with negligible adsorption on BDD surface, at this iodide concentration. Results of the flow-injection analysis show a highly reproducible amperometric response. The linear working range was observed up to 200muM (r(2) = 0.999). The detection limit, as low as 0.01muM (3sigma of blank), was obtained. This method was successfully applied for quantification of iodide contents in nuclear emergency tablets.