Development and characterization of a FIA system for selective assay of L-ascorbic acid in food samples.

An amperometric detector and an enzymatic reaction were combined for the measurement of L-ascorbic acid. The enzyme cell (containing immobilized ascorbate oxidase) was connected to a flow injection analyzer (FIA) system with a glassy carbon electrode as an amperometric detector. During optimization and measurements two sample injectors were used, one before and one after the enzyme cell, thus eliminating the background interferences. Subtraction of the signal area given in the presence of enzyme from the one given in the absence of enzyme was applied for measuring analyte concentrations and calibration at 400 mV. Analysis capacity of system is 25 samples/hour. The relative standard deviation (RSD) was below 5% (5 times repeated, 400 µmol/L conc.), linearity up to 400 µmol/L, limit of detection (LOD) 5 µmol/L, fitting of calibration curve in 25-400 µmol/L range was R (2) = 0.99.

Characterization of the gold-catalyzed deposition of silver on graphite screen-printed electrodes and their application to the development of impedimetric immunosensors.

This paper describes the characterization of the gold-catalyzed deposition of silver on graphite screen-printed electrodes (SPEs) using electrochemical impedance spectroscopy (EIS) and the application of this approach to the development of impedimetric immunosensors. After applying -0.1 V for 45 s, the amount of electrodeposited silver quantitatively changes the magnitude of two elements of the electrical equivalent circuit: the interface capacitance, Ci, and the charge-transfer resistance, R(CT). Better correlations have been found when considering the R(CT) since this parameter is almost exclusively dependent on the amount of deposited silver under these experimental conditions. This approach has been successfully applied to the development of an impedimetric immunosensor for aflatoxin M(1). The R(CT) magnitude shows good correlation with the amount of gold immobilized on the electrode surface after a competitive assay and thus, with the toxin concentration. This approach has been found sensitive in a wide range of concentrations, from 15 to 1000 free-AFM(1) ppt with a limit of detection of 12 ppt.