A batch injection analysis system for ascorbic acid determination using amperometric detection on a sessile mercury drop electrode

A batch injection analysis (BIA) system, which uses an automatic mercury el ectrode (AME) as an amperometric detector, is described. The capillary of t his AME was adapted in an inverted position through the bottom of a simple cylindrical electrochemical cell, in order to generate more stable sessile drops and to grant free access for the tip of the programmable micropipetto r. The BIA system was evaluated by using the electrochemical oxidation of L -ascorbic acid to dehydroascorbic acid on the sessile mercury drop electrod e. Acetate buffer solution (50 mM, pH 4.8) was used as supporting electroly te. The working electrode was operated at +0.230 V (vs. Ag/AgCl). The effec t of the dispensing rate, of the dispensed volume, of the distance between the mercury drop and the pipettor tip, as well as of the solution level in the cell were evaluated. Injections of 50 mu L were suitable to reach a det ection limit of about 2.5 mu M (450 ppb). At the 50 mu M ascorbic acid leve l, an RSD of 1.6% (N = 35) was observed with or without mercury drop renewa l between injections. A frequency of 300 injections per hour may be reached when the drop is renewed sporadically. The new BIA method was applied to t he determination of ascorbic acid in packed/canned tropical fruit juices. S electivity was granted by differential measurements of the peak current bef ore and after exposure of samples to ascorbate oxidase (from cucumber).