Simultaneous spec trophotometric determination of ascorbic acid and glutathione by kinetic-based flow injection analysis

A flow injection (FI) spectrophotometric method for the simultaneous determination of ascorbic acid (AA) and glutathione (GSH) in a single injection (one-shot) is presented. The method is based on the reaction kinetics of a reduction reaction of an iron (III)-1,10-phenanthroline (phen) complex to a red iron (II-phen complex (lambda(max) = 510 nm) with each reducing compound. AA rapidly reduces an iron (III)-phen complex to a red complex, while the reaction rate with GSH is very slow. However, copper(II) greatly accelerates the reaction rate with GSH. Therefore, in the presence of copper(II) the concentration of the produced red complex corresponds to the total concentration of AA and GSH. A double flow cell that has two cell compartments in parallel is installed in a double-beam spectrophotometer. The use of the double flow cell makes it possible to detect simultaneously two analytes from a one-shot, i.e. AA can be detected at 510 nm in the absence of copper when an injected sample zone passes through the first compartment in the double flow cell, and then after the sample zone merges with a copper(II) solution, the total concentration of AA and GSH can be detected in the second compartment. Under the optimum conditions, AA and GSH can be determined simultaneously in the ranges of 5 x 10(-7) similar to 5 x 10(-6) M for AA and of 2 x 10(-6) similar to 1 x 10(-4) M for GSH with a sample throughput rate of 45 h(-1). The proposed FI method was successfully applied to the determination of these compounds in pharmaceutical preparations and a supplement.