Simultaneous flow injection determination of ascorbic acid and cysteine using double flow cell

A spectrophotometric method was developed for the simultaneous determination of ascorbic acid (AA) and cysteine (Cys) by flow-injection (FI) system equipped with a double flow cell, which was installed in a double-beam spectrophotometer. Iron(III) is quantitatively reduced to iron(II) with AA or Cys in the presence of 1,10-phenanthroline (phen), producing an iron(II)phen complex (lambdamax=510nm). It was found that copper(II) greatly accelerated the reduction reaction of iron(III) with Cys, while that with AA was little affected by copper(II). On the basis of the difference in the rate between the two reactions, AA and Cys can be determined in the ranges 2?10-7 to 4?10-5M and 1?10-6 to 9?10-5M, respectively at a rate of 45 samples per hour. The limits of detection (S/N=3) were 1?10-7M for AA and 2?107M for Cys. A batchwise method for the simultaneous determination of both reductants was also studied. The proposed FI method was successfully applied to the simultaneous determination of both analytes in pharmaceutical formulation.