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A procedure is developed for the flow-injection spectrophotometric determination of cysteine in dietary supplements based on the formation of a reduced phosphomolybdic complex by the analyte in mixing a sample solution with a solution of ammonium 18-molybdophosphate. For the flow-injection determination of cysteine, a polymethyl methacrylate chip is fabricated, in the channels of which the analytical form was obtained and detected. The detection limit for cysteine is 3 × 10–6 M and the throughput is 240 determinations per hour. The procedure was tested in the analysis of different samples of dietary supplements; the results obtained were verified by HPLC.

A stepwise injection analysis system was developed to monitor the concentration of l -cysteine in biologically active supplements (BAS) and fodders. It is based on the rapid redox reaction of l -cysteine with 18-molybdo-2-phospahte heteropoly anion (18-MPA) and the detection of the formed heteropoly blue with a spectrophotometry. The method has relatively wide optimal ranges of pH and 18-MPA concentration − 5.0 to 7.3 and 0.4 to 2.0 mM, respectively. Under optimized operating conditions the performance of the SWIA system was linear up to a concentration of l -cysteine of 0.1 mM ( R 2 = 0.994) with a detection limit of 0.003 mM and a sample frequency of 20 h − 1 . The SWIA system was employed to determine the concentration of l -cysteine in BAS and fodder. The obtained data were in good agreement with those measured by a HPLC method.

Simple, rapid and fully automated methods for the manual and automated spectrophotometric determination of epinephrine have been developed by using schemes of stepwise injection (SWIA) and sequential injection analysis (SIA) implemented in the same manifold. The determination is based on the formation of reduced form of 18-molybdodiphosphate heteropoly anion by its reaction with epinephrine. Using of the reaction vessel in the general SWIA configuration instead of a holding and reaction coil in the SIA manifold provides several essential advantages, including higher sensitivity and lower reagent consumption. The linear dependence of the analytical signal on the epinephrine concentration was preserved over the range of 1.5–30, 3.0–30, and 1.5–25 μmol L −1 by using of SWIA, SIA and spectrophotometric analysis, respectively. The relative standard deviation for the SWIA determination of 10 μmol L −1 epinephrine was 1.8% ( n =10).