A relatively simple and sensitive procedure with spectrofluorimetric detection was developed for the determination of cystine and cysteine by flow injection system with sequential determination. This method is based on the reduction of Tl(III) with cysteine in acidic media, producing a fluorescence reagent, TlCl3(2-) (λex = 227 nm, λem = 419 nm). Before injection, the sample solution was divided into two streams. The first stream was treated with Cd reduction column and then joined with the carrier to react with Tl(III) at pH 5.0 and then passed through a 100 cm reaction coil to the flow cell of the spectrofluorimeter, where the fluorescence intensity was measured (λex = 227 nm, λem = 419 nm). This signal is related to cystine and cysteine concentrations. The second stream of sample solution was injected directly into the carrier stream to react with the reagents and then passed through the reaction coil and detector for measuring the fluorescence intensity. The signal in this step is related only to cysteine. Thus, the cystine content was determined directly from difference of the two signals. Cystine and cysteine can be determined in the range of 0.10 to 5.50 µmol L-1 and 0.20 to 8.0 µmol L-1, respectively, at a rate of 20 samples per hour. The limit of detection (3s/k) was 0.10 µmol L-1 for both analytes. The relative standard deviations for ten replicates determination of 4.0 and 3.5 µmol L-1 cystine or cysteine were 1.1% and 1.8%, respectively. The influence of potential interfering substances was studied. The proposed method was successfully applied to the sequential determination of both analytes in pharmaceutical samples.
cysteine; cystine; sequential determination; spectrofluorimetry
