SEQUENTIAL FLOW INJECTION SPECTROFLUORIMETRIC DETERMINATION OF CYSTINE AND CYSTEINE IN PHARMACEUTICAL SAMPLES
Disorders of cysteine metabolism include cystinosis, an antosomal recessive disease produced by a defect in lysosomal transport and cystinuria, a common heritable disorder of amino acids transport. The oxidized derivative of L-cysteine (L-cystinel has additional metabolic roles such as stabilization of the tertiary structure of proteins (via disulfide bonds).Thereby, cysteine and cystine are widely used in many pharmaceutical products and as supplement in some foods [1-3]. A simple and sensitive procedure with spectrofluorimetric detection was developed for the sequential determination of cystine and cysteine by flow injection system. This method is based on the reduction of Tl(III) with cysteine in acidic media, producing fluorescence reagent, TLC[ 32- ()`ex = 227 nm, Xem = 419 nm). Before injection, the sample solution was divided into two streams. The first stream was treated with Cd reduction column and then injected to carrier to react with Tl(III) at pH 5.0 and then passed through a 10o cm reaction coil to the flow cell of the spectrofluorimeter, where the fluorescence intensity was measured (hex = 227 nm, Xem = 419 nm). This signal is related to cystine and cysteine concentration. The second stream of sample solution was injected directly to carrier stream to react with reagents and then passed through reaction coil and detector to measure of the fluorescence intensity. This signal is related only to cysteine. Thus, the cystine content was determined directly by the difference of two signals. Cystine and cysteine can be determined in the range of 0.10 to 5.50 NM and 0.20 to 8.0 pM, respectively, at a rate of 20 samples per hour. The limits of detection (S/N=3) were 0.10 NM for both of analytes. The influence of potential interfering substances was studied. The proposed method was successfully applied to the sequential determination of both analytes in pharmaceutical samples.
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