Kinetic determination of cysteine on flow injection system by utilizing catalytic complexation reaction of Cu(II) with 5,10,15,20-tetrakis (4-N-trimethylammino-phenyl) porphyrin

A kinetic method for the determination of cysteine on flow injection system is described. Cysteine was found as a catalyst for the complexation reaction of Cu(II) with an ultra sensitive colorimetric reagent of 5,10,15,20-tetrakis (4-N-trimethylammino-phenyl) porphyrin (TTMAPP) (ɛ = 4.8 × 105 cm−1 M−1 at 433 nm), which was used as the indicator reaction in this paper. Soret band of either the porphyrin or the produced complex (ɛ = 4.6 × 105 cm−1 M−1 at 411 nm) can be used for detection. The continuous flow injection system greatly enhanced the analytic precision and efficiency of the kinetic method, giving a relative standard deviation of 0.73% for a 0.1 μg ml−1 cysteine with 10 injections at a throughput of 30 h−1. The detection limits (3S/N) in this case was 15 ng ml−1 and the working dynamic range was over 25 ng ml−1 to 1 μg ml−1. Sugars, organic acids and amino acids that are possible in coexistence with cysteine could be tolerated at high concentrations. This method was critically compared with the Ellman's reagent in the determinations of cysteine contents of three pharmaceutical injections for hepatic diseases and one permanent wave agent and showed better applicability in the respect of matrix interferences.