Time measurement-visual analysis of L-cysteine using the autocatalytic sodium sulfite/hydrogen peroxide reaction...

Trace amounts of L-cysteine can function as a trigger, i.e., reaction initiator, in the autocatalytic sodium sulfite/hydrogen peroxide reaction system. Rapidly changing of pH after induction time is visually confirmed by color changing of bromothymol blue in this autocatalytic reaction. Based on this finding, microg L(-1) levels of L-cysteine were measured over time using the autocatalytic reaction system. The determination range using the above method was 5.0 x 10(-8)-2.5 x 10(-6)M, the detection limit (3 sigma) was 1.8 x 10(-8)M (1.94 microg L(-1)), and the relative standard deviation was 2.41% at an l-cysteine concentration of 5 x 10(-7)M (n=5). This method was also applied to length detection-flow injection analysis. The determination range for the flow injection analysis was 2.0 x 10(-7)-1.0 x 10(-5)M. The detection limit (3 sigma) was 1.4 x 10(-7)M (17.0 microg L(-1)), and the relative standard deviation was 0.91% at an initial L-cysteine concentration of 10(-6)M (n=5).