A flow injection analysis (FIA) system comprising a cysteine selective
electrode as detection system was developed for determination of this
amino acid in pharmaceuticals. Several electrodes were constructed
for this purpose, having PVC membranes with different ionic
exchangers and mediator solvents. Better working characteristics
were attained with membranes comprising o-nitrophenyl octyl ether
as mediator solvent and a tetraphenylborate based ionic-sensor. Injection of 500 µL standard solutions into an ionic strength adjuster
carrier (3x10-3 M) of barium chloride flowing at 2.4mL min-1,
showed linearity ranges from 5.0x10-5 to 5.0x10-3 M, with
slopes of 76.4±0.6mV decade-1 and R2>0.9935. Slope decreased
significantly under the requirement of a pH adjustment, selected
at 4.5. Interference of several compounds (sodium, potassium,
magnesium, barium, glucose, fructose, and sucrose) was estimated
by potentiometric selectivity coefficients and considered negligible.
Analysis of real samples were performed and considered accurate,
with a relative error to an independent method of +2.7%
