Selective and sensitive detection of the amino acid tryptophan is of importance in food processing, pharmaceutical formulations and in biological fluids. Electrochemical methods of detection of tryptophan are hampered by sluggish electron transfer kinetics and in complex matrices through overlapping peaks from interferents. This study examines the potential of the cation exchange membrane Nafion (R) to enhance selectivity and sensitivity of this analyte through a seldom explored feature of this membrane: pH manipulation. A detailed examination of the effect of pH on the selectivity afforded by Nafion (R) as a function of the analyte charge is presented. Selective detection of tryptophan and significant increases in sensitivity of its detection was observed in the presence of melatonin, dopamine and other interferents present in a pharmaceutical formulation through manipulation of the pH of the solution. At pH 3.0 at a Nafion (R)-modified electrode, changes in the protonation of melatonin and tryptophan lowered the anodic potential of the analytes in a non-uniform manner increasing the peak resolution and permitting analyses with detection limits of 1.6 +/- 0.1 nM and 1.6 +/- 0.2 nM respectively. (C) 2009 Elsevier Ltd. All rights reserved.