STUDY OF THE ENHANCED CHEMILUMINESCENCE FROM THE LUMINOL HORSERADISH-PEROXIDASE HYDROGEN-PEROXIDE P-COUMARIC ACID SYSTEM AT VERY SHORT TIMES - STOPPED-FLOW SELECTIVE DETERMINATION OF P-COUMARIC ACID IN BEERS

p-Coumaric acid has a greater enhancing effect on the chemiluminescence of the luminol-H2O2-horseradish peroxidase system, at low concentration, than other phenolic acids studied. We have used this effect to study the variations of the chemiluminescent signal with luminol, hydrogen peroxide, p-coumaric acid, horseradish peroxidase concentrations and pH, using the stopped-flow technique, by monitoring the initial reaction rate. The interference effects of other phenolic acids on the enhanced chemiluminescence with p-coumaric acid (25 nM) were negligible at similar concentrations of phenolic acid. We monitored the chemiluminescence intensity at 10 s for the determination of p-coumaric acid in beers. The detection limit was ca. 0.7 nM and the linear range was 0-12.5 nM. The precision of the method, expressed as a relative standard deviation, was 2.5%.