A flow system with a conventional spectrophotometer for the chemiluminescent determination of lactic acid in yoghurt

A flow-based analytical procedure for lactate determination in yoghurt by c hemiluminescence using a conventional UV-Vis spectrophotometer as detector is described. The radiation source was switched off. The flow cell was mach ined in acrylic with a 1 mm path length and a 80 mm(2) surface and was posi tioned 2 mm distal of the photodetector (100 mm(2) sensible area) in order to improve detection. The flow network computer-controlled comprised a set of three-way solenoid valves assembled to implement the multicommutation ap proach. The chemiluminescence was obtained by using the reaction of luminol with hydrogen peroxide, catalyzed by hexacyanoferrate (III) after enzymati c reaction with lactate. The lactate oxidase enzyme was immobilized on poro us silica beads (glass aminopropyl, SIGMA). The signal generated by the spe ctrophotometer reaction was read by the microcomputer and stored as a funct ion of time for further treatment. Immobilization condition, enzyme concent ration, temperature, pH, stability of the enzymatic reactor, and flow rates were investigated. The feasibility of the system was ascertained by analyz ing a set of yoghurt samples. Results were in agreement with those obtained by a conventional method (Boehringer UV-Kit), and no significant differenc e at 95% confidence level was observed. A linear response within 10-125 mg l(-1) L-lactate, a 1.9% standard deviation (n = 10), and an analytical thro ughput of 55 determinations per hour were achieved. (C) 2001 Elsevier Scien ce B.V. All rights reserved.