Applications of Lasers in Bioanalytical Chemistry

ground due to oxygen. This background ECL however, does not prevent one from obtaining peroxide detection limits below I ,aM. The ECL intensity varies with both pH and potential. The net ECL to background ECL ratio is a maximum at pH 10. Gold and glassy carbon electrodes are preferable to platinum, but gold is somewhat better than glassy carbon. Potentials more negative than +0.3 V yield insignificant ECL; potentials of +0.4 to +0.6 V yield maximum ECL intensity. Longterm stability of the ECL signal is improved by use of a potential waveform which alternates between a positive potential where luminol is oxidized and a negative potential (about -0.2 V) where the electrode surface is reduced. If luminol or luminol-labeled species are the analyte, then one needs both peroxide and catalyst as the reagents. Because the catalyst can be achieved electrochemically, it makes sense also to consider electrochemical generation of the peroxide from oxygen and water. In a stream flowing through an electrochemical flow cell, as long as the oxygen concentration, pH, flow rate, and electrode behavior remain constant, the amount of peroxide produced at the electrode per unit time will remain constant so that a constant concentration of peroxide will be achieved in the flowing stream. Use of a glassy carbon generating electrode, poised at about -1.0 V, yields sufficiently high peroxide concentrations over the pH 9.5 to 11 range to be useful with luminol CL; those solution pH values are also compatible with the luminol CL reaction. By using two electrodes in series (an upstream electrode to generate peroxide and a downstream electrode for luminol ECL) we have determined luminol from 0.1 nM to 10 jaM.