Flow injection analysis to measure the production ability of superoxide with chemiluminescence detection in natural waters

In connection with the photo-chemical processes in the aquatic environment, the evaluation system for the production ability of superoxide anion was constructed. The technique was based on the flow-injection method with luminol chemiluminescence detection. The injected sample was first irradiated by a solar simulator in a vortex quartz cell (cell volume = 0.167 mL), whose vortex face was exposed to light after passing through two air-mass filters. The carrier was the aqueous solution at pH 11 adjusted by NaOH. After irradiation was finished, the carrier (with sample) flow was merged with 1.52 mM of the luminol solution and was introduced into a chemiluminescence detector. The results of the laboratory experiment show that the production of superoxide is linearly related to the concentration of humic acid up to 50 ppm, and also to that of dissolved oxygen. In addition, the chemiluminescence intensity (superoxide production) was proportionally related with the irradiation intensity of the solar simulator. By means of changing the flow rate of the carrier, the half-life of superoxide at pH 11 aqueous solution was estimated to be 15 s. ESR was measured for the sample containing 0.5% humic acid, 0.5% NaOH, and 20 μL DMPO (spin trap agent). ESR spectra were obtained after 5 min of irradiation of the solar simulator. In addition to the four sharp peaks due to OH radicals, a broad peak appeared at the middle of the OH signal. The obtained signal cannot conclusively be ascribed to superoxide, but the peak that appeared may be due to the radical produced in the humic acid molecule. The river water was collected at 18 points of the Tamagawa River located in Tokyo. Upstream, the production ability of superoxide was observed, but not downstream or in the estuarine district. Although the concentration of humic acid is much higher in the estuarine sample, some quenching mechanisms work for superoxide production.