Fiber-optic chlorine probe based on fluorescence decay of N-(6-methoxyquinolyl)acetoethyl ester

A fiber-optic probe for measuring chlorine in aqueous solution is evaluated. The probe uses a microporous gas-permeable membrane to trap a small volume of aqueous internal solution made of N-(6-methoxyquinolyl)acetoethyl ester (MQAE) in front of a fiber-optic assembly. The working principle of this probe is based on a combination of fluorescence quenching and redox reaction of MQAE by chloride ion and chlorine, respectively. While the magnitude of total fluorescence decay is independent of the sample concentration, the initial rate of decrease in fluorescence intensity is linearly proportional to the chlorine concentration over the range from 8.4 to 418 microM. Once the probe is exposed to a chlorine sample, the probe tip must be refilled with a fresh aliquot of the internal solution before the next measurement. In this mode, the probe displays excellent reproducibility; the relative standard deviation of the initial rate of fluorescence decay (n = 3) is only 1 percent. This probe can also be used for the measurement of hypochlorous acid (HOCl) in aqueous solution. The response properties are identical to those of the chlorine probe.