Continuous in Situ Cyanide Monitoring Using a Highly Sensitive and Selective FIA System

Abstract

A flow injection analysis (FIA) system incorporating a gas-diffusion membrane was fabricated for the detection of cyanide anion in aqueous samples. The principle of measurement is based on the reaction of o-phthalaldehyde (OPA) and cyanide in the presence of glycine to produce a fluorescent isoindole derivative. The cyanide concentration of the samples is thus proportional to the observed fluorescence intensity. Although extremely low levels of cyanide could be determined using this system (lower detection limit 0.4 ng mL-1 of CN-), measurements were affected by the presence of sulfite ion and thiols. Therefore, a gas-diffusion membrane was incorporated into the system to separate gaseous hydrogen cyanide from interferents in the sample. Consequently, this system displayed high selectivity for cyanide. The presence of sulfite ion (1 μg mL-1), 2-mercaptoethanol (0.1 μg mL-1), or 2-mercaptopropionic acid (10 μg mL-1) did not significantly increase the fluorescence intensity of cyanide solutions (16 ng mL-1). The sensor was then used for the rapid (150 s per measurement) detection of cyanide in samples of river water. A linear response (Kawashima bridge, y = 0.174x − 2.54E-2, r 2 = 0.999, n = 3; Yaba bridge, y = 0.170x + 2.75E-2, r 2 = 0.997, n = 3) was observed between the concentration of cyanide (0.4−40 ng) added to the samples and the response of the sensor. Furthermore, a device based on this FIA system was constructed and used for the continuous, in situ monitoring of cyanide concentrations in river water for 5 months, taking reading every 5 min, convincingly demonstrating the utility of our sensor. Although the sensitivity (mV/[CN-] of the sensor system tended to decrease over time, replacement of the gas-diffusion membrane restored the sensitivity to its initial level. In all cases, calibration curves yielded correlation coefficients ranging from r 2 = 0.991−1.00.