An improved flow system for spectrophotometric determination of anions exploiting multicommutation and multidetection Original Research Article

A multicommutated flow system is proposed for the determination of anions in water samples. The flow set up was assembled with a set of computer-controlled three-way solenoid valves in order to manage the addition of different reagents by binary sampling. An optical-fiber CCD-array spectrophotometer with a tungsten-halogen lamp was employed for multidetection. Water samples were used as carrier and the chromogenic reagents were intermittently added, allowing the sequential determination of nitrate, nitrite, chloride and phosphate with or without in-line concentration by ion exchange. In-line concentration of the analytes was performed during the signal measurements of the other species. In this way, a 180 s loading time was implemented without impairing the sampling rate (estimated as 50 determinations per hour). Under the proposed conditions, the procedure can be used for samples containing 30–300 μg l−1 N-NO2−, 0.1–1.0 mg l−1 N-NO3−, 1.0–10 mg l−1 Cl−, and 0.05–2.5 mg l−1 P-PO43−. Detection limits were estimated as 6 μg l−1 N-NO2−, 40 μg l−1 N-NO3−, 400 μg l−1 Cl− and 30 μg l−1 P-PO43−at 99.7% confidence level. Coefficients of variation were estimated (n=20) as 1.6, 2.2, 2.3 and 1.5% for nitrite, nitrate, chloride and phosphate, respectively. The reagent consumption was reduced from 3- to 40-fold and from 20- to 760-fold regarding the conventional FIA systems and batch procedures, respectively. Results for river water samples agreed with those obtained by single-analyte FIA procedures at the 95% confidence level.