Simultaneous determination of nitrate and nitrite ion by micro-flow injection analysis

To minimize the sample size, reagent consumption and waste, a micro-flow injection analysis (μFIA) was investigated and applied to the simultaneous determination of nitrate and nitrite ion in water samples. A double-plunger micropump was used to propel a carrier and a chromogenic solution; each flow rate was 50 μl min-1, and the delivery volume was 2.5 μl per stroke. Nitrate was reduced to nitrite on line by passing through a mini column (0.53 mm i.d.×5 cm) packed with powdered Cd/Cu (6080 mesh). The reduction efficiency was 99% using this reduction column. Detection was carried out at 538 nm with a light-emitting diode used as an azo dye. The carrier solution contained EDTA and NaCl (pH 8.18.4) and the reagent solution contained sulfanilamide, N-(1-naphthyl)ethylenediamine and HCl. For a simultaneous determination, the sample injected into the carrier stream was split into two streams: one passed through the mini Cd/Cu column ; the other passed through a bypass column packed with glass beads. The sample zone passing through the bypass column gave a first peak derived from nitrite; the other sample zone passing through the reduction column gave the second peak derived from nitrate and nitrite. The detection limits of nitrate and nitrite were about 1×10 -7 M using the proposed simultaneous determination system.