Sequential injection sandwich technique for the simultaneous determination of nitrate and nitrite

An automated sequential injection system for monitoring nitrate and nitrite in water samples is described. The method enables the simultaneous determination of both parameters with a single injection of sample. A sandwich arrangement, in which the sample is placed between two Griess reagent zones, is adopted. Nitrite is determined in one end of the sample zone by diazotization-coupling reactions and spectrophotometric detection of the azo dye formed. In the other end, nitrate is similarly determined after its on-line reduction to nitrite using a copperised cadmium column. The calibration is linear over the range 0.5-40 mu mol l(-1) nitrite and 2-100 mu mol l(-1) nitrate, the mid-range relative standard deviations being <2% in both the cases. Under optimised conditions the system has been applied to the determination of nitrate in tap, mineral and sea waters (the nitrite concentration was below the detection limit) and the results have been compared with those obtained by ion chromatography and a flow injection system based on the same reactions. (C) 1998 Elsevier Science B.V. All rights reserved.