A submersible flow injection-based sensor for the determination of total oxidised nitrogen in coastal waters

The design and construction of a remotely deployed submersible sensor for the determination of total oxidised nitrogen (nitrate plus nitrite) in seawater is described. It was based on the flow injection principle with solid state spectrophotometric detection of the diazotisation product from the reaction of nitrite with N-(1-naphthyl)ethylenediamine dihydrochloride (N1NED) and sulphanilamide. Nitrate was pre-reduced in-line with a copperised cadmium powder column. The limit of detection (3 s) was 0.1 μM nitrate (1.4 μg l−1 N) and the linear range was 0.1–55 μM nitrate with a 20 mm path length flow cell and a 260 μl sample loop. The linear range was modified for different environmental conditions by changing the sample loop size and - or the flow cell path length. Results are presented for preliminary laboratory studies and field trials in the Tamar Estuary and for deployments in the North Sea. The sensor was successfully deployed on three separate occasions for complete tidal cycles (13 h) at a depth of 4 m from the bow of RRS Challenger. Field data are compared with results from a shipboard air segmented analyser and correlated with turbidity and salinity data.