A SUBMERSIBLE FLOW-INJECTION ANALYZER FOR THE IN-SITU DETERMINATION OF NITRITE AND NITRATE IN COASTAL WATERS

An flow-injection system is described for the in-situ determination of nutrient concentrations in rivers and ocean waters. In estuarine and coastal waters, significant interferences may be caused by temperature and salinity variations. Therefore, we used a dual wavelength detecto r to measure simultaneously the reference and sample signals. The devi ce has been used to measure spatial and temporal variations of nitrite and nitrate concentrations in coastal waters exhibiting strong salini ty variations (Bay of Brest and Iroise Sea, France). Our original dete ction system coupled with flow-injection analysis (FIA) allows high-fr equency measurements (40 samples per hour), very good precision (1%) a nd a low detection limit (0.45 mu M NO3). The device can work up to a depth of 300 m, within a temperature range of 2 to 35 degrees C, and w ith salinities varying from 0 to 35. The dynamic range (0-150 mu M NO3 ) can be adapted to the expected concentrations of the study area by u sing flow cells with various path lengths. Intercalibration with sampl es collected by conventional means and analysed with a spectrophotomet ric reference method at the laboratory showed a good agreement between both methods (1.3%).