Measurement Of Low Concentrations Of Nitrate And Nitrite Using Chemiluminescent Technique And Its Application

Nitrite in seawater is generally determined by spectrophotometric method, in which nitrite is reacted with a Sulfanilamide to form a diazonium salt.Then the salt reacts with a second amine N-(1-naphthyl)ethylene-diamine to form an azo dye. Nitrate is determined after the reduction of nitrate to nitrite on an active cadmium metal surface.Although the molar extinction of the aze dye is very large, the detection of nitrate and nitrite in this method is just 100 nmol/L and 20 nmol/L. The concentration of nitrate and nitrite in the oligotrophic area often can't be detected by this method. Then an improved method is needed to detect nitrate and nitrite.The major results are as follows:1,Modify the parameters of the Chemiluminescence Nitrogen Oxides Analyzer Model 200E, make sure it suit the experiment condition of nitrate and nitrite in the laboratory;2,According to Cox, Garside , Braman and Hendrix's report, by exploring various experiment factors, the best experimental conditions with detection limit of 10 nmol/L were set up in the laboratory. Nitrate is reduced to nitric oxide in the condition of 0.1mol/L VCl3,2mol/L HCl,150℃temperature and 480mL/min N2 flow. The relative standard deviation was 5% at the 200nmol/L level and 3% at the 400nmol/L level; Nitrite is reduced at room temperature to nitric oxide in the condition of 2mol/L NaI,15%H3PO4 and 480mL/min N2 flow. The relative standard deviation was 6% at the 200nmol/L level and 5% at the 400nmol/L level.3,The nitrate, nitrite and the total dissolved nitrogen are measured by both chemiluminescence and traditional spectrophotometric method. Chemiluminescence method is proved to be appropriate to determinate the nitrate , nitrite and the total dissolved nitrogen ,especially at low concentration levels. 4,By the surfer tables ,the cruise's nutrient elements'vertical distribution in the ocean is discussed separately. Based on the redfield ratio(Si:DIN:P=16:16:1), the limited element is discussed. As a result, the phytoplankton might be limited by the phosphate rather than nitrogen and silicate along the offshore area of China;5,According to the tables and appendixes, the distrubutions of the low nitrate and nitrite along the offshore area of China are analyzed especially. As a result, we find that the low nitrate levels mainly distribute in the upper 20m in the Yellow Sea and upper 50m in the South China Sea. Nitrite is very low in the whole China Sea except the P3 Stations. In these area, nitrate and nitrite are more appropriate measured by the chemiluminescent technique rather than the spectrophotometric method.