Simultaneous determination of nitrite and nitrate in water by chemiluminescent flow-injection analysis

A sensitive and fully automated flow-injection system for the simultaneous determination of nitrite and nitrate in water is described. The method combines online photolytic conversion of nitrate to nitrite and the chemiluminescent detection of nitrite. Nitrite initially present in the sample is directly determined after oxidation to peroxynitrous acid by the chemiluminescent reaction with luminol. Nitrate in another portion of sample is photolytically reduced to nitrite by absorption of UV light at quartz capillaries and the sum of original and reduced nitrite is detected in the parallel detection system via the same way. Nitrate content is determined from the difference. The detection limit of nitrite (S/N = 3) is 2 x 10(-9) mol/l and the calibration graph is linear in the concentration range 8 x 10(-9) to 1 x 10(-5) M NO2-. The relative standard deviation is 1.8% for 1 x 10(-6) M and 2.9% for 3 x 10(-8) M NO2-, respectively. The detection limit of nitrate (S/N = 3) is 4 x 10(-9) mol/l and the calibration graph is linear in the range 8 x 10(-9) to 1 x 10(-5) M NO3-. The relative standard deviation is 2.1% for 1 x 10(-6) M and 3.6% for 1 x 10(-7) M NO3-, respectively. The interferences of cations are eliminated passing the sample through a cation-exchange column. Common anions do not interfere. Analysis time is 2.5 min. The concentrations of nitrites and nitrates found by presented method in different water samples were in a good agreement with those obtained by spectrophotometric or chromatographic method, respectively. (C) 2003 Elsevier B.V. All rights reserved.