The analysis of nitrate in environmental samples by reversed-phase HPLC

A method is presented for the analysis of nitrate in natural waters and waste water by high-performance liquid chromatography (HPLC) with a reversed-phase octadecyl column, aqueous phosphoric acid/dihydrogen phosphate mobile phase, and UV detector. The optimum nitrate concentration is 0.3 to 3 mg/L as N with linear detector response below 3 mg/L. The detection limit is 0.007 mg/L. Relative standard deviations in the optimum range are consistently less than one percent. Several potential interferences have been investigated; nitrite and organic chromophores are resolved from nitrate and do not interfere. Hexavalent chromium and sulfate are slight positive interferences, negligible at typical environmental concentrations. The method produces results in agreement with the accepted chromotropic acid method except in samples from eutrophic lakes, for which evidence is presented indicating that the chromotropic acid is inaccurate.