Microscale determination of inorganic nitrogen in water and soil extracts

Abstract

Rapid, sensitive analysis of NH₄ ^‐ NO₃ ^‐, and NO₂ ^‐ in 1–150 μL of soil extract or water was achieved using a modified indophenol blue technique adapted to microtiter plate format. The microplate technique was similar to conventional steam distillation in accuracy and precision. By varying aliquot volume, a wide linear dynamic range (0.05 to 1000 mg of NH₄ ⁺‐ or NO₃ ^‐‐NL^‐1) was achieved without the need for sample dilution or concentration. High sample throughput (250–500 NH₄ ⁺ analyses d^‐1) was accomplished manually, but could be significantly increased by automation. Of considerable importance was the very low waste stream produced by the method. All equipment and supplies required are commercially available and need no modifications for this use. The microtiter plate format could be used for other soil colorimetric analyses with little or no down time for equipment setup, a major consideration for commercial soil‐testing laboratories. The method and equipment used are well suited to quality control and quality assurance programs, as required under FIFRA Good Laboratory Practices.