A highly-sensitive microplate fluorimetric method for the high-throughput determination of nitrate ion in aqueous compost extracts

In this paper, a new spectrofluorimetric method for the determination of nitrate in aqueous compost extracts is presented. The microplate procedure is based on the reduction of nitrate to ammonium by means of Zn powder under acidic conditions and the following derivatization of ammonium with o-phthalaldehyde (OPA) and N-acetylcysteine (NAC) to give a fluorescent derivative. Optimization work allowed performing the reduction in 30 min on a small sample volume (150 μL) and using a little quantity of metal (20 mg). The use of strong acids is also avoided. Excellent limit of detection was achieved (1.3 μM = 0.08 mg L−1 NO3−) along with good precision levels (4.2 and 14.3% for 50 and 5 μM, respectively) and a satisfactory linear dynamic range (4-100 μM). A good agreement between data coming from our method and from ion-exchange chromatography was found on real compost samples, indicating a good level of accuracy.