Two rapid and sensitive automated methods for the determination of nitrite and nitrate in soil samples

Spectrophotometric flow injection methods were developed for the individual determination of nitrite or nitrate, and for the simultaneous determination of nitrite and nitrate, in soil samples. Nitrite was determined directly using a modified version of the Griess–Ilosvay diazo-coupling reaction, measuring at 543 nm the absorbance of the azo–dye complex formed. Simultaneous nitrite and nitrate determinations were based on on-line nitrate reduction in a micro column containing copperised cadmium. A single chromogenic reagent containing all the necessary reactants was used in both methods. For determinations, the chemical and instrumental variables were optimised by univariate analysis and simplex chemometric method. The optimised conditions gave a linear calibration range between 0.05 and 1.6 µg m L− 1 for N–NO2− and between 0.05 and 7.0 µg m L− 1 for N–NO3−. The detection limits for nitrite and nitrate were 22 µg L− 1 and 44 µg L− 1 respectively. The proposed methods allowed up to 35–40 samples per hour to be analysed with good precision. The simultaneous method was successfully used for the determination of nitrite and nitrate in soil samples (the results obtained were validated against those obtained by reference methods). The proposed methods are simpler and faster than conventional methods and could be routinely used in environmental monitoring laboratories.