Removal of nitrite impurity from nitrate labeled with nitrogen-15

Potassium nitrate labeled with 15N is often used as a tracer in studies of N dynamics in soil and water systems. Typically, 0.8% NO2- impurity has been found in the batches of K15NO3 enriched to 99 atom % excess 15N that were purchased by our laboratory. Nitrite is an intermediate in several N cycling processes so its addition when adding NO3- could produce misleading results. We have developed a safe, simple, and inexpensive method to remove NO2- impurity from any NO3- solution in a water matrix. The principle is the oxidation of NO2- to NO3- by UV light in the presence of a heterogenous TiO2 catalyst. A NO2- concentration of 0.2 mM in 100 mL of 0.2 M NO3- solution could be oxidized in 12 min using 0.5 g L-1 TiO2 in a specially constructed photoreactor with a 75-W UV facial tanning lamp. For the routine removal of NO2-, use of the same TiO2 concentration in a standard beaker worked equally well when the irradiation time was extended to 2.5 h. After irradiation, the TiO2 is easily and totally removed from the solution by membrane filtration