3.3 Nitrogen and oxygen isotopes test
To determination nitrogen and oxygen isotope of nitrate. First, the pH value of the water sample was determined. The pH value of water samples was adjusted to be within the pH range of 6~8 by hydrochloric acid (10%) and imidazole solution (2 mol/L). The nitrite was removed in advance with sulfonic acid solution, the amount of sulfonic acid added in the water sample was 1.5 times of that of nitrite, reacted at room temperature for at least 30 min, and then reacted in boiling water bath for 15 min to destroy the generated complex and eliminate the interference of nitrite.
After the two-step pre analysis, 40 mL of water sample was taken and put it into a 60 ml headspace bottle. 0.8mL of CdCl2solution (20 g/L), 0.8ml of NH4Cl solution (250 g/L) and 3×10 cm 4N (or 3N) clean zinc tablet (wiped clean with alcohol) were added and shaken on the shaker at a speed of 220 r/min for 20 min (full reaction). Then the zinc tablet was taken out, the empty bottle was closed to completed the nitrate reduction step. 2 mL of NaN3 solution (2mol/L) and 1:1 mixture of CH3COOH (20%) were added into the headspace bottle after nitrate reduction, and shaken violently to mix the sample and reagent. After shaking at 220 r/min for 30 min (full reaction), the azide reaction was ended by adding 1.2 mL NaOH solution (10 mol/L) as the termination agent (the solution was alkaline, which was not conducive to azide reaction).
Nitrate was converted into N2O gas by the above chemical process reaction, and the nitrogen and oxygen isotope values of N2O gas were measured by gas bench stable isotope mass spectrometer. The N2O was carried into Mat 253 isotope ratio mass spectrometer to determine the nitrogen isotope ratio.