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.