Vanadium(III)-chloride (97%, Merck, Darmstadt, Germany) was used to prepare solutions of 0.015 M V(III)Cl3 in 0.3 M HCl (36.5 % -38%, Alfa Aesar, Kandel, Germany). In order to preserve the HPLC pumps, which were constructed from stainless steel and were susceptible to high chloride concentrations, we did not utilize excessive amounts of reducing agents. Heated hydrochloric acid is highly corrosive and can irreparably damage the stainless-steel reactor in the LC-IRMS interface, therefore a deactivated silica capillary was used (see below). The reduction of nitrate to nitric oxide with vanadium(III)-chloride follows the chemical reaction :