Blank injections into the modified interface produced significant nitrogen peaks (Figure ), which also occurred with no reducing agents in the mobile phase and when the reactors were at room temperature. The measured δ 15 N signatures of these peaks were between +2 and +8‰ against the nitrogen reference gas, which was set to zero. We therefore assumed that these blank peaks were either produced by dissolved elemental nitrogen in the sample or by switching the valve position of the 6-port valve from the autosampler. Switching the valves might have introduced small amounts of elemental nitrogen from the surrounding air into the eluent flow of the system. Separation of nitrate from the injection peak and nitrite was therefore crucial to avoid interferences and was achieved by a porous graphitic carbon (PGC) HPLC column (Hypercarb 100 x 2.1 mm, 5 µm, ThermoScientific). We used an elevated column temperatures at around 80°C and diluted sulfuric acid as anionic competitor to elute nitrate from the column.