To evaluate the ability of the copper reactor to reduce the high loads of nitrogen oxides coming from the modified LC-IRMS interface in addition to the oxygen loads coming from the eluent, we consecutively injected 5 µL of a 50 mgL -1 N solution of the USGS 34 international reference material on three following days. The copper reactor was reduced overnight in between measurement days with a 2 mL min -1 flow of 3 vol-% H2 in He gas. The measured δ 15 N values were constant for four to six injections, after which they started to decrease until the 20 th to 25 th injection (see Figure ). This indicated that the reactor performance is stable for the first few injections, but quickly declines afterwards. Overnight regeneration of the reactor increased the measured δ 15 N values on the next day, but not to the same values previously observed. This showed that while regenerating the reactor is feasible, there might still be daily differences in reactor or system performance. Thus, frequent referencing through international or in-house standards, as typically required and practiced in CSIA, minimized the daily influence on the performance. Measuring nitrogen isotope ratios from nitrate with high precision throughout a day or for prolonged sample runs might therefore require a combination of two copper reactors in parallel, where one reactor is used for one or several injections of nitrate containing samples while the other is regenerated with a separate line of hydrogen gas.