Two international nitrate reference materials with certified nitrogen isotope signatures on natural abundance levels (USGS 34 and USGS 35) and one material enriched in its nitrogen isotope ratio (USGS 32) were utilized in this study. Additionally, two in-house nitrate standards (KNO3 and NaNO3) were calibrated on an EA-IRMS system with the three USGS reference materials. The referenced values of all five standard and reference materials are summarized in Table . We injected 10 µL of a nitrate solution containing 50 mg L -1 N-NO3 -of each material in triplicate and plotted the measured δ 15 N values with all standard and reference materials (Figure ) and with materials on natural abundance levels (Figure ). The y-intercept of a linear regression curve between referenced and measured δ 15 N values shows that measured δ 15 N values are -8.1 and -8.2 ‰ lower than the referenced values, but a slope of 0.96 and 0.99 indicates that the differences between measured values of different materials are in good agreement to the differences of referenced values. As mentioned earlier, we occasionally observed small shifts in the m/z 29/28 ratio during measurements, which resulted in higher measured δ 15 N values if the shifts overlapped with either a reference gas peak or a chromatographic peak and caused the observed difference. The reported values were produced throughout continuous measurements on two individual days and the copper reactors were regenerated overnight. The results are therefore also subject to isotope shifts over extended measurement periods and to different copper reactor performance on individual sampling days.