Methods to reduce interference effects in thermal conversion elemental analyzer/continuous flow isotope ratio mass spectrometry δ18O measurements of nitrogen-containing compounds

On-line determination of the oxygen isotopic composition (delta O-18 value) in organic and inorganic samples is commonly performed using a thermal conversion elemental analyzer (TC-EA) linked to a continuous flow isotope ratio mass spectrometry (IRMS) system. Accurate delta O-18 analysis of N-containing compounds (like nitrates) by TC-EA-IRMS may be complicated because of interference of the N-2 peak on the m/z 30 signal of the CO peak. In this study we evaluated the effectiveness of two methods to overcome this interference which do not require any hardware modifications of standard TC-EA-IRMS systems. These methods were (1) reducing the amount of N-2 introduced into the ion source through He dilution of the N-2 peak and (2) an improved background correction on the CO m/z 30 sample peak integration. Our results show that He dilution is as effective as diverting the N2 peak in order to eliminate this interference. We conclude that the He-dilution technique is a viable method for the delta O-18 analysis of nitrates and other N-containing samples (which are not routinely measured using He dilution) using TC-EA-IRMS, since it can easily be programmed in the standard software of IRMS systems. With the He-dilution technique delta O-18 values of the nitrate isotope standards USGS34, IAEA-N3 and USGS35 were measured using the shortest possible traceability chain to the VSMOW-SLAP scale, and the results were -28.1 +/- 0.1%, +25.5 +/- 0.1% and +57.5 +/- 0.2%, respectively. An improved background correction was also an effective method, but required manual correction of the raw data. Copyright (C) 2008 John Wiley & Sons, Ltd.