Elimination of nitrogen interference during online oxygen isotope analysis of nitrogen‐doped organics using the “NiCat” nickel reduction system

RATIONALE Accurate online analysis of the δ 18 O values of nitrogen‐bearing organic compounds is of interest to several emergent fields, including ecology, forensics and paleontology. During online analysis, high‐temperature conversion (HTC) of nitrogen‐bearing organics produces N 2 gas which creates isobaric interference with the isotopic measurement. Specifically, N 2 reacts with trace amounts of oxygen in the mass spectrometer source to form 14 N 16 O ( m/z 30), which prevents accurate evaluation of the sample 12 C 18 O peak ( m/z 30). METHODS We present an alternative system to the conventional HTC, which uses a nickel‐catalyzed (“NiCat”) reduction furnace to convert HTC‐produced CO into CO 2 , allowing for δ 18 O measurement using signal intensities at m/z 44 and 46. RESULTS This system yields identical δ 18 O values for nitrogen‐doped and undoped sucrose and cellulose compounds up to molar yield ratios of N 2 :CO = 0.22. In contrast, our conventional HTC system configured to factory recommendations with the stock gas chromatography (GC) column produced a discrepancy of ~5‰ between nitrogen‐doped and undoped samples. CONCLUSIONS Because of its ability to eliminate isobaric interference, the NiCat system is a viable alternative to conventional HTC for δ 18 O measurement, and can be constructed from relatively inexpensive and readily available materials. As an additional advantage, the CO 2 analyte produced by NiCat may be cryofocused, to allow for oxygen‐isotope determinations on very small amounts of sample substrate. Copyright © 2012 John Wiley & Sons, Ltd.