High precision mass spectrometric analysis of isotopic abundance ratios in nitrous oxide by direct injection of N2O

This paper presents the technical and theoretical background of a technique for directly determining the nitrogen and oxygen isotopic composition of N2O by measuring simultaneously the m/z 44, 45, and 46 beam intensities on a gas source ratio mass spectrometer. Mass interferences are caused by any CO2 contaminant as CO2 has the same molecular masses as N2O. Any small amount of CO2 remaining in the sample can be corrected for by measuring the ratio of m/z 12 to 14 or m/z 12 to 44 ratio in the sample. Under certain source conditions, NO2 is produced in the ion source, causing a mass interference at m/z 46. NO2 production in the source can be corrected for by establishing an NO2 slope correction factor through measuring the ratio of m/z 46 to 44 in the sample at various ion source pressures. The production of NO2 can be totally eliminated by increasing the draw-out voltage in the source to a value similar to that used for hydrogen isotope analysis. Experiments were carried out on both a Finnigan MAT251 and a Finnigan MAT252. Delta values for oxygen and nitrogen isotope abundances were obtained with an internal precision of better than 0.08 permil and 0.02 permil, respectively, on samples as small as 3 μmol on both instruments.