Anal. Chem. 1994,66, 2187-2189
Determination of the CO Contribution to the 15N/14NRatio Measured by Mass Spectrometry Val6rie Beaumont,’*+ Pierre Agrlnier,t Marc Javoy,t and Franqois Robert* Laboratoire de gbchimie des isotopes stables, T 54-64, Universith Paris 7, 2 place Jussieu, 7525 1 Paris C6dex 05,France, and Laboratoire de Minhralogie, Mudum d’Histoire Naturele, CNRS, URA 736, 61 rue Buffon, 75005 Paris, France
The isobaric effect of CO on the determination of 615N has been calibrated for CO/N2 mixtures. An effect of 6.49% on 615Nby percent of CO has been observed; the linearity of the isobaric effect for CO concentrations in the range 11-0.02% (measured manometrically) has been established. Cousequently, a method to determine accurately the CO/N2 ratio (especially for CO amounts lower than 1%)in natural samples, along withtbe615NdeterminatiooinasinglemPasspectroaretric manipulation has been developed. In this aim the relationship between b15N of CO/N2 fixed mixtures and 12/14 current ratios Bas been calibrated. This calibration allows the mass spectrometricdetectionof trace quantitiesof CO in N2 natural samples. Furthermore, a correction of 615N of contaminated samples is possible with a precision between *0.14 and i.0.20% for CO concentrationsbetween 0.01 and 1% respectively.The method is valid for any dynamic mass spectrometer. The measurement of nitrogen isotopic compositions is usually carried out on the N2 molecule. This gas exhibits an isobaric interference with CO which cannot be resolved with a Nier-type gaseous mass spectrometer. A small quantity of CO impurity results in a measured ratio that greatly overestimates the natural 15N isotopic abundance of the sample, and this phenomenon is sometimes overlooked, especially in small nitrogen samples (
