DETERMINATION OF THE CO CONTRIBUTION TO THE N-15/N-14 RATIO MEASURED BY MASS-SPECTROMETRY

The isobaric effect of CO on the determination of delta(15)N has been calibrated for CO/N-2 mixtures. An effect of 6.49 parts per thousand on delta(15)N by percent of CO has been observed; the linearity of the isobaric effect for CO concentrations in the range 11-0.02% (measured manometrically) bag been established. Consequently, a method to determine accurately the CO/N-2 ratio (especially for CO amounts lower than 1%) in natural samples, along with the delta(15)N determination in a single mass spectrometric manipulation has been developed. In this aim the relationship between delta(15)N of CO/N-2 fixed mixtures and 12/14 current ratios has been calibrated. This calibration allows the mass spectrometric detection of trace quantities of CO in N-2 natural samples. Furthermore, a correction of delta(15)N Of contaminated samples is possible with a precision between +/-0.14 and +/-0.20 parts per thousand for CO concentrations between 0.01 and 1%, respectively. The method is valid for any dynamic mass spectrometer.