NUKLEONIKA 2004, 49(2):37-42

During cryogenic separation of CO₂ from atmosphere and soil air for isotopic analyses, N₂O present in the air condenses together with CO₂. The N₂O, having the same molecular masses (44, 45 and 46) as CO₂, seriously affects the results of ¹³C and ¹⁸O isotope analyses of carbon dioxide, making it necessary to introduce appropriate corrections. A simple method of correcting the measured d¹³C and d¹⁸O values of the analysed CO₂ was developed. It requires experimental determination of several parameters of the used mass spectrometer and changes in the computational algorithm. The performance of the method was tested using N₂O-CO₂ mixtures. The experiments showed that the proposed correction method removes the effect of presence of N₂O in the analysed CO₂ sample down to analytical uncertainty of the measurement. This method was adopted in routine analyses of stable isotope composition of carbon dioxide in atmospheric and soil air samples being performed in our laboratory.