The isotope fractionation of O(2) gas during adsorption onto, and desorption from, molecular sieve zeolite (MSZ) has been confirmed. It has been found that the magnitude of fractionation during gas adsorption for the determination of isotope ratio measurements by dual inlet mass spectrometry is significant during desorption but not during adsorption. The isotope shift will take place during insufficient desorption from MSZ and its magnitude varies with the amount of MSZ and its identity. In order to obtain complete desorption and to remove isotope shifts, the use of the minimum amount of 5A MSZ and additional heating at a temperature of 60 degrees C for a period of 10 min just before gas admission into the mass spectrometer are recommended.