Partial pressure dependency of 17O/16O and 18O/16O of molecular oxygen in the mass spectrometer

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Abstract
A method to determine both 17O/16O and 18O/16O ratios for molecular oxygen with high precision by direct introduction into the mass spectrometer without gas separation is presented. Because both 17O/16O and 18O/16O in mixed gases have good linear correlations with their mixing ratios, these isotopic compositions can be determined without a gas‐separation procedure by calibration using prepared standard gases with variable mixed ratios and by monitoring the amounts of fragment ions. Analytical precision for δ17O and δ18O of 45 and 7 per meg, respectively, were obtained. The observed partial pressure dependency of isotopic composition may be caused by isotope fractionation during admission from the ionization chamber into the flight tube of the mass spectrometer. Copyright © 2003 John Wiley & Sons, Ltd.