Natural N-15 abundance measurements and atmospheric nitrogen standard calibration

Two different mass spectrometers have been used to assess the magnitude of the ability of argon present in purified air samples to modify the apparent isotope ratio of atmospheric N2. This 'argon effect' seems to be essentially an instrumental, presumably pressure-related, effect. Corrections must therefore be applied to measurements of the dN-15 of atmospheric N2 when spectrometers showing this argon effect are used. The effect may be modified through tuning of the ion-source focusing.