Natural ¹⁵N abundance measurements and atmospheric nitrogen standard calibration

Abstract

Small differences in natural isotope ratios between two compounds can be measured easily and with high precision by using mass spectrometers fitted with double (or triple) ion collectors and dual inlet systems equipped for rapid switching between reference and sample^1–3. The parameter used to describe these differences in natural ¹⁵N abundance is:

where R is the isotopic ratio of m/z = 29(¹⁴N¹⁵N⁺) to m/z = 28(¹⁴N¹⁴N⁺). Although an international standard for nitrogen has not yet been defined, atmospheric N₂ had been chosen through practical experience, even before the worldwide homogeneity of its isotopic composition was demonstrated^4–7. However, the presence of argon (≃1%) in purified air samples may, in some circumstances, modify the apparent isotope ratio of atmospheric N₂. Two different mass spectrometers in our laboratory have been used to assess the magnitude of this ‘argon effect’, which seems to be essentially an instrumental, presumably pressure-related, effect. Therefore, on mass spectrometers showing this argon effect it is imperative that a correction be applied to measurements of δ¹⁵N of atmospheric N₂. However, in some circumstances (different mass spectrometers, different tuning of the same instrument) this effect can be non-existent or negligible and might require no correction whatsoever.