Analytical Sciences

Precise Isotopic Measurements of Nitrogen at the Sub-Nanomole Level

Naoto TAKAHATA*, Yoshiro NISHIO**, Naohiro YOSHIDA*** and Yuji SANO *

*Department of Earth and Planetary Sciences, Faculty of Science, Hiroshima University, Kagamiyama, Higashi Hiroshima 739-8526, Japan
**Geological Institute, School of Science, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
***Institute for Hydrospheric-Atmospheric Sciences, Nagoya University, Chikusa, Nagoya 464-8602, Japan

A system for the isotopic measurement of nitrogen at the sub-nanomole level has been developed using a modified noble gas mass spectrometer, allowing 40 Ar/36 Ar, N2/40 Ar, 4 He/40 Ar and C/N ratios as well as the d 15 N value to be measured on one sample. Moreover, carbon dioxide can be collected for a d 13 C measurement. The experimental details concerning the analytical system are presented here, including the nitrogen purification procedure and its performance. The analytical errors concerning the d 15 N values and 28 N2/40 Ar ratios were estimated to be about 0.3%₀ and 3%₀ for actual samples. Using six basalt glasses, the ability of the instrument to make accurate nitrogen isotope measurements has been evaluated. The observed values of the nitrogen content, nitrogen isotopes and nitrogen/argon ratios are compatible with the data in the lit-erature. The results of some basic experiments and the observation of basalt glasses demonstrate that the new system can be applied to measuring nitrogen isotopes in geochemical and environmental samples. (Keywords: d 15 N, nitrogen isotope, argon isotope, N2/40 Ar ratio, sub-nanomole)

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