Analytical Sciences

Carbon Isotope Measurement of Extremely Low Amounts of CH₄: Application to Volcanic Gases from Satsuma-Iwojima, Japan

Masanori SATO* , Urumu TSUNOGAI, Jun-ichiro ISHIBASHI, Kenji NOTSU and Hiroshi WAKITA

Laboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo, Tokyo 113-0033, Japan

A preconcentration system attached to a GC/C/MS (gas chromatography/combustion/mass spectrometer) was developed to determine the carbon-isotope composition of methane in volcanic gases with extremely low concentration. With this system, we can analyze the carbon isotope ratio (delta¹³C/¹²C) of 0.7 - 6 nmol methane with a precision of +-1.6 permill, unlike other conventional methods without preconcentration (150 - 1000 nmol CH₄ with a precision of +-0.2 permill). This system was tested with volcanic gas samples from Satsuma-Iwojima volcano, suggesting a mixture of magmatic and thermogenic origins for methane. (Keywords: Carbon isotope, methane, volcanic gas, preconcentration, GC/C/MS (gas chromatography/combustion/mass spectrometer))

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