The use of static mass spectrometry to determine the combined stable isotopic composition of small samples of atmospheric methane

Abstract

Global budgets of atmospheric trace gases are increasingly being constrained by means of stable isotope measurements. Published analytical techniques for studying the parallel stable isotopic composition of methane (δ13C and δD) require prohibitively large quantities of methane for analysis, making them unsuitable for studies where sample size is small, e.g. soil methane fluxes. A highly sensitive static mass spectrometer has been developed which uniquely uses CH4 as the analyte. The method requires only 8 ng of CH4 for analysis (<10 mL ambient air), making replicated measurements of the isotopic composition of CH4 in small samples feasible for the first time. This paper provides the first detailed description of the instrumentation and the analytical technique. The technique has been used to analyse small samples of air collected in Snowdonia over 21 months. The combined stable isotopic composition (δ17M) ranged from 29.5 to 35.5‰, with an average value of 32.2‰, and was strongly correlated with wind direction (p <0.01, r2 = 0.71). Copyright © 1999 John Wiley & Sons, Ltd.