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

Global budgets of atmospheric trace gases are increasingly being constraine d by means of stable isotope measurements. Published analytical techniques for studying the parallel stable isotopic composition of methane (delta(13) C and delta D) require prohibitively large quantities of methane for analys is, making them unsuitable for studies where sample size is small, e.g. soi l methane fluxes, A highly sensitive static mass spectrometer has been deve loped which uniquely uses CH4 as the analyte, The method requires only 8 ng of CH4 for analysis (<10 mL ambient air), making replicated measurements o f the isotopic composition of CH4 in small samples feasible for the first t ime. This paper provides the first detailed description of the instrumentat ion and the analytical technique. The technique has been used to analyse sm all samples of air collected in Snowdonia over 21 months. The combined stab le isotopic composition (delta(17)M) ranged from 29.5 to 35.5 parts per tho usand, with an average value of 32.2 parts per thousand, and was strongly c orrelated with wind direction (p <0.01, r(2) = 0.71). Copyright (C) 1999 Jo hn Wiley & Sons, Ltd.