Fig. 2. The isotopic composition of methane plotted versus the ln of the fraction of methane remaining at a number of time points in closed-system incubations of methane in air over landfill soil. M represents the concentration of methane at time t and M_o represents the concentration at the initial time. Using the slopes of the lines fit to the data from different incubations (represented by the different symbols), the fractionation factor () was calculated using Eq. [3], where 1000(1/ - 1) is the slope of the line fit to the data. Solid symbols represent incubations of soil from the Hökhuvud landfill while open symbols represent soil from the Falköping landfill