For each pulse valve, the number of molecules per pulse was calibrated by following the pressure drop in the gas supply tank when pulsing Ar with a high frequency through an empty micro-reactor at different pre-pressures. For the reactants (CO, CO2, H2, and C 18 O2) we calculated the pulse size at a certain pre-pressure via their molar masses, assuming that the impingement rate on the valve aperture determines the pulse size. Relative values of the crosssections for the detection of different gases (specific cross-sections) in the mass spectrometer were determined regularly by admitting the respective gas via an additional calibrated leak gas line from the supply tank, at a defined pre-pressure in the tank, directly into the analysis chamber. Absolute values for Ar were determined from the pulse size derived via the pressure drop in the tank as described above. For H2, CH4, H2O, and C 16 O 18 O, they were derived from the experimentally determined sensitivity factor of Ar and the tabulated sensitivity factors of these gases relative to the sensitivity factor of Ar.