Average of the mass spectra that were recorded during the methyl acetate injection to form the ice film (orange histogram), which trace the QMS mass fragmentation pattern of pure methyl acetate. We also show the average of a few spectra thatwere recorded at the beginning of the ice-film irradiation (blue histogram). Both mass spectra are scaled to mass 74, which corresponds to ionised methyl acetate for comparison. This shows that masses 74, 59, and 43 can be used to monitor methyl acetate because the radiolysis with the ion produces most contributions to peaks at lower masses. See text for details.