The methodology used to generate optical images of inked fingerprints is fully reported in H. Reed, et al., The Reed-Stanton press rig for the generation of reproducible fingermarks: Towards a standardised methodology for fingermark research, Sci. Justice (2015). One donor was used throughout. For the generation of marks undergoing ageing and analysed by MALDI MSI (subsequently statistically treated) no human donor was used; instead flat fingermarks” were employed. These are flat and linear patterns produced by a silicone master stamp in the press rig. The marks were generated using a latent print reference pad containing sebaceous oil secretions.