The present invention refers to a method that comprises the following steps: Providing an electrically conductive tip with a terminal shelf which has between three and twenty atoms, generating a first ion beam by interacting a gas with the electrically conductive tip, providing an ion optical system, eliminating by the ion optical system some of the ions in the first ion beam to generate a second ion beam comprising ions 70% or more of which are generated via interaction of the gas with one atom of the terminal shelf of the electrically conductive tip, and interacting the second ion beam with an activating gas to promote a chemical reaction at a surface of a sample. The figure shows a gas field ion microscope system (100).