Mass spectrometry. Physicist holding a mass spectrometer used in G-SIMS (gentle secondary ion mass spectrometry) research. SIMSs are processes that analyse the surface of a solid material by firing a beam of ions to ionise it. Ions are atoms that are positively or negatively charged due to losing or gaining an electron. G-SIMS differs from other SIMS techniques in that it allows the material to be analysed under different conditions, such as using different types of ion beams. Photographed at the National Physical Laboratory in Teddington, UK.