Complex sentence: It is one of the two principal modes of operation of SIMS, which is the mass spectrometry of ionized particles emitted by a solid (or sometimes liquid) surface upon bombardment by energetic primary particles.
Split into simpler sentences: It is one of the two principal modes of operation of SIMS. SIMS is the mass spectrometry of ionized particles which are emitted when a surface, usually a solid, though sometimes a liquid, is bombarded by energetic primary particles.