Metastable ions play important role in the background formation; therefore, the knowledge of the specific features of their emergence is necessary. In this work, we report the study of the reasons and laws of the formation of ions of this type. A procedure is proposed for the search for precursor and product ions corresponding to metastable ions that decomposed in the field-free zone between the exit from the ion source and an entrance of the magnetic mass-analyzer, and also of ions formed in the accelerating zone of the ion source. Using this procedure we could identify peaks in the mass spectrum unknown previously. © Pleiades Publishing, Ltd., 2011.