field-free region (FFR)

In the flight path of an ion from the source of the mass spectrometer through the mass analyzer to the detector, the ion may pass through regions in which there is no specific magnetic or electric field. These are field-free regions, as in the portion of the ion path between the electric and magnetic analyzers in a double-focusing mass spectrometer, or between the source and the first sector of both single- and double-focusing mass spectrometers. Unimolecular ion dissociations in these regions can occur to give rise to signals in the mass spectrum recorded under normal conditions (metastable  ions), or can be specifically investigated.