a tandem mass spectrometer consisting of two quadrupole mass spectrometers in series, with a (non mass-resolving) radio frequency (RF) only quadrupole between them to act as a collision cell for collision-induced dissociation. The first (Q1) and third (Q3) quadrupoles serve as mass filters, whereas the middle (q2) quadrupole serves as a collision cell. This collision cell is an RF only quadrupole (non-mass filtering) using an inert gas such as Ar, He or N2 gas to provide collision-induced dissociation of a selected precursor ion that is selected in Q1. Subsequent fragments are passed through to Q3 where they may be filtered or scanned.