Comparison of the fragmentations of uracil and 5-fluorouracil following double ionisation by 70 eV electron impact

We have performed coincidence mass spectrometry of the fragmentations of the RNA base uracil and the radiosensitizer 5-fluorouracil by 70 eV electron impact. Ionized fragments have been mass selected using a reflectron time-of-flight mass spectrometer, and a field programmable gate array has been used for the timing of the electron and ion extraction pulses and for the event-by-event detection of the ions. Double ionization results in a number of prominent fragmentations producing two singly-charged fragments with kinetic energies of up to a few eV. In most fragmentations the Coulomb explosion is accompanied by a secondary dissociation involving the loss of a neutral from one (or sometimes both) of the fragments. For both uracil and fluorouracil the strongest fragmentation produces 28 u and 43 u ions in coincidence, involving the initial loss of a CH or CF neutral, followed by the Coulomb explosion producing 56 u and 43 u ions, followed by the secondary dissociation of