Regular Article

Fragmentation processes of ionized 5-fluorouracil in the gas phase and within clusters^★

¹ Department of Experimental Physics, National University of Ireland Maynooth Maynooth, Co. Kildare, Ireland
² School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
³ Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Fsica, FCT – Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal
⁴ CIMAP UMR 6252 (CEA/CNRS/ENSICAEN/Université de Caen Normandie), Boulevard Becquerel, 14070 Caen Cedex 5, France

^a e-mail: peter.vanderburgt@mu.ie

Received: 26 February 2019
Revised: 14 June 2019
Published online: 27 August 2019

Abstract

We have measured mass spectra for positive ions produced from neutral 5-fluorouracil by electron impact at energies from 0 to 100 eV. Fragment ion appearance energies of this (radio-)chemotherapy agent have been determined for the first time and we have identified several new fragment ions of low abundance. The main fragmentations are similar to uracil, involving HNCO loss and subsequent HCN loss, CO loss, or FCCO loss. The features adjacent to these prominent peaks in the mass spectra are attributed to tautomerization preceding the fragmentation and/or the loss of one or two additional hydrogen atoms. A few fragmentions are distinct for 5-fluorouracil compared to uracil, most notably the production of the reactive moiety CF⁺. Finally, multiphoton ionization mass spectra are compared for 5-fluorouracil from a laser thermal desorption source and from a supersonic expansion source. The detection of a new fragment ion at 114 u in the supersonic expansion experiments provides the first evidence for a clustering effect on the radiation response of 5-fluorouracil. By analogy with previous experiments and calculations on protonated uracil, this is assigned to NH₃ loss from protonated 5-fluorouracil.