Mass spectrometry (fragmentation ratios) of DNA base molecules following 80Â keV proton impact with separation of direct ionization and electron capture processes

The first fragmentation ratios are presented for the ionization of gas-phase DNA bases by 80 keV (1.8v0 in Bohr velocity units) proton impact. Event-by-event determination of the projectile charge state post-collision enables branching ratios to be determined for electron capture (EC) by the projectile and for direct ionization (DI) of the target molecule (without projectile neutralization). Results are compared with similar experiments on uracil [Tabet et al., Phys. Rev. A 81 (2010) 012711] and water [Gobet et al., Phys. Rev. A 70 (2004) 062716]. In all cases, whereas both processes (EC and DI) produce the same fragment ion groups, greater fragmentation ratios are observed for EC than for DI. Moreover the fragmentation ratio is greater for thymine than for adenine, cytosine, and uracil.