Neutral-particle emission in collisions of electrons with biomolecular ions in an electrostatic storage ring

Electron-biomolecular ion collisions were studied using an electrostatic storage ring with a merging electron beam device. Biomolecular ions produced by an electrospray ion source and accelerated to 20 keV/charge were injected into the ring after being mass-analyzed. The circulating ion beam was then merged with an electron beam. Neutral reaction products in collisions of electrons with ions were detected by a micro-channel plate outside of the ring. Electron-ion collisions were studied for multiply-deprotonated oligonucleotide and peptide anions as well as singly protonated oligonucleotide and peptide cations. For peptide cations, neutrals were resonantly emitted at an electron energy of around 6.5 eV, which was almost independent of the ion masses. This is deduced to come from electron-ion recombination, resulting in the cleavage of a peptide bond. For DNA oligonucleotide cations, resonant neutral particle emission was also observed. In electron and DNA anion collisions, neutrals started to increase from definite threshold energies, where the threshold energies increased in proportion to the ion charge. The same was found for peptide anions. The origin of this phenomenon is discussed.