Secondary ion emission from insulin film bombarded with methane and noble gas cluster ion beams

Recent advances in large cluster projectiles for secondary ion mass spectrometry (SIMS) allow the intact ions of some protein molecules to be detected without a matrix. However, detailed mechanisms of soft-sputtering and ionization of biomolecules remain unknown. Herein we investigate the secondary ion emission from insulin films under argon, krypton, and methane cluster ion bombardment. The intact insulin ion intensity significantly decreases for (CH4)1500+ ion bombardment compared with Ar1500+ ion bombardment at the same energy range of 3.3eV/atom (or molecule), even though collisions with energetic methane clusters should generate numerous protons on the surface, which would enhance the ionization probability through proton attachment. In contrast, the intact ion intensity is almost the same for Ar2500+ and Kr2500+ cluster ion bombardment at the same energy range of 2eV/atom. These observations suggest that detailed mechanisms for the ionization and sputtering by gas cluster ions should be investigated to enhance the intact ion intensity.