Soft-Sputtering SIMS by Using Argon Cluster Ions

Abstract

In secondary ion mass spectrometry (SIMS) of organic molecules, the mass of molecular ions currently detectable is at best only as high as the 1000 Da, which for all practical purposes prevents the technique from being extended to apply to bio molecules with larger mass. We developed SIMS equipment capable of bombardment in which the primary ions were argon cluster ions having a kinetic energy per atom controlled down to 1eV. By applying this equipment to several peptides and proteins, the intensity of fragment ions was decreased by a factor of 10² when the kinetic energy per atom was decreased below 5 eV, and molecular ions of insulin (molecular weight: 5808), cytochrome C (molecular weight: 12327) and chymotrypsin (molecular weight: 25000) were detected without using any matrix. Furthermore, we found that adjusting the kinetic energy per atom can realize site-specific bond breaking within a molecule. Based upon the above results, a future prospect of argon-cluster projectile for SIMS is discussed.