SCANNING FORCE MICROSCOPY STUDIES OF SURFACE-DEFECTS INDUCED BY INCIDENT ENERGETIC MACROMOLECULAR IONS

Scanning force microscopy (SFM) has been employed to study morphological changes associated with single-ion-impacts on SiO2, highly oriented pyrolytic graphite (HOPG), and single-crystals of the amino acid L-valine. Massive biomolecular ions are generated using an electrospray ionization source and a quadrupole mass spectrometer. The target is biased for post-acceleration and resulting impact velocities are around 50 lan/s. Impacts of molecular ions with masses between 5.7 kDa (bovine insulin) and 66.3 kDa (bovine albumin) are studied. Surfaces are probed with SFM operated in both the continuous contact (repulsive force) mode and an intermittent contact mode which eliminates damaging lateral forces during scanning. The use of different SFM modes allows an assessment to be made of the role of artefacts in the obtained images. Surface topological features due to individual ion impacts were identified and had the appearance of low hillocks on SiO2 and HOPG, and shallow craters on L-valine. Results of a study of defect size versus incident projectile molecular mass are presented for approximately constant impact velocity. Slow but energetic massive polyatomic ions are expected to create regions of high energy volume density leading to cratering. Hillock formation may reflect a premature end to the crater formation process due to rapid diffusion of deposited energy into the bulk.