Initial velocity distributions of secondary ions in electronic sputtering of biomolecules

Abstract

Beams of fast heavy ions from the Uppsala EN-tandem accelerator and a time-of-flight mass spectrometer have been used to investigate the initial axial velocity distributions of secondary ions from samples of biomolecules of different sample thickness. The initial “axial” energies, the energy corresponding to the centroid of the initial axial velocity distribution, of desorbed H + and H + 2 ions were found to increase linearly with the sample thickness in the region 0 to 400 Å, whereas the initial “ axial” energy for other secondary ions, including molecular ions (M + H) + of peptides, seem to be independent of the sample layer thickness. An approximation to the time-of-flight equation at high acceleration voltage has been used to “energy” calibrate time-of-flight spectra of high mass biomolecular ions. The initial “axial” energy distribution of molecular ions of bovine insulin is presented.