Relative collision cross sections of organic ions

The relative collision cross sections σc of selected precursor ions that collide with helium at high ion kinetic energies (kiloelectronvolts) have been obtained by monitoring the ion beam attenuation at different collision pressures. A value for the total collision cross section is obtained, which includes contributions from different processes that lead to fragmentation, scattering, charge exchange, charge inversion, and stripping. For n-alkanes and n-alkylbenzenes the σc value increases linearly by 0.1 per methylene group. Linear relationships exist between relative collision cross section and ionization cross section σi, and polarizability α of the corresponding neutral molecule, but different equations are required for the description of σc in each chemical series. A simple relationship, which requires only the chemical formula of the precursor ion, has been determined that can predict all relative collision cross sections with an average error of 3.5%. The total relative collision cross section increases with the size of the ion. The experimental results are in agreement with the predictions of the hard-sphere collision theory. The approach can be applied to determine the relative collision cross sections at different ion kinetic energy regimes and collisions with different target gases