Relative collision cross sections of organic ions.

The relative collision cross sections σ c 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 σ c value increases linearly by 0.1 per methylene group. Linear relationships exist between relative collision cross section and ionization cross section σ i, and polarizability α of the corresponding neutral molecule, but different equations are required for the description of σ c 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.