Study of the dependence of peptide collision cross section on the ion bunch drift velocity in nitrogen

Transport properties of gramicidin S, apamin, melittin and cytochrome C ions [M + nH] ⁿ⁺ were experimentally measured at conditions: T = 300 K, p = 1.9 Pa and in the E/N range from 40 up to 400 Td using gas filled segmented RF-quadrupole with rotational excitation of ions. Modified theoretical model of ion deceleration in collisions with gas molecules was used for the data analysis. The model is valid for hard sphere collisions and for ion mass much higher than mass of gas molecule. Using this model it is possible to simulate ion motion in gas under influence of electric fields in wide ion velocity range. Collision cross sections of the ions were calculated from the experimentally measured drift times. Calculated cross sections for most of the ions showed significant dependence on the ion drift velocity. It was supposed that long-range (polarization) ion-molecular interaction plays an essential role in the observed transport properties of the gas phase peptide ions with molecular weight in the range of 1000–3000 Da and m/z values up to 1000 Th.