Collision cross sections of myoglobin and cytochrome c ions with Ne, Ar, and Kr

The energy losses of ions of myoglobin and cytochrome c in collisions with Ne, Ar, and Kr have been measured with a triple quadrupole mass spectrometer system. The results have been interpreted with drag coefficients, suitable for the case of an object moving through a low density gas, to give collision cross sections or, equivalently, projection areas. For a given charge state, these cross sections with Ne, Ar, and Kr are nearly identical. No evidence is found for substantial contributions to the cross section from ion-induced dipole forces. Comparisons of cross sections obtained with different gases and comparisons to literature cross sections measured by ion mobility suggest that a ''diffuse'' scattering model,suitable for collisions with a rough surface, gives the best description of collisions between protein ions and neutrals. (C) 1997 American Society for Mass Spectrometry.