An aerodynamic drag model for protein ions.

The energy losses of protein ions passing through a collision cell filled with inert gas have been modeled as the aerodynamic drag on a projectile at high Knudsen number. When applied to the energy loss data of Covey and Douglas (J. Am. Soc. Mass Spectrom. 1993, 4, 616-623) with drag coefficients from the gas dynamics literature, derived protein cross sections are ∼0.8 of those found with the simple collision model used by Covey and Douglas.